To assess the regional multi-decadal to multi-centennial climate variability along the
southern Iberian Peninsula during the mid- to late-Holocene record of paleo-environmental
indicators from marine sediments were established for two sites in the Alboran Sea
(ODP-161-976A) and the Gulf of Cádiz (GeoB5901-2). High-resolution records of organic
geochemical properties and planktic foraminiferal assemblages are used to decipher
precipitation and vegetation changes as well as hydrological conditions with respect to
sea surface temperature (SST) and marine primary productivity (MPP). As a proxy for
precipitation change, records of plant-derived n-alkane composition suggest a series of
five distinct dry episodes in southern Iberia at 5.4±0.3 ka cal BP, from 5.1 to
4.9±0.1 ka cal BP, from 4.8 to 4.7±0.1 ka cal BP, from 4.4 to 4.3±0.1 ka cal BP, and at 3.7±0.1 ka cal BP. During each dry episode the
vegetation suffered from reduced water availability. Interestingly, the dry phase from
4.4 to 4.3±0.1 ka cal BP is followed by a rapid shift towards wetter conditions
revealing a more complex pattern in terms of its timing and duration than was described
for the 4.2 ka event in other regions. The series of dry episodes as well as closely
connected hydrological variability in the Alboran Sea were probably driven by NAO-like
(North Atlantic Oscillation) variability. In contrast, surface waters in the Gulf of
Cádiz appear to have responded more directly to North Atlantic cooling associated
with Bond events. In particular, during Bond events 3 and 4, a pronounced increase in
seasonality with summer warming and winter cooling is found.
Introduction
The Holocene climate is considered to be fairly stable in comparison with the large and
abrupt climatic changes during the last glacial and deglacial periods (Martrat et al.,
2007; Rasmussen et al., 2014). For the Mediterranean realm, generally long-term trends in
sea surface temperature (SST) cooling (Kim et al., 2004; Martrat et al., 2014) and
continental aridification (Fletcher and Sánchez Goñi, 2008; Ramos-Román et
al., 2018a) are described, and are superimposed with several short cold and dry
perturbations, e.g. the 8.2 ka event or the North Atlantic Bond events (Alley et al.,
1997; Bond et al., 1997; Mayewski et al., 2004). Another prominent Holocene climate
perturbation is the 4.2 ka event. This event, considered to have had a global impact, is
associated with generally colder and wetter conditions over the North Atlantic as well as
northern and central Europe and is broadly coincident with the onset of neoglacial
glacier advances in Scandinavia and the Alps (Bakke et al., 2010; Le Roy et al., 2017).
Additionally, intense droughts have been ascribed to this event for the mid-latitudes in
northern America and Eurasia including the eastern and central Mediterranean (Booth et
al., 2005; Cheng et al., 2015; Jalut et al., 2000; Magny et al., 2013). In the western
Mediterranean, the 4.2 ka event is so far insufficiently resolved in existing marine and
terrestrial archives. Several marine archives even reveal contrasting (dry vs. wet)
signals (Weinelt et al., 2015). Concerning terrestrial records, a dry event at
4.2 ka cal BP is suggested for southern Iberia (Schröder et al., 2018; Walczak et
al., 2015) while relatively wet conditions were inferred for northern Morocco (Zielhofer
et al., 2019).
Similar to the manifestation of the 4.2 ka event in the western Mediterranean region,
the driving mechanism(s) for it are not well understood. Many studies suggest NAO-like
(North Atlantic Oscillation) atmospheric variability as a potential driver of climatic change
during the mid- to late-Holocene in the area (e.g. Deininger et al., 2017; Wassenburg et
al., 2016). The NAO is the dominant driver for the modern precipitation distribution in
the western Mediterranean and particularly active during winter (Hurrell, 1995). Another
potential driving mechanism, not only of the oceanic variability, is the cyclical Bond
events (Bond et al., 1997), which are believed to weaken the thermohaline oceanic
circulation and associated northward heat transport, and are thus responsible for cooling
of the Northern Hemisphere (Bond et al., 2001; Wanner et al., 2011).
Here, we explore the mid- to late-Holocene climate development in southern Iberia on the
basis of two marine sediment cores from the Gulf of Cádiz (GeoB5901-2) and the
Alboran Sea (ODP-161-976A). Both sites are analysed for changes in terrestrial vegetation
and precipitation as well as for changes in seasonal and annual SST and marine primary
productivity (MPP). For that purpose, terrestrial n-alkane concentrations and the
Norm33 n-alkane ratio from higher-plant leaf waxes are used to decipher the continental
climate change. For the marine conditions, analyses of the alkenone-derived SST index
(U37K′) and planktic foraminiferal assemblages were used to
reconstruct annual mean and seasonal changes in SST by applying the modern analogue
technique (MAT). Changes in MPP are based on fluctuations in the content of alkenones in
the bulk sediment. Altogether, this study aims to provide new insights into the temporal
and spatial manifestation of the 4.2 ka event as well as to discuss potential driving
mechanisms by linking terrestrial and marine surface climate variability in southern
Iberia and adjacent oceans.
Study area
The Iberian Peninsula is influenced by the major Atlantic and Mediterranean atmospheric
regimes (Lionello, 2012). Today, the Atlantic climate is typically marked by relatively
cool annual air temperatures and evenly distributed precipitation throughout the year. In
contrast, the Mediterranean climate is characterized by pronounced seasonal contrasts
with a rainy winter season and a dry and hot summer season. In general, most of the
precipitation at the Iberian Peninsula occurs during the winter season (Fig. 1; Lionello,
2012). The spatial pattern of winter precipitation clearly reflects the two atmospheric
regimes (Fig. 1). The Atlantic regime, which spans along the western and northern coasts,
is characterized by high precipitation of more than 800 mm during winter. It is
associated with the westerly wind belt at the Iberian Peninsula (Zorita et al., 1992) and
to a large extent controlled by the NAO winter conditions (Hernández et al., 2015;
Hurrell, 1995). During positive NAO conditions (i.e. a large pressure difference between
the Azores High and the Icelandic Low), North Atlantic storm tracks and associated
precipitation are directed towards northern Europe, while Iberia receives less rainfall.
On the other hand, during negative NAO conditions (i.e. a small pressure difference)
storm tracks are directed towards the Iberian Peninsula, which then experiences wetter
winters. The pronounced difference in precipitation is also evident in the seasonal
variability in the river discharges in southern Iberia. For example, the average
discharge of the Guadalquivir is below about 100 hm3 month-1 from April to
October and peaks during December and January with values above
400 hm3 month-1 (Fernández-Delgado et al., 2007). Nonetheless, the
central parts of the Iberian Peninsula as well as the eastern and southern coasts under
the influence of the Mediterranean climate remain relatively dry on average with
precipitation less than 600 mm during winter (Fig. 1).
Overview maps. (a) Overview with discussed (black dots) and
shown (white dots) references at the Iberian Peninsula and studied sediment
cores. Cores of this study: GeoB5901-2 (blue dot) and ODP-161-976A (red dot).
Shown references: (1) El Refugio Cave (Walczak et al., 2015), (2) Borreguil
de la Virgen and Borreguil de la Caldera (García-Alix et al., 2018),
(3) MD95-2043 (Cacho et al., 2001; Fletcher and Sánchez Goñi, 2008;
Martrat et al., 2014; Pérez-Folgado et al., 2003), (4) Elx (Burjachs and
Expósito, 2015). Discussed references: (a) Grotte de Piste (Wassenburg et
al., 2016), (b) MD99-2339 (Salgueiro et al., 2014), (c) Lake Medina
(Schröder et al., 2018), (d) Padul (Ramos-Román et al., 2018a, b),
(e) TTR-293G (Rodrigo-Gámiz et al., 2014), (f) San Rafael
(Pantaléon-Cano et al., 2003), (g) Cabo de Gata (Burjachs and
Expósito, 2015), (h) Mazarrón (Navarro-Hervás et al., 2014),
(i) Villaverde (Carrión et al., 2016), and (j) Ejulve cave (Moreno et
al., 2017). The main river system and associated catchment of the
Guadalquivir (hatched blue area) is shown as well as the river catchments of
various small-scale rivers draining the southern Sierra Nevada area (hatched
red area). The river catchments have been downloaded from the web site of the
European Environmental Agency. The colour shading indicates the mean winter
precipitation (October to February) from 1970 to 2000 in the area. The data
were provided by WorldClim version 2 (Fick and Hijmans, 2017).
(b) Annual average precipitation (blue) and air temperature (red) at
Seville airport (1981–2010) show the high seasonality with rainy and cold
winters and hot and dry summers. Data were provided by the Spanish State
Meteorological Agency (AEMET). Mean summer (c) and
winter (d) SSTs for the period 1955–2012 are shown. The data were
provided by the World Ocean Atlas 2013 (Locarnini et al., 2013) and processed
with Ocean Data View 5.1.2 (Schlitzer, 2018). Black arrows indicate the
surface currents in the area including the gyres in the Alboran Sea. Blue and
red dots mark locations of sediment cores GeoB5901-2 and ODP-161-976A,
respectively. AC – Azores Current, IPC – Iberian Poleward Current, PC –
Portugal Current, EAG – East Alboran Gyre, WAG – West Alboran Gyre.
A strong seasonal contrast is also evident in the surface ocean in the Gulf of Cádiz,
which receives warm waters with the eastward Azores Current (AC) that turns into a
poleward surface current (Iberian Poleward Current; IPC) along the western Iberian coast
during winter (Peliz et al., 2005). During summer, however, the surface current is
directed southward and favours coastal upwelling along the western continental margin
limiting the influence of the AC, and thus causing relatively low SSTs (Fig. 1; Haynes et
al., 1993; Peliz et al., 2002). These opposite oceanographic currents limit the
seasonality recognized in the SSTs in the Gulf of Cádiz. The seasonal difference at
the core location of GeoB5901-2 is typically ∼6∘C during modern times
(Locarnini et al., 2013). Atlantic surface water passes the Strait of Gibraltar and
enters the Alboran Sea. Within the Alboran Sea it circulates in two anti-cyclonic gyres
– the West Alboran Gyre (WAG) and the East Alboran Gyre (EAG) (Lionello, 2012). At the
WAG relatively warm and fresh waters become upwelled almost continuously throughout the
year (Sarhan et al., 2000). This upwelling is accompanied by an elevated MPP at the
northern rim of the gyre (Minas et al., 1991). The modern seasonality in the Alboran Sea
is very similar to the one in the Gulf of Cádiz (∼6∘C).
Materials and methodsSediment cores and sampling
For this study two marine sediment cores from the Alboran Sea and the Gulf of Cádiz
were analysed. Sediment core ODP-161-976A (36∘12.32′ N; 4∘18.76′ W;
1108 m water depth) was retrieved in the Alboran Sea during the JOIDES RESOLUTION cruise
in 1995 (Comas et al., 1996). To achieve multi-decadal resolution, the section from 100.0
to 149.0 cm was continuously sampled at 0.5 cm distances in the IODP (International
Ocean Discovery Program) core repository at MARUM (Center for Marine Environmental
Sciences) in Bremen (Germany). The analysed section consists of homogenous clayey and
silty sediments, which appear slightly bioturbated. No hiatus or depositional event (e.g.
turbidite) has been recognized throughout the section (Comas et al., 1996). Sediment core
GeoB5901-2 (36∘22.80′ N; 7∘04.28′ W; 574 m water depth) was
retrieved during the METEOR cruise in 1999 in the Gulf of Cádiz (Schott et al.,
2000). This sediment core, containing hemipelagic mud, was sampled at 0.5 cm resolution
from 1.0 to 50.0 cm in the GeoB core repository at MARUM in Bremen (Germany). The
samples used for organic geochemical analysis were freeze-dried and homogenized prior to
analysis.
