CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-15-253-2019Diatom-oxygen isotope record from high-altitude Lake Petit (2200 m a.s.l.) in
the Mediterranean Alps: shedding light on a climatic pulse at 4.2 kaShedding light on a climatic pulse at 4.2 kaCartierRosinerosine.cartier@geol.lu.seSylvestreFlorencePaillèsChristineSonzogniCorinneCouapelMartinehttps://orcid.org/0000-0001-9742-6881AlexandreAnnehttps://orcid.org/0000-0003-0696-4022MazurJean-CharlesBrissetElodiehttps://orcid.org/0000-0001-8706-0505MiramontCécileGuiterFrédéricAix-Marseille University, CNRS, IRD, Collège de France, INRA, CEREGE, Europôle de l'Arbois, 13545 Aix-en-Provence, FranceAix-Marseille University, CNRS, IRD, Avignon University, IMBE, Europôle de l'Arbois, 13545 Aix-en-Provence, FranceIPHES, Institut Català de Paleoecologia Humana i Evolució Social, Tarragona, SpainÀrea de Prehistòria, Universitat Rovira i Virgili, Tarragona, SpainLund University, Department of Geology, Lund, SwedenRosine Cartier (rosine.cartier@geol.lu.se)7February201915125326315August20183September201830December2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://cp.copernicus.org/articles/15/253/2019/cp-15-253-2019.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/15/253/2019/cp-15-253-2019.pdf
In the Mediterranean area, the
4.2 ka BP event is recorded with contrasting
expressions between regions. In the southern Alps, the high-altitude Lake
Petit (Mercantour Massif, France; 2200 m a.s.l.) offers pollen and
diatom-rich sediments covering the last 4800 years. A multi-proxy analysis
recently revealed a detrital pulse around 4200 cal BP due to increasing
erosion in the lake catchment. The involvement of a rapid climate change leading
to increasing runoff and soil erosion was proposed. Here, in order to clarify
this hypothesis, we measured the oxygen isotope composition of diatom silica
frustules (δ18Odiatom) from the same sedimentary core.
Diatoms were analysed by laser fluorination isotope ratio mass spectrometry
after an inert gas flow dehydration. We additionally enhanced the accuracy of
the age–depth model using the
Bacon R package. The
δ18Odiatom record allows us to identify a 500-year time
lapse, from 4400 to 3900 cal BP, where δ18Odiatom reached its
highest values (>31 ‰). δ18Odiatom was about
3 ‰ higher than the modern values and the shifts at 4400 and
3900 cal BP were of similar amplitude as the seasonal δ18Odiatom shifts occurring today. This period of high δ18Odiatom values can be explained by the intensification of
18O-enriched Mediterranean precipitation events feeding the lake during
the ice-free season. This agrees with other records from the southern Alps
suggesting runoff intensification around 4200 cal BP. Possible changes in
other climatic parameters may have played a concomitant role, including a decrease
in the contribution of 18O-depleted Atlantic winter precipitation to the
lake water due to snow deficit. Data recording the 4.2 ka BP event in the
north-western Mediterranean area are still sparse. In the Lake Petit
watershed, the 4.2 ka BP event translated into a change in precipitation
regime from 4400 to 3900 cal BP. This record contributes to the recent
efforts to characterize and investigate the geographical extent of the
4.2 ka BP event in the Mediterranean area.
Introduction
Since the last glaciation, several abrupt climatic changes, with large
environmental effects, were identified from palaeoclimatic records (Berger
and Guilaine, 2009; Magny et al., 2009), such as the Younger Dryas
(13 500–11 500 cal BP)
at the end of the Late Glacial and the 8.2 ka event at the beginning of the Holocene (Alley et al., 1997; Brauer et al.,
1999; Tinner and Lotter, 2001), for the coldest events. Other Holocene
climatic events were described as less intense or regionally limited but may
have triggered substantial impacts on the environment at the local scale.
