Geochemical and organic petrological characterization of the organic matter of lacustrine Eocene oil shales (Prinz von Hessen,Germany): reconstruction of the depositional environment

Optical and geochemical techniques were applied to sedimentary organic matter from the profundal area of the Eocene Lake Prinz von Hessen, which formed in a pull-apart basin on the Sprendlinger Horst, near Darmstadt, Germany. Variations in total sulphur content (S tot) and total organic carbon content (TOC), hydrogen index (HI), oxygen index (OI) and ¹³C values of the organic matter were used to reconstruct the lakes filling history. Following an initial rapid deepening phase, open lake conditions developed with HI reaching more than 500 mg HC/g TOC and TOC values up to 40%. The productivity of the lake was probably high and organic matter preservation was enhanced by a stratified water column. As the lake began to fill with sediment and became shallower, TOC and HI values declined, as the lake water was better oxygenated and preservation conditions declined. ¹³C values between –31 and –27 are controlled by the mixing of aquatic (algae and microbial mats) and terrigenous organic matter (wood, spores, pollen and cuticles). Following a rapid drop in lake level, shallow lake conditions alternated with swamp deposits (lignites) in the basin center. The organic matter preserved during this stage is strictly terrigenous in nature and experienced oxic degradation (HI 100 mg HC/g TOC). ¹³C values between –26 and –24 are typical for Eocene terrigenous matter. The inferred lake level fluctuations are interpreted to have been controlled by tectonic as well as climatic processes.     

The Late Pleistocene - Holocene palaeolimnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter

Three piston cores from Lake Victoria (East Africa) have been analysed for organic carbon (TOC) and nitrogen (TN) content, stable isotopes (¹³C and ¹⁵N), and Hydrogen Index (HI). These data are combined with published biogenic silica and water content analyses to produce a detailed palaeolimnological history of the lake over the past ca. 17.5 ka. Late Pleistocene desiccation produced a lake-wide discontinuity marked by a vertisol. Sediments below the discontinuity are characterised by relatively low TOC and HI values, and high C/N, ¹³C and ¹⁵N, reflecting the combined influence of abundant terrestrial plant material and generally unfavourable conditions for organic matter preservation. A thin muddy interval with lower ¹³C and higher HI and water content indicates that dry conditions were interrupted by a humid period of a few hundred years duration when the lake was at least 35 m deep. The climate changed to significantly more humid conditions around 15.2 ka when the dry lake floor was rapidly flooded. Abundant macrophytic plant debris and high TOC and ¹³C values at the upper vertisol surface probably reflect a marginal swamp. ¹³C values decrease abruptly and HI begins to increase around 15 ka BP, marking a shift to deeper-water conditions and algal-dominated lake production. C/N values are relatively low during this period, suggesting a generally adequate supply of nitrogen, but increasing ¹⁵N values reflect intense utilisation of the lake's DIN reservoir, probably due to a dramatic rise in productivity as nutrients were released to the lake from the flooded land surface.An abrupt drop in ¹³C and ¹⁵N values around 13.8-13.6 ka reflects a period of deep mixing. Productivity increased due to more efficient nutrient recycling, and ¹³C values fell as ¹²C-rich CO₂ released by bacterial decomposition of the organic material was brought into the epilimnion. A weak drop in HI values suggests greater oxygen supply to the hypolimnion at this time. Better mixing was probably due to increased wind intensity and may mark the onset of the Younger Dryas in the region.After the period of deep mixing, the water column became more stable. TOC, C/N, ¹³C and HI values were at a maximum during the period between 10 and 4 ka, when the lake probably had a stratified water column with anoxic bottom waters. A gradual decrease in values over the last 4000 yrs suggest a change to a more seasonal climate, with periodic mixing of the water column. Rising sediment accumulation rates and a trend to more uniform surface water conditions over the last 2000 yrs are probably a result of increased anthropogenic impact on the lake and its catchment.Following a maximum at the time of the rapid lake-level rise during the terminal Pleistocene, ¹⁵N has remained relatively low and displays a gradual but consistent trend to lower values from the end of the Pleistocene to the present. TN values have risen during the same period. The lack of correlation between ¹³C and ¹⁵N, and the absence of any evidence for isotopic reservoir effects despite the rise in TN, suggests that the atmosphere, rather than the lake's dissolved nitrogen pool has been the principal source of nitrogen throughout the Holocene. The importance of atmospheric N fixation to Lake Victoria's nitrogen cycle thus predates by a very considerable margin any possible anthropogenic eutrophication of the lake.     

