CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-12-1539-2016Holocene hydrological changes in the Rhône River (NW Mediterranean) as
recorded in the marine mud beltBassettiMaria-AngelaBernéSergeSicreMarie-Alexandrinehttps://orcid.org/0000-0002-5015-1400DennielouBernardAlonsoYoannBuscailRoselyneJalaliBassemhttps://orcid.org/0000-0002-4171-1664HebertBertilMennitiChristopheCEFREM UMR5110 CNRS, Université de Perpignan Via Domitia, Perpignan, FranceSorbonne Universités (UPMC, Université Paris 06)-CNRS-IRD-MNHN,
LOCEAN Laboratory, 4 place Jussieu, 75005 Paris, FranceIFREMER, Centre de Brest, Plouzané, FranceGEOGLOB, Université de Sfax, Sfax, TunisiaMaria-Angela Bassetti (maria-angela.bassetti@univ-perp.fr)15July20161271539155317January201625February201618June2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/articles/12/1539/2016/cp-12-1539-2016.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/12/1539/2016/cp-12-1539-2016.pdf
Expanded marine Holocene archives are relatively scarce in the Mediterranean
Sea because most of the sediments were trapped in catchment areas during
this period. Mud belts are the most suitable targets to access expanded Holocene
records. These sedimentary bodies represent excellent archives for the study
of sea–land interactions and notably the impact of the hydrological activity
on sediment accumulation. We retrieved a 7.2 m long sediment core from the
Rhône mud belt in the Gulf of Lions in an area where the average
accumulation rate is ca. 0.70 m 1000 yr-1. This core thus provides a
continuous and high-resolution record of the last 10 ka cal BP. A
multiproxy dataset (XRF core scan, 14C dates, grain size and organic-matter analysis) combined with seismic stratigraphic analysis was used to
document decadal to centennial changes in the Rhône hydrological activity.
Our results show that (1) the early Holocene was characterized by high
sediment delivery likely indicative of local intense (but short-duration)
rainfall events, (2) important sediment delivery around 7 ka cal BP
presumably related to increased river flux, (3) a progressive increase in continental/marine input during the mid-Holocene despite increased distance
from river outlets due to sea-level rise possibly related to higher
atmospheric humidity caused by the southward migration of the storm tracks
in the North Atlantic, (4) multidecadal to centennial humid events took place in the
late Holocene. Some of these events correspond to the cold periods
identified in the North Atlantic (Little Ice Age, LIA; Dark Ages Cold Period) and also
coincide with time intervals of major floods in the northern Alps. Other
humid events are also observed during relatively warm periods (Roman Humid
Period and Medieval Climate Anomaly).
Introduction
The Holocene climate is characterized by centennial-scale climate changes
that punctuated the final deglacial warming after the Younger Dryas.
Renssen et al. (2009), Rogerson et al. (2011) and Wanner et al. (2008,
2014) provided an extensive review of Holocene climate
variability mainly based on chronologically well-constrained continental
temperature time series that emphasize the superimposition of the
insolation-driven climate changes with those induced by other external
forcings such as solar activity, volcanism and greenhouse gases (CH4,
CO2 and NO2). Based on existing data, Holocene climate can be
divided into four periods:
the early Holocene (between 11.7 and 8.2 ka cal BP; Walker et al.,
2012), characterized by a progressive warming-inducing ice cap melting and
outbreaks of freshwater from North America glacial lakes, leading to a
regional cooling in the Northern Hemisphere, i.e. the 8.2 ka cal BP cold event
(Barber et al., 1999);
the warm middle Holocene (between 8.2 and 4.2 ka cal BP; Walker et al.,
2012) that coincides approximately with the Holocene thermal maximum (HTM)
and is punctuated by several cold relapses (CRs) (Wanner et al.,
2011). Events at 6.4, 5.3 and 4.2 ka cal BP are the most significant in
terms of temperature change (Wanner et al., 2011). The 4.2 ka
event corresponds to enhanced dryness in the southern Mediterranean, Asia
and North America that presumably played a role in the collapse of various
civilizations (Magny et al., 2013).
the cold late Holocene (from 4.2 ka cal BP to the mid 19th
century; Walker et al., 2012), which includes the 2.8 ka cal BP cold event possibly
responsible for the collapse of the Late Bronze Age civilization (Do
Carmo and Sanguinetti, 1999; Weiss, 1982) and the Migration Period cooling
around 1.4 ka cal BP (Wanner et al., 2014); the late Holocene
cooling trend culminated during the Little Ice Age (LIA) between the
14th and 19th century (Wanner et al., 2011);
the warm industrial era from 1850 AD onwards (Rogerson et al., 2011;
Wanner et al., 2011).
In contrast to these cool events, the Medieval Climate Anomaly (MCA,
800–1300 AD) is often described as a warm period characterized by intense
dryness in some regions of the Northern Hemisphere, such as for example
Europe and the Mediterranean region, although it is not synchronous worldwide
(PAGES-2k-Consortium, 2013).
The causes of Holocene climate variability are not yet fully understood
despite recent advances achieved through the study of climate archives from
all around the word from both marine and continental settings. There are still open questions: to what extent are these well-known climate events global rather than regional and
what are the driving mechanisms at play? Numerical
modeling allows examining in more detail and on a broader geographical
scale causes of rapid climate changes and the role of natural or
anthropogenic forcings by better integrating data from marine, land and ice
archives. Nonetheless, there are significant discrepancies between proxy
reconstructions and numerical simulations that suggest the need to generate
better chronologically constrained high-resolution proxy records from
continental and marine archives and develop new approaches (Anchukaitis
and Tierney, 2013; Evans et al., 2013). Of particular interest are the
locations that allow developing paleohydrological and paleoenvironmental
investigations at the land–sea interface to better link atmospheric
circulation controlling the precipitation pattern over the continent and
changes in the thermohaline circulation.
