Climate,vegetation and land use as drivers of Holocene sedimentation: A case study from Lake Saint-Point (Jura Mountains,eastern France)

Abstract

A multiproxy approach to a sediment sequence at Lake Saint-Point in the French Jura Mountains gives evidence of a strong coupling between changes in terrestrial and lacustrine ecosystems throughout the Holocene. The early Holocene (11,700–10,200 cal. BP) is characterised by the recovery of terrestrial and lake ecosystems favoured by climatic warming. During the middle Holocene (10,600–6200 cal. BP), the climatic optimum coincided with an extension of deciduous forests into the catchment area, while lake sedimentation is dominated by authigenic carbonates and low detrital inputs. After 6200 cal. BP, the Neoglacial favoured expansion of Abies-Fagus forests and increasing detrital inputs to the lake where ostracod fauna declined and changed in composition. After 1200 cal. BP, human impact was responsible for extensive forest clearings in the catchment area, while the lake basin shows contrasting pictures with increasing detrital input, resuming sedimentation of authigenic carbonates and changes in dominant ostracod species. Orbitally driven climatic variations were the dominant factor of environmental changes until c. 1200 cal. BP. Around 2600 cal. BP, human impact increased and became the major factor in the catchment area and the lake basin from 1200 cal. BP onwards. Finally, the Saint-Point record offers a clear illustration of how gradual changes in insolation or increasing human impact may provoke, even under temperate climatic conditions, abrupt responses in mid-European terrestrial and lake ecosystems, and how differences in the dates of tipping points revealed by proxies suggest specific threshold values depending on the sensitivity of indicators used and on their role in the different compartments of these ecosystems.

Keywords

land use Holocene sedimentation tipping points vegetation west-central Europe

Introduction

Lake sediment archives provide insights into the trajectories followed by terrestrial and lacustrine ecosystems during the present interglacial. The long palaeoclimatic and palaeoenvironmental records established from cores taken in lake basins offer the opportunity to gain long-term perspectives for a better understanding of factors behind the functioning of modern ecosystems and landscapes (Dearing et al., 2008; Giguet-Covex et al., 2011; Millet et al., 2010; Ramrath et al., 1999, 2000). More particularly, with ongoing global change in perspective, they may give evidence of processes associated with a transition, characteristic for the Holocene period, from nature- to human-dominated environmental conditions.

On the basis of a multiproxy approach, recent investigations developed at Lake Saint-Point in the Jura Mountains (eastern France) supplied a continuous palaeoenvironmental record for the Holocene (Leroux, 2010; Leroux et al., 2008). The lake Saint-Point catchment area is located in a region sensitive to changes in climate conditions because of its position in the summit zone of the Jura Mountains and a severe semi-continental climate. Moreover, this area is characterised by a relatively late extension of anthropogenic deforestation during the last millennium (Gauthier, 2004). Thus, Lake Saint-Point offers a suitable site to observe (and disentangle between) the successive impacts of changing climate and land use on lake sedimentation and to study past climate–human–environment relationships. In this domain, it is particularly interesting in the Saint-Point sediment sequence to give evidence that interactions between climate, vegetation and land use may not always develop through univocal straightforward processes, but also through counter-intuitive ones depending on the compartments considered in the ecosystems.

Site and methods

Lake Saint-Point (46°48.7′N, 6°12.2′E) is a 7.6 km long narrow and overdeepened basin of glacial origin and it comprises two sub-basins (Figure 1). The present water depth reaches 41 m and the lake area is 7 km². It is located at an altitude of 850 m a.s.l. in the highest part of the central French Jura Mountains (Figure 1). The catchment area of the lake covers c. 247 km² with a low altitudinal range (summit at 1463 m a.s.l.). The substratum mainly comprises Upper Jurassic and Tertiary limestone (Leroux et al., 2008). The main inlet (as well as the outlet) of the lake is the Doubs River, which supplies the main part of the suspended and dissolved matter. In this context, the lacustrine sedimentation at Saint-Point is clearly dominated by both detrital and authigenic carbonates (hard-water lake; Bichet et al., 1999).

Figure 1.

Location of Lake Saint-Point in the Jura Mountains (eastern France).

The seasonal variations in the water supply are characterised by maxima most often due to autumn rains and spring snowmelt, while minima result from summer droughts (Barbe et al., 1979). The Lake Saint-Point region is marked by severe semi-continental climatic conditions. The daily temperatures show a wide annual amplitude of lower than −30°C in winter and more than 25°C in summer. During severe winters, the lake surface may remain frozen for several successive months. At lake altitude, mean annual precipitation attains c. 1500 to 1900 mm, including frequent summer storms while winter snow fall represents 30% of the total precipitation. The mean annual temperature is c. 10.2°C (2.7°C in the coldest month, and 21.3°C in the warmest month).

Coring site SP05 (Figure 1) is located at 35.5 m water depth in the deepest part of the northern basin of the lake. It was chosen after an extensive seismic reflection survey (seistec IKB high-resolution) to avoid zones possibly disturbed by slumps. In addition to gravity cores to obtain undisturbed cores from the sediment–water interface, two twin cores were taken at point SP05 using an UWITEC piston corer. While a 12.5 m long composite core, which spans the last 16 millennia (Leroux et al., 2008), was taken in 2005, this paper focuses on the Holocene only.

The chronology is based on 24 radiocarbon dates from terrestrial macroremains (Table 1). The calibration curve used was IntCal 09 (Reimer et al., 2009). Given the fact that no matter was available for radiocarbon dating in the lower half of the Saint-Point Holocene sediment sequence, the chronology relies for this time interval on the well-radiocarbon-dated regional pollen stratigraphy (de Beaulieu et al., 1994; Magny et al., 2006). In addition, the geochemical and mineralogical identification of the Laachersee Tephra (LST; Ammann et al., 2000; Brauer et al., 1999), and ¹³⁷Cs and ²¹⁰Pb measurements (Appleby et al., 1991; Smith and Clark, 1986) provide complementary time control levels for the Lateglacial–Holocene transition and the top 20 cm of the core, respectively.

