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Biogeochemical and micro-facial fingerprints of ecosystem response to rapid Late Glacial climatic changes in varved sediments of Meerfelder Maar (Germany)
Palaeogeography, Palaeoclimatology, Palaeoecology 211 (2004) 139 – 155 www.elsevier.com/locate/palaeo
Biogeochemical and micro-facial fingerprints of ecosystem response to rapid Late Glacial climatic changes in varved sediments of Meerfelder Maar (Germany) Andreas Lqckea,*, Achim Brauerb a b
ICG V: Sedimenta¨re Systeme, Isotopengeochemie und Pala¨oklima, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, Germany Sektion 3.3, Klimadynamik und Sedimente, Geoforschungszentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany Received 22 July 2003; received in revised form 29 March 2004; accepted 4 May 2004
Abstract The response of a lacustrine ecosystem to climatic changes from 13,500 to 10,800 BP was studied in a varve dated sediment profile of Lake Meerfelder Maar, western Germany. Bulk biogeochemical parameters, stable carbon and nitrogen isotopes of sedimentary organic matter and micro-facies analysis are used for a detailed investigation of the lake’s development from the Allerbd interstadial (AL) to the Younger Dryas stadial (YD) and into the early Holocene (Preboreal). Varve micro-facies reveal rapid changes in composition and seasonal structure of the depositional environment mainly in temporal accordance with changes of terrestrial biozones. The respective responses of bulk proxy parameters (~bi-decadal resolution) indicate different sensitivities to diverse climatic changes. A prominent transition took place within two decades at the AL/YD boundary (12,690– 12,670 BP). Increased flux of nutrients released from redeposited littoral sediments and shorter YD summer seasons led to an acceleration and concentration of lacustrine primary production with reduced discrimination against 13C. High lacustrine primary production was further favored by relatively warm YD summer temperatures. At 12,240 varve years BP, from 1 year to the next, a regular deposition of detrital layers in spring set in which is related to local hydrologic threshold processes amplifying the response to increased snowmelt discharge. This distinct change in the middle of the YD is clearly detected by varve micro-facies but not equally recorded in organic bulk proxy data. Especially, carbon isotope ratios remain constant and indicate a negligible effect of this process on the productivity of lacustrine algae. The YD/Preboreal transition (11,645–11,585 BP) is marked by a characteristic change in micro-facies. Throughout this transition, stable carbon isotope ratios strongly decline while the accumulation of opal and organic matter (OM) remained constant. This is seen as an increasing importance of spring and autumn for gross primary production at the onset of the Holocene. D 2004 Elsevier B.V. All rights reserved. Keywords: Younger Dryas; Preboreal; Stable isotopes; Micro-facies; Maar lake; Climate change
* Corresponding author. Tel.: +49 246 161 4590; fax: +49 246 161 2484. E-mail address: [email protected] (A. Lqcke). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.05.006
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1. Introduction Lakes of all kinds are sensitive recorders of environmental change (e.g. Schindler et al., 1996). However, their usefulness for palaeoclimatology depends on our ability to recognize their specific response to different elements of a changing climate in the sedimentary record. The Late Glacial is an ideal time to study a bnatural experimentQ of climate change and its impact, respectively imprint into a lacustrine system. Micro-facies and palynological studies of Late Glacial sediments from a maar lake in Germany (Lake Meerfelder Maar, MFM) with varved sediments have previously shown prominent environmental changes (Brauer et al., 1999a; 2000a) but not all involved and relevant processes had been fully understood. Stable isotopes of organic matter buried in lake sediments were previously used as descriptors for lacustrine palaeoproductivity and nutrient cycling (Hodell and Schelske, 1998; Lu¨cke et al., 2003; Meyers, 1997; Talbot, 2001). Therefore, this study combines varve micro-facies analysis (seasonal resolution) with stable isotope and biogeochemical proxies (mean resolution ~15 years) in MFM sediments to shed further light on the response of the lake environment to drastic climatic oscillations, on their expression in the sedimentary record, and on their ecological relevance for lacustrine organisms. Numerous reconstructions have been used to infer climatic conditions of the Late Glacial in Europe and special emphasis has been put on the temperature changes of the Younger Dryas. However, the available information is still ambiguous. Estimates of winter temperature change converge in a reduction of 15 to 20 8C (Isarin et al., 1998), though modelling results by Renssen and Isarin (1998) imply a significantly lower reduction of only 5 8C. Summer temperature estimates based on pollen and cladoceran assemblages revealed a reduction of 2 8C (Lotter et al., 2000), reconstruction from beetle remains in England resulted in a ~3.5 8C depression (Coope et al., 1998) while modelling experiments suggested a reduction of 4.5–5.0 8C (Isarin and Bohncke, 1999; Isarin et al., 1998). Even a temperature reduction of 7–8 8C for July was inferred for The Netherlands from fossil Coleoptera (Walker, 1995) and for Switzerland from a multiproxy data series (Lemdahl, 2000).
Hydrologic changes are also associated with the Younger Dryas stadial (YD) (Taylor et al., 1997) but detailed information in particular on precipitation are sparse and not straightforward. In general, drier conditions have been inferred for the Younger Dryas in Middle Europe (Walker, 1995; Zolitschka et al., 1992). One of the few quantitative reconstructions report a reduction of 25% of precipitation compared to modern conditions in Switzerland (Wohlfarth et al., 1994). In addition to the assumed general decline, there are also indications for changes in the amount of precipitation within the Younger Dryas. However, the course of these changes is still controversially discussed. Goslar et al. (1993) and Brauer et al. (2000a) reported drier conditions in the early part of the Younger Dryas in Poland and western Germany, whereas for The Netherlands, Switzerland and the French Jura a trend from wetter towards drier conditions has been found for the second part (Bohnke et al., 1988; Lotter et al., 1992; Magny and Ruffaldi, 1995). As for temperature and precipitation, information on atmospheric carbon dioxide concentration during the Younger Dryas is ambiguous. Ice core data from Antarctica indicate a moderate increase of atmospheric carbon dioxide during the Younger Dryas of 28 ppmv (Monin et al., 2001) whereas evidence from stomata frequencies of fossil leaves indicate a reduced pCO2atm during the Younger Dryas (76 ppmv) (McElwain et al., 2002). Both approaches agree in an accelerated if not rapid increase in pCO2atm at the Younger Dryas to Preboreal transition. If relatively small amplitude changes are correct, photosynthetic carbon isotope discrimination of terrestrial or lacustrine plants should not have been affected in a detectable manner.
2. Site characteristics Lake Meerfelder Maar is situated in the mountainous Eifel region of Germany (6852V50WE, 50807V05WN) within a steep maar crater about 170 m beneath the surface of the surrounding landscape (Fig. 1). The modern lake is located at 336 m a.s.l. and extends over 1/3 of the crater while 2/3 is filled with delta sediments of the Meerbach. Presently, the Meerbach flows in an artificial bed passing by the lake in the south and leaving the crater at an elevation
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Fig. 1. Location of Lake Meerfelder Maar with depth contour lines of the lake and coring sites. The morphological setting in the crater as well as the location of the delta and the village Meerfeld are indicated.
of about 345 m a.s.l. The present lake is eutrophic and has a maximum depth of 17 m. The lake is thermally stratified from April to October and mixes twice a year. Due to modern intensive agriculture in the lake surroundings, a monimolimnion has developed. The present day mean annual air temperature at station Manderscheid (DWD station No. 2265, 3 km E of the lake) is 8.2 8C. The warmest month, July, has a mean of 17.4 8C and the coldest month, February, has a mean of 0.2 8C. Mean annual precipitation amounts to 950 mm where highest monthly values are reached during the winter in December and January.
3. Material and methods A 2.05-m-long sediment section of the Meerfelder Maar (profile MFM6; coring campaigns 1992 and 1996) from 6.80 to 8.85 m composite depth has been investigated. Sediments consist of organic and clastic/ organic varves and display a high variability in varve micro-facies. Varve counts and thickness measure-
ments as well as micro-facies analyses have been performed on thin sections using a petrographic microscope. Subsamples for isotopic and geochemical analyses were taken continuously from the composite profile in 1 cm slices each and subsequently freeze dried. In total, 200 samples were investigated for chemical element content and stable isotope ratios. Determination of biogenic opal and carbonate was done on a reduced set of 144 samples. Nevertheless, results of single parameters are completely congruent with each other since a single sample was shared for all determinations. The chronology was established through varve counting and thickness measurements obtained from microscopic thin section analyses (Brauer et al., 1999b). An overlapping series of large-scale petrographic thin sections (each covering 10 cm of sediment) has been prepared to continuously cover the whole sediment profile. All samples have been counted at least twice in order to achieve a reliable error estimate (ca. 1%). More details on the varve counting procedure are given in Brauer et al. (1999b). The
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resulting chronology has been further controlled by tephra time markers (Laacher See Tephra, Ulmener Maar Tephra) and radiocarbon dating (Brauer et al., 2000b). Mean time resolution of all samples is 13 years (standard deviation: 5 years), with a maximum of 37 years and a minimum of 5 years. The first part of the Younger Dryas is best resolved with ~10 years while the Preboreal is resolved with ~17 years. All ages given represent varve years BP. The zonation used is based on biostratigraphy (Litt and Stebich, 1999; Stebich, 1999). Total sediment accumulation rates (SAR) are based on annual layer thickness data and dry densities (Brauer et al., 1999b). Specific accumulation rates of organic matter (OM-SAR) and biogenic Opal (Opal-SAR) are calculated from the total sediment accumulation rates and the respective elemental contents. TOC values are converted to organic matter (OM) using the Redfield ratio of 2.8. The residuum (SAR (OM-SAR+OpalSAR)) is taken as representative of the minerogenic component (siliciclastic matter, Min-SAR) since the mass contribution of carbonates was found to be negligible for most of the studied time interval. However, during the late Allerbd siderite was a major component of the autumn/winter sub-layer. Authigenic calcite has formed in larger quantities in summer layers during a 15-year period (i.e. one bulk sample) about 280 years before the beginning of the Younger Dryas. Prior to measurements, sample material was mildly homogenized using a spatula. This was found to be sufficient to guarantee good sample reproducibility for stable isotope analysis. Elemental and isotopic analyses of organic matter were performed with an elemental analyser (EA 3000, Euro Vector) interfaced in continuous flow mode to an isotope ratio mass spectrometer (IRMS; IsoPrime, Micromass). Analysis of organic carbon content and carbon isotope ratios was done using decarbonized sample aliquots (5% HCL, freeze dried). Samples were weighed to provide 100 Ag of carbon and packed in tin foil boats. Analyses of nitrogen content (TN) and nitrogen isotopes were done on the untreated freeze dried samples weighed to provide 75 Ag of nitrogen and packed in tin foil boats. Samples were burned in excess oxygen in an elemental analyser and the analytical gas was flushed into the IRMS by a helium carrier-gas flow. The isotopic signal was also used to integrate the elemental content of nitrogen. Results were calibrated using certified elemental standards and international isotope stand-
ards (IAEA, Vienna). Isotope values of carbon and nitrogen are reported as d sample =[(R sample / R standard) 1]1000 in per mil (x). R represents the isotope ratio (13C/12C, 15N/14N) of the sample and the standard, respectively. Carbon isotope values are given relative to VPDB, whereas nitrogen isotopes are reported relative to atmospheric nitrogen (AIR). Reproducibility for replicate analyses of samples is F0.2% relative for elemental contents, F0.1x for carbon isotope ratios and F0.2x for nitrogen isotope ratios.