Age model
Age models of both cores are based on existing and new AMS 14C dates, all measured
at the Leibniz Laboratory at Kiel University (Table 1). Published data of cores
ODP-161-976A and GeoB5901-2 were taken from Combourieu Nebout et al. (2002) and Kim et
al. (2004), respectively. In addition, we measured six new AMS 14C dates on
monospecific planktic foraminiferal samples of Globigerinoides ruber
white + pink or Globigerina bulloides larger than 150 µm from
sediment core ODP-161-976A as well as seven new dates from sediment core GeoB5901-2. A
section from 116.25 to 124.75 cm in ODP-161-976A included three samples yielding the
same AMS 14C age (see Table 1). Since there is no evidence for strong bioturbation,
hiatuses, or turbidites, we assume a rather continuous sedimentation rate throughout the
core, and thus decided to take the date at 120.25 cm as the only age control point for
this depth interval since it was dated twice on two different planktic foraminifera. The
dates at 116.25 and 124.75 cm were not considered for the final age model in order to
avoid artificially induced calculations of extraordinary high sedimentation rates in this
thin sediment increment. The final age models have been calculated using Bacon (Blaauw
and Christen, 2011). During age model processing, all AMS 14C dates have been
calibrated using the marine13 calibration curve including a global mean reservoir
correction of 400 years (Reimer et al., 2013). Based on the AMS 14C dates, we assume
a relatively high accumulation rate for ODP-161-976A. Accordingly, we set the mean
accumulation parameter of the model to 50 yr cm-1 and run the model for 2.5 cm
thick sections to allow for a certain variability in the accumulation rates throughout
the section. The accumulation rate of GeoB5901-2 is expected to be lower in the analysed
section based on AMS 14C data. Thus, the model parameters have been set to
100 yr cm-1 and 5 cm thick sections. In the case of sediment core ODP-161-976A
the initial age model of Jiménez-Amat and Zahn (2015) was shifted significantly
within the studied section by about 700 years. The reason for this shift are the new AMS
14C dates accomplished during this study emphasizing the need of a dense dating
strategy. According to the final age model, sedimentation rates range between 9.8 and
50.0 cm kyr-1 in core ODP-161-976A and between 4.8 and 33.3 cm kyr-1 in
core GeoB5901-2, respectively. With 0.5 cm sampling intervals decadal to multi-decadal
resolution is achieved. The studied section of sediment core ODP-161-976A dates between
approximately 5.4 to 3.0 ka cal BP exhibiting a temporal resolution between 10 to
57 years per sample with continuous high resolution between 5.4 and 4.6 ka cal BP. The
analysed section of sediment core GeoB5901-2 dates between approximately 6.2 and
1.8 ka cal BP resulting in a temporal resolution between 15 and 104 years per sample.
Age models and sedimentation rates for both sediment cores are shown in Figs. 2 and 3,
respectively.
Age model of sediment cores ODP-161-976 and GeoB5901-2. All dates
from previous studies have been re-calibrated. Not considered dates are shown
in bold.
Sediment coreLab. No.DepthAMS 14C ageCal. ageDated materialReference(cm)(BP) ±σ(cal BP) ±2σGeoB5901-2KIA-145222.001840±351392±96planktic mixKim et al. (2004)GeoB5901-2KIA-530068.252485±252162±110G. ruber w+pthis studyGeoB5901-2KIA-5300512.253185±272973±106G. ruber w+pthis studyGeoB5901-2KIA-5300214.753545±263436±82G. ruber w+pthis studyGeoB5901-2KIA-1452116.003685±353590±108planktic mixKim et al. (2004)GeoB5901-2KIA-5300316.753789±273730±95G. ruber w+pthis studyGeoB5901-2KIA-5300418.253852±273801±100G. ruber w+pthis studyGeoB5901-2KIA-1452024.004500±404689±123planktic mixKim et al. (2004)GeoB5901-2KIA-5266526.254820±355116±138G. ruber w+pthis studyGeoB5901-2KIA-1451830.005035±405384±101planktic mixKim et al. (2004)GeoB5901-2KIA-5266630.755130±405486±99G. ruber w+pthis studyGeoB5901-2KIA-1451690.007035±557518±97planktic mixKim et al. (2004)GeoB5901-2KIA-13704120.007495±507955±121planktic mixKim et al. (2004)ODP-161-976CKIA-643561.001710±401259±82G. bulloidesCombourieu Nebout et al. (2002)ODP-161-976CKIA-6436103.003235±303050±106G. bulloidesCombourieu Nebout et al. (2002)ODP-161-976AKIA-53326116.254435±354636±138G. bulloidesthis studyODP-161-976AKIA-53235120.254435±304619±117G. ruber w+pthis studyODP-161-976AKIA-53327120.254480±404671±130G. bulloidesthis studyODP-161-976AKIA-53236124.754435±354636±138G. ruberw+pthis studyODP-161-976AKIA-53234132.754650±354885±84G. ruber w+pthis studyODP-161-976AKIA-53325132.754700±504945±134G. bulloidesthis studyODP-161-976CKIA-6437214.007010±507500±84G. bulloidesCombourieu Nebout et al. (2002)
Age model. The age model from ODP-161-976A (red) is based on AMS 14C dates
(red dots) from Combourieu Nebout et al. (2002) and new AMS 14C dates are from this
study (orange dots). Black crosses mark AMS 14C dates considered outliers. Black
dotted line indicates the shift of the previous age model from Jiménez-Amat and
Zahn (2015). The Age model of GeoB5901-2 (blue) is based on AMS 14C datings done by
Kim et al. (2004) (dark blue dots) and new AMS 14C dates accomplished during this
study (light blue dots). The red and blue shaded area indicates the 95 % probability
of the age model for ODP-161-976A and GeoB5901-2, respectively. All AMS 14C dates
are listed in Table 1.
Data of marine sediment cores ODP-161-976A (red) and GeoB5901-2 (blue).
(a) Terrestrial n-alkane concentration (ΣC27–33)
indicative of precipitation change. Pearson's correlation coefficient has been calculated
based on 100-year time slices for two periods (see Supplement for further information).
(b) Norm33 n-alkane ratio showing plant water stress. (c) Alkenone
and planktic-foraminifera-based sea surface temperature (SST) reconstruction from
ODP-161-976A (black – summer; grey – winter; red – annual mean). Error bars indicate
the uncertainty in the calibration for the annual mean (red) and seasonal (black)
reconstructions. (d) Sedimentation rate of both cores are shown for comparison.
Coloured dots at the bottom show AMS 14C age control points and associated 2σ errors.
Organic geochemical analysis and calculations
Lipids were extracted from the freeze-dried and finely ground sediment samples with an
accelerated solvent extractor (ASE-200, Dionex) at 100 bar and 100 ∘C using a
9:1 (v/v) mixture of dichloromethane (DCM) and methanol. After extraction samples
were de-sulfured by stirring for 30 min with activated copper. The de-sulfured lipids
were subsequently separated by silica gel column chromatography using activated silica
gel (450 ∘C for 4 h) into neutral (hexane) and polar (DCM) fractions containing
n-alkanes and alkenones, respectively. The neutral fraction was further separated using
silver nitrate (AgNO3) coated silica gel. Samples were then left at room
temperature for approximately 24 h for homogenization.
Afterwards, n-alkanes were analysed by gas chromatography (GC) using an Agilent 6890N
gas chromatograph equipped with a Restek XTI-5 capillary column
(30 m × 320 µm × 0.25 µm) and a flame ionization
detector (FID) at the Institute of Geosciences, Kiel University. Example chromatograms
are provided in the Supplement of this paper. n-Alkanes were identified by the
comparison of their retention times with an external standard containing a series of
n-alkane homologues of known concentration. On this basis, n-alkanes were also
quantified using the FID peak areas calibrated against the external standard. For
environmental interpretation the sum of terrestrial-sourced odd n-alkane homologues
C27 to C33 is used. The mean analytical error (2σ) is 7.0 ng g-1
sediment based on replicate analyses (n=62).
Various studies used ratios among individual n-alkane homologues as the environmental
sensitive parameter, e.g. the Norm33 ratio (e.g. Herrmann et al., 2016). The Norm33 ratio
is calculated by the following equation:
Norm33=C33/C29+C33,
where CX is the peak area of the n-alkane in the chromatogram with x being the
number of carbon atoms. The mean analytical error (2σ) is 0.004 based on replicate
analyses (n=62).
Alkenones were analysed by multi-dimensional double gas column chromatography (MD-GC) set
up with two Agilent 6890 gas chromatographs. The compounds (C37:2 and
C37:3) were quantified by calibration to an external standard. The alkenone
concentration is derived from the sum of the C37:2 and C37:3 isomers
with a mean analytical error (2σ) of 6.9 ng g-1 sediment. The alkenone
unsaturation index (U37K′) was obtained by using the peak areas of
the aforementioned compounds by applying the equation of Prahl and Wakeham (1987):
U37K′=C37:2/C37:2+C37:3,
where CX represents the respective peak area in the chromatogram. The
U37K′ index was subsequently transferred into annual
mean SST using the calibration of Müller et al. (1998):
SST(∘C)=U37K′-0.044/0.033.
The laboratory internal analytical error is approximately 0.12 ∘C,
while the error of the calibration is 1.5 ∘C.
Planktic foraminiferal analysis and the modern analogue technique
For foraminiferal analysis sediment samples of approximately 10 cm3 were washed over
63 µm sieves, dried at 40 ∘C, and subsequently dry-sieved for larger
fractions. Planktic foraminifera assemblages were analysed in the size fractions >150µm enabling the application of commonly used transfer techniques for SST
reconstructions based on relative abundances of 26 taxonomic categories within the
assemblage, following the concept by Pflaumann et al. (1996). For reliable assemblage
counts samples were dry-split into aliquots of at least 300 specimens with a Kiel dry
sample splitter. For SST estimates we used the SIMMAX non-distance-weighted modern
analogue technique, using a similarity index of >0.963 and based on 10 closest
analogues (Pflaumann et al., 2003). Because the study sites are influenced by Atlantic
and Mediterranean ocean circulation, we combined an updated North Atlantic core-top
database (Kucera et al., 2005a, b; Salgueiro et al., 2014) and the Mediterranean database
(Hayes et al., 2005) with modern temperature at 10 m water depth taken from the World
Ocean Atlas 1998 (Salgueiro et al., 2014). Seasonal temperatures are averaged for
Northern Hemisphere summer (July to September) and winter (December to February). This
method, applied on 1212 core-top samples from the Atlantic and the Mediterranean, yields
a root mean square error predicted accuracy of ±1.3∘C for summer and
winter seasons.