Table 1.

Radiocarbon dates obtained from core SP05.

Depth	Material	Lab code	C yr BP	Cal. yr BP
(cm)			(±1σ)	(2σ range)
125	Leaf	Poz-17040	1120±30	956–1088
126.5	Leaf	Poz-17041	1100±30	952–1062
127.5	Leaf	Poz-17041	1155±30	979–1150
157.4	Leaf	Poz-17043	1775±35	1604–1817
157.6	Leaf	Poz-17051	1785±35	1613–1818
185	Leaf	Poz-17055	2145±35	2033–2305
190.5	Leaf	Poz-17057	2230±35	2152–2336
192.5	Leaf	Poz-17068	2255±35	2155–2344
193.5	Leaf	Poz-17053	2195±35	2124–2324
198.5	Twig	Poz-17054	2490±30	2457–2725
219.5	Leaf	Poz-17067	2800±35	2839–2993
220	Leaf	Poz-17065	2975±35	3059–3319
246.5	Leaf	Poz-17064	3360±35	3483–3689
250.5	Leaf	Poz-17062	3500±35	3688–3871
271.5	Needle	Poz-17063	3740±35	3982–4160
278.5	Leaf	Poz-17061	3935±35	4281–4513
290.5	Leaf	Poz-17060	4130±35	4566–4 821
306.5	Leaf	Poz-17059	4460±40	4960–5294
337.5	Leaf	Poz-18354	4850±40	5476–5657
352.5	Leaf	Poz-18356	5150±50	5748–5995
483	Leaf	Poz-18353	6500±50	7308–7507
484	Leaf	Poz-18357	6470±50	7276–7461
501	Leaf	Poz-18315	6980±50	7695–7932
683	Leaf	Poz-18360	11,730±60	13,426–13,733

Regarding the sedimentological analyses, magnetic susceptibilty was measured at 5 mm steps using a Barlington MS2E point sensor adapted on a GEOTEK Multi Sensor Core Logger System. Previous studies in carbonate lakes in England and France have shown that MS maxima may reflect intense detrital events in response to (1) climate cooling and changes in vegetation cover during the Lateglacial period or (2) forest clearings and land use history (Higgitt et al., 1991; Nolan et al., 1999). Grain-size measurements were performed on H₂O₂ pretreated samples by laser diffraction using a Beckman-Coulter LS230.

The bulk mineral composition was determined by x-ray diffraction (XRD), using an X’Pert Philips diffractometer with a cobalt anticathode. The diffractometer settings were 5s per step from 3° to 72° ²Th for a bulk characterisation of non-oriented powdered samples (Holtzappfel, 1985). Given that samples reflect variable degrees of mineral mixing, the area of the major peak of each mineral was measured on the diffractograms. The semi-quantitative mineral composition was expressed as a percentage (Amorosi et al., 2002; Moore and Reynolds, 1997). In Figure 3, the mineralogical fraction ‘Others’ mainly corresponds to oxides associated with organic matter (soils from the catchment area) in addition to silicates (plagioclases, K feldspars and dolomite, residual from weathering of the catchment rocks).

The organic carbon (C_org), carbonate and silicate contents were calculated through combustion at 1000°C (loss-on-ignition, LOI) and major elements measurements performed by XRD. Assuming that the Ca is mainly associated with the carbonate fraction, it is possible to work out the total carbonate content. The organic carbon content is calculated from LOI measurement after correction of CaCO₃ content and the silicate fraction was deducted as the remaining fraction.

In this carbonated catchment and hard-water lake context, the main difficulty is to distinguish detrital from authigenic carbonates. To this end, assuming that the Lake Saint-Point water composition only enables the precipitation of pure calcite or very low-Mg calcite (Calmels, 2007), the Ca-Mg content in samples is used to determine proportions of each fraction through a two-component mixture calculation. As extensively explained in Leroux et al. (2008), the calculation is defined by a linear regression of Ca-Mg contents between a detrital pool (highest Mg contents and catchment rocks Ca-Mg references) and a pure calcitic pool (highest Ca and no Mg contents).

The Total Organic Content (TOC) (wt.% of dry sediment) was determined by Rock-Eval pyrolysis with a model 6 device (Vinci Technologies; Espitalié et al., 1985). The Hydrogen and Oxygen indexes (HI and OI, respectively) were used as indicators of organic matter quality (Disnar et al., 2003, 2008); they reflect the origin (lacustrine versus terrestrial) of organic matter and its state of preservation (Espitalié et al., 1985).

Regarding biotic indicators, samples for pollen analysis were treated by standard methods, including HCl, HF, NAOH, and acetolysis. Pollen preservation was good and concentrations were always sufficient to count over 500 pollen grains per slide. Pollen percentages are based on the pollen sum of arboreal (AP) and non-arboreal (NAP) pollen grains, excluding spores, Alnus, Cyperaceae, aquatic and hygrophilous taxa. Human impact is studied through various pollen Anthropogenic Indicators (AI, i.e. Ceralia type, Secale, Fagopyrum, Papaver, Centaurea cyanus, Plantago lanceolata, Plantago major-media, Rumex, Artemisia, Chenopodiaceae, Urticaeae, Polygonum aviculare, and Convolvulus) (Gauthier, 2004). Samples for ostracod analysis were disaggregated by submersion in a 10% hydrogen peroxide solution for about 2 h, and washed through a 125 µm sieve. Ostracod shells were picked using a stereomicroscope and identified according to Meisch (2000). Relative abundances of species (in percentages) as well as absolute abundance of valves were calculated.

Results

Figure 2 presents the age–depth model established from (1) radiocarbon- and short-lived radio-isotopes, (2) radiocarbon-dated regional pollen stratigraphy, and (3) LST tephra horizon. The radiocarbon dates and the ages based on the regional pollen stratigraphy and on the LST horizon provide consistent ages for the sections where they may be combined. The Younger Dryas/Holocene transition has been fixed with reference to the end of a period which was characterised by a minimum of Pinus and a maximum of Poaceae and directly followed the LST deposition.