One of them, the “4.2 ka BP event”, was recognized as an abrupt climate
change (Bond et al., 1997; Booth et al., 2005; Huang et al., 2011; Thompson
et al., 2002; Staubwasser et al., 2003) and is now commonly used as a marker
of Holocene stratigraphy (Walker et al., 2012). In the Mediterranean area,
the 4.2 ka BP event is recorded with contrasting expressions between regions
(Bruneton et al., 2002; Digerfeldt et al., 1997; Drysdale et al., 2006;
Kharbouch, 2000; Magny et al., 2009; Miramont et al., 2008; Zanchetta et
al., 2011). In the eastern Mediterranean area, this climatic event is
assumed to have been responsible for severe droughts and involved in the
fall of the Akkadian civilization (Weiss et al., 1993; Cullen et al., 2000; Dean et
al., 2015). In the central Mediterranean area, speleothem isotope records
suggest a reduction in cave recharges from ca. 4500 cal BP to 4100 cal BP
at Corchia Cave (Isola et al., 2019) and ca. 4500 cal BP to 4100 cal BP at
Renella Cave (Zanchetta et al., 2016), linked to annual and/or winter dry
conditions. In the Alps (northern Italy), an opposite trend has been
described, annual cool and wet conditions being assigned to the period
around 4.2 ka (Magny et al., 2012; Zanchetta et al., 2016). Sedimentary
records of past lake levels also mirror different climatic expression
between regions. At Lake Ledro and Lake Accesa, in Italy, the transition
from mid- to late Holocene surrounding 4.2 ka shows a shift from low to
high lake levels. Pollen-based precipitation reconstructions, although
showing high variability from 5000 to 3000 cal BP, suggest no significant
change in the amount of annual precipitation but increasing summer
precipitation (Peyron et al., 2013). The high-resolution record from Lake
Accesa (Italy) allowed us to interpret the 4.2 ka BP climatic event as a
tripartite climatic oscillation with a phase of drier conditions from 4100
to 3950 cal BP bracketed by two phases of wetter conditions (Magny et al.,
2009). Overall, palaeoclimatic records from the Mediterranean area highlight
climatic features spatially heterogeneous around 4200 cal BP, which makes it
difficult to assign a general pattern. Further studies from different
geomorphological contexts are required for a better characterization of the
4.2 ka BP climatic event in the area.
In the southern Alps, the high-altitude Lake Petit (Mercantour Massif,
France; 2200 m a.s.l.) offers pollen and diatom-rich sediments covering the
last 5000 years. A multi-proxy analysis, including sedimentological and
geochemical measurements, pollen and diatom morphological analyses, revealed
a detrital pulse around 4200 cal BP due to increasing erosion in the lake
catchment (Brisset et al., 2012, 2013), followed by an abrupt change in
diatom assemblages. The replacement of the dominant diatom Staurosirella pinnata by
Pseudostaurosira spp. responded to a change in lacustrine living conditions (e.g. nutrient
availability, turbidity) following the detrital input (Cartier et al.,
2015). The hypothesis of a massive deforestation in the catchment to explain
the detrital pulse was rejected as the vegetation surrounding the lake
stayed open over the last 5000 years. Therefore, the involvement of a rapid
change either in precipitation regime or temperature, leading to increasing
soil erosion and runoff around 4200 cal BP, was proposed (Brisset et al.,
2012, 2013; Cartier et al., 2015).
Here, in order to clarify this hypothesis, we measured the oxygen isotope
composition of diatom silica frustules (δ18Odiatom) from
the Lake Petit sedimentary core previously used for the
multi-proxy analyses and covering the last 5000 years (Brisset et al., 2013). δ18Odiatom
records are commonly used for paleoclimatic reconstructions (e.g. Barker et
al., 2001; Leng and Barker, 2006; Quesada et al., 2015). The δ18Odiatom value is controlled by the lake water isotope
composition (δ18Olake) and the temperature of silica
polymerization. The δ18Olake value is itself influenced by
the δ18O signatures of precipitation (δ18Oprecipitation) and other waters reaching and leaving the lake
(groundwater, surface water) and the extent of the lake water evaporation.
Lastly, the δ18Oprecipitation is controlled by the isotope
composition of its water vapour source and Rayleigh fractionation processes
occurring during the vapour transport and rain drop formation. Changes in
Lake Petit δ18Odiatom values are discussed according to
these parameters, and assumptions characterizing the abrupt climate change
that may have occurred around 4200 cal BP in the lake catchment area, and
more broadly in the southern Alps, are presented.