4. Biogeochemical and stable isotope proxies Total organic carbon (TOC) contents range from 2% to 7% by weight (mean 3.4%, n=200) (Fig. 2b). Total nitrogen (TN) values of the untreated sediments vary between 0.3% and 0.8% by weight (mean 0.55%, n=200). Biogenic silica, i.e. diatom opal with negligible contributions of chrysophyte cysts, varied from 7% to 46% by weight (mean 26.4%, n=144). For the calculation of TOC/ON ratios, the amount of inorganic nitrogen was estimated with 0.16% according to Talbot and Johannessen (1992) and, assuming this to be a constant, was subtracted from TN to determine the amount of organic nitrogen (Fig. 3). Calculated TOC/ON ratios (based on weight percent) vary between 6 and 13 with a mean of 8.8 (Fig. 2b) which is generally thought to indicate a major contribution of planktonic organisms to total organic matter. Carbon isotope ratios of bulk organic matter (d 13COM) show large amplitude variations between 36.1x and 29.3x. This signature is below that for higher plant matter (Fig. 4c). The minimum is determined for the Late Allerbd, while highest values occur during the Younger Dryas with a maximum in the first part. Decadal variability during the Younger Dryas is low. Nitrogen isotope ratios vary between 1.1x and 4.1x representing values commonly observed in land plants and freshwater sediments (e.g. Talbot, 2001). 4.1. Organic matter derived proxies and diagenetic alteration Early diagenesis reduces the amount of deposited organic matter in the upper centimetres of a sediment
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Fig. 2. Late Glacial and early Holocene characteristics of the Meerfelder Maar sedimentary record. Sedimentation rates (a), amount of organic carbon (TOC) and organic carbon to organic nitrogen ratios (TOC/ON) (b) and the ratios of organic matter to biogenic opal (OM-SAR/OpalSAR) as well as biogenic opal to minerogenic matter (Opal-SAR/Min-SAR) (c). Vertical grey bars indicate micro-facies transition zones. Biozones in the top panel are indicated according to Litt and Stebich (1999) and Stebich (1999).
column while selective degradation of specific organic compounds may lead to alteration of the primary isotopic signature (Harvey et al., 1995; Spiker and Hatcher, 1984). Incubation experiments over several weeks regarding carbon and nitrogen gave contradictory results. Experiments with fresh phytoplankton matter showed an initial depletion of both isotope ratios followed by a subsequent stabilization of the values (Lehmann et al., 2002). Similar experiments using surface sediments revealed no isotopic alteration (Macko et al., 1994). Thus, the primary isotopic signature of organic matter from fresh phytoplankton might be depleted during and after sedimentation but the relative isotope variations are effectively preserved, especially under anoxic conditions (Hodell and Schelske, 1998; Meyers and
Lallier-Verge`s, 1999) which are pervasive in Lake Meerfelder Maar. 4.2. Organic matter sources and contributions The major shifts in the depositional system of Meerfelder Maar at the onset and the termination of the Younger Dryas are not equally reflected in parameters that describe as a first approximation amount and type of organic matter like TOC and TOC/ON ratio (Fig. 2b). The relative contributions of OM derived from external sources (with markedly higher TOC/ON ratios than algae) and OM derived from autochthonous production (phytoplankton) did not change remarkably, thus the OMterrigeneous/OMlacustrine ratio is approximately constant. Consequently, a relative increase in
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However, only in a few discrete layers do thin section analyses reveal large abundances of macrophytes. Thus, d 13COM was only affected in the few samples which include such layers, e.g. that at 13,180 BP (Fig. 4c). 4.3. Inorganic carbon sources
Fig. 3. Cross plot of total nitrogen (TN) against total organic carbon (TOC). Assuming a strictly linear relationship between organic nitrogen (ON) and organic carbon, the amount of inorganic nitrogen (IN) is defined as the intercept with the ordinate (IN=0.16%).
organic matter derived from soil erosion (TOC/ ONN20) can be excluded. Instead, the thick autumn/ winter sub-layer contains abundant reworked shallow water sediment from the littoral. This is further supported by the occurrence of epiphytic diatom species within these layers. The Opal-SAR/Min-SAR ratio (Fig. 2c) describes the relation between diatoms (autochthonous lacustrine) and detrital sediments (minerogenic matter). While in the first part of the Younger Dryas (12,680– 12,240 BP), the ratio shows a remarkable increase, dilution of the lacustrine component is obvious for the second half of the Younger Dryas (12,240–11,650 BP). Assuming a constant SiO2 to organic matter ratio for diatomaceous algae, the ratio of OM-SAR/OpalSAR describes the relation between non-siliceous and siliceous algae (Fig. 2c). No significant change has been found in this ratio except for higher amplitudes of short-term variability in the Allerbd. If this assumption holds true, diatoms remained an important source for autochthonous production throughout the investigated period. It might be expected that submersed and/or emersed macrophytes remains are included within such reworked littoral sediments, possibly altering the planktonic d 13C signature (Lu¨cke et al., 2003).
Calcite precipitation should have had no influence on the carbon cycle in the photic zone since Meerfelder Maar is a soft-water lake with generally low TIC contents (b1% CaO equivalents). Modern lake water is about neutral and no indications of lasting changes in pH of the lake waters in the past are evident. Thus, the relative availability of inorganic carbon species (CO2, HCO3 ) for primary production should not have changed over time. Only for phases of exceptionally high productivity, a temporary use of bicarbonate for assimilation cannot be completely excluded. The carbon isotope minimum phase at the end of the Allerbd might be influenced by carbonate formation since higher contents of calcium carbonate (up to 8%) have been measured there in single samples and abundant siderite in autumn/winter layers has been observed in thin sections. Despite atmospheric carbon dioxide, remineralisation of organic matter at the sediment surface can be another source of inorganic carbon for the lake. However, the isotopic signature of such carbon dioxide is almost identical to the isotopic composition of the respective OM source and would only lead to a further depletion of the lake’s d 13CDIC. 4.4. Nitrogen isotope variations and significance The absolute level and the relative stability of nitrogen isotope ratios give indications about the lakes ecology (Fig. 4c). Since d 15N values higher than 6– 8x are interpreted as an indicator of pollution and eutrophication (Hodell and Schelske, 1998; Hollander, 1989), the mean nitrogen isotope composition of 1.6x might be interpreted as an indication of oligotrophic to mesotrophic nutrient conditions (Gu et al., 1996). The pool of inorganic nitrogen seems not to have been significantly altered by primary producers, since d 15N does not show large changes (Meyers and Teranes, 2001). Most prominent changes occur immediately before the respective transitions. In this
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Fig. 4. Specific accumulation rates of Meerfelder Maar Late Glacial and early Holocene sediments differentiated in minerogenic (a), organic matter and biogenic silica accumulation (b) together with respective changes of stable carbon and nitrogen isotopes of sedimentary organic matter (c). Vertical grey bars indicate micro-facies transition zones.
respect, the positive nitrogen isotope excursion within the YD is to a certain extent a precursor of the midYD change. Negative d 15N excursions at 12,750 and 11,700 BP might be seen as due to temporal increases of cyanoprocaryota, or increased discrimination with improved nitrate availability, while positive d 15N excursions at 12,400 and 11,200–11,000 BP might indicate the reverse development. However, it is unlikely that nitrogen has become the limiting nutrient during the investigated period. Also no fundamental change from common nitrate nitrogen using plankton, e.g. siliceous microalgae, towards an increased proportion of atmospheric nitrogen fixing cyanobacteria with depleted isotopic signature can be stated (Talbot, 2001). This further supports the idea of relative stable algal associations despite a changing climate during the period studied. Despite the stability
of the planktonic community with respect to the competition between different classes of algae, the diatom association underwent considerable changes. For example, the mass occurrence of the genus Stephanodiscus starting at the AL/YD boundary is reduced during the second part of the YD and only recovers at the beginning of the Holocene.