Figure 2.

Age–depth model for core SP05 from radiocarbon dates (see Table 1), radiocarbon-dated regional pollen stratigraphy, tephra horizon (LST: Laachersee tephra), and ¹³⁷Cs and ¹²⁰Pb measurements (see inset).

The chronology for the last 100 years was established with ²¹⁰Pb and ¹³⁷Cs dating. Two successive ¹³⁷Cs peaks at 9–10 cm depth correspond to the main fall-out interval of ad 1963–1966 in the Northern Hemisphere resulting from atmospheric nuclear weapon tests (Pennington et al., 1973). The ¹³⁷Cs age–depth model is in close agreement with the ²¹⁰Pb chronology, which allows a high-resolution age–depth model for the entire last century. The derived curve of sedimentation rate shows increases between c. 10,000 and 6000 cal. BP, and during the last millennium. The Younger Dryas–Holocene transition corresponds to level 645 cm and on the basis of the age–depth model, and the mean temporal resolution for analyses ranges from 100 to 200 years per sample.

Regarding the lithology, the sediment sequence SP05 highlights four main sediment units for the Holocene as follows.

Below level 637 cm, unit 4 is composed of dark green-grey silts with scarce laminations.

Unit 3 (637–328 cm depth) corresponds to an unlaminated light-coloured carbonate lake-marl.

Unit 2 (328–117 cm depth) is formed by dark green-grey silts with irregular laminations including frequent organic macroremains which made radiocarbon dating easier.

At the top, unit 1 (above 117 cm depth) may be distinguished from unit 2 by a lighter colour.

Figure 3 presents results for abiotic sedimentological indicators analysed in core SP05. Regarding the magnetic susceptibility measurements, four main successive phases may be recognised. After a first early-Holocene maximum, the MS curve shows an abrupt decrease around 11,200 cal. BP, followed by minimum values until c. 6200 cal. BP, a slight increase between 6200 and 5500 cal. BP, a relative stability until c. 1400 cal. BP, and finally a maximum during the last six centuries.

Figure 3.

Abiotic sedimentological indicators analysed in core SP05. MS: magnetic suceptibility; TOC: total organic carbon; OI/HI: oxygen index/hydrogen index (Rock-Eval pyrolysis); XRD: x-ray diffraction.

The grain-size analysis distinguishes between four successive periods which, broadly, correspond to sediment units 4 to 1. At the base of the Holocene sequence (unit 4), the coarsest particles (> 63 µm) mark only short-lived limited peaks. Generally, results obtained from units 4 and 3 show relatively well-sorted sediments with a dominance (around 58%) of fine particles (< 4 µm). In contrast, units 2 and 1 show evidence of an increase in coarse particles while sorting decreases. The fraction < 4 µm decreases by around 10% while the fraction > 63 µm increases. The 4.6 µm mode is maintained while an additional 121 µm mode develops. After 750 cal. BP, fraction > 63 µm displays a slight increase and is maintained at around 10%.

Mineralogy exhibits a clear domination of calcite throughout the Holocene sediment sequence, while phyllosilicates display variable percentages (up to 17%). Quartz rapidly disappears after 11,000 cal. BP; it marks a short-lived peak around 8600 cal. BP and finally reappears from c. 6500 cal. BP onwards to remain continuously during the second half of the Holocene at c. 2%. Quartz and phyllosilicates are produced by soil erosion and correspond to insoluble residue from rocks in the catchment area.

Expressed in fluxes (mg/cm² per yr), the total of detrital carbonates and silicates, which may be used as indicators of erosion inputs from the catchment area, shows a rapid decrease after 11,200 cal. BP to values below 40 mg/cm² per yr. After an increase between 7000 and 6300 cal. BP, the values remain above 40 mg/cm ² per yr during a large part of the second half of the Holocene. They undergo a rapid increase after 2600 cal. BP and reach more than 100 mg/cm² per yr during the last millennium. However, one observes that, during the last 1200 years, the silicate flux marks a stabilisation (values generally around 30 mg/cm² per yr) while detrital carbonates display a continuous increase.

TOC appears to maintain relatively low approximate values (below 4%) throughout the Holocene. A clear bipartition of the sediment sequence may be observed with values below 1% during the first half of the Holocene, and above 2% during the second half (over 3% from c. 4000 to 1000 cal. BP). The last millennium is characterised by a rapid TOC decrease from c. 4% to c. 1%. HI values appear to be relatively stable during the entire Holocene and maintain around 200 mg HC/g TOC. These low values suggest that the contribution of the lake productivity to TOC is rather weak and that the TOC content mainly reflects the riverine runoff. This interpretation is supported by a strong correlation coefficient (i.e. r = 0.81) between TOC and silicates throughout the Holocene in core SP05 (Leroux, 2010). The values of OI show a rapid increase at the beginning of the Holocene, a marked maximum (around 800 mg CO₂/g TOC between 10,200 and 7000 cal. BP), and finally a return to values generally around 300 mg CO₂/g TOC after 5300 cal. BP.

Regarding the biotic indicators (Figure 4), the frequency of ostracod valves also points to a clear division of the Holocene, with the first half characterised by (1) a rapid development of ostracod fauna (total number of valves most often found between 2000 and 3000) and (2) a domination by Limnocythere sanctipatricii, and the second half marked by a low frequency of valves and a domination by Cypria ophtalmica. Finally, the last 750 years show a marked increase in the frequency of valves (more than 4000), while Cypria ophtalmica decreases.

Figure 4.

Biotic indicators (ostracod and pollen) analysed in core SP05 and compared with (1) TOC and detrital flux (see Figure 3) and (2) variations in summer insolation (Berger and Loutre, 1991). AI: anthropogenic indicators; AC: Artemisia + Chenopodiaceae. Horizontal grey bands indicate periods of rapid changes (tipping points).