Site settings
Lake Petit (2200 m a.s.l.; 44∘06.789 N, 7∘11.342 E) is
a small circular body of water, 150 m in diameter, located in the southern
French Alps about 60 km from the Mediterranean Sea. The 6 km2lake
catchment culminates at 2600 m a.s.l. It is composed of crystalline bedrock
(gneiss and migmatites) and is largely covered by alpine meadows. The upper
tree line (Larix sp.) is located at about 2100 m a.s.l. Lake Petit is at the
lowest elevation of a chain of five lakes that were partly formed by glacier
retreat (Fig. 1). The five lakes are connected in spring during meltwater
but remain unconnected for the rest of the year. The lake surface is usually
frozen from October to April. The water depth of Lake Petit reaches 7 m in
the wake of the snowmelt and is about 1 m lower at the end of summer.
The lake is open during snowmelt but has no outlet during summer. Water
inputs are thus represented by snowmelt in spring and precipitation during
the ice-free season. Water outputs mainly consist of evaporation,
infiltration likely being very low due to the geological characteristics of
the catchment. Today, diatoms are mainly benthic but tychoplanktonic diatoms
are also present. These diatoms develop mainly during the ice-free season,
even if some species (e.g. Achnanthes, Fragilaria spp.)
are expected to continue to grow under the
ice during winter as observed in other alpine lakes (Lotter and Bigler, 2000).
Localization map of Lake Petit: (a) mean
δ18O in
precipitation (δ18Op)
(in ‰ vs. V-SMOW) in the
western Mediterranean region (IAEA/WMO, 2018; period 1960–2009)
and selected
palaeoclimatic studies: a – Ecrins-Pelvoux Massif (Le Roy et al.,
2017); b – Buca della Renella (Zanchetta et al., 2016); c –
Accesa Lake (Magny et al.,
2009); d – Preola Lake (Magny et al., 2012).
(b) GNIP (Global Networks of Isotopes in Precipitation) stations (IAEA/WMO,
2018) in black squares: (1) Thonon-les-bains, (2) Draix, (3) Malaussène, (4)
Monaco. (c) Watershed characteristics: (1) glacial cirque, (2) glacial step, (3)
moraine, (4) polished bedrock, (5) active debris slope, (6) dam built in 1947.
In the Mercantour Massif, alpine and mediterranean influences produce a
climate marked by mild winters and dry summers. Mean annual air temperature
at 1800 m a.s.l. is 5 ∘C, varying from 0.3 ∘C in
winter to 9.9 ∘C in summer. Mean annual precipitation is 1340 mm
at 1800 m a.s.l. Snow depths in winter are relatively important (150 to
250 cm at 2400 m a.s.l.) and snow cover duration is about 185 days at
2100 m a.s.l. mainly from November to April (Durand et al., 2009a, b). Because it is
located in the extreme south-western part of the Alps, Lake Petit is
strongly influenced by precipitation originating from the Mediterranean
region during the summer, while winter snowfalls are essentially associated
with north-west Atlantic atmospheric flows (Bolle, 2003; Lionello et al.,
2006, 2012).
In southern France, precipitation is mostly generated by the
clash between the warm, humid air of Mediterranean or mixed
Atlantic–Mediterranean origin and cool air masses coming from the north.
Today, 54 % of precipitation in southern France (average for six
meteorological stations) strictly come from the Mediterranean area, 12 %
from the Atlantic and 34 % have a mixed Mediterranean–Atlantic influence
(Celle-Jeanton, 2001). In spring and autumn the advection of air masses from the
Mediterranean can produce strong storms. Altogether, the Mediterranean
influence remains predominant today, with high Mediterranean δ18Oprecipitation values compared to Atlantic δ18Oprecipitation values: from April 1997 to March 1999, at
Avignon (IAEA/WMO, 43∘57 N) precipitation of Mediterranean origin
had a weighted annual mean δ18Oprecipitation of
-4.33 ‰ (SD=1.72 ‰),
whereas precipitation from the Atlantic had a δ18Oprecipitation of -8.48 ‰
(SD=3.51 ‰) (Celle-Jeanton et al., 2004). Added to
changes in temperature, changes in precipitation sources explain the current
seasonal weighted δ18Oprecipitation values in the Alps,
lower from October to March than from April to September
(Fig. 1, period
1960–2009; IAEA/WMO, 2018; Terzer et al., 2013). Different precipitation
sources also explain the δ18Oprecipitation values obtained
for the same period at the meteorological stations close to Lake Petit
(Fig. 2a; IAEA/WMO, 2018).