ResultsODP-161-976A
Terrestrial proxies. The terrestrial n-alkane concentration varies between 67
and 714 ng g-1 sediment exhibiting no long-term trend across the covered time
period from 5.4 to 3.0 ka cal BP (Fig. 3). Three distinct concentration minima below
200 ng g-1 sediment can be observed at 5.4 ka cal BP, from 5.0 to
4.9 ka cal BP, and from about 4.8 to 4.7 ka cal BP. Less distinct minima are
observed between 4.4 and 4.3 ka cal BP and at about 3.7 ka cal BP. A sharp increase
towards high concentrations of up to 617 ng g-1 sediment is evident at about
4.2 ka cal BP. n-Alkane concentrations appear to remain generally above
450 ng g-1 sediment until 3.8 ka cal BP, when concentrations sharply decrease
towards 209 ng g-1 sediment at 3.7 ka cal BP. Afterwards, n-alkane
concentrations reveal a slightly increasing trend towards 3.0 ka cal BP. The Norm33
ratio exhibits no trends and varies between 0.29 and 0.49 (Fig. 3). Four sharp increases
towards values greater than 0.42 can be recognized at about 5.4 ka cal BP, from 5.0 to
4.9 ka cal BP, from 4.8 to 4.7 ka cal BP, and at about 4.6 ka cal BP.
Apparently, the Norm33 maxima parallel the n-alkane concentration minima between 5.4
and 4.7 ka cal BP.
Marine proxies. The alkenone derived annual mean SST remains quite stable
between 18.9 and 20.0 ∘C (Fig. 3). No trends are visible in the analysed time
period. Only weak maxima in the annual mean SST of 0.5 to 1.0 ∘C in amplitude
are apparent at about 5.4, 3.3, and 3.0 ka cal BP as well as between 5.1 and
4.6 ka cal BP at about 100-year intervals (Fig. 4). All foraminifera-derived seasonal
SST reconstructions suggest similar stable temperature conditions without any obvious
trends between 5.4 and 3.0 ka cal BP. Summer SSTs vary around 22.8±0.1∘C with several cooling episodes of up to 1 ∘C at about 5.0, 4.6,
4.3, 3.6, 3.3, and 3.2 ka cal BP. Winter SSTs generally vary around 15.0±0.1∘C with weak warming episodes of up to 1 ∘C at about 5.0, 4.6,
4.3, 3.6, 3.3, and 3.2 ka cal BP. Thus, the winter SST maxima and summer SST minima
are contemporaneous and result in a decreasing seasonal SST difference during these
periods. Notably, the seasonal SST maxima or minima, respectively, do not parallel the
SST maxima observed in the annual mean reconstruction. The alkenone concentration varies
between about 224 and 440 ng g-1 sediment and shows an increasing trend from 5.4
to 4.0 ka cal BP followed by a decreasing trend towards 3.0 ka cal BP (Fig. 4).
These trends are superimposed by several minima from 5.4 to 5.3 ka cal BP; around 5.2,
5.0, and 4.8 ka cal BP; from 4.7 to 4.6 ka cal BP; at 3.8 ka cal BP; and from
3.1 to 3.0 ka cal BP. The alkenone concentration is to some degree correlated (r=0.61) to the annual mean SST across the studied time period (see Supplement) with warmer
annual mean SSTs paralleled by decreased alkenones content at about 5.4, 5.2, 4.9, 4.7,
and 4.6 ka cal BP, and from 3.1 to 3.0 ka cal BP (Fig. 4).
Potential driver of atmospheric and Alboran Sea climate variability.
(a) NAO reconstruction (Olsen et al., 2012). (b) Terrestrial n-alkane
concentrations from ODP-161-976A (red) and GeoB5901-2 (blue, this study).
(c) Annual mean SST from ODP-161-976A (this study). (d) Alkenone
concentration from ODP-161-976A indicative of MPP variability (this study). Thick red
lines show the 5-point running means. Orange bars indicate dry phases observed in this
study, and grey bars indicate periods of annual mean SST maxima and MPP minima at the
core location of ODP-161-976A.
GeoB5901-2
Terrestrial proxies. The terrestrial n-alkane concentration varies between 9
and 535 ng g-1 sediment and has an increasing trend towards younger ages between
5.4 and 3.0 ka cal BP (Fig. 3). During the studied time period at least three periods
of decreasing n-alkane concentration below 100 ng g-1 sediment are found from
5.1 to 4.7 ka cal BP, from 4.4 to 4.3 ka cal BP, and at about 3.7 ka cal BP. The
oldest concentration minimum reveals a W-shaped pattern with increasing concentrations of
up to 110 ng g-1 sediment at about 4.9 ka cal BP. An additional concentration
minimum (46 ng g-1 sediment) at 5.2 ka cal BP is only corroborated by a single
data point, and thus not robust. The Norm33 varies without any trends between 0.31 and
0.51 showing high amplitude maxima with values above 0.42 at about 5.0, 4.8, 4.3, and
3.7 ka cal BP (Fig. 3). Notably, the Norm33 reveals maxima during periods of low
terrestrial n-alkane concentration.
Marine proxies. Alkenone-derived annual mean SSTs vary without any trends
between 5.5 and 3.0 ka cal BP and reveal SSTs around 20.4±0.3∘C with
the exception of a period from about 4.3 to 3.9 ka cal BP, where SST vary around 21.6±0.6∘C with maximum SST being 22.7 ∘C (Fig. 5).
Foraminifera-derived summer SSTs reveal a slight decreasing trend towards the present
between 6.2 and 1.8 ka cal BP. Notably, between about 6.2 and 4.0 ka cal BP the
variability in summer SST is quite high with a mean of 22.4±0.7∘C
superimposed by several warm events of up to 23.9 ∘C that occur at 6.0, 5.8,
5.5, 4.7, and 4.1 ka cal BP. Also, the variability and the amplitudes of the warm
events decrease towards the present. After 4.0 ka cal BP summer SSTs vary around 21.7±0.2∘C. Winter SSTs reveal a very similar pattern compared to the summer
SSTs with high variability (16.0±0.7∘C) until 4.0 ka cal BP and less
(16.8±0.2∘C) afterwards. Overall, there is a warming trend from about 16
to 17 ∘C towards younger ages with major cooling episodes at 6.0, 5.8, 5.5, 4.7,
and 4.1 ka cal BP. Winter SSTs decrease below 15.0 ∘C during most of these
periods. The observed SST events in the seasonal reconstructions (summer and winter) are
contemporaneous and result in an increasing seasonal difference of up to 9.3 ∘C
during these events (Fig. 5). Overall, the seasonal difference is decreasing towards
younger ages from 6.4 to 4.7 ∘C. SST events within the seasonal reconstructions
do not parallel any events in the alkenone-derived annual mean SST. The warm period
observed in the annual mean data between 4.3 and 3.9 ka cal BP is not visible in the
seasonal data. Also, the annual mean SST is very close to summer conditions, even
exceeding those around 4.2 ka cal BP.
Oceanic variability in the Gulf of Cádiz. (a) Stacked
hematite-stained grain percentages from marine sediment cores in the North
Atlantic showing Bond events (BEs) 2 to 4 (Bond et al., 2001).
(b) Seasonal SST difference (winter – summer) deduced from the
GeoB5901-2 foraminiferal data. (c) Summer insolation at
35∘ N (Laskar et al., 2004). (d) Alkenone and
planktic-foraminifera-based SST reconstruction from GeoB5901-2 (black –
summer; grey – winter; blue – annual mean). Error bars indicate the
uncertainty in the calibration for the annual mean (blue) and seasonal
(black) reconstructions.
Comparison of both sediment cores
In general, a good agreement between the n-alkane concentration in both sediment cores
is apparent (Fig. 3). This is corroborated by the high correlation coefficient (r=0.82) for the period between 5.5 and 3.6 ka cal BP. Since the correlation calculation
is based on 100-year time slices (see Supplement), there is not such a robust correlation
in the younger part between 3.6 and 3.0 ka cal BP (r=0.04). Nonetheless, visual
inspection implies a good agreement between both records when only the more pronounced
minima and maxima are considered for this younger interval. The n-alkane concentration
minima observed at about 4.3 and 3.7 ka cal BP appear to be contemporaneous in both
regions. Also, the two events from about 5.0 to 4.9 ka cal BP and from 4.8 to
4.7 ka cal BP in ODP-161-976A as well as the W-shaped event between 5.1 and
4.7 ka cal BP in GeoB5901-2 correspond to a large extent. Only the drop towards low
n-alkane concentrations between 5.1 and 5.0 ka cal BP is offset by about 100 years
(Fig. 3). Bearing in mind the chronological uncertainties around 5.0 ka cal BP of ±110 years and ±160 years in the age model of GeoB5901-2 and ODP-161-976A,
respectively, this offset is likely chronological bias. The Norm33 maxima of both
sediment cores correspond well between 5.4 and 4.7 ka cal BP (Fig. 3).
With respect to absolute SST values and trends in the temperature reconstructions,
differences between both cores are larger than for the terrestrial biomarker records. The
alkenone-based SSTs as well as the foraminifera-derived winter SSTs were about
1 ∘C warmer in the Gulf of Cádiz, while summer SSTs were about 1 ∘C
warmer in the Alboran Sea. The most apparent difference between both regions is
noticeable in the seasonal warm/cold events, which do not agree in their timing, as is
the same for the much more moderate events observed in the alkenone-based SST records.
Also, the overall trend in the difference in seasonal SST estimates is not similar in
both regions. While in the Gulf of Cádiz (GeoB5901-2) the seasonal difference
increases (i.e. summer warming and winter cooling), the seasonal difference in the
Alboran Sea record (ODP-161-976A) does not change significantly.
DiscussionTerrestrial climate conditions in southern Iberia
Terrestrial plants synthesize long-chain n-alkanes in their leaves as coatings for
protection against water loss (Eglinton and Hamilton, 1967). These long-chain n-alkanes
are either eroded directly from the leaves by wind or deposited in the soils in autumn.
Afterwards, n-alkanes are removed from the atmosphere or eroded from the soils by rain
and are further transported into the marine realm via aeolian and riverine transport
(e.g. Bird et al., 1995; Conte and Weber, 2002; Schreuder et al., 2018). The fraction of
the riverine input in coastal areas is usually much higher compared to the aeolian input.
Therefore, terrestrial n-alkane concentrations have already been successfully applied
to study the riverine input in marine settings in the Mediterranean (Abrantes et al.,
2017; Cortina et al., 2016; Jalali et al., 2016, 2017). More specifically,
Rodrigo-Gámiz et al. (2015) have shown from radiogenic isotopes that in the Alboran
Sea the riverine dominates the aeolian input during the studied time period. Because of
the dominant riverine transport mechanism, it is assumed that both proxies integrate
spatially over the river catchment areas shown in Fig. 1, and thus indicate climate
conditions for the entire southernmost Iberian Peninsula. The dominant catchment for
sediment core GeoB5901-2 is the one of the Guadalquivir river draining into the Gulf of
Cádiz. The smaller mountainous rivers draining the southern Sierra Nevada area are
considered more relevant for sediment core ODP-161-976A. A delivery of material from the
Guadalquivir with the inflow of the Atlantic water through the Strait of Gibraltar cannot
be excluded though. Since the river discharge and also the plant growing season is
strongly coupled to precipitation and the rainy season at the Iberian Peninsula in winter
(Fig. 1; Lionello, 2012), the n-alkane proxies are probably biased towards the winter
season.