As illustrated in Figure 4, changes in the vegetation cover exhibit a similar partition of the Holocene, with (1) a rapid disappearance of Pinus forests after 10,500 cal. BP, (2) a domination by deciduous trees (successively Corylus and Quercus) between 10,400 and 6300 cal. BP, and (3) a rapid development of Abies-Fagus forests after 6300 cal. BP. Anthropogenic indicators and NAP suggest that human impact remains relatively low until 1200 cal. BP, despite early first Neolithic forest clearing in the region (Gauthier, 2004; Richard, 1997, 2004) followed by a weak but regular human impact. In addition, one observes a clear (even if limited and irregular) increase in NAP values as early as 2600 cal. BP, i.e. at the Bronze/Iron age transition. However, it is also possible that the poor signal of human impact until the Medieval times may be partly explained by the large size of the lake, which makes it less sensitive to slash and burn practices better recorded in narrow sites. Nevertheless, without excluding such processes, the pollen data obtained from core SP05 are in full agreement with regional archaeological and historical data (Gauthier, 2004; Pétrequin et al., 2005).

Discussion

Changes in lake sediment composition are expected to reflect variations in the lake basin and its catchment area in response to changes in climate and human impact. The study of the sediment sequence of Lake Saint-Point is based on a multiproxy approach (see above, section Results) which allows us to show, in the following discussion, how the trajectories of the terrestrial and lacustrine ecosystems have been tightly coupled throughout the Holocene. Thus, by means of biotic and abiotic indicators, four successive periods may be distinguished in the Saint-Point Holocene record as follows (Figures 3 and 4).

11,700–10,600 cal. BP: The early Holocene

This period corresponds to unit 4 and to the base of unit 3, and from a palaeoenvironmental point of view, to the recovery of terrestrial and lacustrine ecosystems after the severe impact of the Younger Dryas cold event (Magny et al., 2006). Regarding the vegetation, AP values increase from 45% to more than 95%. The restoration of forest cover seems to have taken a substantially longer time (c. 1100 years in order to attain 90% AP) than evaluated at Lake Lautrey (c. 300 years) situated at an equivalent elevation, i.e. 788 m a.s.l. (Magny et al., 2006). However, the catchment area of Lake Saint-Point is 200 times larger than that of Lake Lautrey and includes areas at a higher elevation. In response to this reforestation, the erosion in the catchment area decreases as early as 11,200 cal. BP as shown by (1) the reduction in the flux of detrital carbonates and silicates, and (2) the quasi-disappearance of quartz. At c. 10,800 cal. BP, favoured by the early-Holocene climate warming, the abrupt transition from sediment unit 4 to unit 3 points to the rapid development of authigenic carbonate lake-marl in the lake sedimentation. In the lake water, the ostracods quickly developed. The short-lived but well-marked peak of TOC dated to c. 11,300 cal. BP and synchronous with a peak of Artemisia and Chenopodiaceae may correspond to a cooling event equivalent to the Preboreal Oscillation (PBO; Björck et al., 1997) and may be responsible for an erosion increase in the Saint-Point region. Previous studies have demonstrated how the PBO was synchronous with wetter climatic conditions (increasing runoff) in the Jura Mountains (Magny, 2004, 2007; Magny et al., 2007). The curve of NAP reconstructed at Lake Saint-Point suggests a more complex pattern with possible successive cooling events within the PBO as shown by other records in the North Atlantic area (e.g. Björck and Wastergard, 1999; Magny et al., 2006).

10,600–6200 cal. BP: The middle Holocene

This period is mainly documented by unit 3. In the catchment area, it corresponds to the domination by deciduous forests (Corylus followed by Quercus), which benefit from the insolation maximum (Figure 4) and replace Pinus forests. The high values of OI prevailing during this time interval in core SP05 suggest a transport of well-oxydated and biodegraded organic particles from soils of deciduous forests. However, the runoff remains low, as shown by low TOC values. This is in agreement with (and may explain) the scarcity of terrestrial macroremains available for radiocarbon-dating. In the lake basin, warm climate conditions favoured the formation of authigenic carbonates (deposition of unit 3) which explain higher sedimentation rates (with two maximums around 9500 and 6500 cal. BP as shown in Figure 2). Both favourable climate conditions and authigenic carbonate formation may have favoured the development of ostracod fauna (maximum of valves) which is dominated by Limnocythere sanctipatricii. Within the dating uncertainty, the peak of TOC dated to c. 8500 cal. BP may reflect an impact of the 8.2 ka event (Alley et al., 1997). In addition, Rohling and Pälike (2005) have shown how the 8.2 ka event may be a part of a climatic deterioration spanning 400 to 600 years between c. 8600 and 8000 cal. BP. As shown by Magny et al. (2003), the 8.2 ka event in the mid-European latitudes may have been responsible for wetter climatic conditions and increasing runoff and erosion in the Saint-Point region (Magny, 2004; Magny et al., 2003).

6200–1200 cal. BP: The late Holocene

This period is recorded by unit 2 and the upper part of unit 3. In the catchment area, except for limited and short-lived peaks, generally low values of NAP suggest a weak influence of human impact on the vegetation until c. 2600 cal. BP. However, around 6300–5800 cal. BP, in response to a decrease in insolation (Figure 4), the regional vegetation cover is characterised by a rapid decline of Quercus and an expansion of mixed Abies-Fagus forests. This vegetation change seems to favour increasing erosion as shown by the flux of detrital carbonates and silicates which is multiplied by two. In addition to changes in the vegetation cover, such a reinforcement of erosion may also reflect a general trend toward wetter climatic conditions in relation with the onset of the Neoglacial (Magny et al., 2006). Quartz reappears in the sediment composition and points to increasing detritism, while the grain size highlights coarser particles (increasing runoff). During this phase, OI rapidly decreases from 800 to c. 300 mg CO₂/g TOC. This probably results from changes in soil types under changing vegetation cover (less oxygenated and biodegraded organic matter associated with more acid soils under coniferous forests). HI remains stable at low values and indicates that the multiplication of TOC by three may be another consequence of increasing erosion. In the lake basin, in addition to a probable slight decrease in the water pH because of the extension of coniferous forests into the catchment area (more acid soils), the cooling trend of climate provokes a marked decrease in the authigenic carbonate formation (transition from unit 3 to unit 2) resulting in a reduction in the sedimentation rate. The ostracod population declines as shown by the number of valves, while Limnocythere sanctipatricii nearly disappears and Cypria ophtalmica becomes the dominant species possibly favoured by cooler conditions (Moreno et al., 2011). From 2600 to 1200 cal. BP, one observes a clear reinforcement in the flux of detrital carbonates and silicates, the values of which progressively rise from c. 50 to 80 mg/cm² per yr. This may reflect (1) an increase in humidity and erosion at the Sub-Boreal–Sub-Atlantic transition (van Geel et al., 1996) and (2) an increase in the human impact as shown by NAP and anthropogenic indicators at the Bronze–Iron age transition. However, while the total detrital flux gives evidence of progressive regular processes since 2600 cal. BP, the NAP curve highlights discontinuous and more chaotic phenomena. In addition, a clear increase in NAP values occurred only after 1400 cal. BP. This suggests that, between 2600 and 1400 cal. BP, human impact still remains a forcing factor of second order in comparison to climate.