Sediment drifts fed by water streams connected to the deep sea such as the
Var (Bonneau et al., 2014) or mid-shelf mud belts are interesting
locations to recover sedimentary archives where both continental and marine
proxies can be analyzed. Mid-shelf mud belts, in particular, are depot
centers fed by streams that result from various processes including
diffusion under the influence of storms, advection by currents and transport
by gravity flows (Hill et al., 2007). They often
form elongated sediment bodies, at between 10–30 and 60–100 m water depth,
roughly parallel to the coastline. Such sediment bodies can reach several
tens of meters in thickness when they are associated with large streams and
form infralittoral prograding prisms (sometimes called subaqueous deltas) as
for instance along the Italian Adriatic coast (Cattaneo et al., 2003).
In some ways, they are shallow-water equivalents to contourites, but they
generally display higher accumulation rates, making them ideal targets for
paleoenvironmental reconstructions.
In this study, we present a continuous record of the Holocene climate
obtained from a 7.03 m long sediment core retrieved from the Rhône mud belt
in the Gulf of Lions. Owing to the high sedimentation rate of this
environmental setting, we were able to generate sedimentological data on a
decadal-scale resolution for sediment grain size and semiquantitative chemical
composition obtained by means of continuous X-ray fluorescence. Organic-matter parameters and the overall seismic architecture of the mud belt were
also used to reconstruct the terrigenous flux and the degree of alteration
of land-derived material for investigating the relationship between detritic
fluxes and the paleohydrology of peri-Mediterranean rivers. Based on the
comparison of available data, we explored the linkages between rapid climate
changes and continental paleohydrology with a focus on the Rhône River
flood activity.
Environmental and climatic frameworkThe Gulf of Lions geological and oceanographic settings
The Gulf of Lions (GoL) is a passive and prograding continental margin with
a relatively constant subsidence and a high sediment supply
(Berné and Gorini, 2005). Located in the northwest sector of the
Mediterranean Sea, the GoL is bounded to the west and to the east by the
Pyrenean and Alpine orogenic belts, and comprises a crescent-shaped
continental shelf with a maximum width of 72 km near the mouth of the Rhône
(Berné et al., 2004). The general oceanic circulation is
dominated by the geostrophic Liguro-Provençal or Northern Current
(Millot, 1990), which is the northern branch of the general cyclonic
circulation in the western Mediterranean basin. This current flows
southwestward along the continental slope and intrudes on the
continental shelf during northwesterly winds events (Millot, 1990;
Petrenko, 2003). Surface water circulation in the GoL shelf is
wind-dependent (Millot, 1990). Different wind patterns affect the
circulation and transport of suspended particles on the shelf and produce
distinctive wave regimes. The continental cold and dry winds known as the mistral and tramontane, blowing from the N and NW through the passages between the Pyrenees,
the Massif Central and the Alps, are associated with a short fetch that
generates small waves on the inner shelf. During winter, these winds induce strong
cooling and mixing of the shelf waters, triggering dense-water formation
(Estournel et al., 2003) and locally generating
upwelling (Millot, 1990). Episodic and brief E–SE (marin or maritime regime)
winds are associated with a long fetch and large swells. This wind regime
induces a rise in sea level along the shore and intense cyclonic circulation
on the shelf (Ulses et al., 2008), producing alongshore
currents and downwelling (Monaco et al., 1990). Transport
of humid marine air masses over the coastal relief induces abundant
precipitation often accompanied by river flooding.
The main source of sediment in the GoL is the Rhône River
(Pont et al., 2002) and to a lesser extent small rivers
of the Languedoc-Roussillon region (Hérault, Orb, Aude, Agly, Têt,
Tech) (Fig. 1). The latter experience episodic discharges (flash floods
in spring and fall) that are difficult to quantify. The terrigenous sediment
supply originating from the Rhône River represents 80 % of the total
sediment deposited on the shelf (Aloisi et al., 1977). The Rhône River drains
a largely mountainous catchment area of 97 800 km2, incising a
geologically heterogeneous substrate, consisting of siliciclastic and
carbonate sedimentary rocks in valley infills and a crystalline (plutonic and
metamorphic from the Alpine domain) bedrock. The mean annual water discharge
measured at the Beaucaire gauging station, downstream of the last confluence, is
1701 m3 s-1 (mean for 1961–1996); the solid discharge
varies between 2 and 20 × 106 tons yr-1 (Eyrolle et al.,
2012; Pont et al., 2002).
Bathymetric map of the Gulf of Lions and position of core KSGC31.
The approximate extent of the Rhône mud belt is represented in green; the
arrow represents the direction of dominant transport of suspended sediments.
Bathymetric map based on Berné et al. (2007). Contour lines every 5 m on
the shelf. The dotted line corresponds to the retreat path of the Rhône
during the deglacial period (based on Gensous and Tesson, 2003; Berné et al.,
2007; Jouet, 2007; Fanget et al., 2014; Lombo Tombo et al., 2015). ERDC:
Early Rhône Deltaic Complex.
Seismic profile across the Rhône mud belt at the position of core
KSGC31 (position in Fig. 1). SB: sequence boundary surface formed by
continental erosion during the Last Glacial Maximum (LGM). RS: ravinement surface
formed by wave erosion during sea-level rise. It corresponds to the coarse
interval at the base of the core. MFS: maximum flooding surface. It
corresponds to the phase of transition between coastal retrogradation and
coastal progradation. It is dated here to ca. 7.5 ka cal BP (i.e. the period
of global sea-level stabilization). S1 and S2 are seismic surfaces used as
time lines on the basis of the age model in Fig. 3 (respectively
4.2 ± 0.5 and 2.5 ± 0.5 ka cal BP). Horizontal bars every
meter along the core. Vertical scale in milliseconds two-way travel time (ms TWTT).