Consequently, intervals of very low n-alkane concentration in the studied cores are
interpreted as periods of dry winters, which occurred in southern Iberia at 5.4±0.3 ka cal BP, from 5.1 to 4.9±0.1 ka cal BP, from 4.8 to 4.7±0.1 ka cal BP, from 4.4 to 4.3±0.1 ka cal BP, and at 3.7±0.1 ka cal BP (Fig. 3). Drier conditions at about 5.5 ka cal BP are well in phase
with the end of the African Humid Period (deMenocal et al., 2000). This transition
appears to be marked by an aridity event as evidenced by speleothem data from El Refugio
Cave and Grotte de Piste (Walczak et al., 2015; Wassenburg et al., 2016) and high
charcoal concentration in Cabo de Gata between about 5.4 and 5.3 ka cal BP. Also, a
contemporaneous remarkable decrease in Mediterranean forest was noticed by
Ramos-Román et al. (2018b) from 5.5 to 5.4 ka cal BP. Schröder et al. (2018)
describe a drought event centred at about 5.3 ka cal BP in Lake Medina (SW Iberia).
The dry phases between about 5.1 and 4.9 and from 4.8 to 4.7 ka cal BP observed in
this study are corroborated by regional speleothem records (Moreno et al., 2017;
Wassenburg et al., 2016). A dramatic forest decline occurred between 5.0 and
4.5 ka cal BP in SE Iberia (Pantaléon-Cano et al., 2003) along with high charcoal
concentrations at Cabo de Gata indicating more frequent wild fires (Burjachs and
Expósito, 2015). Moderate forest declines are found in pollen records from the
Alboran Sea and Elx sequence (Fig. 6; Burjachs and Expósito, 2015; Fletcher and
Sánchez Goñi, 2008). The following period between 4.4 and 3.8 ka cal BP
comprising the 4.2 ka event is discussed in more detail in Sect. 4.2. The youngest dry
phase found in this study at about 3.7 ka cal BP has also been inferred from pollen
data at Elx and Villaverde (Burjachs and Expósito, 2015; Carrión et al., 2016),
while pollen data from the Alboran Sea indicate a moderate forest decline (Fletcher and
Sánchez Goñi, 2008). Additional evidence for a severe dry phase around
3.7 ka cal BP again stems from speleothem data (Moreno et al., 2017; Walczak et al.,
2015).
Proxy data from the Iberian Peninsula. (a) Speleothem density data from
El Refugio Cave (Walczak et al., 2015). (b) Pollen data from Elx sequence
(Burjachs and Expósito, 2015; Burjachs et al., 1997) and marine sediment core
MD95-2043 (Fletcher and Sánchez Goñi, 2008). (c) Norm33 n-alkane
ratios from Borreguil de la Caldera and Borreguil de la Virgen. These data have been
calculated on the basis of n-alkane raw data from García-Alix et al. (2018).
(d) Norm33 n-alkane ratios from ODP-161-976A (red) and GeoB5901-2 (blue, this
study). (e) Terrestrial n-alkane concentrations from ODP-161-976A (red) and
GeoB5901-2 (blue, this study). The orange bars indicate dry phases observed in this
study. The locations of all the references are shown in Fig. 1. CT number (HU) are
computed tomography numbers (Hounsfield units).
Notably, in the GeoB5901-2 data all dry episodes are paralleled by Norm33 maxima, while
in the ODP-161-976A record the dry phases at about 5.4, 4.9, and 4.7 ka cal BP
coincide with Norm33 maxima (Fig. 3). A tendency towards higher n-alkane chain lengths,
as revealed by increasing Norm33 values, is an indication for warmer and/or drier
environmental conditions (Bush and McInerney, 2013; Leider et al., 2013; Rommerskirchen
et al., 2006). A local study from southern Iberia further shows that increasing chain
lengths is a function of decreasing water availability (García-Alix et al., 2017).
Consequently, the Norm33 maxima observed in this study indicate high water-stress for the
plants in relation to the dry climate episodes. This is supported by Sierra Nevada bog
sediments Borreguil de la Virgen and Borreguil de la Caldera (Fig. 6; García-Alix et
al., 2018), which also show Norm33 maxima during such dry episodes. It has further been
shown that Mediterranean forests declined while scrubs expanded in southern Iberia during
the respective dry phases (Fletcher et al., 2007; Ramos-Román et al., 2018a, b).
Accordingly, the dry episodes caused a noticeable response in the vegetation, which
suffered from decreasing water availability. This further implies that the vegetation in
southern Iberia at that time was very sensitive to winter precipitation changes.
The 4.2 ka event
The manifestation of the 4.2 ka event across the Mediterranean is currently intensively
debated with a particular focus on the western part. In the eastern and central
Mediterranean many studies suggest a dry phase in the time window between 4.4 and
3.8 ka cal BP (e.g. Calò et al., 2012; Cheng et al., 2015; Finné et al., 2017;
Zanchetta et al., 2016). In the western Mediterranean evidence for a dry phase associated
with the 4.2 ka event comes from northern Algeria (Ruan et al., 2016) and pollen data
from southern Iberia (Ramos-Román et al., 2018a). Furthermore, a drastic forest
opening in SE Iberia is indicated from pollen sequences at Cabo de Gata around
4.4 ka cal BP and at Elx at about 4.3 ka cal BP (Fig. 6; Burjachs and Expósito,
2015). Additional indications for a dry phase coinciding with the 4.2 ka event come from
lithological analyses in SE Iberia (Navarro-Hervás et al., 2014), a hiatus in pollen
data from SW Iberia (Schröder et al., 2018), and speleothem data (Moreno et al.,
2017; Walczak et al., 2015; Wassenburg et al., 2016).
Compared to the prolonged dry phase between about 4.4 and 3.8 ka cal BP recorded in
speleothems from the Italian Peninsula and Algeria (Ruan et al., 2016; Zanchetta et al.,
2016), our data indicate a more complex environmental pattern around the 4.2 ka event.
The terrestrial n-alkane concentrations suggest a brief dry period from about 4.4 to
4.3 ka cal BP followed by a rapid return to relatively moister conditions after about
4.2 ka cal BP, which lasted until approximately 3.8 ka cal BP. The Norm33 data,
further, shows a peak in the GeoB5901-2 at about 4.3 ka cal BP indicating that plants
in the Guadalquivir basin were suffering from water stress. Contrastingly, the
intermediate drop in n-alkane concentration in ODP-161-976A from the Alboran Sea is not
paralleled by a Norm33 maximum. This might imply that the dry episode between about 4.4
and 4.3 ka cal BP was more severely manifested in the lowlands of the Guadalquivir
basin compared to the high altitudes of the Sierra Nevada. Altogether, our
reconstructions show that the onset of the dry phase at about 4.4 ka cal BP is in line
with proxy data from the central Mediterranean, while the rapid return to relatively
moister conditions at about 4.2 ka cal BP lasting until about 3.8 ka cal BP
highlights regional differences in the duration of the 4.2 ka event compared to the
prolonged dry phase associated with the 4.2 ka event in Italian and Algerian speleothems
(Ruan et al., 2016; Zanchetta et al., 2016). This interpretation is corroborated by dry
conditions at 4.4 ka cal BP and wet conditions at 4.2 ka cal BP found in ostracod
shells from northern Morocco (Zielhofer et al., 2017, 2019).
Hydrological conditions in the Gulf of Cádiz and the Alboran Sea
In contrast to the terrestrial climate variability our marine reconstructions suggest
fairly stable conditions between 5.4 and 3.0 ka cal BP in the Alboran Sea and between
6.2 and 1.8 ka cal BP the Gulf of Cádiz. The maximum range in the annual mean
values based on alkenones is about 1 ∘C, and thus within the calibration
uncertainty. The low temporal resolution of previously studied cores from the area (below
about 115 and 250 years for annual mean and seasonal data, respectively) further hampers
the comparison with observed SST events. Therefore, a careful interpretation of the
alkenone-based SST results is required.
Alkenone-derived SSTs from sediment core ODP-161-976A suggest annual mean temperatures
between 18.9 and 20.0 ∘C (Fig. 4), which exceed the modern values of about
18.0 ∘C (Locarnini et al., 2013). An overall cooling trend over the course of
the mid- to late-Holocene in response to decreasing insolation (Fig. 5) is known on the
regional as well as on the global scale (e.g. Cacho et al., 2001; NGRIP-Members, 2004),
thus corroborating warmer SSTs during the mid-Holocene compared to modern SSTs. Annual
mean SST values around 19 ∘C at that time were also reconstructed by other
studies from the Alboran Sea (Ausín et al., 2015; Cacho et al., 2001; Martrat et
al., 2014; Pérez-Folgado et al., 2003; Rodrigo-Gámiz et al., 2014). Moreover,
Cacho et al. (2001) described a cold SST event of about 1.5 ∘C in the Alboran
Sea between 5.94 and 4.75 ka cal BP. Our data from the Alboran Sea shows no cooling
event of similar magnitude between 5.4 and 4.7 ka cal BP. Bearing in mind the error of
the calibration there might be a cooling at about 5.3 ka cal BP in the order of just
0.5 ∘C, but surface water conditions between 5.4 and 4.7 ka cal BP appear to
have even been warmer compared to the period afterwards. In the Gulf of Cádiz
alkenone-based SST values vary between 20.0 to 22.7 ∘C and are warmer compared
to recent annual mean temperatures (18.7 ∘C; Locarnini et al., 2013). Higher
SSTs during the mid-Holocene might be partly a consequence of higher insolation during
this period. However, previous alkenone analyses of Kim et al. (2004) on the same
sediment core yielded SST values between 19 and 20 ∘C using a different SST
calibration of Prahl et al. (1988), which would result in slightly cooler SSTs for our
data as well. Additionally, annual mean SSTs around 20 ∘C in the Gulf of
Cádiz were also found by Cacho et al. (2001).
Summer temperatures. Reconstructed summer SSTs based on foraminiferal assemblage
variations vary around 22.8 ∘C in the Alboran Sea and around 22.4 and
21.7 ∘C before and after approximately 4.0 ka cal BP, respectively, in the
Gulf of Cádiz. Alboran Sea summer SSTs are exceeding modern ones (21.4 ∘C;
Locarnini et al., 2013) likely due to higher insolation forcing during the mid-Holocene.
This is even better demonstrated by the summer SST record in the Gulf of Cádiz. Here,
summer SST estimates are progressively cooling towards mean summer SSTs around
21.7 ∘C, which perfectly agree with modern summer temperatures in the Gulf of
Cádiz (21.7 ∘C; Locarnini et al., 2013). Reconstructed Alboran Sea summer
SSTs are supported by Rodrigo-Gámiz et al. (2014), who estimated summer SSTs varying
around 22 ∘C between 5.5 and 3.0 ka cal BP. Pérez-Folgado et al. (2003)
found even warmer summer SSTs between 24 and 25 ∘C in the Alboran Sea. This
difference may be a result of the Pérez-Folgado et al. (2003) data calibrated against
August temperatures only, while our method used the mean of the summer season
(July–September). These authors also found a 0.5 ∘C cooling between about 5.0
and 4.5 ka cal BP, when our data implies two cold events of approximately
1 ∘C at about 5.0 and 4.6 ka cal BP. In the Gulf of Cádiz, in sediment
core MD99-2339 (Salgueiro et al., 2014), summer SSTs vary between approximately 22 and
24 ∘C, exhibiting a cooling trend towards the present, which is in harmony with
our summer SST reconstructions deduced from GeoB5901-2.