5. Varve micro-facies The composition and seasonal structure of varves show several marked changes during the studied time interval which are not similarly represented in sedimentation rates (Fig. 2a). Except for an extremely sharp mid-Younger Dryas shift, all major changes in varve micro-facies occurred in gradual transitions over
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Table 1 Time and duration of transitions between types of Late Glacial varve micro-facies as determined by varve counting Transition zone
Time (in varve years BP)
Duration (years)
Older Dryas–Allerbd Allerbd–Younger Dryas Mid-Younger Dryas change Younger Dryas–Preboreal
13,380–13,350 12,690–12,670 12,240–12,239 11,645–11,585
30 20 1 60
decades (20–60 years). Their lengths were determined by varve counting (Table 1). These changes occurred contemporaneous with biozone boundaries. The transition zones marked in Figs. 2, 4 and 6) are based on these varve counts. Development of varve micro-facies in the course of the Allerbd is rather complex. The main characteristic is a general trend from an organic-clastic to an organic-sideritic varve facies (Plate Ia–d). This succession is well expressed in decreasing Min-SAR values (Fig. 4a). The small peak in the middle of the Allerbd reflects a period of 35 years (from 13,100 to 13,135 years BP) with thicker detrital winter layers interrupting the general trend. The organic-clastic facies is composed of mainly three seasonal sub-layers, (1) a diatom layer deposited during spring and summer, often subdivided in a succession of a lower Stephanodiscus sp. layer and an upper Cyclotella sp. layer, (2) an autumn layer of organic detritus and amorphous organic matter in which vivianite often has formed massively, and (3) a winter clay-layer which in some cases also contains fine silty siliciclastic matter. However, additional sublayers in single varves (individual years) are common,
displaying a high micro-facies variability. These are, for example, layers of detrital siliciclastic matter between the winter clay and the spring diatoms (interpreted as runoff deposits during the snowmelt period), or biochemically formed calcite layers in summer interrupting or preceding diatom layers. The occasional formation of these calcite layers occurred either in single years or in phases of several years (one to two decades) and is not well understood. The organic-sideritic facies of the late Allerbd is composed of only two sub-layers, (1) a summer layer composed of amorphous organic matter with only few scattered diatom frustules (no discrete diatom layer) and, (2) an autumn/winter layer of organic detritus and authigenic siderite often associated with vivianite. This facies does not contain any significant amount of detrital siliciclastic matter. The seasonal sub-layers are extremely thin, resulting in lowest sedimentation rates of the whole profile (Fig. 2a) The absence of discrete diatom layers is also expressed as the minimum of the Opal-SAR curve. The five-fold increase in varve thickness at the Allerbd to Younger Dryas transition is a clear sign for a prominent facies change (Fig. 2a). Varves of the early Younger Dryas (Plate Ie, f) are composed of two sub-layers, (1) monospecific diatom layers (Stephanodiscus sp.) reflecting massive diatom blooms at the base followed by (2) thick mixed layers including amorphous and particular organic matter, a large variety of diatom species with a major contribution of epiphytic species and dispersed detrital silt and clay. This composition clearly indicates partial reworking of shallow water sediments. The thickness of these reworked layers varies not as much as diatom
Plate I. Succession of varve micro-facies from the early Allerbd (a–b), late Allerbd (c–d), early Younger Dryas (e–f), late Younger Dryas (g–h) and Preboreal (i–j) in petrographic thin section images. Images on the left side (a, c, e, g, i) display the same sediment section as the ones on the right side (b, d, f, h, j) but with different optical tools. The left row is seen under plain parallel light while for the right one cross-polarized light has been used. In between the left and right rows, the number of years represented is indicated. On the right side, seasonal sub-layers are marked with numbers as in the text. Boundaries between the sub-layers are often transitional and indicated by dotted lines. Early Allerbd varves are composed of (1) summer diatom layers, (2) detrital autumn layers with abundant vivianite and, (3) winter clay/silt layers. Additional sub-layers which do not occur in each year are best recognizable in the polarized light image (b); these are (dccT) a biochemically precipitated calcite layer within the spring–summer diatom succession and a thin detrital layer below the spring diatoms (ddlT). Authigenic vivianite in some cases forms distinct sub-layers in the autumn layer and is indicated by dvT. At shown magnification, the detrital and vivianite layers appear to have similar optical characteristics. At higher magnification, both layers are clearly distinguishable. Late Allerbd varves are composed of a (1) mainly amorphous organic layer and, (2) a winter layer rich in siderite. Early Younger Dryas varves are couplets of (1) monospecific diatom frustules and (2) mixed clastic-organic detritus sometimes containing vivianite (dvT in image f). Late Younger Dryas varves are characterized by (1) a detrital snowmelt layer in spring, (2) mixed layer with diatom frustules and, (3) a winter clay/silt layer. Preboreal varves are build up of three sub-layers, (1) a diatom layer from spring/early summer deposition, (2) a mixed organic layer with diatoms (late summer/autumn) and, (3) a winter clay layer. Nest of vivianite (image k) formed in (2) and (3).
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layer thickness which exhibits a large interannual variability. Discrete detrital layers at the base of an annual succession which would indicate deposition after snowmelt runoff have not been observed. Only in some of the diatom layers a few silty siliciclastic particles are dispersed, indicating wind-blown rather than melt water transport. This mode of deposition during the early Younger Dryas changes within 1 year (Plate II) at 12,240 varve years BP. From this time on, sedimentation in every year started with a layer of siliciclastic detritus. These layers clearly resemble snowmelt layers as observed during spring in modern lakes in Finland (Ojala and Francus, 2002). During the first 150–200 years of this type of deposition, these snowmelt layers in general are not very thick and diatom layers continued to settle, but not in every year. Thereafter, the typical varve micro-facies of the second part of Younger Dryas has developed (Plate Ig–h). At the base, (1) a prominent detrital siliciclastic layer (snowmelt) is deposited, followed by (2) a faint layer enriched in organic matter including diatoms (mainly Cyclotella sp.), the latter not in every year. The end of the annual cycle (3) is formed by a prominent winter clay/silt layer. During the Younger Dryas/Preboreal transition, varve preservation is rather poor preventing varve thickness measurements. At the beginning of the transition, regular deposition of snowmelt layers ceased resulting in lower amounts of minerogenic detritus (see also Fig. 2a, Min-SAR). Preboreal varve micro-facies (Plate Ii–j) is comparable to early Allerbd varves and characterized by three seasonal sub-layers, (1) a first diatom bloom consisting of Stephanodiscus sp. followed by (2) a second bloom mainly of Cyclotella sp. which is mixed with organic matter interpreted as late summer/autumn deposition and, (3) a thin winter clay layer. Transitions between (2) and (3) are not very sharp and commonly authigenic vivianite has formed within these sub-layers.
6. Lacustrine productivity Enriched carbon isotope ratios during the YD (Fig. 4c) are interpreted as a change of discrimination by the planktonic community (Brenner et al., 1999; Gu et al., 1996). Resuspended littoral material bears an older (decades to centuries) isotopic signature and clearly could not induce the observed rapid change in isotopic composition of profundal varves. As larger changes in atmospheric carbon dioxide ( pCO2, d 13CO2) and in the photosynthetically used dissolved inorganic carbon source are unlikely, the observed positive isotope shift of 4.0x is a measure of an increased carbon demand per time of the phytoplankton cells, i.e. a reduced discrimination against 13C (Herczeg and Fairbanks, 1987). Several processes can lead to such a development: (1) raised absolute carbon demand during the vegetation period exceeding the replenishment of the carbon dioxide pool through the air–water interface (gross primary production); (2) increased rate of inorganic carbon demand and uptake by single cells, i.e. growth rate; (3) enlarged zone of diffusive resistance at the boundary layer (cell–water interface) caused by the growth of larger specimen, i.e. a local CO2 diss minimum zone. As the relevant factor for the observed reduction of discrimination, we propose a reduced length of the lacustrine vegetation period. In consequence, all biological processes must have taken place in a shorter time with increased biological turnover rates in order to support the observed massive increase of primary productivity during the YD. This argument is supported as biogenic opal accumulation is significantly correlated with carbon isotope ratios (r=0.66, R 2=0.44, Pb0.0001, n=143) where discrimination against 13C generally decreases when Opal-SAR increases (Fig. 5f). During the YD an asymptotic behaviour is indicated and discrimination seems to
Plate II. Thin section image profiles from the mid-Younger Dryas micro-facies change composed of a succession of five single images. Both profiles display 11 varves from the same sediment section. For the left profile, plain parallel light has been used while the right is under crosspolarized light. Varves in the lower part are better distinguishable under plain parallel light whereas the upper varves are better recognized under cross-polarized light. Artificial cracks (indicated as dCrT) are due to the freeze-drying process and occur preferably in diatom layers. Detrital snowmelt layers appear under cross-polarized light as light bands and are indicated on the right side by hatched bars. Vivianite (dvT) also appears as thin light bands or patches but is easily distinguishable at higher magnification. The alternating thin black and white bars on the right side mark single varves. Varve boundaries are further indicated by lines in between the left and right profile. The arrow at the transition from varve number 12,239 to 12,240 (years BP) marks the onset of regular deposition of detrital snowmelt layers in spring. In the first year (varve 12,240), this layer still is rather thin but thickness progressively increased in the following years.
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converge to a minimum, defined by parameters other than photosynthetic carbon demand. It cannot be evaluated if primary production was limited by a single nutrient other than silica, like, for example, phosphorus. However, sharp upper boundaries in the ratios between OM-SAR and Opal-SAR to Min-SAR (Fig. 5a, b) may indicate the respective maxima of productivity obtainable by the algae while using the available nutrient pool.