The last 1200 years

The last millennium is characterised in the Saint-Point region by a strong increase in human impact (Gauthier, 2004): after 750 cal. BP, peaks of NAP reach more than 50%. However, possible erosion indicators offer contrasting pictures: TOC values are divided by four, the flux of silicates stabilises, while that of detrital carbonates continuously increases and MS marks maximum values. During the last 750 years, marked changes are also observed in the ostracod fauna: the number of valves shows a large increase while Cypria ophtalmica declines and Limnocythere sanctipatricii slightly resumes. This suggests that, in addition to the increase in flux of detrital carbonates, the strong increase in the sedimentation rate during the last millennium (Figure 2) partly reflects a new development of authigenic carbonates responsible for the lighter-coloured unit 1. Differences observed between possible erosion indicators also suggest possible differences between types of land use (agriculture versus pastures and forestry) and/or parts of the catchment area affected by human activities (calcareous summits and slopes versus valley floors). In the Saint-Point region, higher elevated areas (Upper Jurassic substratum characterised by low content in insoluble residue) are mainly characterised by the development of pastures and that of forestry with a growing need for charcoal in the glass industry during Modern Times, while agriculture mainly develops on valley floors (Cretaceous marl-limestone with higher content in insoluble residue). Thus, the development of pastures, responsible for forest clearing or disappearance, may have contributed to an increase in fluxes of detrital carbonates, but also to a greater stabilisation of soils by meadow vegetation than under coniferous forests. A better organisation of cultivated areas on valley floors could have limited erosion and led to similar results. Taken together, these new conditions during the last seven centuries could explain contrasting pictures with a stabilisation of silicates, a rapid reduction of TOC, a resumption of authigenic carbonate sedimentation and, by extension, that of the ostracod population (Figures 3 and 4).

As a first concluding remark, the comparisons illustrated by Figure 5 show that the palaeoenvironmental record reconstructed at Lake Saint-Point is consistent with the regional pattern of palaeohydrological changes established for the Holocene in the Jura Mountains and in west-central Europe. The sediment sequence studied at Lake Chaillexon, a lake located on the Doubs River downstream from Lake Saint-Point and formed by a natural rockfall dam (Figure 1), exhibits low detritism during the first half of the Holocene, and has also been characterised by the deposition of authigenic carbonates, and an increasing detristism since c. 6800 cal. BP (Bichet et al., 1999). This is in agreement with the pattern of water-table fluctuations reconstructed at Lake Cerin in the southern Jura Mountains (Magny et al., 2011a) as well as with the increasing sedimentation rate observed at Lake Anterne (French Alps, 2063 m a.s.l.; Giguet-Covex et al., 2011). Both of these records reflect the imprint of orbital factors resulting from changes in insolation (Berger and Loutre, 1991). Lake Bourget in the French northern pre-Alps presents a slightly different pattern of palaeohydrological changes with a relatively stable signal from 7200 to 2700 cal. BP followed by a clear increase in detritism during the last 2700 years (Arnaud et al., 2005). On the southern slope of the Alps, the record established at Lake Iseo gives evidence of a marked increase in the sedimentation rate around 4500 cal. BP (Lauterbach et al., 2012), in agreement with a later beginning of the Neoglacial in the north-central Mediterranean region (Giraudi et al., 2011). Thus, the tipping point of change in the Holocene palaeohydrological regime and associated sedimentation rate appears to depend both on the location and the sensitivity of the lake under consideration.

Figure 5.

Comparaison of records from Lakes Saint-Point (this study), Chaillexon (Bichet et al., 1999), Cerin (Magny et al., 2011a), Anterne (Giguet-Covex et al., 2011), and Mezzano (Ramrath et al., 2000).

The second remark points to the fact that core SP05 mainly reflects the impact of orbital factors on the regional climate responsible for a clear bipartition of the Holocene, with the first phase characterised by a climatic optimum and the second marked by cooler and wetter Neoglacial climatic conditions, with more rapid changes around 6200 cal. BP. Except for two short-lived events (i.e. the PBO and the 8.2 ka event), the higher frequency climatic oscillations do not appear clearly on the Saint-Point record, in contrast to other palaeoclimatic records established from deep cores in large lakes (Arnaud et al., 2005; Lauterbach et al., 2012). Such a weakened signal may result from the location of the coring point being located in the northern sub-basin of Lake Saint-Point, i.e. in a distal position from the main inlet situated at the southern extremity of the lake. This interpretation is supported by studies of present-day surface sediments which show that the detritic input from the Doubs River concentrates in the southern sub-basin of the lake close to the main inlet (Figure 1) (Barbe et al., 1979). Moreover, the weaker sensitivity of Lake Saint-Point to high frequency climatic oscillations may also originate from characteristics of its catchment area with limited altitudinal variability and gentle slopes.