Most of the sediment delivered by the Rhône is trapped on the inner shelf,
mainly in prodeltas (Fanget et al., 2013; Ulses et al., 2008), but
redistribution processes operating along the shelf create mid-shelf
depocenters of fine sediments. The sediment accumulation rate varies from 20
to 50 cm yr-1 at the present Roustan mouth of the Rhône River and
strongly decreases with the distance from the river. Sediment is exported
seaward by several turbid layers: the surface nepheloid layer, related to
river plume; an intermediate nepheloid layer that forms during periods of
water-column stratification; and a persistent bottom nepheloid layer, whose
influence decreases from the river mouth to the outer shelf (Calmet and
Fernandez, 1990; Naudin et al., 1997). The surficial plume is typically a
few meters thick close to the mouth but rapidly thins seaward to a few
centimeters (Millot, 1990); it is deflected southwestward by the surface
water circulation on the GoL shelf. The predominance of the Rhône River in
the sediment supply and the continental shelf circulation allow the
identification of several zones in the GoL (Durrieu De Madron
et al., 2000): (i) the deltaic and prodeltaic sediment units where most of
the sediments are trapped, (ii) the mid-shelf mud belt between 20 and 50–90 m
depth resulting from sediment transport under the influence of the main
cyclonic westward circulation, and (iii) the outer shelf where fine-grained
sedimentation is presently very low and where relict fine sands are
episodically reworked during extreme meteorological events
(Bassetti et al., 2006).
Holocene paleohydrology in the western Mediterranean
The hydrological budget in the Mediterranean borderlands depends on the
seasonality of precipitation as well as the catchment geology, vegetation
type and geomorphology of the region. In northwestern Mediterranean the most
important fluvial discharges occur in spring and autumn, while minimum flow
is observed in summer (Thornes et al., 2009). On a Holocene timescale, the Mediterranean fluvial hydrology is characterized by the
alternation of wet and dry episodes related to changes in atmospheric
circulation leading to a north–south hydrological contrast in the
Mediterranean region with climate reversal occurring at about 40∘ N
(Magny et al., 2013). Complex climate regimes result from external forcing
(orbital, solar activity, volcanism) as well as from internal modes of
atmospheric variability such as the North Atlantic Oscillation, east
Atlantic, east-Atlantic–west-Russian or Scandinavian modes (Josey et al.,
2011; Magny et al., 2013).
In the NW Mediterranean, the Holocene fluvial hydrology has been
reconstructed using major hydrological events (extreme floods and lake
levels) recorded in lake and fluvial sediments (Arnaud et al., 2012;
Benito et al., 2015; Magny et al., 2013; Wirth et al., 2013). Overall, the
early Holocene climate was generally dry except for short pulses of higher
fluvial activity reported in the Durance and southern Alps rivers
(Arnaud-Fassetta et al., 2010). A marked cooling trend is
observed with a major change around 7500 a cal BP (Fletcher
and Sánchez Goñi, 2008) corresponding to humid conditions in the
Iberian peninsula (Benito et al., 2015). The mid-Holocene
(from ca. 7000 to 5000 a cal BP) also records low torrential activity but
increasing flood frequency between 6000 and 4500 a cal BP in Spain,
Tunisia and southern France (Arnaud-Fassetta, 2004; Benito et al., 2003;
Faust et al., 2004), which evolves in the late Holocene to a general increase
in fluvial activity, at least in the Rhône Basin catchment and north Alps
domain (Wirth et al., 2013). In addition, anthropogenic
activities (agriculture and deforestation) over the last 5000 years have
modified the erosional rate in the catchment area, resulting in
increased/decreased sediment delivery to the sea depending on the
deforestation/forestation phases related to the agricultural development
(Arnaud-Fassetta et al., 2000; van der Leeuw, 2005).
Deglacial and Holocene history of the Rhône Delta
During the last ca. 20 kyr, the morphology of the Rhône Delta strongly
evolved in response to sea-level and climate changes. At the end of the Last
Glacial Maximum, the Rhône reached the shelf edge and directly fed the Petit
Rhône Canyon (Fig. 1) (Lombo Tombo et al., 2015).
The disconnection between the river and the canyon head is dated to 19 ka cal BP in response to rapid sea-level rise (Lombo Tombo et al., 2015). The landward retreat path of
the estuary mouth on the shelf has been tracked through the mapping and
dating of paleodelta lobes (Berné et al., 2007; Fanget et al., 2014;
Gensous and Tesson, 2003; Jouet, 2007; Lombo Tombo et al., 2015) and,
onshore, through the study of ancestral beach ridges (Arnaud-Fassetta,
1998; L'Homer et al., 1981; Vella and Provansal, 2000). During the Younger
Dryas, an “Early Rhône Deltaic Complex” (ERDC) formed at depths between -50 and -40 m below the present sea level (Berné et
al., 2007). The estuary then shifted to the NW as the sea level rose during the
early Holocene (Fanget et al., 2014). The period of maximum
flooding in the delta (the turnaround between coastal retrogradation and
coastal progradation) is dated to ca. 8500–7500 a cal BP
(Arnaud-Fassetta, 1998). Around this time, the mouth of the Rhône was
situated about 15 km north of its present position. Between this period and
the Roman Age (approximately 20 BC–390 AD in western Europe), the position
of the Rhône outlet(s) are not precisely known and many distributaries, with
their associated deltaic lobes, have been identified. However, there is a
general consensus on the eastward migration of the delta from the St Ferréol
Distributary, which occupied the position of the modern Petit Rhône between
ca. 6000 and 2500 a cal BP, and the modern Grand Rhône, rearranged for navigation at the end
of the 19th century. To the west of the Rhône, a mud belt and subaqueous
delta, about 150 km in length and up to 20 m thick, is observed (Fig. 1). So
far, little attention has been paid to this sediment body, and neither
seismic data nor detailed core analysis were available.
Material and methods
The gravity core KSGC-31 (7.03 m long) was retrieved from the Rhône mud belt
(43∘0′23′′ N, 3∘17′56′′ E; water depth 60 m) during the
GM02-Carnac cruise in 2002 on the R/V Le Suroît. Seismic data were
acquired in 2015 aboard R/V Néréis during the Madho1 cruise, using
an SIG™ sparker. The shooting rate was 1 s. Data were loaded on a
Kingdom™ workstation. An average seismic velocity of 1550 m s-1
(based on measurements of sonic velocity with a Geotek™ core logger)
was used to position the core data on seismic profiles. The uncertainty in
the position of time lines on the seismic profile at the core position is on
the order of ±0.5 m, taking into account the resolution of the
seismic source, the errors in positioning and sound velocity calculation.