Winter temperatures. Winter SSTs in the Alboran Sea vary around ca.
15.0 ∘C between 5.4 and 3.0 ka cal BP and agree with modern conditions
(15.4 ∘C; Locarnini et al., 2013). Moreover, winter SSTs (February) around
14.5 ∘C during the mid- to late-Holocene reported by Pérez-Folgado et
al. (2003) support our results. Winter SSTs between 16 and 17 ∘C between 6.2 and
1.8 ka cal BP in the Gulf of Cádiz are also in good agreement with the modern
winter SSTs of 16.0 ∘C (Locarnini et al., 2013). Slightly colder winter SSTs
between 15 and 16 ∘C were reconstructed from dinocyst assemblages in the
neighbouring core MD99-2339 (Penaud et al., 2016).
Potential drivers of terrestrial and oceanic climate
variability
Under modern conditions the NAO is responsible for much of the variability in winter
precipitation at the Iberian Peninsula (Hurrell, 1995; Zorita et al., 1992). Many
paleo-climatic studies provide evidence for NAO-like climate variability since the
mid-Holocene (Abrantes et al., 2017; Deininger et al., 2017; Olsen et al., 2012). We thus
compare the observed sequence of dry climate episodes to a reconstruction of NAO-like
variability from Lake SS1220 in Greenland (Fig. 4; Olsen et al., 2012). All dry phases
observed in this study can be associated with positive excursions in the NAO
reconstruction. Moreover, it is evident that a wetter period between about 4.2 and
3.8 ka cal BP coincides with a period of more stable, less positive NAO-like
conditions. Accordingly, we conclude that the dry phases observed between 5.4 and
3.0 ka cal BP in southern Iberia were caused by more dominant positive NAO-like
conditions. Yet, other atmospheric circulation patterns like the Scandinavian and the
east Atlantic patterns also influence precipitation variability at the Iberian Peninsula
(Abrantes et al., 2017; Hernández et al., 2015). This may explain why our records do
not reveal dry episodes between 4.7 and 4.5 ka cal BP and at about 3.1 ka cal BP
despite the NAO reconstruction suggesting a strong positive circulation mode for these
times. Interestingly, at around 5.2, 4.6, and 3.0 ka cal BP we found contemporaneous
minima in the MPP and maxima in annual mean SSTs in the Alboran Sea (Fig. 6). This
linkage is also visible in the dry phases at about 5.4 ka cal BP and between 5.0 and
4.8 ka cal BP, while during those at about 4.3 and 3.7 ka cal BP the warming of the
annual mean SSTs was more pronounced. This linkage was previously described by Ausín
et al. (2015) since 7.7 ka cal BP in the Alboran Sea and can also be related to NAO
forcing. Currently, the inflow of the Atlantic Waters through the Strait of Gibraltar is
strong under negative NAO conditions, and thereby its flow within the Alboran Sea is
shifted southwards. This promotes intensified upwelling of cold and nutrient-rich waters,
which also result in higher MPP (Sarhan et al., 2000). During positive NAO conditions, on
the other hand, the inflow of the Atlantic Waters is weakened resulting in a more stable
water column with limited upwelling, and thus low MPP (Sarhan et al., 2000). Despite the
calibration uncertainty in the annual mean SST reconstructions, a similar mechanism is
also indicated by our data during the mid- to late-Holocene. Accordingly, we conclude
that a NAO-like variability was coupled to the terrestrial variability in southern Iberia
as well as for the surface water variability in the Alboran Sea between 5.4 and
3.0 ka cal BP.
It has further been suggested that NAO-like variability in the past was responsible for
oceanic changes in the subtropical Atlantic such as the position of the Azores Front
(Goslin et al., 2018; Repschläger et al., 2017) as well as for increased upwelling
and related SST changes along the western Iberian margin (Abrantes et al., 2017). In more
detail, Abrantes et al. (2017) suggest that warm winters and cool summers during the
Medieval Climate Anomaly were related to positive NAO-like conditions. For the time
interval between 6.2 and 1.8 ka cal BP we cannot confirm a general link between the
NAO and the hydrological conditions in the Gulf of Cádiz. Interestingly, we observe
two notable differences between the hydrological conditions in the Gulf of Cádiz and
the Alboran Sea: (1) the summer/winter SST variability in the Gulf of Cádiz is
opposite, resulting in a greater seasonality during seasonal SST events, while
seasonality appears to decrease during summer/winter events in the Alboran Sea within
methodological error, and (2) the observed seasonal warming/cooling events in both
adjacent basins are not contemporaneous. This suggests that different mechanisms are
driving the hydrological variability in the Gulf of Cádiz and the Alboran Sea. The
pronounced seasonal SST variability in the Gulf of Cádiz is in general agreement with
the stacked ice-rafted debris (IRD) records for the North Atlantic (Bond et al., 2001;
Fig. 5). This implies that larger seasonal SST contrasts in the Gulf of Cádiz relate
to periods of enhanced iceberg discharge in the North Atlantic during Bond events 3 and
4. During Bond events, cold polar water masses have been advected as far south as the
latitude of Britain (Bond et al., 1997). Our data suggests that this regime resulted in
colder winter temperatures in the Gulf of Cádiz as well. Moreover, the Bond events
likely weakened the deep water formation in the North Atlantic by limiting the oceanic
heat transport towards the north (Bond et al., 2001; Wanner et al., 2011).
Repschläger et al. (2017) further proposed that early-Holocene freshwater forcing
from the Laurentide ice sheet may have resulted in a weak subsurface heat transport from
the subtropical gyre (STG) towards the North Atlantic, and subsequently in a subsurface
heat storage within the STG. Following these mechanisms, we hypothesize that during Bond
events 3 and 4 the northward heat transport was blocked due to freshwater forcing in the
North Atlantic, which in turn results in a heat storage within the STG. During summer,
when the Intertropical Convergence Zone (ITCZ) moved northwards, these heated water
masses probably reached the Gulf of Cádiz via the Azores Current resulting in warmer
summer temperatures. These warmer water masses could also reach the Alboran Sea through
the Strait of Gibraltar, but are not recorded within our data because summer temperatures
in the Alboran Sea were generally warmer by about 1 ∘C during the studied
period. Interestingly, it seems that annual mean SSTs in the Gulf of Cádiz warmed by
up to 2 ∘C during Bond event 3 (Fig. 5). This might rather imply a growing
season shift of the alkenones producing coccolithophores from spring to summer rather
than a warming of the annual mean SSTs. Such a reaction of the coccolithophores might be
also indicated to a lesser extent during Bond event 2 when annual mean SSTs warmed by
approximately 1 ∘C at about 3.2 ka cal BP. Our data does not cover the whole
interval though. Also, Bond event 4 is not entirely covered so that this interpretation
remains hypothetical. But all in all, the influence of cold water from the North Atlantic
during winter along with the influence of warmer subtropical water masses during summer
likely resulted in a pronounced seasonality during Bond events 3 and 4 (Fig. 5). Notably,
the amplitude of the seasonal events and their accordingly high seasonality as well as
the overall seasonality in the Gulf of Cádiz were decreasing towards the present. It
is, moreover, interesting to note that Bond event 2 is not visible in our seasonal SST
records at all (Fig. 5). The general decrease in seasonality in the Gulf of Cádiz can
be attributed to decreasing summer insolation. We hypothesize that this is also true for
the decreasing amplitude of the seasonal SST events. During Bond event 4, when summer
insolation was high, the more northward position of the ITCZ during summer allowed for a
stronger inflow of warmer subtropical water masses into the Gulf of Cádiz. On the
other hand, during winter the ITCZ was much further south compared to its present
position, allowing enhanced southward flow of the colder water masses from the north.
During Bond event 3 the summer and winter positions of the ITCZ were already less
extreme, thus weakening the influence of either warm and cold water masses during summer
or winter, respectively. Later, during Bond event 2, seasonal movements of the ITCZ were
even more limited such that no influence of either cold water masses during winter or
warm subtropical water masses during summer is recognizable by a pronounced seasonal
temperature difference in the Gulf of Cádiz.
Conclusion
Two marine sediment cores from the Gulf of Cádiz and the Alboran Sea have been
studied by aiming at exploring the climatic variability with respect to precipitation and
vegetation change in southern Iberia as well as resolving seasonal hydrological
conditions during the mid- to late-Holocene. The following conclusions can be drawn from
this study.
Based on terrestrial n-alkane concentrations we found five major dry
climate episodes in southern Iberia at 5.4±0.3 ka cal BP, from 5.1 to 4.9±0.1 ka cal BP, from 4.8 to 4.7±0.1 ka cal BP, from 4.4 to 4.3±0.1 ka cal BP, and at 3.7±0.1 ka cal BP. These dry phases also impacted the
vegetation, which suffered from environmental stress due to reduced water availability.
The manifestation of the 4.2 ka event in southern Iberia appears to be
shorter compared to the prolonged dry episode observed in other regions. Instead, our
data suggests a distinct dry phase from 4.4 to 4.3±0.1 ka cal BP followed by a
rapid shift towards relatively moister conditions at about 4.2 ka cal BP, which lasted
until approximately 3.8 ka cal BP.
SST reconstructions suggest fairly stable annual mean temperature conditions
in the Alboran Sea and the Gulf of Cádiz with estimated values varying within the
range of uncertainty of the calibration. Based on summer and winter SST estimates, the
new records imply different mechanisms driving the seasonal variability in these two
oceanic basins. While in the Gulf of Cádiz opposite seasonal SST events (i.e. summer
warming and winter cooling) of up to 2 ∘C amplitude enhanced the seasonal SST
difference, only slight seasonal SST variations (i.e. summer cooling and winter warming)
within the methodological uncertainty have been found in the Alboran Sea at different
times.
The new records further suggest that variability in precipitation and
vegetation in southern Iberia and probably the hydrological variability in
the Alboran Sea can be well explained by a NAO-like atmospheric circulation.
Dominant positive NAO-like conditions appear contemporaneous with the
observed dry phases over southern Iberia associated with slightly warmer
annual mean SSTs and decreased MPP in the Alboran Sea.
The variability in the seasonal SST contrasts in the Gulf of Cádiz seems to
be closely related to North Atlantic Bond events. We observe increasing seasonality with
summer warming and winter cooling during Bond events 3 and 4. We propose that during
winter the southward transport of cold water masses from the North Atlantic affected the
Gulf of Cádiz, while during summer warm water masses originating from the STG reached
our study site. Bond event 2 is not reflected by a larger difference in our seasonal SST
reconstructions. This is likely due to decreasing summer insolation that also influences
the seasonal movements of the subtropical Azores Front, which in the Gulf of Cádiz
can also limit either the inflow of cold or warm water masses during winter and summer,
respectively.
Data availability
The data reported in this paper are archived in PANGAEA
and available at https://doi.pangaea.de/10.1594/PANGAEA.899720 (Schirrmacher et al., 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/cp-15-617-2019-supplement.