7. Evolution of environment and climate 7.1. The Allerød interstadial While iniatially minerogenic accumulation gradually diminishes, most of the Allerbd interstadial (AL) is characterized by relatively stable accumu-
lation rates of biogenic silica and OM and larger carbon isotope variations (Fig. 4b, c). This situation changed for about the last 200 years of the Allerbd after the deposition of the 7–10-cm-thick ash layer of the Laacher See volcano dated at 12,880 BP. Biogenic silica accumulation reached its absolute minimum about 40 years after the eruption and both isotopic markers, d 13C and d 15N, decreased by about 2x. Since Birks and Lotter (1994) have demonstrated on the sediment record from the adjacent Lake Holzmaar that the ash fall had no significant impact on the diatom assemblages, a causal link between the tephra and the reduced diatom blooms is also unlikely for MFM. Probably, a starvation situation for diatoms has been caused by reduced nutrient input into the lake leading to a diminished utilization of the silica and inorganic carbon and nitrogen pools. One may speculate on a
Fig. 5. Cross plots of specific accumulation rates and stable carbon isotopes. (a) Organic matter accumulation (OM-SAR) against minerogenic matter accumulation (Min-SAR), (b) biogenic silica accumulation (Opal-SAR) against Min-SAR, (c) OM-SAR against Opal-SAR with the 1:3 line, (d) carbon isotopes (d 13COM) against Min-SAR, (e) d 13COM against OM-SAR and (f) d 13COM against Opal-SAR where the Younger Dryas is indicated by a box.
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causal link between reduced nutrient fluxes and deposition of organic-sideritic varves with extremely low contents of detrital siliciclastic matter in the last 200–300 years of the Allerbd. 7.2. Transition to the Younger Dryas The length of the transition AL/YD as observed in varve micro-facies can be determined within 20–25 years. Responses of parameters describing different lake compartments are synchronous but articulated in different amplitudes. Minerogenic accumulation increases moderately reaching levels comparable to the early Allerbd while an almost explosive development of the biological system took place, i.e. diatom sedimentation increases by 1.9 mg cm 2 year 1 per decade. Together, increased minerogenic influx and increased diatom productivity resulted in a five-fold increase in annual sedimentation rates (Fig. 2a). The OM-SAR/Opal-SAR ratio falls back to the early Allerbd level, while the Opal-SAR/MIN-SAR ratio almost doubles despite the increase in minerogenic accumulation (Figs. 2c and 4a). These drastic changes in the depositional and biological systems are the result of a combination of various processes. Firstly, an assumed falling lake level results in a higher catchment area/lake surface ratio and a decreased water volume leading to an increase in nutrient concentrations in the water body. An increase in nutrient fluxes is further indicated by massive layers of reworked shallow water sediment. These layers were deposited after thick (300– 500 Am) monospecific diatom layers of Stephanodiscus sp. Secondly, a drop of winter temperatures and longer winter seasons resulted in markedly reduced vegetation periods with consequences for lacustrine plankton organisms. The start of the vegetation period was delayed by longer ice-cover leading to a coincidence of a still undiminished nutrient pool with already high seasonal irradiances (higher levels than at present). Thus, photosynthetic activity, respectively plankton growth and possibly cell size (Frenette et al., 1998), supported by improved replenishment of nutrients from reworked littoral sediments, was enhanced and is expressed by massive blooms of diatoms and strongly reduced discrimination against 13 C (e.g. Goericke et al., 1994; Riebesell et al., 1993). Most probably, nutrient cycling in the water body was also enhanced by reduced duration and strength of
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summer stagnation, further increasing available nutrient concentrations (Salmaso, 2002). All these processes led to a concentration of biomass production during the relative warm summer months and summer temperatures must have remained high enough to enable such massive increase of biological activity. This confirms anomalously mild summers reported from southern Greenland that have been suggested to be a consequence of regional climatic differences (Bjo¨rck et al., 2002). Our data show that this seasonal climate characteristic probably was more widespread than expected. 7.3. The Younger Dryas stadial A two-fold division of YD has been reported earlier (e.g. Walker, 1995) but mostly the differences between the subunits were not very distinct. This is different for the Meerfelder Maar record, where at 12,240 varve years BP the deposition pattern in the lake distinctly changed between two different varve types from 1 year to the next (Plate II). During the first 440 years of the Younger Dryas, annual deposition was controlled by a monospecific diatom layer of Stephanodiscus sp. followed by a massive layer of reworked littoral sediments. During the last 650 years of the Younger Dryas, annual deposition started with a discrete detrital snowmelt layer followed by occasional diatom layers (mainly Cyclotella sp.). These micro-facies changes are well expressed in a sharp rise of Min-SAR (Fig. 4a) and a drop in the Opal-SAR/Min-SAR ratio (Fig. 2c). Within the second, Min-SAR rich, part of the YD the frequency and thickness of diatom layers decreased and detrital layers became dominant. However, the planktonic community and their fractionation against 13C remained stable over this marked change in the depositional system (Figs. 2c and 4b). Only between ca. 12,100 and 11,800 varve years BP when clastic sedimentation was slightly lower, Opal-SAR and carbon isotope ratios indicate small changes of the planktonic productivity. Both curves do not respond to the distinct drop of clastic accumulation at 11,645 BP. The observed difference between stable carbon isotopes and Min-SAR at the mid-YD shift in depositional pattern suggests an environmental factor affecting only the inorganic sedimentation.
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Our hypothesis is that changes in lake and catchment hydrology triggered the observed shift. It can be argued that if surface runoff and transport capacity have increased (Richter, 1998), more clastic matter would have been transported by the Meerbach and probably led to stronger channel erosion and incision into the floodplain. Higher surface runoff could have been triggered by increased rates of snowmelt which would imply mainly higher winter (snow) precipitation (Dearing, 1991). The observed abrupt onset of regular snowmelt layer deposition indicates some kind of threshold process rather than a gradual change of the system. A rapid increase in minerogenic deposition could have been caused by a change in the course of the Meerbach. During the early YD, the Meerbach might have passed the lake in the Southeast but discharged into the lake since 12,240 BP when a morphological threshold (e.g. levee deposits) was removed due to higher discharge and increased channel erosion. 7.4. Termination of the Younger Dryas Deposition of regular snowmelt layers ceased at about 11,645 varve years BP at the same time when a first maximum of Juniper pollen appeared (Brauer et al., 1999a). From this, it is concluded that a denser vegetation cover led to lower erosion rates and, consequently, diminished minerogenic matter flux. This change in the depositional pattern marks the onset of a ca. 60-year period of gradual micro-facies change. The sharp decrease of d 13COM, indicating rapidly changing conditions for lacustrine primary production, occurs at the end of this 60-year transitional period and coincides with the YD-Preboreal biozone boundary and the onset of Preboreal varve micro-facies. OM-SAR and Opal-SAR remain almost constant during the transition and the following Preboreal. It might be hypothesized that Preboreal warming led to shorter cold seasons allowing lake productivity to start earlier and end later in the year. This would not change primary production but would cause a depletion of carbon isotope values since a higher portion of production takes place at lower temperature and higher CO2 solubility conditions in spring and autumn. Increasing atmospheric carbon dioxide concentrations might have contributed to this development (McElwain et al., 2002). A switch of a
lacustrine system forth and back between two different states must, thus, not necessarily be expressed completely identical by the respective algal community (Anneville et al., 2002) and, in turn, also identical proxy signals cannot be taken as proof for identical forcings.
8. Synthesis and conclusion The significant correlation found between OpalSAR and carbon isotope ratios confirms carbon isotopes of lacustrine organic matter as tracer of palaeoproductivity. However, this interpretation is only valid during times where autochthonous production by far exceeds allochthonous input and as long as photosynthetic discrimination by algae is not limited by physiological effects. Micro-facies transitions and respective reactions in the isotopic composition of organic matter occurred rapidly in the Meerfelder Maar. Within a single year or a few decades, transitions are completed implying either a sudden response to abrupt climate change or a respective threshold reaction. It is striking that at the AL/YD transition micro-facies changes were completed faster than at the respective transition to the Holocene indicating that the response of lacustrine systems (lake and catchment) to warming might be slightly slower than to cooling. Taking d 13COM as a proxy for lacustrine primary production and Min-SAR as proxy for erosion and transport processes in the catchment of Meerfelder Maar provides independent information for different environmental processes which are directly influenced by climate (Fig. 6). The almost equivalent carbon isotope shift at the beginning and the end of the YD represent different climatic influences with strong effects on seasonality and the length of the vegetation period. The increasing length of winter seasons at the AL/YD transition led to shortened growing periods for lacustrine algae but increased primary production due to higher nutrient availabilities during the remaining summer months. Temperatures during YD growing seasons must have been sufficiently warm to support high primary production and probably no substantial change occurred at all. The development at the transition Younger Dryas– Holocene is seen as the consequence of seasonal
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Fig. 6. Comparison of indicators of different forcings on the lake, respectively different transfer of climatic information into the lake and its catchment. As a descriptor of direct climate forcing for the lake carbon isotopes are taken while catchment processes seen as indirect climatic forcing on the system are described by minerogenic accumulation. While the former one may be seen as seasonality descriptor, the latter one may be seen as hydrological descriptor. See text for further discussion. Vertical grey bars indicate micro-facies transition zones.
climatic warming leading to longer growing seasons probably with higher maximum temperatures but approximately constant gross productivity. The relative the contribution of algal biomass from spring and autumn increased due to improved growth conditions in these months. The distinct increase of the seasonal detrital influx at 12,240 BP is not reflected in the carbon isotope signal and can thus not be considered as a major controlling factor for lacustrine algae. This increased Min-SAR values are interpreted as increased snowmelt run-off leading to higher erosion and transport of clastic matter into the lake probably in conjunction with the crossing of a morphological threshold. This study has demonstrated that multiproxy data sets offer the opportunity to describe different subsystems of the lake leading to an improved understanding of lacustrine responses to climate change. The role of seasonality and changes therein for ecosystems as demonstrated for MFM is especially emphasized. This implies that experimental data of climate model runs should not only reproduce annual means of climate parameters but also the seasonal amplitudes.
Acknowledgements The authors would like to thank J.F.W. Negendank who made the sedimentary archive of MFM available, and S. Bjfrck and one anonymous reviewer for constructive comments. AL wants to thank G.H. Schleser and H. Vos for stimulating discussions and encouragement, and H. Wissel for analytical work. AL gratefully acknowledges financial support by the German Ministry for Education and Research (BMBF, Grant 01LD0001). This is a contribution to the German Climate Research Program DEKLIM.