Due to the shallow water depth, core deformation by cable stretching is
considered negligible.
Grain size analyses were carried out by mean of a Malvern™ Mastersize
3000 laser diffraction particle size analyzer using a HydroEV dispersing
module, which measures particle grain sizes between 0.04 and
3000 µm. Samples were dispersed in a solution of (NaPO3)6 (1.5 g L-1 of
distilled water) for 1 h in order to better disaggregate the sediment.
Before each measurement, the sample was stirred on a rotating mixer for 20 min. Grain size parameters were measured all along the core every centimeter.
Three size ranges were used to classify the grains: clay (< 8 µm,
as recommended by Konert and Vandenberghe (1997), coarse silt (> 8 and
< 63 µm) and sand (> 63 and < 250 µm).
The D50, representing the maximum diameter of 50 % of the
sediment sample, was calculated.
Core KSGC31 was analyzed using an Avaatech XRF core scanner at IFREMER
(Brest, France). This nondestructive method provides semiquantitative
analyses of major and minor elements by scanning split sediment cores
(Richter et al., 2006). Measurements were performed every 1 cm
with a counting time of 20 s and a 10 and 30 kV acceleration
intensity. Resulting element abundances are expressed as element-to-element
ratio. Three ratios are used in this work:
Ca / Ti ratio, to account for
two end-members in the sediment composition. The Ca is supposedly mostly
derived from biogenic carbonates, while Ti is commonly used for tracking
terrigenous sediments, even if usually found in small amounts. Nonetheless,
it is worthwhile remembering that calcite of detritic origin, generated by
erosion of calcareous massifs in the catchment area, represents an important
component of the fluvial Rhône water sediment. This type of calcite is
transported into the sea, but it is mainly accumulated in the sand fraction,
trapped in the proximal deltaic sediments. In the mud belt, where deposits
are mostly pelitic, the detritic calcite quickly decreases seaward of the
river mouth, with only a very small fraction being preserved in the clay
fraction (Chamley, 1971). On the other hand, calcite of biogenic marine
origin (bioclasts) is usually abundant. Benthic (rare planktonic)
foraminifera, ostracods, fragmented mollusk shells and debris from bryozoan
and echinoids can be observed under the binocular microscope. Thus, the Ca
content in the core KSGC31 is considered to be related to biogenic marine
productivity.
Zr / Rb reflects changes in grain size, with higher values in
the relatively coarse-grained sediments. Zr is enriched in heavy minerals
and commonly associated with the relatively coarse-grained (silt–sand)
sediment fraction (highest Zr values are found in sandstones), whereas Rb
is associated with the fine-grained fraction, including clay minerals and
micas (Dypvik and Harris, 2001).
K / Ti values can be related
to illite content. Illite is formed by the weathering of K-feldspars under
subaerial conditions, and most of the K leached from the rocks is adsorbed by
the clay minerals and organic material before it reaches the ocean
(Weaver, 1967). In the case of the GoL, the Rhône waters
deliver mainly illite and chlorite to the Mediterranean Sea, whereas rivers
flowing from Massif Central, Corbières and Pyrenees mainly carry
illite and montmorillonite (Chamley, 1971). Thus, illite (K) is thought
to be abundant in fluvial waters ending in the GoL, and thus K relative
abundances can be used as a proxy for sediment continental provenance.
Because illite may be depleted in K upon pedogenetic processes, the K / Ti
ratio can be considered an indirect proxy for the intensity of chemical
weathering (Arnaud et al., 2012).
The XRF raw data were smoothed using a five-point moving average to remove
background noise. In addition, semiquantitative bulk geochemical parameters such as total
carbon (TC), organic carbon (OC) and total nitrogen (TN) were determined
from freezed-dried homogenized and precisely weighed subsamples of sediment
using the Elementar Vario MAX CN automatic elemental analyzer. Prior to the
OC analyses, samples were acidified with 2 M HCl overnight at 50 ∘C
in order to remove carbonates (Cauwet et al., 1990). The
precision of TC, OC and TN measurements was 5 and 10 %. The calcium
carbonate content of the sediments was calculated from TC - OC using the
molecular mass ratio (CaCO3: C = 100 : 12). Results are expressed as the
weight percent of dry sediment (% d.w.). The atomic C : N ratio (C : Na)
was calculated and used as a qualitative descriptor of organic matter (OM).
Moloney and Field (1991) proposed C : Na= 6 for OM of
marine origin because of the high protein content of organisms such as
phytoplankton and zooplankton. Higher plant-derived OM of terrestrial origin
have higher C : Na ratios (> 20) than marine organisms because
of a high percentage of non-protein constituents (Meyers and
Ishiwatari, 1993). In marine sediment, C : Na ratios are usually higher
than phytoplankton. C : Na ratios between 6 and 10 are
indicative of degraded organic detritus resulting from the breakdown of the
more labile nitrogenous compounds, and values of C : Na ratio > 13 indicate a significant contribution of terrestrial organic matter
(Goñi et al., 2003).
The age model is based on 21 radiocarbon dates (Table 2) obtained by
an accelerator mass spectrometer (AMS) at the Laboratoire de Mesure du Carbone
14, Saclay (France). The two uppermost dates were obtained at the Beta Analytic
Radiocarbon Dating Laboratory and indicate post-bomb values (AD 1950). The
14C dates were converted into 1σ calendar years using Calib7.1
(Stuiver and Reimer, 1993) and the MARINE 13 calibration dataset
including the global marine reservoir age (400 years) (Charmasson
et al., 1998). We used a local marine reservoir age correction of ΔR= 23 ± 71 years (http://calib.qub.ac.uk/marine/regioncalc.php). The age model was obtained
by polynomial interpolation between 14C dates excluding the minor
reversal at 18.5 cm (350 ± 78 years) and the two post-bomb dates. Timing
and uncertainty for the main events is estimated using the Bayesian approach
of OxCal 4.2 (Ramsey and Lee, 2013) (Table 3). We used the same age model
as in Jalali et al. (2016). Age inversions are not used
in the estimation of the sedimentation rate (SR) (Table 2).