Author contributions
JS, the main author of this study (with
contributions from all co-authors), performed the geochemical analyses for reconstruction
of precipitation, vegetation, and alkenone-based temperature. JS also established the new
age models for the two sediment cores. Planktic foraminiferal analyses for seasonal SST
estimates were provided by MW. ES compiled the calibration data sets for the SIMMAX MAT
analyses and computed SIMMAX SST. TB was responsible for alkenone analysis and quality
control.
Competing interests
The authors declare that they have no conflict of
interest.
Special issue statement
This article is part of the special issue “The 4.2 ka BP
climatic event”. It is a result of “The 4.2 ka BP Event: An International
Workshop”, Pisa, Italy, 10–12 January 2018.
Acknowledgements
This research was performed in the framework of the CRC 1266 “Scales of
transformation” (project number: 2901391021) funded by the DFG (German
Research Foundation). Emília Salgueiro was funded by Fundação
para a Ciência e Tecnologia (fellowship: SFRH/BPD/26525/2006 &
SFRH/BPD/111433/2015). Sample material has been provided by the GeoB core
repository and the IODP core repository at the MARUM – Center for Marine
Environmental Sciences, University of Bremen, Germany. In this respect, kind
support is acknowledged provided by Jürgen Pätzold, Vera B. Bender,
and Alex Wülbers. The authors acknowledge Francesc Burjachs for his kind
data supply. We are also very grateful for enormous support from Silvia Koch
with the geochemical analyses. Additionally, we thank Ingo Feeser for
assistance with the Bayesian age modelling and Wolfgang Hamer for helping
with the statistical analyses. We also thank the four anonymous referees as
well as Antonio Garcia-Alix for their helpful comments on improving the
manuscript.
Review statement
This paper was edited by Harvey Weiss and reviewed by four
anonymous referees.
ReferencesAbrantes, F., Rodrigues, T., Rufino, M., Salgueiro, E., Oliveira, D., Gomes,
S., Oliveira, P., Costa, A., Mil-Homens, M., Drago, T., and Naughton, F.: The
climate of the Common Era off the Iberian Peninsula, Clim. Past, 13,
1901–1918, 10.5194/cp-13-1901-2017, 2017.
Alley, R. B., Mayewski, P. A., Sowers, T., Stuiver, M., Taylor, K. C., and
Clark, P. U.: Geology: Holocene climatic instability: A prominent, widespread
event 8200 yr ago, Geology, 25, 483–486, 1997.Ausín, B., Flores, J. A., Sierro, F. J., Cacho, I.,
Hernández-Almeida, I., Martrat, B., and Grimalt, J. O.: Atmospheric
patterns driving Holocene productivity in the Alboran Sea (Western
Mediterranean): A multiproxy approach, Holocene, 25, 583–595,
10.1177/0959683614565952, 2015.Bakke, J., Dahl, S. O., Paasche, Ø., Riis Simonsen, J., Kvisvik, B.,
Bakke, K., and Nesje, A.: A complete record of Holocene glacier variability
at Austre Okstindbreen, northern Norway: An integrated approach, Quaternary
Sci. Rev., 29, 1246–1262, 10.1016/j.quascirev.2010.02.012, 2010.Bird, M. I., Summons, R. E., Gagan, M. K., Roksandic, Z., Dowling, L., Head,
J., Keith Fifield, L., Cresswell, R. G., and Johnson, D. P.: Terrestrial
vegetation change inferred from n-alkane σ13C analysis in the
marine environment, Geochim. Cosmochim. Ac., 59, 2853–2857,
10.1016/0016-7037(95)00160-2, 1995.Blaauw, M. and Christen, J. A.: Flexible paleoclimate age-depth models using
an autoregressive gamma process, Bayesian Anal., 6, 457–474,
10.1214/11-BA618, 2011.Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P.,
Priore, P., Cullen, H., Hajdas, I., and Bonani, G.: A Pervasive
Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates,
Science, 278, 1257–1266, 10.1126/science.278.5341.1257, 1997.Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M. N., Showers, W.,
Hoffmann, S., Lotti-Bond, R., Hajdas, I., and Bonani, G.: Persistent solar
influence on North Atlantic climate during the Holocene, Science, 294,
2130–2136, 10.1126/science.1065680, 2001.Booth, R. K., Jackson, S. T., Forman, S. L., Kutzbach, J. E., Bettis, E. A.,
Kreigs, J., and Wright, D. K.: A severe centennial-scale drought in
midcontinental North America 4200 years ago and apparent global linkages,
Holocene, 15, 321–328, 10.1191/0959683605hl825ft, 2005.Burjachs, F. and Expósito, I.: Charcoal and pollen analysis: Examples of
Holocene fire dynamics in Mediterranean Iberian Peninsula, Catena, 135,
340–349, 10.1016/j.catena.2014.10.006, 2015.
Burjachs, F., Giralt, S., Roca, J. R., Seret, G., and Julià, R.:
Palinología holocénica y desertización en el Mediterráneo
occidental, in: El paisaje mediterráneo a través del espacio y del
tiempo: Implicaiones en la desertificación, edited by: Ibañez, J. J.,
Valero, B. L., and Machado, C., Geoforma Editores, Logroño, 379–394,
1997.Bush, R. T. and McInerney, F. A.: Leaf wax n-alkane distributions in and
across modern plants: Implications for paleoecology and chemotaxonomy,
Geochim. Cosmochim. Ac., 117, 161–179, 10.1016/j.gca.2013.04.016, 2013.Cacho, I., Grimalt, J. O., Canals, M., Sbaffi, L., Shackleton, N. J.,
Schönfeld, J., and Zahn, R.: Variability of the western Mediterranean Sea
surface temperature during the last 25,000 years and its connection with the
Northern Hemisphere climatic changes, Paleoceanography, 16, 40–52,
10.1029/2000PA000502, 2001.Calò, C., Henne, P. D., Curry, B., Magny, M., Vescovi, E., La Mantia, T.,
Pasta, S., Vannière, B., and Tinner, W.: Spatio-temporal patterns of
Holocene environmental change in southern Sicily, Palaeogeogr. Palaeocl.,
323–325, 110–122, 10.1016/j.palaeo.2012.01.038, 2012.Carrión, J. S., Sánchez-Gómez, P., Mota, J. F., Yll, R., and
Chaín, C.: Holocene vegetation dynamics, fire and grazing in the Sierra
de Gádor, southern Spain, Holocene, 13, 839–849,
10.1191/0959683603hl662rp, 2016.Cheng, H., Sinha, A., Verheyden, S., Nader, F. H., Li, X. L., Zhang, P. Z.,
Yin, J. J., Yi, L., Peng, Y. B., Rao, Z. G., Ning, Y. F., and Edwards, R. L.:
The climate variability in northern Levant over the past 20,000 years,
Geophys. Res. Lett., 42, 8641–8650, 10.1002/2015GL065397, 2015.Comas, M. C., Zahn, R., and Klaus, A., et al.: Proc. ODP, Init. Repts., 161:
College Station, TX (Ocean Drilling Program),
10.2973/odp.proc.ir.161.1996, 1996.Combourieu Nebout, N., Turon, J. L., Zahn, R., Capotondi, L., Londeix, L.,
and Pahnke, K.: Enhanced aridity and atmospheric high-pressure stability over
the western Mediterranean during the North Atlantic cold events of the past
50 k.y, Geology, 30, 863,
10.1130/0091-7613(2002)030<0863:EAAAHP>2.0.CO;2, 2002.Conte, M. H. and Weber, J. C.: Long-range atmospheric transport of
terrestrial biomarkers to the western North Atlantic, Global Biogeochem. Cy.,
16, 89-1–89-17, 10.1029/2002GB001922, 2002.Cortina, A., Grimalt, J. O., Rigual-Hernández, A., Ballegeer, A.-M.,
Martrat, B., Sierro, F. J., and Flores, J. A.: The impact of ice-sheet
dynamics in western Mediterranean environmental conditions during
Terminations. An approach based on terrestrial long chain n-alkanes deposited
in the upper slope of the Gulf of Lions, Chem. Geol., 430, 21–33,
10.1016/j.chemgeo.2016.03.015, 2016.Deininger, M., McDermott, F., Mudelsee, M., Werner, M., Frank, N., and
Mangini, A.: Coherency of late Holocene European speleothem δ18O records linked to North Atlantic Ocean circulation, Clim. Dynam., 49,
595–618, 10.1007/s00382-016-3360-8, 2017.deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker,
L., and Yarusinsky, M.: Abrupt onset and termination of the African Humid
Period, Quaternary Sci. Rev., 19, 347–361,
10.1016/S0277-3791(99)00081-5, 2000.Eglinton, G. and Hamilton, R. J.: Leaf Epicuticular Waxes, Science, 156,
1322–1335, 10.1126/science.156.3780.1322, 1967.Fernández-Delgado, C., Baldó, F., Vilas, C.,
García-González, D., Cuesta, J. A., González-Ortegón, E.,
and Drake, P.: Effects of the river discharge management on the nursery
function of the Guadalquivir river estuary (SW Spain), Hydrobiologia, 587,
125–136, 10.1007/s10750-007-0691-9, 2007.Fick, S. E. and Hijmans, R. J.: WorldClim 2: New 1-km spatial resolution
climate surfaces for global land areas, Int. J. Climatol., 37, 4302–4315,
10.1002/joc.5086, 2017.Finné, M., Holmgren, K., Shen, C.-C., Hu, H.-M., Boyd, M., and Stocker,
S.: Late Bronze Age climate change and the destruction of the Mycenaean
Palace of Nestor at Pylos, PloS one, 12, e0189447,
10.1371/journal.pone.0189447, 2017.Fletcher, W. J. and Sánchez Goñi, M. F.: Orbital- and
sub-orbital-scale climate impacts on vegetation of the western Mediterranean
basin over the last 48,000 yr, Quaternary Res., 70, 451–464,
10.1016/j.yqres.2008.07.002, 2008.Fletcher, W. J., Boski, T., and Moura, D.: Palynological evidence for
environmental and climatic change in the lower Guadiana valley, Portugal,
during the last 13 000 years, Holocene, 17, 481–494,
10.1177/0959683607077027, 2007.García-Alix, A., Jiménez-Espejo, F. J., Toney, J. L.,
Jiménez-Moreno, G., Ramos-Román, M. J., Anderson, R. S., Ruano, P.,
Queralt, I., Delgado Huertas, A., and Kuroda, J.: Alpine bogs of southern
Spain show human-induced environmental change superimposed on long-term
natural variations, Sci. Rep.-UK, 7, 7439, 10.1038/s41598-017-07854-w,
2017.García-Alix, A., Jiménez-Espejo, F. J., Jiménez-Moreno, G.,
Toney, J. L., Ramos-Román, M. J., Camuera, J., Anderson, R. S.,
Delgado-Huertas, A., Martínez-Ruiz, F., and Queralt, I.: Holocene
geochemical footprint from Semi-arid alpine wetlands in southern Spain,
Scientific Data, 5, 180024, 10.1038/sdata.2018.24, 2018.Goslin, J., Fruergaard, M., Sander, L., Gałka, M., Menviel, L.,
Monkenbusch, J., Thibault, N., and Clemmensen, L. B.: Holocene centennial to
millennial shifts in North-Atlantic storminess and ocean dynamics, Sci.