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (¹³C and ¹⁵N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four ²¹⁰Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and ²²⁶Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low ¹³C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher ¹³C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the ¹⁵N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both ¹³C and ¹⁵N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that ¹³C and ¹⁵N reflect shifts in historic lake trophic state.     

Climatic and anthropogenic influence on the stable isotope record from bulk carbonates and ostracodes in Lake Neuchâ, Switzerland, during the last two millennia

Lake Neuchåtel is a medium sized, hard-water lake, lacking varved sediments, situated in the western Swiss Lowlands at the foot of the Jura Mountains. Stable isotope data (18O and 13C) from both bulk carbonate and ostracode calcite in an 81 cm long, radiocarbon-dated sediment core represent the last 1500 years of Lake Neuchåtel's environmental history. Comparison between this isotopic and other palaeolimnologic data (mineralogical, geochemical, palynological, etc.) helps to differentiate between anthropogenic and natural factors most recently affecting the lake. An increase in lacustrine productivity (450–650AD ca), inferred from the positive trend in 13C values of bulk carbonate, is related to medieval forest clearances and the associated nutrient budget changes. A negative trend in both the bulk carbonate and ostracode calcite 18O values between approximately 1300 and 1500AD, is tentatively interpreted as due to a cooling in mean air temperature at the transition from the Medieval Warm Period to the Little Ice Age. Negative trends in bulk carbonate 18O and 13C values through the uppermost sediments, which have no equivalent in ostracode calcite isotopic values, are concomitant with the recent onset of eutrophication in the lake. Isotopic disequilibrium during calcite precipitation, probably due to kinetic factors in periods of high productivity is postulated as the mechanism to explain the associated negative isotopic trends, although the effect of a shift of the calcite precipitation towards the warmer months cannot be excluded.     

A reconnaissance study of oxygen, hydrogen and strontium isotopes in geochemically diverse lakes, Western Nebraska, USA

Reconnaissance ¹⁸O,, D, and ⁸⁷Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of ⁸⁷Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the ⁸⁷Sr values. Based on the mutual variations of ¹⁸O and D, the lakes can be divided into three groups. In Group 1, both ¹⁸O and D values increase from spring to fall. The ¹⁸O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the _a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.     

Lacustrine ostracodes as stable isotope recorders of late-glacial and Holocene environmental dynamics and climate

Oxygen- and carbon-isotopic signatures of benthic ostracodes from lake sediments from climate sensitive regions in the Alpine region, Central Europe, the north-central USA, the Chilean Altiplano and Patagonia, Argentina, are used to characterize lake system processes and to reconstruct climate patterns of the past 16,000 years. The case studies provide examples that highlight different aspects of the broad application of isotope stratigraphies, and provide keys for the interpretation of complex lacustrine records.The integration of stable-isotope stratigraphy, sedimentology, and ecological information from ostracode assemblages is a new tool that acquires climate information from the indirect views of climate series provided by lake sediments. This tool (1) identifies lake system characteristics, (2) confines which isotopic signatures are controlled by which processes in the lake system and/or in the catchment, and (3) defines which signatures are ultimately controlled by climate change. If sudden shifts in the isotopic composition occur concomitantly with changes between sedimentological units, then the isotopes reflect first of all changes in catchment hydrology that may be ultimately controlled by climate. Also, if ostracode ¹⁸O and ¹³C values show the same timing and direction of shifts, then this indicates a major change in the hydrological budget of the lake.The case studies presented here show that coupled isotopic signatures may be used to track hydrological changes related to meltwater and deglaciation, shifting rivers and ground water sources, and changes in precipitation mechanisms and patterns. Values of ¹⁸O from large lakes with short water residence time, low evaporation rates and homothermic bottom waters provide records of past temperatures of precipitation. The ¹³C values reflect changes in the ratio of C₃:C₄ plants in the catchment. They indicate shifts in modes of organic decay in the surface sediments that can be linked to a change in hydrodynamics within a lake. The ¹³C values also allow detection of the input of volcanically charged ground waters providing large quantities of ¹⁴C-free CO₂ that hinders accurate ¹⁴C chronology. General climate trends for the sites in the Americas indicate a dry mid-Holocene punctuated by moist spells, and show a general increase in moisture during the past approximately 4000 years, interrupted by recurring droughts. This hints at an interhemispheric connection and a common driving mechanism.Environmental isotopes from high-resolution lake sediments thus provide an ideal tool to identify and characterize the regional impact and magnitude of global climate change. This tool contributes to a better understanding of regional climate change and its driving mechanisms and thus provides the type of information needed to improve climate models. Environmental isotopes provide more information than just moisture balance and airmass history if they are integrated with the detailed sedimentological and ostracode ecological evidence, and understanding for the component system. Thus environmental isotopes serve to a better understanding of the climate signal archived in lake records and represent an essential contribution to Global Change research and Earth System Science.     