Chronology of Holocene cold relapses (CRs) based on existing
literature.
EventTime slice (ka)ReferencesCR08.2Barber et al. (1999)CR16.4–6.2Wanner et al. (2011)CR25.3–5.0Magny and Haas (2004),Roberts et al. (2011)CR34.2–3.9Walker et al. (2012)CR42.8–3.1Chambers et al. (2007),Swindles et al. (2007)CR51.45–1.65Wanner et al. (2011)CR60.55–0.15Wanner et al. (2011)
a Post-bomb radiocarbon ages, obtained using OxCal 4.2 (Ramsey and Lee, 2013), not used for the interpolation. b Reversal
date, not used for the interpolation.
ResultsAge model and sedimentological core description
Core KSGC31 was retrieved at the seaward edge of the Rhône mud belt. The
seismic profile at the position of the core displays the architecture of
this mud belt that drapes Pliocene rocks and continental deposits of the
Last Glacial Maximum (Fig. 2). The bottom of the core corresponds to the
ravinement surface (RS in Fig. 2) that was formed by wave erosion at the time of marine
flooding during the deglacial period. This 20 cm thick heterolytic interval
includes fluvial and coastal sands and gravels mixed with marine shells in a
muddy matrix. At the position of core KSGC31, it is postdated by the
overlying muds immediately above (ca. 10 000 a cal BP). The period of “turn
around” between coastal retrogradation and coastal progradation is well
marked on the seismic profile by a downlap surface dated to ca. 7.5 ka cal BP
at the position of the core. It corresponds to the maximum flooding surface in the sense of Posamantier and Allen (1999). Two other
distinct seismic surfaces (higher amplitude, slightly erosional) can be
recognized in the upper part of the wedge (Fig. 2); they are dated to ca. 4.2
and 2.5 ka cal BP from the core.
Based on the 21 14C dates, the average SR has been estimated to be ∼ 0.70 m 1000 yr-1. The absolute chronology allows
identifying three stratigraphic intervals corresponding to the formal
subdivision of the Holocene epoch proposed by Walker et al. (2012). The well-known
cold events (CRs) are defined on the basis of this chronology
(Fig. 3, Table 1) and used in this paper to highlight possible correlation
with local conditions.
Correlation age–depth in core KSGC31. The Holocene time is divided
into early, middle and late Holocene according to Walker et al. (2012).
General core lithology is shown through the distribution of three grain size
classes: < 8, 8–63 and > 63 µm.
The core is predominantly composed of silt (60–70 %) and clay. The clay
content is highly variable but no more than 50 % between 10 000 and 4000 a cal BP and between 50 and 60 % in its upper 350 cm, corresponding to the
last 4000 years (Fig. 3). Small-size shell debris is randomly mixed with
the clayey silt but becomes more abundant between 400 and 500 cm depth.
Abundant and well-preserved Turritella sp. shells that have certainly not been reworked are found
between 680 and 640 cm. The sand fraction is generally very low (0.5–5 %)
except for the lowermost 30 cm (50 %, Fig. 3). At visual inspection, the
thin sandy base (between 703 and 690 cm) contains very abundant shell
debris. Weak bioturbation is visible on the X-ray images as well as the
occurrence of sparse articulated shells.
Elemental and geochemical distribution
Ca / Ti, K / Ti and Zr / Rb ratios were generated and cross-analyzed with
grain size (clay content D50 computed curve) and C : Na to assess
changes in geochemical composition.
In the early Holocene, the Ca / Ti ratio is fairly constant and relatively
high. The carbonate content is high (> 45 % CaCO3; Fig. 4b), whereas C : Na values fluctuate greatly between values of 20
(∼ 10 ka) and lower values of 13 towards the mid-Holocene
(Fig. 4a). Zr / Rb ratios gradually decrease, while K / Ti shows relatively
stable behavior. Between 7000 and 9000 a cal BP, K / Ti and Zr / Rb indicate
lower values, but with a peak in the mid-interval, around 8200–8300 a cal BP (Fig. 4d,
e). Throughout the period, clay content is approximately between 24 and 52 % (Fig. 5c); D50 is generally
> 10 µm and variable (Fig. 4f). A significant drop in Ca / Ti and D50 is observed in the 7000–6400 a cal BP interval (Fig. 4c,
f). Similar trends are observed for the K / Ti and Zr/Rb, but the most abrupt
drop occurs between 6500 and 6400 a cal BP. No significant changes are
detected in the main lithology (mostly clayey; Fig. 3). C : Na ratios
decrease (< 13) due to a better preservation of nitrogen in clay
deposits.
KSGC31 geochemical and sedimentological proxies. (a) C : Na
atomic ratio is used as qualitative descriptor of organic matter nature.
Values of C : Na ratio > 13 indicate significant amount of
terrestrial organic matter, according to Goñi et al. (2003). (b) CaCO3
content (%) calculated from TC - OC using the molecular mass
ratio (CaCO3: C = 100 : 12). (c) Ca / Ti ratio is used for estimating the
degree of detritism, since Ti is commonly found in terrigenous sediments.
(d) K / Ti ratio can be related to illite content, formed by weathering of
K-feldspars. Illite may be depleted in K upon pedogenetic processes; the
K / Ti ratio can be considered an indirect proxy for the intensity of
chemical weathering (Arnaud et al., 2012). (e) Zr / Rb is known to reflect
changes in grain size; Zr is commonly associated with the relatively
coarse-grained fraction of fine-grained sediments, whereas Rb is associated
with the fine-grained fraction. (f) D50 represents the maximum diameter of
50 % of the sediment grain size. These plots are correlated to the reconstruction of North Atlantic Oscillation (NAO) (g) from a lake in Greenland (Olsen et al., 2012) and
reconstructed SST (C∘) from alkenones (h) in the same core
(Jalali et al., 2016). Blue bands correspond to CR0-6 chronology; dotted
black lines highlight the main wet events that may be observed in sediment
records in the late Holocene (Table 3).