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Biogeochemical and micro-facial fingerprints of ecosystem response to rapid Late Glacial climatic changes in varved sediments of Meerfelder Maar (Germany) Andreas Lqckea,*, Achim Brauerb a b
ICG V: Sedimenta¨re Systeme, Isotopengeochemie und Pala¨oklima, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, Germany Sektion 3.3, Klimadynamik und Sedimente, Geoforschungszentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany Received 22 July 2003; received in revised form 29 March 2004; accepted 4 May 2004
Abstract The response of a lacustrine ecosystem to climatic changes from 13,500 to 10,800 BP was studied in a varve dated sediment profile of Lake Meerfelder Maar, western Germany. Bulk biogeochemical parameters, stable carbon and nitrogen isotopes of sedimentary organic matter and micro-facies analysis are used for a detailed investigation of the lake’s development from the Allerbd interstadial (AL) to the Younger Dryas stadial (YD) and into the early Holocene (Preboreal). Varve micro-facies reveal rapid changes in composition and seasonal structure of the depositional environment mainly in temporal accordance with changes of terrestrial biozones. The respective responses of bulk proxy parameters (~bi-decadal resolution) indicate different sensitivities to diverse climatic changes. A prominent transition took place within two decades at the AL/YD boundary (12,690– 12,670 BP). Increased flux of nutrients released from redeposited littoral sediments and shorter YD summer seasons led to an acceleration and concentration of lacustrine primary production with reduced discrimination against 13C. High lacustrine primary production was further favored by relatively warm YD summer temperatures. At 12,240 varve years BP, from 1 year to the next, a regular deposition of detrital layers in spring set in which is related to local hydrologic threshold processes amplifying the response to increased snowmelt discharge. This distinct change in the middle of the YD is clearly detected by varve micro-facies but not equally recorded in organic bulk proxy data. Especially, carbon isotope ratios remain constant and indicate a negligible effect of this process on the productivity of lacustrine algae. The YD/Preboreal transition (11,645–11,585 BP) is marked by a characteristic change in micro-facies. Throughout this transition, stable carbon isotope ratios strongly decline while the accumulation of opal and organic matter (OM) remained constant. This is seen as an increasing importance of spring and autumn for gross primary production at the onset of the Holocene. D 2004 Elsevier B.V. All rights reserved. Keywords: Younger Dryas; Preboreal; Stable isotopes; Micro-facies; Maar lake; Climate change
* Corresponding author. Tel.: +49 246 161 4590; fax: +49 246 161 2484. E-mail address: [email protected] (A. Lqcke). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.05.006
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1. Introduction Lakes of all kinds are sensitive recorders of environmental change (e.g. Schindler et al., 1996). However, their usefulness for palaeoclimatology depends on our ability to recognize their specific response to different elements of a changing climate in the sedimentary record. The Late Glacial is an ideal time to study a bnatural experimentQ of climate change and its impact, respectively imprint into a lacustrine system. Micro-facies and palynological studies of Late Glacial sediments from a maar lake in Germany (Lake Meerfelder Maar, MFM) with varved sediments have previously shown prominent environmental changes (Brauer et al., 1999a; 2000a) but not all involved and relevant processes had been fully understood. Stable isotopes of organic matter buried in lake sediments were previously used as descriptors for lacustrine palaeoproductivity and nutrient cycling (Hodell and Schelske, 1998; Lu¨cke et al., 2003; Meyers, 1997; Talbot, 2001). Therefore, this study combines varve micro-facies analysis (seasonal resolution) with stable isotope and biogeochemical proxies (mean resolution ~15 years) in MFM sediments to shed further light on the response of the lake environment to drastic climatic oscillations, on their expression in the sedimentary record, and on their ecological relevance for lacustrine organisms. Numerous reconstructions have been used to infer climatic conditions of the Late Glacial in Europe and special emphasis has been put on the temperature changes of the Younger Dryas. However, the available information is still ambiguous. Estimates of winter temperature change converge in a reduction of 15 to 20 8C (Isarin et al., 1998), though modelling results by Renssen and Isarin (1998) imply a significantly lower reduction of only 5 8C. Summer temperature estimates based on pollen and cladoceran assemblages revealed a reduction of 2 8C (Lotter et al., 2000), reconstruction from beetle remains in England resulted in a ~3.5 8C depression (Coope et al., 1998) while modelling experiments suggested a reduction of 4.5–5.0 8C (Isarin and Bohncke, 1999; Isarin et al., 1998). Even a temperature reduction of 7–8 8C for July was inferred for The Netherlands from fossil Coleoptera (Walker, 1995) and for Switzerland from a multiproxy data series (Lemdahl, 2000).
Hydrologic changes are also associated with the Younger Dryas stadial (YD) (Taylor et al., 1997) but detailed information in particular on precipitation are sparse and not straightforward. In general, drier conditions have been inferred for the Younger Dryas in Middle Europe (Walker, 1995; Zolitschka et al., 1992). One of the few quantitative reconstructions report a reduction of 25% of precipitation compared to modern conditions in Switzerland (Wohlfarth et al., 1994). In addition to the assumed general decline, there are also indications for changes in the amount of precipitation within the Younger Dryas. However, the course of these changes is still controversially discussed. Goslar et al. (1993) and Brauer et al. (2000a) reported drier conditions in the early part of the Younger Dryas in Poland and western Germany, whereas for The Netherlands, Switzerland and the French Jura a trend from wetter towards drier conditions has been found for the second part (Bohnke et al., 1988; Lotter et al., 1992; Magny and Ruffaldi, 1995). As for temperature and precipitation, information on atmospheric carbon dioxide concentration during the Younger Dryas is ambiguous. Ice core data from Antarctica indicate a moderate increase of atmospheric carbon dioxide during the Younger Dryas of 28 ppmv (Monin et al., 2001) whereas evidence from stomata frequencies of fossil leaves indicate a reduced pCO2atm during the Younger Dryas (76 ppmv) (McElwain et al., 2002). Both approaches agree in an accelerated if not rapid increase in pCO2atm at the Younger Dryas to Preboreal transition. If relatively small amplitude changes are correct, photosynthetic carbon isotope discrimination of terrestrial or lacustrine plants should not have been affected in a detectable manner.
2. Site characteristics Lake Meerfelder Maar is situated in the mountainous Eifel region of Germany (6852V50WE, 50807V05WN) within a steep maar crater about 170 m beneath the surface of the surrounding landscape (Fig. 1). The modern lake is located at 336 m a.s.l. and extends over 1/3 of the crater while 2/3 is filled with delta sediments of the Meerbach. Presently, the Meerbach flows in an artificial bed passing by the lake in the south and leaving the crater at an elevation
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Fig. 1. Location of Lake Meerfelder Maar with depth contour lines of the lake and coring sites. The morphological setting in the crater as well as the location of the delta and the village Meerfeld are indicated.
of about 345 m a.s.l. The present lake is eutrophic and has a maximum depth of 17 m. The lake is thermally stratified from April to October and mixes twice a year. Due to modern intensive agriculture in the lake surroundings, a monimolimnion has developed. The present day mean annual air temperature at station Manderscheid (DWD station No. 2265, 3 km E of the lake) is 8.2 8C. The warmest month, July, has a mean of 17.4 8C and the coldest month, February, has a mean of 0.2 8C. Mean annual precipitation amounts to 950 mm where highest monthly values are reached during the winter in December and January.
3. Material and methods A 2.05-m-long sediment section of the Meerfelder Maar (profile MFM6; coring campaigns 1992 and 1996) from 6.80 to 8.85 m composite depth has been investigated. Sediments consist of organic and clastic/ organic varves and display a high variability in varve micro-facies. Varve counts and thickness measure-
ments as well as micro-facies analyses have been performed on thin sections using a petrographic microscope. Subsamples for isotopic and geochemical analyses were taken continuously from the composite profile in 1 cm slices each and subsequently freeze dried. In total, 200 samples were investigated for chemical element content and stable isotope ratios. Determination of biogenic opal and carbonate was done on a reduced set of 144 samples. Nevertheless, results of single parameters are completely congruent with each other since a single sample was shared for all determinations. The chronology was established through varve counting and thickness measurements obtained from microscopic thin section analyses (Brauer et al., 1999b). An overlapping series of large-scale petrographic thin sections (each covering 10 cm of sediment) has been prepared to continuously cover the whole sediment profile. All samples have been counted at least twice in order to achieve a reliable error estimate (ca. 1%). More details on the varve counting procedure are given in Brauer et al. (1999b). The
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resulting chronology has been further controlled by tephra time markers (Laacher See Tephra, Ulmener Maar Tephra) and radiocarbon dating (Brauer et al., 2000b). Mean time resolution of all samples is 13 years (standard deviation: 5 years), with a maximum of 37 years and a minimum of 5 years. The first part of the Younger Dryas is best resolved with ~10 years while the Preboreal is resolved with ~17 years. All ages given represent varve years BP. The zonation used is based on biostratigraphy (Litt and Stebich, 1999; Stebich, 1999). Total sediment accumulation rates (SAR) are based on annual layer thickness data and dry densities (Brauer et al., 1999b). Specific accumulation rates of organic matter (OM-SAR) and biogenic Opal (Opal-SAR) are calculated from the total sediment accumulation rates and the respective elemental contents. TOC values are converted to organic matter (OM) using the Redfield ratio of 2.8. The residuum (SAR (OM-SAR+OpalSAR)) is taken as representative of the minerogenic component (siliciclastic matter, Min-SAR) since the mass contribution of carbonates was found to be negligible for most of the studied time interval. However, during the late Allerbd siderite was a major component of the autumn/winter sub-layer. Authigenic calcite has formed in larger quantities in summer layers during a 15-year period (i.e. one bulk sample) about 280 years before the beginning of the Younger Dryas. Prior to measurements, sample material was mildly homogenized using a spatula. This was found to be sufficient to guarantee good sample reproducibility for stable isotope analysis. Elemental and isotopic analyses of organic matter were performed with an elemental analyser (EA 3000, Euro Vector) interfaced in continuous flow mode to an isotope ratio mass spectrometer (IRMS; IsoPrime, Micromass). Analysis of organic carbon content and carbon isotope ratios was done using decarbonized sample aliquots (5% HCL, freeze dried). Samples were weighed to provide 100 Ag of carbon and packed in tin foil boats. Analyses of nitrogen content (TN) and nitrogen isotopes were done on the untreated freeze dried samples weighed to provide 75 Ag of nitrogen and packed in tin foil boats. Samples were burned in excess oxygen in an elemental analyser and the analytical gas was flushed into the IRMS by a helium carrier-gas flow. The isotopic signal was also used to integrate the elemental content of nitrogen. Results were calibrated using certified elemental standards and international isotope stand-
ards (IAEA, Vienna). Isotope values of carbon and nitrogen are reported as d sample =[(R sample / R standard) 1]1000 in per mil (x). R represents the isotope ratio (13C/12C, 15N/14N) of the sample and the standard, respectively. Carbon isotope values are given relative to VPDB, whereas nitrogen isotopes are reported relative to atmospheric nitrogen (AIR). Reproducibility for replicate analyses of samples is F0.2% relative for elemental contents, F0.1x for carbon isotope ratios and F0.2x for nitrogen isotope ratios.