(a)δ18Op (in ‰ vs. V-SMOW) from GNIP
stations (IAEA/WMO, 2018) and from Lake Petit (in red) at two key times of
the year (empty red square –
17 May 2011; full red square – 17 September 2011) plotted
across the global meteoric water line (black line). Locations of GNIP
stations are shown in Fig. 1. Mean weighted average of δ18Op for
each station is represented by black filled markers for summer months (April
to September) and black empty markers for winter months (October to March):
Thonon-les-bains (black diamonds),
Malaussène (white squares), Monaco
(black triangles),
Draix (black circles).
(b) Average annual distribution of precipitation (mm)
at the meteorological station Malaussène by month for the year 1997 and
1998; no data for the month of January (IAEA/WMO, 2018). (c) Profile of water
temperature (∘C) as a function of water depth (m) in Lake Petit at
two locations on 17 May 2012.
Material and methods
Sediment core PET09P2 (144 cm long) was sampled in 2009 in the deepest part
of the lake using a UWITEC gravity corer. Core PET09P2 is organic-rich
(total organic carbon represents 9 % of the dry weight on average), and
biogenic silica is abundant (averaging 65 % of the dry weight) (Brisset
et al., 2013). The core is composed of homogeneous yellow to greenish
diatomaceous sediments with millimetre-thick brownish diatom–clay
laminations. The sediments consist of biogenic silica (diatoms), organic
compounds (essentially algal as the hydrogen index comprised between 450 and
575 HC (hydrocarbon) / TOC (total organic carbon) and a terrigenous clay fraction (Brisset et al., 2012, 2013).
The different lithological units are presented in Fig. 4. Diatoms (D)
represent the major contribution of biogenic silica in the sedimentary
record. Only a few cysts of Chrysophyceae (C) were identified (C/D ratio = 0.01).
The age–depth model covering the last 4800 years is based on
short-lived 210Pb and 137Cs radionuclide data and seven
14C ages obtained from terrestrial macro-remains (Brisset et al., 2013, for
further details). For this study, we recalculated the age–depth model using
the Bacon R package (Blaauw and Christen, 2011) and implemented the function
proxy.ghost (square resolution: 200) in order to highlight the
chronological uncertainties of the age–depth model and to estimate the
duration of the 4.2 ka BP event recorded at Lake Petit. Figure 3b shows a
range of possible ages for each sample depth.
(a) Oxygen isotope composition of diatoms (δ18O diatom
expressed in ‰ vs. V-SMOW) from Lake Petit sediments; (b)δ18O diatom (vs. V-SMOW) taking into account the age
uncertainties (the darkest grey is assigned to the most likely value within
the entire core (normalized to 1); lower age probabilities are coloured in
lighter grey); (c) SEM image of a cleaned diatom sample from 127 cm depth
using a scanning electron microscope.
Multi-proxy comparison of environmental responses to the 4.2 ka BP
event at Lake Petit including the lithological units (1: pure diatomaceous
sediments; 2: diatomaceous–clay sediments; 3: clay–diatomaceous sediments;
4: diffuse laminations, Brisset et al., 2013), oxygen isotope measurements
on diatoms (δ18O diatom, ‰ vs. V-SMOW; this
study), the detrital fraction (% dry weight; Brisset et al., 2013),
biogenic silica fluxes (g cm2 yr) and dominant diatom species (relative
abundance (%) of Staurosirella Pinnata, Pseudostaurosira robusta)
(Cartier et al., 2015).
Twenty diatom samples (1 cm3) were subsampled from core PET09P2. Each
diatom sample includes on average 36 years (min: 11 years; max: 55 years) of
sedimentation according to the age–depth model. Diatom samples were weighed
after drying at 50 ∘C. To remove carbonates and organic matter,
the samples were first treated using standard procedures (bathed in a
1:1 mixture of 33 % H2O2 : water, a 1:1 mixture of 10 %
HCl : water, and repeatedly rinsed in distilled water). Following these steps, the
identification and counting of diatom species for palaeoenvironmental
reconstruction were performed. The data were reported in Cartier et al. (2015).
Then, diatom silica frustules were cleaned from remaining detrital
particles by following a protocol based on chemical oxidation and
densimetric separation previously detailed in Crespin et al. (2008). The
purity of each sample was checked using optical and scanning electron
microscopy (SEM). Micro-X-ray fluorescence (XRF) measurements (five measurements per sample) were additionally made using a HORIBA XGT-5000177
microscope equipped with an X-ray guide tube capable of producing a focused,
high-intensity beam with a 100 µm spot size (detection limit: 2 ppm).