Rep.-UK, 8, 12778, 10.1038/s41598-018-29949-8, 2018.Hayes, A., Kucera, M., Kallel, N., Sbaffi, L., and Rohling, E. J.: Glacial
Mediterranean sea surface temperatures based on planktonic foraminiferal
assemblages, Quaternary Sci. Rev., 24, 999–1016,
10.1016/j.quascirev.2004.02.018, 2005.Haynes, R., Barton, E. D., and Pilling, I.: Development, persistence, and
variability of upwelling filaments off the Atlantic coast of the Iberian
Peninsula, J. Geophys. Res., 98, 22681, 10.1029/93JC02016, 1993.Hernández, A., Trigo, R. M., Pla-Rabes, S., Valero-Garcés, B. L.,
Jerez, S., Rico-Herrero, M., Vega, J. C., Jambrina-Enríquez, M., and
Giralt, S.: Sensitivity of two Iberian lakes to North Atlantic atmospheric
circulation modes, Clim. Dynam., 45, 3403–3417,
10.1007/s00382-015-2547-8, 2015.Herrmann, N., Boom, A., Carr, A. S., Chase, B. M., Granger, R., Hahn, A.,
Zabel, M., and Schefuß, E.: Sources, transport and deposition of
terrestrial organic material: A case study from southwestern Africa,
Quaternary Sci. Rev., 149, 215–229, 10.1016/j.quascirev.2016.07.028,
2016.Hurrell, J. W.: Decadal trends in the north atlantic oscillation: regional
temperatures and precipitation, Science, 269, 676–679,
10.1126/science.269.5224.676, 1995.Jalali, B., Sicre, M.-A., Bassetti, M.-A., and Kallel, N.: Holocene climate
variability in the North-Western Mediterranean Sea (Gulf of Lions), Clim.
Past, 12, 91–101, 10.5194/cp-12-91-2016, 2016.Jalali, B., Sicre, M.-A., Kallel, N., Azuara, J., Combourieu-Nebout, N.,
Bassetti, M.-A., and Klein, V.: High-resolution Holocene climate and
hydrological variability from two major Mediterranean deltas (Nile and
Rhone), Holocene, 27, 1158–1168, 10.1177/0959683616683258, 2017.Jalut, G., Esteban Amat, A., Bonnet, L., Gauquelin, T., and Fontugne, M.:
Holocene climatic changes in the Western Mediterranean, from south-east
France to south-east Spain, Palaeogeogr. Palaeocl., 160, 255–290,
10.1016/S0031-0182(00)00075-4, 2000.Jiménez-Amat, P. and Zahn, R.: Offset timing of climate oscillations
during the last two glacial-interglacial transitions connected with
large-scale freshwater perturbation, Paleoceanography, 30, 768–788,
10.1002/2014PA002710, 2015.Kim, J.-H., Rimbu, N., Lorenz, S. J., Lohmann, G., Nam, S.-I., Schouten, S.,
Rühlemann, C., and Schneider, R. R.: North Pacific and North Atlantic
sea-surface temperature variability during the Holocene, Quaternary Sci.
Rev., 23, 2141–2154, 10.1016/j.quascirev.2004.08.010, 2004.Kucera, M., Rosell-Melé, A., Schneider, R., Waelbroeck, C., and Weinelt,
M.: Multiproxy approach for the reconstruction of the glacial ocean surface
(MARGO), Quaternary Sci. Rev., 24, 813–819,
10.1016/j.quascirev.2004.07.017, 2005a.Kucera, M., Weinelt, M., Kiefer, T., Pflaumann, U., Hayes, A., Weinelt, M.,
Chen, M.-T., Mix, A. C., Barrows, T. T., Cortijo, E., Duprat, J., Juggins,
S., and Waelbroeck, C.: Reconstruction of sea-surface temperatures from
assemblages of planktonic foraminifera: Multi-technique approach based on
geographically constrained calibration data sets and its application to
glacial Atlantic and Pacific Oceans, Quaternary Sci. Rev., 24, 951–998,
10.1016/j.quascirev.2004.07.014, 2005b.Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A. C. M., and
Levrard, B.: A long-term numerical solution for the insolation quantities of
the Earth, Astron. Astrophys., 428, 261–285, 10.1051/0004-6361:20041335,
2004.Leider, A., Hinrichs, K.-U., Schefuß, E., and Versteegh, G. J.M.:
Distribution and stable isotopes of plant wax derived n-alkanes in
lacustrine, fluvial and marine surface sediments along an Eastern Italian
transect and their potential to reconstruct the hydrological cycle, Geochim.
Cosmochim. Ac., 117, 16–32, 10.1016/j.gca.2013.04.018, 2013.Le Roy, M., Deline, P., Carcaillet, J., Schimmelpfennig, I., and Ermini, M.:
10Be exposure dating of the timing of Neoglacial glacier advances
in the Ecrins-Pelvoux massif, southern French Alps, Quaternary Sci. Rev.,
178, 118–138, 10.1016/j.quascirev.2017.10.010, 2017.
Lionello, P. (Ed.): The climate of the Mediterranean region: From the past to
the future, 1st Edn., Elsevier Insights, Elsevier Science, Amsterdam, 502
pp., 2012.
Locarnini, R. A., Mishonov, A. V., Antonov, J. I., Boyer, T. P., Garcia,
H. E., Baranova, O. K., Zweng, M. M., Paver, C. R., Reagan, J. R., Johnson, D. R.,
Hamilton, M., and Seidov, D.: World OceanAtlas 2013, Volume 1: Temperature,
S.Levitus, edited by: Mishonov, A., Technical Ed., NOAA Atlas NESDIS 73, 40
pp., National Oceanographic Data Center User Services TeamNOAA/NESDIS
E/OC1SSMCIII, Silver Spring, MD, 2013.Magny, M., Combourieu-Nebout, N., de Beaulieu, J. L., Bout-Roumazeilles, V.,
Colombaroli, D., Desprat, S., Francke, A., Joannin, S., Ortu, E., Peyron, O.,
Revel, M., Sadori, L., Siani, G., Sicre, M. A., Samartin, S., Simonneau, A.,
Tinner, W., Vannière, B., Wagner, B., Zanchetta, G., Anselmetti, F.,
Brugiapaglia, E., Chapron, E., Debret, M., Desmet, M., Didier, J., Essallami,
L., Galop, D., Gilli, A., Haas, J. N., Kallel, N., Millet, L., Stock, A.,
Turon, J. L., and Wirth, S.: North–south palaeohydrological contrasts in the
central Mediterranean during the Holocene: tentative synthesis and working
hypotheses, Clim. Past, 9, 2043–2071,
10.5194/cp-9-2043-2013, 2013.Martrat, B., Grimalt, J. O., Shackleton, N. J., de Abreu, L., Hutterli, M.
A., and Stocker, T. F.: Four climate cycles of recurring deep and surface
water destabilizations on the Iberian margin, Science, 317, 502–507,
10.1126/science.1139994, 2007.Martrat, B., Jiménez-Amat, P., Zahn, R., and Grimalt, J. O.: Similarities
and dissimilarities between the last two deglaciations and interglaciations
in the North Atlantic region, Quaternary Sci. Rev., 99, 122–134,
10.1016/j.quascirev.2014.06.016, 2014.Mayewski, P. A., Rohling, E. E., Curt Stager, J., Karlén, W., Maasch, K.
A., Meeker, L. D., Meyerson, E. A., Gasse, F., van Kreveld, S., Holmgren, K.,
Lee-Thorp, J., Rosqvist, G., Rack, F., Staubwasser, M., Schneider, R. R., and
Steig, E. J.: Holocene Climate Variability, Quaternary Res., 62, 243–255,
10.1016/j.yqres.2004.07.001, 2004.Minas, H. J., Coste, B., Le Corre, P., Minas, M., and Raimbault, P.:
Biological and geochemical signatures associated with the water circulation
through the Strait of Gibraltar and in the western Alboran Sea, J. Geophys.
Res., 96, 8755, 10.1029/91JC00360, 1991.Moreno, A., Pérez-Mejías, C., Bartolomé, M., Sancho, C., Cacho,
I., Stoll, H., Delgado-Huertas, A., Hellstrom, J., Edwards, R. L., and Cheng,
H.: New speleothem data from Molinos and Ejulve caves reveal Holocene
hydrological variability in northeast Iberia, Quaternary Res., 88, 223–233,
10.1017/qua.2017.39, 2017.Müller, P. J., Kirst, G., Ruhland, G., von Storch, I., and
Rosell-Melé, A.: Calibration of the alkenone paleotemperature index
U37K′ based on core-tops from the eastern South Atlantic
and the global ocean (60∘ N–60∘ S), Geochim. Cosmochim.
Ac., 62, 1757–1772, 10.1016/S0016-7037(98)00097-0, 1998.Navarro-Hervás, F., Ros-Salas, M.-M., Rodríguez-Estrella, T.,
Fierro-Enrique, E., Carrión, J.-S., García-Veigas, J., Flores,
J.-A., Bárcena, M. Á., and García, M. S.: Evaporite evidence of
a mid-Holocene (c 4550–4400 cal. yr BP) aridity crisis in southwestern
Europe and palaeoenvironmental consequences, Holocene, 24, 489–502,
10.1177/0959683613520260, 2014.NGRIP-Members: High-resolution record of Northern Hemisphere climate
extending into the last interglacial period, Nature, 431, 147–151,
10.1038/nature02805, 2004.Olsen, J., Anderson, N. J., and Knudsen, M. F.: Variability of the North
Atlantic Oscillation over the past 5,200 years, Nat. Geosci., 5, 808–812,
10.1038/ngeo1589, 2012.Pantaléon-Cano, J., Yll, E.-I., Pérez-Obiol, R., and Roure, J. M.:
Palynological evidence for vegetational history in semi-arid areas of the
western Mediterranean (Almería, Spain), Holocene, 13, 109–119,
10.1191/0959683603hl598rp, 2003.Peliz, Á., Rosa, T. L., Santos, A. M. P., and Pissarra, J. L.: Fronts,
jets, and counter-flows in the Western Iberian upwelling system, J. Marine
Syst., 35, 61–77, 10.1016/S0924-7963(02)00076-3, 2002.Peliz, Á., Dubert, J., Santos, A. M. P., Oliveira, P. B., and Le Cann,
B.: Winter upper ocean circulation in the Western Iberian Basin – Fronts,
Eddies and Poleward Flows: An overview, Deep-Sea Res. Pt I, 52, 621–646,
10.1016/j.dsr.2004.11.005, 2005.Penaud, A., Eynaud, F., Voelker, A. H. L., and Turon, J.-L.:
Palaeohydrological changes over the last 50 ky in the central Gulf of Cadiz:
complex forcing mechanisms mixing multi-scale processes, Biogeosciences, 13,
5357–5377, 10.5194/bg-13-5357-2016, 2016.Pérez-Folgado, M., Sierro, F. J., Flores, J. A., Cacho, I., Grimalt, J.