4. Biogeochemical and stable isotope proxies Total organic carbon (TOC) contents range from 2% to 7% by weight (mean 3.4%, n=200) (Fig. 2b). Total nitrogen (TN) values of the untreated sediments vary between 0.3% and 0.8% by weight (mean 0.55%, n=200). Biogenic silica, i.e. diatom opal with negligible contributions of chrysophyte cysts, varied from 7% to 46% by weight (mean 26.4%, n=144). For the calculation of TOC/ON ratios, the amount of inorganic nitrogen was estimated with 0.16% according to Talbot and Johannessen (1992) and, assuming this to be a constant, was subtracted from TN to determine the amount of organic nitrogen (Fig. 3). Calculated TOC/ON ratios (based on weight percent) vary between 6 and 13 with a mean of 8.8 (Fig. 2b) which is generally thought to indicate a major contribution of planktonic organisms to total organic matter. Carbon isotope ratios of bulk organic matter (d 13COM) show large amplitude variations between 36.1x and 29.3x. This signature is below that for higher plant matter (Fig. 4c). The minimum is determined for the Late Allerbd, while highest values occur during the Younger Dryas with a maximum in the first part. Decadal variability during the Younger Dryas is low. Nitrogen isotope ratios vary between 1.1x and 4.1x representing values commonly observed in land plants and freshwater sediments (e.g. Talbot, 2001). 4.1. Organic matter derived proxies and diagenetic alteration Early diagenesis reduces the amount of deposited organic matter in the upper centimetres of a sediment
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Fig. 2. Late Glacial and early Holocene characteristics of the Meerfelder Maar sedimentary record. Sedimentation rates (a), amount of organic carbon (TOC) and organic carbon to organic nitrogen ratios (TOC/ON) (b) and the ratios of organic matter to biogenic opal (OM-SAR/OpalSAR) as well as biogenic opal to minerogenic matter (Opal-SAR/Min-SAR) (c). Vertical grey bars indicate micro-facies transition zones. Biozones in the top panel are indicated according to Litt and Stebich (1999) and Stebich (1999).
column while selective degradation of specific organic compounds may lead to alteration of the primary isotopic signature (Harvey et al., 1995; Spiker and Hatcher, 1984). Incubation experiments over several weeks regarding carbon and nitrogen gave contradictory results. Experiments with fresh phytoplankton matter showed an initial depletion of both isotope ratios followed by a subsequent stabilization of the values (Lehmann et al., 2002). Similar experiments using surface sediments revealed no isotopic alteration (Macko et al., 1994). Thus, the primary isotopic signature of organic matter from fresh phytoplankton might be depleted during and after sedimentation but the relative isotope variations are effectively preserved, especially under anoxic conditions (Hodell and Schelske, 1998; Meyers and
Lallier-Verge`s, 1999) which are pervasive in Lake Meerfelder Maar. 4.2. Organic matter sources and contributions The major shifts in the depositional system of Meerfelder Maar at the onset and the termination of the Younger Dryas are not equally reflected in parameters that describe as a first approximation amount and type of organic matter like TOC and TOC/ON ratio (Fig. 2b). The relative contributions of OM derived from external sources (with markedly higher TOC/ON ratios than algae) and OM derived from autochthonous production (phytoplankton) did not change remarkably, thus the OMterrigeneous/OMlacustrine ratio is approximately constant. Consequently, a relative increase in
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However, only in a few discrete layers do thin section analyses reveal large abundances of macrophytes. Thus, d 13COM was only affected in the few samples which include such layers, e.g. that at 13,180 BP (Fig. 4c). 4.3. Inorganic carbon sources
Fig. 3. Cross plot of total nitrogen (TN) against total organic carbon (TOC). Assuming a strictly linear relationship between organic nitrogen (ON) and organic carbon, the amount of inorganic nitrogen (IN) is defined as the intercept with the ordinate (IN=0.16%).
organic matter derived from soil erosion (TOC/ ONN20) can be excluded. Instead, the thick autumn/ winter sub-layer contains abundant reworked shallow water sediment from the littoral. This is further supported by the occurrence of epiphytic diatom species within these layers. The Opal-SAR/Min-SAR ratio (Fig. 2c) describes the relation between diatoms (autochthonous lacustrine) and detrital sediments (minerogenic matter). While in the first part of the Younger Dryas (12,680– 12,240 BP), the ratio shows a remarkable increase, dilution of the lacustrine component is obvious for the second half of the Younger Dryas (12,240–11,650 BP). Assuming a constant SiO2 to organic matter ratio for diatomaceous algae, the ratio of OM-SAR/OpalSAR describes the relation between non-siliceous and siliceous algae (Fig. 2c). No significant change has been found in this ratio except for higher amplitudes of short-term variability in the Allerbd. If this assumption holds true, diatoms remained an important source for autochthonous production throughout the investigated period. It might be expected that submersed and/or emersed macrophytes remains are included within such reworked littoral sediments, possibly altering the planktonic d 13C signature (Lu¨cke et al., 2003).
Calcite precipitation should have had no influence on the carbon cycle in the photic zone since Meerfelder Maar is a soft-water lake with generally low TIC contents (b1% CaO equivalents). Modern lake water is about neutral and no indications of lasting changes in pH of the lake waters in the past are evident. Thus, the relative availability of inorganic carbon species (CO2, HCO3 ) for primary production should not have changed over time. Only for phases of exceptionally high productivity, a temporary use of bicarbonate for assimilation cannot be completely excluded. The carbon isotope minimum phase at the end of the Allerbd might be influenced by carbonate formation since higher contents of calcium carbonate (up to 8%) have been measured there in single samples and abundant siderite in autumn/winter layers has been observed in thin sections. Despite atmospheric carbon dioxide, remineralisation of organic matter at the sediment surface can be another source of inorganic carbon for the lake. However, the isotopic signature of such carbon dioxide is almost identical to the isotopic composition of the respective OM source and would only lead to a further depletion of the lake’s d 13CDIC. 4.4. Nitrogen isotope variations and significance The absolute level and the relative stability of nitrogen isotope ratios give indications about the lakes ecology (Fig. 4c). Since d 15N values higher than 6– 8x are interpreted as an indicator of pollution and eutrophication (Hodell and Schelske, 1998; Hollander, 1989), the mean nitrogen isotope composition of 1.6x might be interpreted as an indication of oligotrophic to mesotrophic nutrient conditions (Gu et al., 1996). The pool of inorganic nitrogen seems not to have been significantly altered by primary producers, since d 15N does not show large changes (Meyers and Teranes, 2001). Most prominent changes occur immediately before the respective transitions. In this
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Fig. 4. Specific accumulation rates of Meerfelder Maar Late Glacial and early Holocene sediments differentiated in minerogenic (a), organic matter and biogenic silica accumulation (b) together with respective changes of stable carbon and nitrogen isotopes of sedimentary organic matter (c). Vertical grey bars indicate micro-facies transition zones.
respect, the positive nitrogen isotope excursion within the YD is to a certain extent a precursor of the midYD change. Negative d 15N excursions at 12,750 and 11,700 BP might be seen as due to temporal increases of cyanoprocaryota, or increased discrimination with improved nitrate availability, while positive d 15N excursions at 12,400 and 11,200–11,000 BP might indicate the reverse development. However, it is unlikely that nitrogen has become the limiting nutrient during the investigated period. Also no fundamental change from common nitrate nitrogen using plankton, e.g. siliceous microalgae, towards an increased proportion of atmospheric nitrogen fixing cyanobacteria with depleted isotopic signature can be stated (Talbot, 2001). This further supports the idea of relative stable algal associations despite a changing climate during the period studied. Despite the stability
of the planktonic community with respect to the competition between different classes of algae, the diatom association underwent considerable changes. For example, the mass occurrence of the genus Stephanodiscus starting at the AL/YD boundary is reduced during the second part of the YD and only recovers at the beginning of the Holocene.