The following compounds were detected via XRF: SiO2, Al2O3,
K2O, CaO, TiO2, Fe2O3 and Br2O. The samples are on
average composed of 97.2 % (SD = 1.8 %) of SiO2.
Measurements of oxygen isotopes from diatoms were performed at the CEREGE
Stable Isotope laboratory (Aix-en-Provence, France). The samples were
dehydrated and dehydroxylated under a flow of N2 (Chapligin et al.,
2010). Oxygen extraction was performed using the infrared
(IR) laser-heating
fluorination technique (Alexandre et al., 2006; Crespin et al., 2008). No
ejection occurred during the analysis. The oxygen gas samples were sent
directly to and analysed by a dual-inlet mass spectrometer (ThermoQuest
Finnigan Delta Plus). Measured δ18O values were corrected on a
daily basis using a quartz lab standard (δ18OBoulangé50-100µm) calibrated on NBS28
(9.6±0.3 ‰; n=11). The values are expressed in the
standard δ notation relative to V-SMOW. The long-term precision of
the quartz lab standard is ±0.2 ‰ (1 s; n=50).
The δ18Odiatom values presented here are averages of two
replicates. The reproducibility was better than ±0.2 ‰.
Two surficial lake water samples were collected in the first metres of depth in
spring (17 May 2011) after the snowmelt, and at the end of the
summer (17 September 2011). They were analysed for δ18O
and δD by isotope ratio mass spectrometry (IRMS) and the data were
normalized on the V-SMOW (Vienna Standard Mean Ocean Water)/SLAP (Standard Light Antarctic Precipitation) scale. The values are expressed in the standard
δ notation relative to V-SMOW. Temperature in the water column was
measured in spring (17 May 2012) at two locations (44∘1133 N,
7∘1894 E; 44∘1134 N, 7∘1889 E), every
25 cm, down to the bottom of the lake.
Results
The δ18O and δD compositions of the sampled lake water
were -11.35 ‰ and -80.36 ‰,
respectively, after the snowmelt and -10.19 ‰ and
-72.6 ‰, respectively, at the end of summer 2011. They
plot on the global meteoric water line (Fig. 2a). The distribution of
precipitation over the year at the closest meteorological station to Lake
Petit is presented in Fig. 2b (station Malaussène, period 1997–1998;
IAEA/WMO, 2018). Water temperature measured at two points of Lake Petit in
17 May 2012 varied from 5.4 to 4.9 ∘C and from 6.8 to 5 ∘C
(from surface to bottom) (Fig. 2c).
Because the dissolution of the diatom frustules during sedimentation may occur
and induce kinetic isotope fractionation (Dodd et al., 2017), the samples
were checked under SEM. The diatoms themselves were very well preserved. No
significant dissolution features were observed, as shown in Fig. 3c.
δ18Odiatom values measured on the 20 sedimentary diatom
samples (Table 1) are plotted against ages (cal BP) and presented in Fig. 3a.
δ18Odiatom values range from 26.6 to 32 ‰
with a mean standard deviation (SD) of 0.18 ‰. From the bottom of the core
(4800 cal BP) to 4400 cal BP, the δ18Odiatom average value is
30.3 ‰ ± 0.14 ‰ and the lowest
value (28.97 ‰) occurs at 4750 cal BP. Then, a period
stands out in the record with the highest values of δ18Odiatom for the last 4800 years. At 4400 cal BP, δ18Odiatom increases quickly and reaches its maximum value of
31 ‰. δ18Odiatom remains high (on average 31.3 ‰ ± 0.21 ‰)
between 4400 and 3900 cal BP and decreases, afterwards, to values below
those observed at the base of the core. The period from 3900 to 700 cal BP
shows low-amplitude variations in δ18Odiatom with an
average value of 29.6 ‰ ± 0.13 ‰. After
700 cal BP, the δ18Odiatom
falls sharply to its lowest value over the study period (26.6 ‰
at 309 cal BP). In 1986 CE δ18Odiatom increases
again to reach 27.8 ‰ (Fig. 3a).
Oxygen isotope measurements in diatoms (in ‰ vs. V-SMOW) for the core PET09P2.