Rapid Holocene hydrologic change along boreal treeline revealed by δ13C and δ18O in organic lake sediments, Northwest Territories, Canada

Analysis of ¹⁸O_cellulose, ¹³C_{organic matter}, and ¹³C_cellulose at about 100 year intervals from organic matter deposited in Toronto Lake, Northwest Territories, Canada, revealed an 8000-year history of rapid, post-glacial hydrologic change at the treeline zone. Several mid-Holocene phases of enriched ¹³C_org and ¹³C_cell, caused by elevated lake productivity, declining [CO_2(aq)], and closed basin conditions, were abruptly terminated by intervals of open hydrology recorded by sharply depleted ¹⁸O_cell. Two of these events, at 5000 and 4500 BP, are correlated with increased total organic content and Picea mariana pollen concentration, which indicate that high levels of productivity were also accompanied by northern treeline advances. A third treeline advance at about 2500 BP is also marked by an apparent outflow event from Toronto Lake, but this was not associated with ¹³C_org/cell enrichment in the sediment record because rapid and substantial lake water renewal probably prevented productivity-driven enrichment of the dissolved inorganic carbon and replenished the CO_2(aq) supply to thriving phytoplankton. However, high sediment organic content during this period suggests increased productivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but the prevailing climate during these periods apparently did not favour appreciable northward treeline migration or changes in lake productivity.This is the 14th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers     

Oxygen isotope ratios of organic matter in arctic lakes as a paleoclimate proxy: field and laboratory investigations

Paleoclimate research based on the stable isotopic composition of lake sediments is often hampered by the lack of preservation of suitable material for isotopic analysis. We examined organic material as a proxy for past water isotopic composition in a series of experiments. First, we cultured aquatic moss under constant illumination, temperature, and water ¹⁸O, and show that new cellulose records source water ¹⁸O precisely (r² = 0.9997). Second, we analyzed paired lakewater and vegetation samples collected from sites spanning strong climatic gradients. In field conditions, the relationship between organic ¹⁸O and water ¹⁸O is more variable, though it is still controlled by environmental water isotopic composition. However, terrestrial mosses in the arctic are often significantly enriched in ¹⁸O relative to aquatic mosses in nearby lakes due to their use of different water sources. Third, we measured ¹⁸O of cellulose extracted from disseminated sedimentary organic material. In the majority of the middle- to high-arctic lakes in this study, the ¹⁸O of disseminated sediment cellulose is greatly enriched relative to the expected values based on lakewater ¹⁸O, suggesting a significant component of terrestrial cellulose. This interpretation is supported by radiocarbon dates from a Holocene sediment core in which ¹⁴C ages of sediment cellulose are 700-5000 yrs older than the enclosing sediments. We conclude that aquatic cellulose can be used as a reliable tracer of lakewater isotope ratios, but terrestrial cellulose often dominates the sedimentary cellulose pool in places such as Baffin Island where sedimentation rates are low enough to allow the degradation of aquatic cellulose. Care must be taken when interpreting sediment cellulose ¹⁸O records where diagenesis has played a role, because terrestrial cellulose is more resistant to degradation, and therefore can predominate in environments with low organic carbon burial.     