5. Varve micro-facies The composition and seasonal structure of varves show several marked changes during the studied time interval which are not similarly represented in sedimentation rates (Fig. 2a). Except for an extremely sharp mid-Younger Dryas shift, all major changes in varve micro-facies occurred in gradual transitions over
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Table 1 Time and duration of transitions between types of Late Glacial varve micro-facies as determined by varve counting Transition zone
Time (in varve years BP)
Duration (years)
Older Dryas–Allerbd Allerbd–Younger Dryas Mid-Younger Dryas change Younger Dryas–Preboreal
13,380–13,350 12,690–12,670 12,240–12,239 11,645–11,585
30 20 1 60
decades (20–60 years). Their lengths were determined by varve counting (Table 1). These changes occurred contemporaneous with biozone boundaries. The transition zones marked in Figs. 2, 4 and 6) are based on these varve counts. Development of varve micro-facies in the course of the Allerbd is rather complex. The main characteristic is a general trend from an organic-clastic to an organic-sideritic varve facies (Plate Ia–d). This succession is well expressed in decreasing Min-SAR values (Fig. 4a). The small peak in the middle of the Allerbd reflects a period of 35 years (from 13,100 to 13,135 years BP) with thicker detrital winter layers interrupting the general trend. The organic-clastic facies is composed of mainly three seasonal sub-layers, (1) a diatom layer deposited during spring and summer, often subdivided in a succession of a lower Stephanodiscus sp. layer and an upper Cyclotella sp. layer, (2) an autumn layer of organic detritus and amorphous organic matter in which vivianite often has formed massively, and (3) a winter clay-layer which in some cases also contains fine silty siliciclastic matter. However, additional sublayers in single varves (individual years) are common,
displaying a high micro-facies variability. These are, for example, layers of detrital siliciclastic matter between the winter clay and the spring diatoms (interpreted as runoff deposits during the snowmelt period), or biochemically formed calcite layers in summer interrupting or preceding diatom layers. The occasional formation of these calcite layers occurred either in single years or in phases of several years (one to two decades) and is not well understood. The organic-sideritic facies of the late Allerbd is composed of only two sub-layers, (1) a summer layer composed of amorphous organic matter with only few scattered diatom frustules (no discrete diatom layer) and, (2) an autumn/winter layer of organic detritus and authigenic siderite often associated with vivianite. This facies does not contain any significant amount of detrital siliciclastic matter. The seasonal sub-layers are extremely thin, resulting in lowest sedimentation rates of the whole profile (Fig. 2a) The absence of discrete diatom layers is also expressed as the minimum of the Opal-SAR curve. The five-fold increase in varve thickness at the Allerbd to Younger Dryas transition is a clear sign for a prominent facies change (Fig. 2a). Varves of the early Younger Dryas (Plate Ie, f) are composed of two sub-layers, (1) monospecific diatom layers (Stephanodiscus sp.) reflecting massive diatom blooms at the base followed by (2) thick mixed layers including amorphous and particular organic matter, a large variety of diatom species with a major contribution of epiphytic species and dispersed detrital silt and clay. This composition clearly indicates partial reworking of shallow water sediments. The thickness of these reworked layers varies not as much as diatom
Plate I. Succession of varve micro-facies from the early Allerbd (a–b), late Allerbd (c–d), early Younger Dryas (e–f), late Younger Dryas (g–h) and Preboreal (i–j) in petrographic thin section images. Images on the left side (a, c, e, g, i) display the same sediment section as the ones on the right side (b, d, f, h, j) but with different optical tools. The left row is seen under plain parallel light while for the right one cross-polarized light has been used. In between the left and right rows, the number of years represented is indicated. On the right side, seasonal sub-layers are marked with numbers as in the text. Boundaries between the sub-layers are often transitional and indicated by dotted lines. Early Allerbd varves are composed of (1) summer diatom layers, (2) detrital autumn layers with abundant vivianite and, (3) winter clay/silt layers. Additional sub-layers which do not occur in each year are best recognizable in the polarized light image (b); these are (dccT) a biochemically precipitated calcite layer within the spring–summer diatom succession and a thin detrital layer below the spring diatoms (ddlT). Authigenic vivianite in some cases forms distinct sub-layers in the autumn layer and is indicated by dvT. At shown magnification, the detrital and vivianite layers appear to have similar optical characteristics. At higher magnification, both layers are clearly distinguishable. Late Allerbd varves are composed of a (1) mainly amorphous organic layer and, (2) a winter layer rich in siderite. Early Younger Dryas varves are couplets of (1) monospecific diatom frustules and (2) mixed clastic-organic detritus sometimes containing vivianite (dvT in image f). Late Younger Dryas varves are characterized by (1) a detrital snowmelt layer in spring, (2) mixed layer with diatom frustules and, (3) a winter clay/silt layer. Preboreal varves are build up of three sub-layers, (1) a diatom layer from spring/early summer deposition, (2) a mixed organic layer with diatoms (late summer/autumn) and, (3) a winter clay layer. Nest of vivianite (image k) formed in (2) and (3).
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layer thickness which exhibits a large interannual variability. Discrete detrital layers at the base of an annual succession which would indicate deposition after snowmelt runoff have not been observed. Only in some of the diatom layers a few silty siliciclastic particles are dispersed, indicating wind-blown rather than melt water transport. This mode of deposition during the early Younger Dryas changes within 1 year (Plate II) at 12,240 varve years BP. From this time on, sedimentation in every year started with a layer of siliciclastic detritus. These layers clearly resemble snowmelt layers as observed during spring in modern lakes in Finland (Ojala and Francus, 2002). During the first 150–200 years of this type of deposition, these snowmelt layers in general are not very thick and diatom layers continued to settle, but not in every year. Thereafter, the typical varve micro-facies of the second part of Younger Dryas has developed (Plate Ig–h). At the base, (1) a prominent detrital siliciclastic layer (snowmelt) is deposited, followed by (2) a faint layer enriched in organic matter including diatoms (mainly Cyclotella sp.), the latter not in every year. The end of the annual cycle (3) is formed by a prominent winter clay/silt layer. During the Younger Dryas/Preboreal transition, varve preservation is rather poor preventing varve thickness measurements. At the beginning of the transition, regular deposition of snowmelt layers ceased resulting in lower amounts of minerogenic detritus (see also Fig. 2a, Min-SAR). Preboreal varve micro-facies (Plate Ii–j) is comparable to early Allerbd varves and characterized by three seasonal sub-layers, (1) a first diatom bloom consisting of Stephanodiscus sp. followed by (2) a second bloom mainly of Cyclotella sp. which is mixed with organic matter interpreted as late summer/autumn deposition and, (3) a thin winter clay layer. Transitions between (2) and (3) are not very sharp and commonly authigenic vivianite has formed within these sub-layers.
6. Lacustrine productivity Enriched carbon isotope ratios during the YD (Fig. 4c) are interpreted as a change of discrimination by the planktonic community (Brenner et al., 1999; Gu et al., 1996). Resuspended littoral material bears an older (decades to centuries) isotopic signature and clearly could not induce the observed rapid change in isotopic composition of profundal varves. As larger changes in atmospheric carbon dioxide ( pCO2, d 13CO2) and in the photosynthetically used dissolved inorganic carbon source are unlikely, the observed positive isotope shift of 4.0x is a measure of an increased carbon demand per time of the phytoplankton cells, i.e. a reduced discrimination against 13C (Herczeg and Fairbanks, 1987). Several processes can lead to such a development: (1) raised absolute carbon demand during the vegetation period exceeding the replenishment of the carbon dioxide pool through the air–water interface (gross primary production); (2) increased rate of inorganic carbon demand and uptake by single cells, i.e. growth rate; (3) enlarged zone of diffusive resistance at the boundary layer (cell–water interface) caused by the growth of larger specimen, i.e. a local CO2 diss minimum zone. As the relevant factor for the observed reduction of discrimination, we propose a reduced length of the lacustrine vegetation period. In consequence, all biological processes must have taken place in a shorter time with increased biological turnover rates in order to support the observed massive increase of primary productivity during the YD. This argument is supported as biogenic opal accumulation is significantly correlated with carbon isotope ratios (r=0.66, R 2=0.44, Pb0.0001, n=143) where discrimination against 13C generally decreases when Opal-SAR increases (Fig. 5f). During the YD an asymptotic behaviour is indicated and discrimination seems to
Plate II. Thin section image profiles from the mid-Younger Dryas micro-facies change composed of a succession of five single images. Both profiles display 11 varves from the same sediment section. For the left profile, plain parallel light has been used while the right is under crosspolarized light. Varves in the lower part are better distinguishable under plain parallel light whereas the upper varves are better recognized under cross-polarized light. Artificial cracks (indicated as dCrT) are due to the freeze-drying process and occur preferably in diatom layers. Detrital snowmelt layers appear under cross-polarized light as light bands and are indicated on the right side by hatched bars. Vivianite (dvT) also appears as thin light bands or patches but is easily distinguishable at higher magnification. The alternating thin black and white bars on the right side mark single varves. Varve boundaries are further indicated by lines in between the left and right profile. The arrow at the transition from varve number 12,239 to 12,240 (years BP) marks the onset of regular deposition of detrital snowmelt layers in spring. In the first year (varve 12,240), this layer still is rather thin but thickness progressively increased in the following years.
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converge to a minimum, defined by parameters other than photosynthetic carbon demand. It cannot be evaluated if primary production was limited by a single nutrient other than silica, like, for example, phosphorus. However, sharp upper boundaries in the ratios between OM-SAR and Opal-SAR to Min-SAR (Fig. 5a, b) may indicate the respective maxima of productivity obtainable by the algae while using the available nutrient pool.