After 6.4 ka cal BP, Ca / Ti displays a constant decreasing trend until
4200 a cal BP. On the other hand, C : Na between 6400 and 4200 a cal BP
reveals two prominent peaks (> 15) culminating at 5700 and 4800 a cal BP
(Fig. 4a) that roughly correspond to low K / Ti and Zr/Rb values
(Fig. 4d, e) and higher clay (Fig. 5c) and lower carbonate sediment
contents (Fig. 4b). The most pronounced changes in the elemental ratio are
observed after 4200 a cal BP (Figs. 4 and 5). Millennial-scale
oscillations are discernible in the Ca / Ti record (Fig. 4c) and coherent
with changes in K / Ti and Zr / Rb ratios (Fig. 4d, e) and, to some extent,
with the D50 values (Fig. 4f). Six main episodes of high terrigenous
inputs (lowest Ca / Ti) are clearly expressed in the XRF data at
∼ 3500 ± 170, ∼ 2840 ± 172,
∼ 2200 ± 145, ∼ 1500 ± 124,
∼ 1010 ± 75 and ∼ 720 ± 72 a cal BP
(Fig. 4, Table 3). Considering the age uncertainty, only some of those
events may coincide with CRs (CR6, CR5, CR4; Fig. 4). The peaks in the
clay content correspond to low Ca / Ti ratios of variable amplitude. The clay
content of the 2840 and 2200 a cal BP events is among the highest
(∼ 35 %; Fig. 5e). From 4200 a cal BP to the present, the
C : Na values decrease gradually. Between ∼ 4200
and ∼ 3200 a cal BP, some values exceed 13. Thereafter, the
C : Na values range between 9 and 10 (Fig. 4a). The late Holocene is
also characterized by decreasing carbonate content with a drastic drop
around 2000 a cal BP (Fig. 4b). The SR is also higher than during the
mid-Holocene lying between 0.5 and 1 mm yr-1.
Time uncertainty (1σ) of “wet” events identified by the
Ca / Ti and K / Ti ratios (Fig. 4).
DatesStartMaximum±1σEndProxyin the text(a cal BP)(a cal BP)uncertainty(a cal BP)0.72 ka64572072800Ca / Ti1.01 ka10001015751070Ca / Ti1.5 ka140015001241640Ca / Ti2.2 ka208022001452300Ca / Ti2.84 ka270028401722900Ca / Ti3.5 ka335035001703615Ca / Ti4.8 ka467048001504960K / Ti5.7 ka553057001625770K / TiInterpretation and discussion
Numerous forcing factors (sea level, ice cap extent, forest cover, volcanic
activity, etc.) may account for the climate variability in the Holocene.
Statistical analysis of proxy time series in both the Northern and the Southern
Hemisphere (Wanner et al., 2011) have demonstrated that
multidecadal to multicentury CRs interrupted periods of
relatively stable climate conditions. They are demonstrated to exist at least
in the North Atlantic (Bond, 1997) and the surrounding land areas.
However, there is a general agreement about the different local expressions
and timing offset of these rapid climate changes according to geographical
position or geomorphological setting. In a way, these events cannot be
considered truly global, but they nonetheless represent significant
milestones in the Holocene climate history. In this paper, we use the
correlation with CRs known from the literature (Table 3) in order to
highlight possible differences in features and chronology of rapid events
between the Atlantic and the western Mediterranean during the early, middle and late
Holocene.
Ca / Ti ratio (a) and percentage of fine-grained
(< 8 µm) sediment (c) compared to (b) periods of intense fluvial activity based on
hydromorphological and paleohydrological changes in the Rhône Delta
(Arnaud-Fassetta et al., 2010). (d) Holocene flood frequency (%) in
southern and northern Alps estimated on the basis on lake flood records by
Wirth et al. (2013). (e–g) Lake level fluctuations in central and
eastern Europe during the Holocene (Magny et al., 2013). Blue bands
correspond to CR0-6 chronology; dotted black lines highlight the main
wet events that may be observed in sediment records in the late Holocene
(Table 3).
Early Holocene (11.7–8.2 ka cal BP)
The lower 20 cm of the core are made up of heterolithic coarse-grained
sediments of continental origin mixed with abundant shell debris.
This interval corresponds to the ravinement surface (RS) seen on seismic profiles (Fig. 2); it was formed by
transgressive erosion when the relative sea level was -30/40 m lower than today.
It is unconformably overlaid by fine-grained sediments that represent the
initiation of the mud belt around 9000 a cal BP. The ∼ 9–8.2 ka interval is marked by highest SR values and high terrestrial supply, as
also indicated by the high C : Na ratio (> 13) (Fig. 4a)
(Buscail and Germain, 1997; Buscail et al., 1990; Gordon and Goñi,
2003; Kim et al., 2006). The C : Na ratios ∼ 20, indicative of even larger enrichment in organic material originating from
soils or plant debris in the coarse deposit at the very bottom of the core
(700 cm) (Hedges and Oades, 1997; Meyers and Ishiwatari, 1993), are of note. A layer
of high Turritella abundances is identified in the fine-grained sediments just above
the sandy interval (680–640 cm, i.e. 8500–8000 a cal BP) (Fig. 3). Then
Turritella shells disappear gradually towards the top of the core, suggesting an
upward deepening environment. The high Turritella level could indicate a change in
Northern Hemisphere climate and can be hypothetically related to the
“Turritella Layer” described by Naughton et al. (2007) on the NW Atlantic
shelf, therefore suggesting a regional change between 8700 and 8400 a cal BP,
possibly in relation to the southward migration of the boreal
biogeographical zone. The maximum flooding surface (MFS) is dated around
7500 a cal BP (Fig. 2). This age may vary at different locations because
it depends upon the ratio between sediment delivery and accommodation space,
but it matches well the age of delta initiations observed worldwide by
Stanley and Warne (1994).