The third point well-illustrated by the Saint-Point record is how gradual changes in orbitally driven insolation may result in progressive changes but also provoke rapid climatic and palaeoenvironmental responses, as exemplified in subtropical North Africa around 5500 cal. BP (deMenocal et al., 2000), in northern South America and in the central Mediterranean around 4500–4000 cal. BP (Haug et al., 2001; Magny et al., 2011b). This suggests strongly non-linear feedback processes with threshold values in both climate mechanisms and ecosystems. This also suggests that such rapid responses are not limited to assumed sensitive tropical and arctic regions (deMenocal et al., 2000) but also developed in the temperate zones of west-central Europe. Thus, Figures 3 and 4 exhibit similar rapid changes in vegetation cover, ostracod fauna or lake sedimentation in response to gradual changes in insolation or to relatively progressive changes in human impact. These changes may show synchronous or distinct successive events probably in relation to the proxies considered, their sensitivity and on their role in the functioning of lacustrine and terrestrial ecosystems.

Thus, regarding the ostracod fauna, it is noteworthy that the number of valves shows relatively progressive changes (except for the last centuries) while abrupt variations in the species assemblages may be observed at c. 10,600 cal. BP (strong increase in Limnocythere sanctipatricii) and 5500 cal. BP (strong increase in Cypria ophtalmica). Regarding the feedback processes behind the reconstructed changes in terrestrial and lacustrine environments, it may be hypothesized that changes in the vegetation cover in the catchment area associated with changes in climatic parameters may have induced changes in the sensitivity to erosion and in the runoff intensity. In turn, this may have resulted in changes in both the quantity and composition of detritic input (including nutriments) flowing into the lake, and finally changes in the lake sediment deposition and lacustrine fauna.

Moreover, results obtained at Lake Saint-Point suggest complex interactions between terrestrial and lacustrine environments with different times of response to forcing factors. Thus, at the early- to mid-Holocene transition, the strong decrease in silicates percentage values at c. 11,200 cal. BP well reflects a rapid decrease in MS values (i.e. decreasing detritic input) during a phase of forest restoration (i.e. decreasing sensitivity of the catchment area to runoff and erosion). In contrast, the relatively delayed increase in Limnocythere sanctipatricii at c. 10,600 cal. BP may have been favoured by changes in nutrients provoked by the replacement of Pinus forests (more acid soils) by deciduous forests. Further investigations including (1) a better understanding of limiting factors regulating the behaviour of species in the lacustrine fauna and (2) model simulations are needed to explore the complex interactions between climatic parameters, and terrestrial as well as lacustrine ecosystems.

Finally, it is worth noting the original picture provided by the sediment archives of Lake Saint-Point for the curve of TOC in comparison with others reconstructed from Lake Anterne in the French Alps (Giguet-Covex et al., 2012) and Lake Mezzano in central Italy (Ramrath et al., 2000). At Lake Saint-Point, the increase in TOC values is associated with the development of Neoglacial conditions which resulted in increasing erosion in the catchment area and a maximum of TOC in the lake. At Lake Anterne (Giguet-Covex et al., 2011), a high-elevation subalpine lake (2063 m a.s.l.), the maximum of TOC measured in lake sediments occurred during the first half of the Holocene when the production of organic matter in the catchment area was favoured by the Holocene climatic optimum. At Lake Mezzano (Ramrath et al., 2000), a small maar lake located at 452 m a.s.l., the curve of TOC also shows a maximum during the first half of the Holocene but is mainly the result of the aquatic productivity, while the role of the catchment area (only 1 km²) is quite limited. This is supported by similar results obtained at Lake Alabano, another crater lake in central Italy (Ariztegui et al., 2001).

Thus, the comparison between the three records from Lakes Saint-Point, Anterne and Mezzano (Figure 5) suggests three distinct types of lake functioning even if all the records reflect similar orbital factors resulting in a similar bi-partitioned Holocene. As illustrated by the present study, the Saint-Point TOC record is mainly the result of detritic input from the catchment area. It increased during the second half of Holocene owing to both wetter climatic conditions and changes in the vegetation cover. Regarding the oligotrophic subalpine Lake Anterne, the lake productivity is small because of its high-elevated location. Consequently, the TOC content mainly reflects input from its catchment area. But, in contrast to Lake Saint-Point, because of the high elevation of Lake Anterne, the production of organic matter in the catchment area (and, consequently, of TOC input to the lake) reached a maximum during the Holocene thermal maximum while cooler climatic conditions led to a production minimum of organic matter in the catchment (and of TOC input to the lake) during the second part of the Holocene. Finally, Lake Mezzano, located at a low elevation in the Mediterranean area, benefits from more favourable climatic conditions. In addition, the catchment-to-lake surface ratio is small (around 2). These general conditions explain that the Mezzano TOC record mainly reflects lake productivity modulated by the orbital factor, resulting in a TOC maximum during the Holocene thermal optimum, and in a TOC decrease during the cooler Neoglacial period.

Conclusions

A multiproxy approach to a sediment sequence at Lake Saint-Point, a carbonate lake in the French Jura Mountains, shows four successive phases in response to major Holocene climatic oscillations and to anthropogenic impact as follows.

The early Holocene (11,700–10,200 cal. BP) is characterised by the recovery of terrestrial and lacustrine ecosystems favoured by climatic warming.

During the middle Holocene (10,600–6200 cal. BP), the optimum of climatic conditions favoured the extension of deciduous forests to the catchment area while the lake sedimentation is dominated by authigenic carbonates and low detrital inputs.

The development of cooler and wetter climatic conditions after 6200 cal. BP (Neoglacial) led to the extension of Abies-Fagus forests and to increasing detrital inputs to the lake where ostracod fauna declined.

After 1200 cal. BP, associated with an increase in human impact, forest clearing extended into the catchment area, while the lake basin shows contrasting pictures with increasing detrital input, resuming sedimentation of authigenic carbonates, and changes in dominant ostracod species.

Throughout the Holocene period, changes in the terrestrial and lacustrine ecosystems appear to have been strongly coupled. Orbitally driven climatic variations were the dominant factor in these environmental changes until c. 1200 cal. BP. Around 2600 cal. BP, human impact increased and became the major factor in the lake basin and catchment area from 1200 cal. BP onwards.