The new age–depth model performed with the Bacon R package is presented in
the Supplement Fig. S1. A zoom on the 4800 to 3000 cal BP period is
presented in Fig. 3b. Four 14C ages (Fig. 3a) obtained for this time
interval, yield an age–depth model precision of ca. 320 years. It supports
that at Lake Petit, the 4.2 ka BP event is actually a 500-year period that
occurred from 4400 to 3900 cal BP.
According to age uncertainties, the
4.2 ka BP event cannot be instantaneous in time, and its time range is, at a
confidence interval of 95 % of probability, a minimum of 117 years and
a maximum of 755 years.
Discussion
The 4400 to 3900 cal BP δ18Odiatom values are about
3 ‰ higher than the modern ones (27.8 ‰
in 1986 CE) and correspond to a 1.6 ‰ increase from 4800
to 4400 cal BP and a 1.5 ‰ decrease from 3900 cal BP.
Figure 4 shows that the high δ18Odiatom period is
contemporaneous with the detrital pulse followed by a shift in diatom
species previously evidenced (Brisset et al., 2013; Cartier et al., 2015).
This suggests the occurrence of a climatic pulse that impacted the whole
catchment. This climatic pulse can be further characterized by comparing the
δ18Odiatom signal to the present isotope composition of
the lake water (δ18Olake water) and by assessing the
physical parameters possibly responsible for an increase in δ18Odiatom.
Present δ18Olake water
The hydrological regime of Lake Petit alternates between two states: an open
system when the outlet is active during snowmelt and a closed system during the
remaining time. The 2011 one-off δ18Olake water
measurements indicate that from the beginning of the frost-free season to the
end, the lake water gets heavier by 1.1 ‰. The decrease
in water depth at the same time can be interpreted as a signal of
evaporation. However, in the δD vs. δ18O diagram presented
in Fig. 2a, the lake water samples plot on the global meteoric water
line, which suggests that evaporation has a limited effect on δ18Olake water. The 1.1 ‰ shift may rather be
explained by the drastic decrease in meltwater input at the end of spring.
The oxygen isotope composition of meltwater fed by winter precipitation is
expected to be lower than δ18Oprecipitation during summer,
due to its Atlantic origin and the low temperature at which snow forms.
Post-depositional fractionating processes affecting the snow (including
evaporation, sublimation, ablation, meltwater percolation and drifting) that
may lead to 18O enrichment of meltwater are likely limited. Indeed,
the Lake Petit catchment is small and located under the mountain crest
without any glacier supplying the watershed (Stichler and Schotterer, 2000).
The seasonal shift occurring today in δ18Olake water
has a similar amplitude as the δ18Odiatom shift at 4200 cal BP,
which suggests similar controls.
Paleo-climatic interpretation of the δ18Odiatom record
Diatom blooms in alpine lakes occur mainly after the snowmelt in spring and during autumn. However, sediment traps placed in a lake in
Switzerland located at 2339 m a.s.l. provide evidence that some diatom species
(e.g. Achnanthes, Fragilaria spp.) can continue to grow under the ice
when the lake is frozen (Rautio et al., 2000; Lotter and Bigler, 2000). With the omnipresence of
Fragilaria spp. in the sedimentary record and the absence of any detailed dynamic of
the population over the year, the isotope signal from Lake Petit is
considered to be an annual signal mostly influenced by diatoms growing
during the ice-free season.
The polymerization of the siliceous frustule from the lake water occurs at
equilibrium and the resulting isotope fractionation is thus
thermo-dependent. The equilibrium fractionation coefficient previously
measured for different silica–water couples range from
-0.2 ‰ ∘C-1 to
-0.4 ‰ ∘C-1 (synthesis in Alexandre et al., 2012;
Sharp et al., 2016). According to this range, if the 1.6 ‰ positive
shift in δ18Odiatom around
4400 cal BP was only controlled by the lake water temperature change, this
would require a negative shift in water temperature of 4 to 8 ∘C
during the ice-free season, when most of diatoms grow. A very high
contribution of snowmelt water may lead to a drastic decrease in the lake
water temperature. However, snowmelt is fed by winter precipitation that is
18O depleted, which would counterbalance the effect of low water
temperature on δ18Odiatom.