The increase in K / Ti between 9000 and 7000 a cal BP may reflect the
gradual decrease in the contribution of weathered material from the river
catchment areas, which can thus be interpreted as a signal of weaker
pedogenetic processes and lower soil erosion due to a dry climate in the European
Alps (Fig. 4d) (Arnaud et al., 2012).
The period between ∼ 12 000 and 7000 a cal BP is marked by a
continuous retreat of Arctic continental ice sheets until the complete
disappearance of the Fennoscandian and Laurentide ice cap (Tornqvist and
Hijma, 2012; Ullman et al., 2015). Ice sheet melting is seen in the general
sea-level rise and also manifested by short-lived water releases into the
ocean, occasionally perturbing the North Atlantic Ocean circulation and
climate over Europe (for example the 8200 cal BP event, here CR0). It is
worth noting that around CR0, the K / Ti ratio shows a peak within an
interval of low values between approximately 7900 and 8300 a cal BP. This
peak would identify an increase in continental supply and low chemical
weathering corresponding to cold (weak soil formation) and wet (high
physical erosion) conditions over midlatitude Europe in response to the
8200 a cal BP cooling (Arnaud et al., 2012; Magny et al., 2003). This
phenomenon is also attested to by higher lake levels in western Europe (Fig. 5d,
f) concurrent with CR0 (Magny et al., 2013). Note that no clear
temperature drop in the alkenone-derived sea surface temperature (SST) generated in the core
has been detected (Jalali et al., 2016).
Mid-Holocene (8.2–4.2 ka cal BP)
Values of C : Na ratios are mainly > 13 (Fig. 4a) between 6.3
and 4.4 ka cal BP, while Ca / Ti shows a slight progressive decrease that can
be interpreted as an increase in terrestrial inputs during the mid-Holocene,
despite the increasing distance of the KSGC31 site from river outlets due to
sea-level rise and the progressive shift of the Rhône Delta to the east
(Fanget et al., 2014).
The mid-Holocene is described as a period of relatively mild and high
atmospheric moisture balance (Cheddadi et al., 1998) that
favored the maximum expansion of the mesophytic forest leading to maximum
land cover over Europe. Nonetheless, two main short-lived climate anomalies
are reported at 6600–5700 a cal BP (CR1, Tables 1 and 3) and 5300–5000 a cal BP
(CR2, Tables 1 and 3) over the North Atlantic (Wanner et al., 2011) and in Europe (Magny and Haas, 2004;
Robert et al., 2011), at the time of global cooling. CR1 is associated with a drying climate in eastern Europe and Asia and has been related to the
weakening of the Asian monsoon and the decrease in summer insolation
(Gasse et al., 1991), while CR2 coincides with weaker solar
activity as indicated by maximum atmospheric 14C around 5600–5200 a cal BP
(Stuiver et al., 2006), lower tree lines (Magny
and Haas, 2004) and colder sea surface temperatures (Jalali et al., 2016).
According to our data, during CR1 and CR2, chemical weathering was weak as
suggested by high K / Ti values (Fig. 4d) and mean SR was generally low
(< 1 mm yr-1 on average; Table 2), but there was no significant change
in terrigenous inputs (Ca / Ti ratio; Fig. 4c). The C : Na ratios
indicate better preservation of nitrogen organic compounds preferentially
adsorbed in the clay fraction (Fig. 4a). The reduction in the vegetation
cover in the river catchment, combined with lower (1–1.5 ∘C)
temperatures in the European Alps (Haas et al., 1998), may explain
the low chemical degradation state of the illite minerals. A drop in sea
surface temperature is also recorded by alkenones in the core, as illustrated
in Jalali et al. (2016), confirming the impact of the cold relapses in the
Mediterranean area in the mid-Holocene (Fig. 4h).
Late Holocene (4.2–0 ka cal BP)
Multidecadal to century-scale wet episodes are evidenced from
∼ 4200 a cal BP, which marks the mid–late Holocene transition
(Fig. 3). In the KSGC31 core, wetter intervals are expressed by highly
fluctuating Ca / Ti, Zr / Rb and K / Ti ratios, C : Na, and grain size values
(Fig. 4 and Table 3). From the present to 3.5 ka cal BP, the C : Na ratio
shows an increase from 9 to 11 (Fig. 4), testifying to active diagenetic
processes due to preferential degradation of nitrogen relative to carbon
during burial. Some values > 13 are still observed between 4200
and 3500 a cal BP, indicating enhanced terrestrial inputs. Episodes of
enhanced terrigenous inputs (during floods, for instance) are detected by
low Ca / Ti ratios that also coincide with low Zr / Rb and low D50 values,
indicating general, smaller-size terrigenous grains as also suggested by high
clay content (Fig. 5c). Indeed, after the stabilization of the sea level, only
the finest sediment fraction (clay) transported by the river plume reaches
the mud belt at the core site.
An exception to this pattern is observed for the LIA, when quite high Ca / Ti
would suggest relatively dry conditions (Fig. 4c, f). The qualitative
observation under the binocular microscope of the coarse (> 63 µm)
fraction reveals the presence (only in this specific interval) of
abundant bryozoans and Elphidium crispum (coastal benthic foraminifer) tests together with
rare grains of quartz. The biogenic debris can explain the high Ca content
and presence of quartz grains, the peak of Zr / Rb (Fig. 4e). The
accumulation of this material is maybe due to the concomitant occurrence of
river floods (Fig. 5d) and storms, which may have remobilized coarse
material from a coastal setting (Bourrin et al., 2015).
Thus, intensified hydrological activity associated with high terrestrial
inputs would have prevailed during the late Holocene, as also suggested by
higher SR (Table 2). The enhanced terrestrial inputs are inferred from XRF
ratios and discussed in this work, but the biomarker data in Jalali et al. (2016)
also highlighted enhanced flood activity during the late Holocene.