Finally, one of the main interests of the Saint-Point record is that it offers a clear illustration of how gradual changes in insolation or relatively progressive increases in human impact may provoke abrupt responses in terrestrial and lake ecosystems of west-central Europe, and how differences in the dates of tipping points revealed by proxies suggest specific threshold values depending on the sensitivity of the indicators in use and on their role in the different compartments of these ecosystems.

Footnotes

Acknowledgements

The authors express their sincere thanks to J Olsen for his help with the English language, to Professor Marc Desmet for his help with fieldwork, to Jean-Louis Reiss for the ¹³⁷Cs and ²¹⁰Pb measurements, and to Professors C Di Giovanni and JR Disnar (University of Orléans, France) for fruitful discussions. Thorough reviews and constructive comments by two anonymous referees greatly helped to improve the manuscript.

Funding

Financial support for this study was provided by the French CNRS (GDR 2992 ‘JurAlp’), the University of Franche-Comté, and the city of Besançon (France).

References

Alley

Mayewski

Sowers

. (1997) Holocene climatic instability: A prominent, widespread event 8200 yr ago. Geology 25: 483–486.

Ammann

Birks

HJB

Brooks

. (2000) Quantification of biotic responses to rapid climatic changes around the Younger Dryas – A synthesis. Palaeogeography, Palaeoclimatology, Palaeoecology 159: 313–349.

Amorosi

Centineo

Dinelli

. (2002) Geochemical and mineralogical variations as indicators of provenance changes in Late Quaternary Deposits of SE Po Plain. Sedimentary Geology 151: 273–292.

Appleby

Richardson

Nolan

(1991) ²⁴¹Am dating of lake sediments. Hydrobiologia 214: 35–42.

Ariztegui

Chondrogianni

Lami

. (2001) Lacustrine organic matter and the Holocene paleoenvironmental record of Lake Albano (central Italy). Journal of Paleolimnology 26: 283–292.

Arnaud

Revel

Chapron

. (2005) 7200 years of Rhône river flooding activity in Lake Le Bourget, France: A high-resolution sediment record of NW Alps hydrology. The Holocene 15: 420–428.

Barbe

Faessel

Lafont

. (1979) Etude écologique des lacs de Saint-Point et Remoray. Service Régional d’Aménagement des Eaux Report.

Berger

Loutre

(1991) Insolation values for the climate of the last 10 million years. Quaternary Science Reviews 10: 297–317.

Bichet

Campy

Buoncristiani

. (1999) Variations in sediment yield from the Upper Doubs River carbonate watershed (Jura, France) since the Late-Glacial period. Quaternary Research 51: 267–279.

10.

Björck

Wastergard

(1999) Climate oscillations and tephrochronology in eastern middle Sweden during the last glacial–interglacial transition. Journal of Quaternary Science 14: 399–410.

11.

Björck

Rundgren

Ingolfsson

. (1997) The Preboreal oscillation around the Nordic seas: Terrestrial and lacustrine responses. Journal of Quaternary Science 12: 455–465.

12.

Brauer

Endres

Günter

. (1999) High resolution sediment and vegetation responses to Younger Dryas climate change in varved lake sediments from Meerfelder Maar, Germany. Quaternary Science Reviews 18: 321–329.

13.

Calmels

(2007) Altération chimique des carbonates: Influence des sources d’acidité sur les bilans globaux. PhD Thesis, Université Paris Diderot.

14.

Dearing

Jones

Shen

. (2008) Using multiple archives to understand past and present climate–human–environement interactions: The lake Erhai catchment, Yunnan Province, China. Journal of Paleolimnology 40: 3–31.

15.

De Beaulieu

J-L

Richard

Ruffaldi

. (1994) History of vegetation, climate and human action in the French Alps and the Jura over the last 15 000 years. Dissertationes Botanicae 234: 253–276.

16.

deMenocal

Ortiz

Guilderson

. (2000) Abrupt onset and termination of the African Huimid Period: Rapid climate responses to gradual insolation forcing. Quaternary Science Reviews 19: 347–361.

17.

Disnar

Guillet

Keravis

. (2003) Soil organic matter (SOM) characterisation by Rock-Eval pyrolysis: Scope and limitations. Organic Geochemistry 34: 327–343.

18.

Disnar

Jacob

Morched-Issa

. (2008) Assessment of peat quality by molecular and bulk geochemical analysis: application to the Holocene record of the Chautagne marsh (Haute Savoie, France). Chemical Geology 254: 101–112.

19.

Espitalié

Deroo

Marquis

(1985) La pyrolyse Rock-Eval et ses applications: Première partie. Revue de l’Institut Français du Pétrole 40: 563–579.

20.

Gauthier

(2004) Forêts et agriculteurs du Jura. Les quatre derniers millénaires. Presses Universitaires franc-Comtoises.

21.

Giguet-Covex

Arnaud

Enters

. (2012) Frequency and intensity of high-altitude floods over the last 3.5 ka in nrthwestern French Alps (Lake Anterne). Quaternary Research 77: 12–22.

22.

Giguet-Covex

Arnaud

Poulenard

. (2011) Changes in erosion patterns during the Holocene in a currentless subalpine catchment inferred from lake sediment geochemistry (Lake Anterne, 2063 m a.s.l., NW French Alps): The role of climate and human activities. The Holocene 21: 651–665.

23.

Giraudi

Magny

Zanchetta

. (2011) The Holocene climatic evolution of the Mediterranean Italy: A review of the geological continental data. The Holocene 21: 105–115.

24.

Higgitt

Oldfield

Appleby

(1991) The record of land-use change and soil erosion in the late Holocene sediments of the Petit Lac Annecy, eastern France. The Holocene 1: 14–28.

25.

Haug

Hughen

Sigman

. (2001) Southward migration of the Intertropical Convergence Zone through the Holocene. Science 293: 1304–1308.

26.