Air cooling during the ice-free season may also be invoked. Air cooling
during the 4.2 ka BP event in response to a positive North Atlantic
Oscillation (NAO) was previously suggested for central Italy (Isola et al.,
2019). In the Alps, moraine dating showed moderate glacier advances in
northern and western Alps but not in the Mediterranean Alps (Federici and
Stefanini, 2001; Ribolini et al., 2007; Ivy-Ochs et al., 2009; Le Roy, 2012,
2017; Brisset et al., 2015). The recent synthesis of Bini et al. (2018) for
the Mediterranean region also suggests a possible cooling anomaly at some
sites but temperature data are sparse and not uniform. Moreover, the reconstruction of temperature based on chironomids and pollen assemblages
from the Swiss Alps and Europe suggest that air temperature variations
(likely larger than water temperature variations) did not exceed 2 ∘C
during the Holocene (Davis et al., 2003; Heiri et al., 2003).
At the very least, a decrease in air temperature would decrease δ18Oprecipitation and δ18Olake water during
the ice-free period, which would counterbalance the temperature effect on
δ18Odiatom. Therefore, a decrease in air and/or lake water
temperature cannot be referred to as the dominant control on the increase in
δ18Odiatom over the 4400–3900 cal period.
An increase in the contribution of 18O-enriched Mediterranean
precipitation during the ice-free season or, inversely, a decrease in the
contribution of 18O-depleted Atlantic winter precipitation (due to a winter snow deficit) to the lake water may explain an increase in δ18Olake water at Lake Petit around 4400 cal BP. The other
proxies analysed from the studied core instead support the first hypothesis
as developed below.
From 4800 to 4350 cal BP, low detrital supply and high chemical weathering
suggest the presence of developed acid soils on the catchment slopes
(Brisset et al., 2013). From 4350 to 4000, a maximum of clay detrital supply
highlights the dismantling of the former developed weathered soils. The
sediments deposited during this period are characterized by high terrigenous
fluxes, while the diatom-organic component drops to lower but still
significant concentrations (20 %). Added to the over-representation of
low-dispersal alpine meadow plants, these features argue for an
intensification of runoff on the catchment slopes during the ice-free season
(Brisset et al., 2013; our Fig. 4). For the same period, high percentages of
grassland pollen were recorded in Lake Grenouilles (southern Alps) located
close to Lake Petit (Kharbouch, 2000), and detrital events occurred at other
sites of the Alps, for example at Lake Bourget (Arnaud et al., 2005, 2012).
In addition, a cluster of landslide events was identified in the southern
Alps around 4200 cal BP (Zerathe et al., 2014). All these features suggest
that runoff intensified in the southern Alps around 4200 cal BP, likely due
to increasing intense precipitation events, today occurring in autumn (Llasat
et al., 2010). At a broader scale, records are less in agreement.
Reconstructions of past lake levels suggest wetter conditions from 4500 to
3000 cal BP at Lake St-Léger (Alpes-de-Haute-Provence; Digerfeldt et
al., 1997), Lake Ledro (southern Alps) and Lake Accesa (central Italy)
(Magny et al., 2013) (Fig. 5). A high lake level was also reconstructed at
Lake Cerin (Jura massif). However, by contrast, a trend towards
aridification has been suggested at Lake Preola in Sicily (Magny et al.,
2012) or at Renella and Corchia caves in Italy (Drysdale et al., 2006;
Zanchetta et al., 2016) (Fig. 5).
Oxygen isotope measurements in diatoms (δ18Odiatoms ‰ vs. V-SMOW; this work), detrital
fraction (%) and conc. (concentration in pollen of) Botrychium (nb mL-1) (Brisset et al., 2015) at Lake Petit
compared to the palaeoclimatic record from the RL4 flowstone at Buca della Renella expressed in per
mil with reference to Vienna Pee Dee Belemnite (VPDB) standard (northern Italy,
Zanchetta et al., 2016) and Lake level at Accesa (central Italy,
Magny et al., 2007).
A winter snow deficit might have been superimposed on an increase in intense
precipitation events during the ice-free season at Lake Petit. In the
Italian Apennine, oxygen isotope records from speleothems at Corchia Cave
suggested a reduced water cave recharge from ca. 4500 to 4100 cal BP (Isola
et al., 2019). This was interpreted as a weakening of the cyclone centre
located in the Gulf of Genoa in response to reduced advection of air masses
from the Atlantic during winter.
At the very least, evaporation higher than the modern one may also be considered to
explain the 18O enrichment of the Lake Petit water around 4.2 cal BP.