The TERR-alkane (alkenones and high-molecular-weight odd-carbon numbered n-alkanes) concentrations are among the highest of the entire Holocene
record, with maxima recorded during Common Era (last 2000 years).
A similar signature of continental runoff in marine sediments (low Ca / Ti
ratio) during the past ∼ 6500 years has been reported in the
central Mediterranean and related to climatically driven wet periods
(Goudeau et al., 2014). In the KSGC31, these events (∼ 2840 a,
∼ 1500 and ∼ 720 a cal BP; Fig. 4) barely
coincide with the cold events in the North Atlantic but are concomitant with
periods of increasing flood frequency in the northern Alps as reconstructed
by Wirth et al. (2013) (Fig. 5d) and punctuated by overall warm (and dry)
periods such as the MCA at ∼ 3.500, ∼ 2.200, ∼ 1.000 and ∼ 0.72 a cal BP (Fig. 4). This
pattern suggests different causes for enhanced precipitation in the late
Holocene.
A possible control of the North Atlantic Oscillation (NAO) on the amount of
precipitation in the Mediterranean land areas may be put forward. The NAO
exerts a strong influence on the precipitation pattern in Europe and the NW
Mediterranean region. Today, precipitation in the western Mediterranean
region and southern France is lower during positive NAO. Rainfall increases
under negative NAO due to the southern shift of the Atlantic storm tracks
leading to enhanced cyclogenesis in the Mediterranean Sea (Trigo
et al., 2000). The position of the ITCZ is also important in the
precipitation pattern of the Mediterranean region, and its southernmost
position is the probable cause for extremely dry conditions between 2500
and 2000 a cal BP (Schimmelpfennig et al., 2012). The
reconstructed NAO index (Olsen et al., 2012) indicates a
predominance of positive states between 5000–4500 and 2000–550 a cal BP
(Fig. 4g), in agreement with (a) an increased frequency of floods in
the northern Alps (Fig. 5d; Wirth et al., 2013) and (b) higher lake levels at
Accesa (central Italy), at Ledro (northern Italy) and in central-western Europe
(Magny et al., 2013), all together suggesting more humid conditions in
west-central Europe (Fig. 5d, f).
Late Holocene human settlements along the Rhône Valley and southern France may also have had an impact on the origin and amounts of eroded sediments in the
river catchment areas. The chemical signature of KSGC 31 sediments shows that low K / Ti ratios, reflecting soil weathering due to terrain degradation, are coeval with wet events (Fig. 4d). It could be interpreted as the
result of widespread deforestation by agropastoral activities in this area
since the end of the Neolithic. The most extensive erosion episodes in the
Rhône Valley correspond to (1) the end of the Neolithic (∼ 4000 BC;
6000 a cal BP) after the first phase of human expansion linked to the
development of agriculture, 2) the end of the Bronze Age (∼ 2000 BC; 4000 a cal BP), and 3) the Roman period, when a rapid
transformation of landscape is operated by deforestation and the replacement of forest by intensively cultivated agricultural land (van
der Leeuw, 2005).
Disentangling human impact from climate control on environmental changes in
the late Holocene is not an easy task and requires the study of other river
catchment basins to confirm the regional character of these observations.
However, assuming that climate variability is the major factor influencing
soil pedogenesis, we can hypothesize that the elemental composition of
marine sediments reflects continental erosion and transport because of a
good correspondence with temperature variability in the Mediterranean Sea
along the same core (Jalali et al., 2016) and because, on a regional scale,
both marine and continental climate proxies indicate coeval signals
(Arnaud et al., 2012; Goudeau et al., 2014) . Despite the fact that the
characteristics in amplitude and duration of these climate intervals differ slightly geographically, there seems to be a general agreement on
their origin and the role of solar forcing and large-scale atmospheric
circulation. However, the amplification of soil degradation following waves of
human occupation should be further explored through the accurate correlation
of archeological data and paleoenvironmental proxies in order to better
evaluate the importance of land use for sedimentary signals.
Conclusions
This work represents the first attempt to detect and decipher the linkages
between rapid climate changes and continental paleohydrology in the NW
Mediterranean shallow marine setting during the Holocene.
Based on the combination of sedimentological and geochemical proxies we
could demonstrate that between 11 and 4 ka cal BP, terrigenous input broadly
increased. A Turritella-rich interval is observed in the 8.5–8 ka cal BP interval,
which could correspond to a change in Northern Hemisphere climate and can be
correlated to the “Turritella Layer” described in the NW Atlantic shelf, possibly in
relation to the southward migration of the boreal biogeographical zone.
From ca. 4000 a cal BP to present, the sediment flux proxies indicate
enhanced variability in the amount of land-derived material delivered to the
Mediterranean by the Rhône River input. We suggest that this late Holocene
variability is due to changes in large-scale atmospheric circulation and
rainfall patterns in western Europe, including the increased variability of
extension and retreat of Alpine glaciers. Anthropogenic impacts such as
deforestation, resulting in higher sediment flux into the Gulf of Lions, are
also likely and should be taken into account better in the future.
Acknowledgements
We thank MISTRALS/PALEOMEX for financial support and the crew operating the
GMO2-Carnac (R/V Le Suroît) and GolHo (R/V Néréis)
cruises. Nabil Sultan and the crew and science parties aboard
R/V Suroit (IFREMER) retrieved core KSGC31 during the GMO2-Carnac cruise. The captain
and crew of R/V Néréis (Observatoire Océanologique de Banyuls) as well as Olivier Raynal and Raphael Certain (CEFREM) are thanked for their
assistance during the MADHO 1 cruise. ARTEMIS (Saclay, France) program is
acknowledged for performing the 14C measurements. Two anonymous
reviewers are acknowledged for providing suggestions that allowed us to improve
the quality of the manuscript. S. Luening is thanked for commenting on the
manuscript during the open discussion.
Edited by: B. Martrat
Reviewed by: two anonymous referees
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