7. Evolution of environment and climate 7.1. The Allerød interstadial While iniatially minerogenic accumulation gradually diminishes, most of the Allerbd interstadial (AL) is characterized by relatively stable accumu-
lation rates of biogenic silica and OM and larger carbon isotope variations (Fig. 4b, c). This situation changed for about the last 200 years of the Allerbd after the deposition of the 7–10-cm-thick ash layer of the Laacher See volcano dated at 12,880 BP. Biogenic silica accumulation reached its absolute minimum about 40 years after the eruption and both isotopic markers, d 13C and d 15N, decreased by about 2x. Since Birks and Lotter (1994) have demonstrated on the sediment record from the adjacent Lake Holzmaar that the ash fall had no significant impact on the diatom assemblages, a causal link between the tephra and the reduced diatom blooms is also unlikely for MFM. Probably, a starvation situation for diatoms has been caused by reduced nutrient input into the lake leading to a diminished utilization of the silica and inorganic carbon and nitrogen pools. One may speculate on a
Fig. 5. Cross plots of specific accumulation rates and stable carbon isotopes. (a) Organic matter accumulation (OM-SAR) against minerogenic matter accumulation (Min-SAR), (b) biogenic silica accumulation (Opal-SAR) against Min-SAR, (c) OM-SAR against Opal-SAR with the 1:3 line, (d) carbon isotopes (d 13COM) against Min-SAR, (e) d 13COM against OM-SAR and (f) d 13COM against Opal-SAR where the Younger Dryas is indicated by a box.
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causal link between reduced nutrient fluxes and deposition of organic-sideritic varves with extremely low contents of detrital siliciclastic matter in the last 200–300 years of the Allerbd. 7.2. Transition to the Younger Dryas The length of the transition AL/YD as observed in varve micro-facies can be determined within 20–25 years. Responses of parameters describing different lake compartments are synchronous but articulated in different amplitudes. Minerogenic accumulation increases moderately reaching levels comparable to the early Allerbd while an almost explosive development of the biological system took place, i.e. diatom sedimentation increases by 1.9 mg cm 2 year 1 per decade. Together, increased minerogenic influx and increased diatom productivity resulted in a five-fold increase in annual sedimentation rates (Fig. 2a). The OM-SAR/Opal-SAR ratio falls back to the early Allerbd level, while the Opal-SAR/MIN-SAR ratio almost doubles despite the increase in minerogenic accumulation (Figs. 2c and 4a). These drastic changes in the depositional and biological systems are the result of a combination of various processes. Firstly, an assumed falling lake level results in a higher catchment area/lake surface ratio and a decreased water volume leading to an increase in nutrient concentrations in the water body. An increase in nutrient fluxes is further indicated by massive layers of reworked shallow water sediment. These layers were deposited after thick (300– 500 Am) monospecific diatom layers of Stephanodiscus sp. Secondly, a drop of winter temperatures and longer winter seasons resulted in markedly reduced vegetation periods with consequences for lacustrine plankton organisms. The start of the vegetation period was delayed by longer ice-cover leading to a coincidence of a still undiminished nutrient pool with already high seasonal irradiances (higher levels than at present). Thus, photosynthetic activity, respectively plankton growth and possibly cell size (Frenette et al., 1998), supported by improved replenishment of nutrients from reworked littoral sediments, was enhanced and is expressed by massive blooms of diatoms and strongly reduced discrimination against 13 C (e.g. Goericke et al., 1994; Riebesell et al., 1993). Most probably, nutrient cycling in the water body was also enhanced by reduced duration and strength of
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summer stagnation, further increasing available nutrient concentrations (Salmaso, 2002). All these processes led to a concentration of biomass production during the relative warm summer months and summer temperatures must have remained high enough to enable such massive increase of biological activity. This confirms anomalously mild summers reported from southern Greenland that have been suggested to be a consequence of regional climatic differences (Bjo¨rck et al., 2002). Our data show that this seasonal climate characteristic probably was more widespread than expected. 7.3. The Younger Dryas stadial A two-fold division of YD has been reported earlier (e.g. Walker, 1995) but mostly the differences between the subunits were not very distinct. This is different for the Meerfelder Maar record, where at 12,240 varve years BP the deposition pattern in the lake distinctly changed between two different varve types from 1 year to the next (Plate II). During the first 440 years of the Younger Dryas, annual deposition was controlled by a monospecific diatom layer of Stephanodiscus sp. followed by a massive layer of reworked littoral sediments. During the last 650 years of the Younger Dryas, annual deposition started with a discrete detrital snowmelt layer followed by occasional diatom layers (mainly Cyclotella sp.). These micro-facies changes are well expressed in a sharp rise of Min-SAR (Fig. 4a) and a drop in the Opal-SAR/Min-SAR ratio (Fig. 2c). Within the second, Min-SAR rich, part of the YD the frequency and thickness of diatom layers decreased and detrital layers became dominant. However, the planktonic community and their fractionation against 13C remained stable over this marked change in the depositional system (Figs. 2c and 4b). Only between ca. 12,100 and 11,800 varve years BP when clastic sedimentation was slightly lower, Opal-SAR and carbon isotope ratios indicate small changes of the planktonic productivity. Both curves do not respond to the distinct drop of clastic accumulation at 11,645 BP. The observed difference between stable carbon isotopes and Min-SAR at the mid-YD shift in depositional pattern suggests an environmental factor affecting only the inorganic sedimentation.
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Our hypothesis is that changes in lake and catchment hydrology triggered the observed shift. It can be argued that if surface runoff and transport capacity have increased (Richter, 1998), more clastic matter would have been transported by the Meerbach and probably led to stronger channel erosion and incision into the floodplain. Higher surface runoff could have been triggered by increased rates of snowmelt which would imply mainly higher winter (snow) precipitation (Dearing, 1991). The observed abrupt onset of regular snowmelt layer deposition indicates some kind of threshold process rather than a gradual change of the system. A rapid increase in minerogenic deposition could have been caused by a change in the course of the Meerbach. During the early YD, the Meerbach might have passed the lake in the Southeast but discharged into the lake since 12,240 BP when a morphological threshold (e.g. levee deposits) was removed due to higher discharge and increased channel erosion. 7.4. Termination of the Younger Dryas Deposition of regular snowmelt layers ceased at about 11,645 varve years BP at the same time when a first maximum of Juniper pollen appeared (Brauer et al., 1999a). From this, it is concluded that a denser vegetation cover led to lower erosion rates and, consequently, diminished minerogenic matter flux. This change in the depositional pattern marks the onset of a ca. 60-year period of gradual micro-facies change. The sharp decrease of d 13COM, indicating rapidly changing conditions for lacustrine primary production, occurs at the end of this 60-year transitional period and coincides with the YD-Preboreal biozone boundary and the onset of Preboreal varve micro-facies. OM-SAR and Opal-SAR remain almost constant during the transition and the following Preboreal. It might be hypothesized that Preboreal warming led to shorter cold seasons allowing lake productivity to start earlier and end later in the year. This would not change primary production but would cause a depletion of carbon isotope values since a higher portion of production takes place at lower temperature and higher CO2 solubility conditions in spring and autumn. Increasing atmospheric carbon dioxide concentrations might have contributed to this development (McElwain et al., 2002). A switch of a
lacustrine system forth and back between two different states must, thus, not necessarily be expressed completely identical by the respective algal community (Anneville et al., 2002) and, in turn, also identical proxy signals cannot be taken as proof for identical forcings.
8. Synthesis and conclusion The significant correlation found between OpalSAR and carbon isotope ratios confirms carbon isotopes of lacustrine organic matter as tracer of palaeoproductivity. However, this interpretation is only valid during times where autochthonous production by far exceeds allochthonous input and as long as photosynthetic discrimination by algae is not limited by physiological effects. Micro-facies transitions and respective reactions in the isotopic composition of organic matter occurred rapidly in the Meerfelder Maar. Within a single year or a few decades, transitions are completed implying either a sudden response to abrupt climate change or a respective threshold reaction. It is striking that at the AL/YD transition micro-facies changes were completed faster than at the respective transition to the Holocene indicating that the response of lacustrine systems (lake and catchment) to warming might be slightly slower than to cooling. Taking d 13COM as a proxy for lacustrine primary production and Min-SAR as proxy for erosion and transport processes in the catchment of Meerfelder Maar provides independent information for different environmental processes which are directly influenced by climate (Fig. 6). The almost equivalent carbon isotope shift at the beginning and the end of the YD represent different climatic influences with strong effects on seasonality and the length of the vegetation period. The increasing length of winter seasons at the AL/YD transition led to shortened growing periods for lacustrine algae but increased primary production due to higher nutrient availabilities during the remaining summer months. Temperatures during YD growing seasons must have been sufficiently warm to support high primary production and probably no substantial change occurred at all. The development at the transition Younger Dryas– Holocene is seen as the consequence of seasonal
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Fig. 6. Comparison of indicators of different forcings on the lake, respectively different transfer of climatic information into the lake and its catchment. As a descriptor of direct climate forcing for the lake carbon isotopes are taken while catchment processes seen as indirect climatic forcing on the system are described by minerogenic accumulation. While the former one may be seen as seasonality descriptor, the latter one may be seen as hydrological descriptor. See text for further discussion. Vertical grey bars indicate micro-facies transition zones.
climatic warming leading to longer growing seasons probably with higher maximum temperatures but approximately constant gross productivity. The relative the contribution of algal biomass from spring and autumn increased due to improved growth conditions in these months. The distinct increase of the seasonal detrital influx at 12,240 BP is not reflected in the carbon isotope signal and can thus not be considered as a major controlling factor for lacustrine algae. This increased Min-SAR values are interpreted as increased snowmelt run-off leading to higher erosion and transport of clastic matter into the lake probably in conjunction with the crossing of a morphological threshold. This study has demonstrated that multiproxy data sets offer the opportunity to describe different subsystems of the lake leading to an improved understanding of lacustrine responses to climate change. The role of seasonality and changes therein for ecosystems as demonstrated for MFM is especially emphasized. This implies that experimental data of climate model runs should not only reproduce annual means of climate parameters but also the seasonal amplitudes.
Acknowledgements The authors would like to thank J.F.W. Negendank who made the sedimentary archive of MFM available, and S. Bjfrck and one anonymous reviewer for constructive comments. AL wants to thank G.H. Schleser and H. Vos for stimulating discussions and encouragement, and H. Wissel for analytical work. AL gratefully acknowledges financial support by the German Ministry for Education and Research (BMBF, Grant 01LD0001). This is a contribution to the German Climate Research Program DEKLIM.
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