O., Zahn, R., and Shackleton, N.: Western Mediterranean planktonic
foraminifera events and millennial climatic variability during the last
70 kyr, Mar. Micropaleontol., 48, 49–70, 10.1016/S0377-8398(02)00160-3,
2003.Pflaumann, U., Duprat, J., Pujol, C., and Labeyrie, L. D.: SIMMAX: A modern
analog technique to deduce Atlantic sea surface temperatures from planktonic
foraminifera in deep-sea sediments, Paleoceanography, 11, 15–35,
10.1029/95PA01743, 1996.Pflaumann, U., Sarnthein, M., Chapman, M., d'Abreu, L., Funnell, B., Huels,
M., Kiefer, T., Maslin, M., Schulz, H., Swallow, J., van Kreveld, S.,
Vautravers, M., Vogelsang, E., and Weinelt, M.: Glacial North Atlantic:
Sea-surface conditions reconstructed by GLAMAP 2000, Paleoceanography, 18,
1065, 10.1029/2002PA000774, 2003.Prahl, F. G. and Wakeham, S. G.: Calibration of unsaturation patterns in
long-chain ketone compositions for palaeotemperature assessment, Nature, 330,
367–369, 10.1038/330367a0, 1987.Prahl, F. G., Muehlhausen, L. A., and Zahnle, D. L.: Further evaluation of
long-chain alkenones as indicators of paleoceanographic conditions, Geochim.
Cosmochim. Ac., 52, 2303–2310, 10.1016/0016-7037(88)90132-9, 1988.Ramos-Román, M. J., Jiménez-Moreno, G., Camuera, J., García-Alix,
A., Anderson, R. S., Jiménez-Espejo, F. J., and Carrión, J. S.:
Holocene climate aridification trend and human impact interrupted by
millennial- and centennial-scale climate fluctuations from a new sedimentary
record from Padul (Sierra Nevada, southern Iberian Peninsula), Clim. Past,
14, 117–137, 10.5194/cp-14-117-2018, 2018a.Ramos-Román, M. J., Jiménez-Moreno, G., Camuera, J.,
García-Alix, A., Scott Anderson, R., Jiménez-Espejo, F. J., Sachse,
D., Toney, J. L., Carrión, J. S., Webster, C., and Yanes, Y.:
Millennial-scale cyclical environment and climate variability during the
Holocene in the western Mediterranean region deduced from a new multi-proxy
analysis from the Padul record (Sierra Nevada, Spain), Global Planet. Change,
168, 35–53, 10.1016/j.gloplacha.2018.06.003, 2018b.Rasmussen, S. O., Bigler, M., Blockley, S. P., Blunier, T., Buchardt, S. L.,
Clausen, H. B., Cvijanovic, I., Dahl-Jensen, D., Johnsen, S. J., Fischer, H.,
Gkinis, V., Guillevic, M., Hoek, W. Z., Lowe, J. J., Pedro, J. B., Popp, T.,
Seierstad, I. K., Steffensen, J. P., Svensson, A. M., Vallelonga, P.,
Vinther, B. M., Walker, M. J. C., Wheatley, J. J., and Winstrup, M.: A
stratigraphic framework for abrupt climatic changes during the Last Glacial
period based on three synchronized Greenland ice-core records: Refining and
extending the INTIMATE event stratigraphy, Quaternary Sci. Rev., 106, 14–28,
10.1016/j.quascirev.2014.09.007, 2014.Reimer, P. J., Bard, E., Bayliss, A., Beck, J. W., Blackwell, P. G., Ramsey,
C. B., Buck, C. E., Cheng, H., Edwards, R. L., Friedrich, M., Grootes, P. M.,
Guilderson, T. P., Haflidason, H., Hajdas, I., Hatté, C., Heaton, T. J.,
Hoffmann, D. L., Hogg, A. G., Hughen, K. A., Kaiser, K. F., Kromer, B.,
Manning, S. W., Niu, M., Reimer, R. W., Richards, D. A., Scott, E. M.,
Southon, J. R., Staff, R. A., Turney, C. S. M., and van der Plicht, J.:
IntCal13 and Marine13 Radiocarbon Age Calibration Curves
0–50,000 Years cal BP, Radiocarbon, 55, 1869–1887,
10.2458/azu_js_rc.55.16947, 2013.Repschläger, J., Garbe-Schönberg, D., Weinelt, M., and Schneider, R.:
Holocene evolution of the North Atlantic subsurface transport, Clim. Past,
13, 333–344, 10.5194/cp-13-333-2017, 2017.Rodrigo-Gámiz, M., Martínez-Ruiz, F., Rampen, S. W., Schouten, S.,
and Sinninghe Damsté, J. S.: Sea surface temperature variations in the
western Mediterranean Sea over the last 20 kyr: A dual-organic proxy
(U37K′ and LDI) approach, Paleoceanography, 29, 87–98,
10.1002/2013PA002466, 2014.Rodrigo-Gámiz, M., Martínez-Ruiz, F., Chiaradia, M.,
Jiménez-Espejo, F. J., and Ariztegui, D.: Radiogenic isotopes for
deciphering terrigenous input provenance in the western Mediterranean, Chem.
Geol., 410, 237–250, 10.1016/j.chemgeo.2015.06.004, 2015.Rommerskirchen, F., Plader, A., Eglinton, G., Chikaraishi, Y., and
Rullkötter, J.: Chemotaxonomic significance of distribution and stable
carbon isotopic composition of long-chain alkanes and alkan-1-ols in C4
grass waxes, Org. Geochem., 37, 1303–1332,
10.1016/j.orggeochem.2005.12.013, 2006.Ruan, J., Kherbouche, F., Genty, D., Blamart, D., Cheng, H., Dewilde, F.,
Hachi, S., Edwards, R. L., Régnier, E., and Michelot, J.-L.: Evidence of
a prolonged drought ca. 4200 yr BP correlated with prehistoric settlement
abandonment from the Gueldaman GLD1 Cave, Northern Algeria, Clim. Past, 12,
1–14, 10.5194/cp-12-1-2016, 2016.Salgueiro, E., Naughton, F., Voelker, A. H. L., Abreu, L. de, Alberto, A.,
Rossignol, L., Duprat, J., Magalhães, V. H., Vaqueiro, S., Turon, J.-L.,
and Abrantes, F.: Past circulation along the western Iberian margin: A time
slice vision from the Last Glacial to the Holocene, Quaternary Sci. Rev.,
106, 316–329, 10.1016/j.quascirev.2014.09.001, 2014.Sarhan, T., Lafuente, J. G., Vargas, M., Vargas, J. M., and Plaza, F.:
Upwelling mechanisms in the northwestern Alboran Sea, J. Marine Syst., 23,
317–331, 10.1016/S0924-7963(99)00068-8, 2000.Schirrmacher, J., Weinelt, M., Blanz, T., Andersen, N., Salgueiro, E., and
Schneider, R. R.: Organic geochemical and foraminiferal MAT data for mid- to
late-Holocene sediments of the Gulf of Cadiz and Alboran Sea, PANGAEA,
10.1594/PANGAEA.899720, 2019.Schlitzer, R.: Ocean Data View, available at: https://odv.awi.de (last
access: 24 January 2019), 2018.Schott, F., Meincke, J., Meinecke, G., Neuer, S., and Zenk, W.: North
Atlantic 1999 – Cruise No. M45, 18 May–4 November 1999, Malaga, Spain–Las
Palmas, Spain, METEOR-Berichte, M45, 161 pp., DFG-Senatskommission für
Ozeanographie, 10.2312/cr_m45, 2000.Schreuder, L. T., Stuut, J.-B. W., Korte, L. F., Sinninghe Damsté, J. S.,
and Schouten, S.: Aeolian transport and deposition of plant wax n-alkanes
across the tropical North Atlantic Ocean, Org. Geochem., 115, 113–123,
10.1016/j.orggeochem.2017.10.010, 2018.Schröder, T., van't Hoff, J., López-Sáez, J. A., Viehberg, F.,
Melles, M., and Reicherter, K.: Holocene climatic and environmental evolution
on the southwestern Iberian Peninsula: A high-resolution multi-proxy study
from Lake Medina (Cádiz, SW Spain), Quaternary Sci. Rev., 198, 208–225,
10.1016/j.quascirev.2018.08.030, 2018.Walczak, I. W., Baldini, J. U. L., Baldini, L. M., McDermott, F., Marsden,
S., Standish, C. D., Richards, D. A., Andreo, B., and Slater, J.:
Reconstructing high-resolution climate using CT scanning of unsectioned
stalagmites: A case study identifying the mid-Holocene onset of the
Mediterranean climate in southern Iberia, Quaternary Sci. Rev., 127,
117–128, 10.1016/j.quascirev.2015.06.013, 2015.Wanner, H., Solomina, O., Grosjean, M., Ritz, S. P., and Jetel, M.: Structure
and origin of Holocene cold events, Quaternary Sci. Rev., 30, 3109–3123,
10.1016/j.quascirev.2011.07.010, 2011.Wassenburg, J. A., Dietrich, S., Fietzke, J., Fohlmeister, J., Jochum, K. P.,
Scholz, D., Richter, D. K., Sabaoui, A., Spötl, C., Lohmann, G., Andreae,
M. O., and Immenhauser, A.: Reorganization of the North Atlantic Oscillation
during early Holocene deglaciation, Nat. Geosci., 9, 602–605,
10.1038/ngeo2767, 2016.
Weinelt, M., Schwab, C., Kneisel, J., and Hinz, M.: Climate and societal
change in the western Mediterranean area around 4.2 ka BP, in: 2200 BC,
ein Klimasturz als Ursache für den Zerfall der alten Welt?: 7.
Mitteldeutscher Archäologentag vom 23. bis 26. Oktober 2014 in Halle
(Saale), edited by: Meller, H., Arz, H. W., Jung, R., Risch, R., Tagungen des
Landesmuseums für Vorgeschichte Halle, Band 12,1, Landesamt für
Denkmalpflege und Archäologie Sachsen-Anhalt, Landesmuseum für
Vorgeschichte, Halle (Saale), 461–480, 2015.Zanchetta, G., Regattieri, E., Isola, I., Drysdale, R. N., Bini, M.,
Baneschi, I., and Hellstrom, J. C.: The so-called “4.2 event” in the
Central Mediterranean and its climatic teleconnections, Alpine and
Mediterranean Quaternary, 29, 5–17, 2016.
Zielhofer, C., Fletcher, W. J., Mischke, S., de Batist, M., Campbell, J. F.
E., Joannin, S., Tjallingii, R., El Hamouti, N., Junginger, A., Stele, A.,
Bussmann, J., Schneider, B., Lauer, T., Spitzer, K., Strupler, M., Brachert,
T., and Mikdad, A.: Atlantic forcing of Western Mediterranean winter rain
minima during the last 12,000 years, Quaternary Sci. Rev., 157, 29–51,
10.1016/j.quascirev.2016.11.037, 2017.Zielhofer, C., Köhler, A., Mischke, S., Benkaddour, A., Mikdad, A., and
Fletcher, W. J.: Western Mediterranean hydro-climatic consequences of
Holocene ice-rafted debris (Bond) events, Clim. Past, 15, 463–475,
10.5194/cp-15-463-2019, 2019.Zorita, E., Kharin, V., and von Storch, H.: The Atmospheric Circulation and
Sea Surface Temperature in the North Atlantic Area in Winter: Their
Interaction and Relevance for Iberian Precipitation, J. Climate, 5,
1097–1108, 10.1175/1520-0442(1992)005<1097:TACASS>2.0.CO;2, 1992.