Holtzappfel

(1985) Les minéraux argileux. Préparation. Analyse diffractométrique et détermination. Annales de la Société Géologique du Nord 12, 136 pp.

27.

Lauterbach

Chapron

Brauer

. (2012) A sedimentary record of Holocene surface runoff events and earthquake activity from Lake Iseo (southern Alps, Italy). The Holocene 22: 749–760.

28.

Leroux

(2010) Caractérisation et evolution des flux détritiques et authigènes en contexte lacustre carbonate au cours du Tardiglaciaire et de l’Holocène (Lac Saint-Point, Haute-Chaîne du Jura): implications paléoclimatiques et paléoenvironnementales. PhD Thesis, Université de Franche-Comté.

29.

Leroux

Bichet

Walter-Simonnet

. (2008) Late-Glacial Holocene sequence of Lake Saint-Point (Jura Mountains, France): Detrital inputs as records of climate change and anthropic impact. Compte-Rendus Geosciences 340: 883–892.

30.

Magny

(2004) Holocene climatic variability as reflected by mid-European lake-level fluctuations, and its probable impact on prehistoric human settlements. Quaternary International 113: 65–79.

31.

Magny

(2007) Holocene fluctuations of lake levels in west-central Europe: Methods of reconstruction, regional pattern, palaeoclimatic significance and forcing factors. In: Elias

(ed.) Encyclopedia of Quaternary Geology. Elsevier, pp. 1389–1399.

32.

Magny

Aalbersberg

Bégeot

. (2006) Environmental and climatic changes in the Jura mountains (eastern France) during the Lateglacial–Holocene transition: A multi-proxy record from Lake Lautrey. Quaternary Science Reviews 25: 414–445.

33.

Magny

Bégeot

Guiot

. (2003) Contrasting patterns of hydrological changes in Europe in response to Holocene climate cooling phases. Quaternary Science Reviews 22: 1589–1596.

34.

Magny

Bossuet

Ruffaldi

. (2011a) Orbital imprint on Holocene palaeohydrological variations in west-central Europe as reflected by lake-level changes at Cerin (Jura Mountains, eastern France). Journal of Quaternary Science 26: 171–177.

35.

Magny

Leuzinger

Bortenschlager

. (2006) Tripartite climate reversal in Central Europe 5600–5300 years ago. Quaternary Research 65: 3–19.

36.

Magny

Vannière

Calo

. (2011b) Holocene hydrological changes in south-western Mediterranean as recorded by lake-level fluctuations at Lago Preola, a coastal lake in southern Sicily, Italy. Quaternary Science Reviews 30: 2459–2475.

37.

Magny

Vannière

de Beaulieu

. (2007) Early-Holocene climatic oscillations recorded by lake-level fluctuations in west-central Europe and in central Italy. Quaternary Science Reviews 26: 1951–1964.

38.

Meisch

(2000) Freshwater Ostracoda of western and central Europe. In: Schwoerbel

Zwick

(eds) Süsswasserfauna von Mitteleuropa 8/3. Heidelberg/Berlin: Spektrum Akademischer Verlag, 522 pp.

39.

Millet

Giguet-Covex

Verneaux

. (2010) Reconstruction of the recent history of a large deep prealpine lake (Lake Bourget, France) using subfossil chironomids, diatoms, and organic matter analysis: Towards a definition of a lake-specific reference state. Journal of Paleolimnology 44: 963–978.

40.

Moore

Reynolds

RCJ

(1997) X-ray Diffraction and the Identification and Analysis of Clay Minerals. Second edition. New-York: Oxford University Press.

41.

Moreno

Lopez-Merino

Leira

. (2011) Revealing the last 13500 years of environmental history from the multiproxy record of a mountain lake (Lago Enol, northern Iberian Peninsula). Journal of Paleolimnology 46: 327–349.

42.

Nolan

Bloemendal

Boyle

. (1999) Mineral magnetic and geochemical records of late Glacial climatic change from two northwest European carbonate lakes. Journal of Paleolimnology 22: 97–107.

43.

Pennington

Tutin

Cambray

. (1973) Observations on lake sediments using fallout ¹³⁷Cs as a tracer. Nature 242: 324–326.

44.

Pétrequin

Magny

Bailly

(2005) Habitat lacustre, densité de population et climat. L’exemple du Jura français. In: Della Casa

Trachsel

(eds) WES’O4, Wetland Economies and Societies. Proceedings of the International Conference in Zürich, 10–13 March 2004. Collectio Archaeologica 3, pp. 140–168.

45.

Ramrath

Nowaczyk

Negendank

(1999) Sedimentological evidence for environmental changes since 34 000 years BP from Lago di Mezzano, central Italy. Journal of Paleolimnology 21: 423–435.

46.

Ramrath

Sadori

Negendank

JFW

(2000) Sediments from Lago di Mezzano, central Italy: A record of Lateglacial/Holocene climatic variations and anthropogenic impact. The Holocene 10: 87–95.

47.

Reimer

Baillie

MGL

Bard

. (2009) IntCal09 and Marine09 radiocarbon age calibration curves, 0–50000 years cal. BP. Radiocarbon 51: 1111–1150.

48.

Richard

(1997) Pollen evidence of an early neolithic presence on the Jura range at the VIth et Vth millennia. Quaternaire 8: 55–62.

49.

Richard

(ed.) (2004) Néolithisation précoce. Premières traces d’anthropisation du couvert végétal à partir des données polliniques. Besançon: Presses Universitaires Franc-Comtoises, 2004, (Annales Littéraires; 777. Série ‘Environnement, sociétés et archéologie’; 7).

50.

Rohling

Pälike

(2005) Centennial-scale climate cooling with a sudden cold event around 8200 years ago. Science 435: 975–979.

51.

Smith

Clark

(1986) Radionuclide deposition from the Chernobyl cloud. Nature 322: 690–691.

52.

van Geel

Buurman

Waterbolk

(1996) Archaeological and palaeoecological indications of an abrupt climate change in The Netherlands, and evidence for climatological teleconnections around 2650 BP. Journal of Quaternary Science 11: 451–460.