However, at a yearly scale, the effect of the previous summer's evaporation
is expected to be partially or (greatly) offset by the runoff from snowmelt
(Ito et al., 1998), the same as may happen today. Moreover, this would
contradict the assumption of a higher precipitation amount during the ice-free
season.
In summary, at Lake Petit, the high δ18Olake water
values recorded from 4400 to 3900 cal BP support an increase in intense
18O-enriched Mediterranean precipitation events during the ice-free
season, in agreement with other proxies from the same core and other records
from the southern Alps. A reduction in snow may have been superimposed.
However, additional evidence is needed to further assess this hypothesis.
At 3900 cal BP, δ18Odiatom values decreased and remained
relatively constant for 3300 years during the neoglacial period. Although
the low resolution of the record limits the determination of short-term
events, a 2.7 ‰ decrease in the δ18Odiatom values can be identified around 310 cal BP (Fig. 3a).
This is concomitant with a strong decrease in δ18O measured on
ostracods from Lake Allos sediments (Cartier, 2016), suggesting regional
climate change. Conversely to what may have happened during the time
interval 4400–3900 cal BP, an increase in snowmelt contributing to the lake
may have triggered a decrease in δ18Olake water and
δ18Odiatom. This time span falls within the Little Ice Age
(450–50 cal BP). The Little Ice Age is recorded as a cold and humid period
in the southern Alps as shown in tree-ring records (Corona et al., 2010),
fluvial activity reconstructions (Miramont et al., 1998) and glacial tongue
advances (Holzhauser et al., 2005; Ivy-ochs et al., 2009). These records are
thus in agreement with an increase in snowmelt water contribution to Lake Petit.
Conclusions
The location of Lake Petit above the local tree line, at the head of a small
Alpine watershed, as well as its semi-closed lacustrine system lead to the
high responsiveness of the lake to changes in precipitation regime. Thanks
to a robust and accurate age model, the last 4800-year δ18Odiatom record allowed us to identify a 500-year time lapse, from
4400 to 3900 cal BP, where δ18Odiatom reached its highest
values. This period of high δ18Odiatom values can be
explained by intense 18O-enriched Mediterranean precipitation events
feeding the lake during the ice-free season. This agrees with previous
reconstructions from the same core (Brisset et al., 2012, 2013) and other
records from the southern Alps suggesting runoff intensification around
4200 cal BP. Possible changes in other climatic parameters may have played
a concomitant role, including a decrease in the contribution of 18O-depleted
Atlantic winter precipitation to the lake water due to snow deficit. However, additional evidence is needed to further assess this hypothesis.
Data recording the 4.2 ka BP event in the north-western Mediterranean area
are still sparse. In the Lake Petit watershed, a climatic pulse translated
into a change in precipitation regime occurred from 4400 to 3900 cal BP.
This record contributes to the recent efforts to characterize and
investigate the geographical extent of the 4.2 ka BP event in the
Mediterranean area.
Oxygen isotopes values are presented in this paper. The
age–depth model is available in the Supplement. Radiocarbon ages are
already published in the journal The Holocene as Brisset et al. (2013):
10.1177/0959683613508158.
The supplement related to this article is available online at: https://doi.org/10.5194/cp-15-253-2019-supplement.
RC wrote the manuscript and performed analysis with
FS. CP, FG and CM provided funding support and material. AA, EB and FG helped
improve the manuscript. CS, MC and JCM worked on analysing
samples. All co-authors gave their comments and agreement during the
writing process.
The authors declare that they have no conflict of interest.
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 work was supported by the ECCOREV research federation (HOMERE program
led by Frédéric Guiter and Christine Paillès). The PhD thesis
work of Rosine Cartier
(Aix-Marseille University) was funded by the French Ministry of
Education.
We thank Christine Vallet-Coulomb (CEREGE, France) for the isotope analysis of
modern Lake Petit waters and Pauline Chaurand (CEREGE, France) for providing help
with the micro-XRF measurements. Thanks to Alain Tonetto (Aix-Marseille
University) for managing the SEM in Marseille. Coring of Lake Petit (in 2009
and 2012) was made possible thanks to Fabien Arnaud (EDYTEM), Charline Giguet-Covex
(EDYTEM), Emmanuel Malet (EDYTEM), Johan Pansu (Princeton University), Jérome Poulenard
(EDYTEM) and Bruno Wilhelm (LTHE).
Edited by: Giovanni Zanchetta
Reviewed by: two anonymous referees
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