Water isotopic and hydrochemical evolution of a lake chain in the northern Great Plains and its paleoclimatic implications

Oxygen isotopes and geochemistry from lake sediments are commonly used as proxies of past hydrologic and climatic conditions, but the importance of present-day hydrologic processes in controlling these proxies are sometimes not well established and understood. Here we use present-day hydrochemical data from 13 lakes in a hydrologically connected lake chain in the northern Great Plains (NGP) to investigate isotopic and solute evolution along a hydrologic gradient. The ¹⁸O and ²H of water from the chain of lakes, when plotted in ²H - ¹⁸O space, form a line with a slope of 5.9, indicating that these waters fall on an evaporation trend. However, 10 of the 13 lakes are isotopically similar (¹⁸O = –6 ± 1 VSMOW) and show no correlation with salinity (which ranges from 1 to 65). The lack of correlation implies that the isotopic composition of various source waters rather than in-lake evaporation is the main control of the ¹⁸O of the lakes. Groundwater, an important input in the water budget of this chain of lakes, has a lower ¹⁸O value (–16.7 in 1998) than that of mean annual precipitation (–11) owing to selective recharge from snow melt. For the lakes in this chain with salinity < 15, the water Mg/Ca ratios are strongly correlated with salinity, whereas Sr/Ca is not. The poor correlation between Sr/Ca and salinity results from uptake of Sr by endogenic aragonite. These new results indicate that ¹⁸O records may not be interpreted simply in term of climate in the NGP, and that local hydrology needs to be adequately investigated before a meaningful interpretation of sedimentary records can be reached.     

Hydrologic control on the oxygen-isotope relation between sediment cellulose and lake water, western Taimyr Peninsula, Russia: Implications for the use of surface-sediment calibrations in paleolimnology

Systematic variability occurs between the oxygen isotopic composition of lake water sampled in mid-summer 1993 and cellulose extracted from surficial sediments of a suite of lakes spanning the forest-tundra transition near Noril'sk, Russia. Some tundra and all forest-tundra lakes show greater deviation from expected cellulose-water isotopic separation than forest lakes, apparently because of greater sensitivity to ¹⁸O-depleted snowmelt contributions. Cellulose derived from aquatic plants naturally integrates fluctuations in lake water ¹⁸O, providing a signal that is inherently more representative of average thaw season lake water ¹⁸O than the measure of instantaneous ¹⁸O obtained from an individual sample of lake water. Thus, indiscriminate use of empirical cellulose-water relations derived from calibration samples could lead to erroneous assessment of paleohydrology from the oxygen-isotope stratigraphy of sediment cores from arctic lakes. However, deviation from the expected cellulose-water fractionation is a source of lake-specific hydrologic information useful for qualifying paleoenvironmental interpretations and possibly constraining non-isotopic methods that rely on surface-sediment calibrations.     

Multi-component carbon isotope evidence of early Holocene environmental change and carbon-flow pathways from a hard-water lake in northern Sweden

A 9000-year carbonate-rich sediment sequence from a small hard-water lake in northernmost Sweden was studied by means of multi-component stable carbon isotope analysis. Radiocarbon dating of different sediment fractions provides chronologic control and reveals a rather constant hard-water effect through time, suggesting that the lake has remained hydrologically open throughout the Holocene. Successive depletion of ¹³C in fine-grained calcite and carbonate shells during the early Holocene correlate with a change in catchment vegetation from pioneer herb communities to boreal forest. The vegetational change and associated soil development likely gave rise to an increased supply of ¹³C-depleted carbon dioxide in groundwater recharging the lake. This process is therefore believed to be the main cause of decreasing values of ¹³C in dissolved inorganic carbon of the lake and thereby in limnic carbonates. Strongly ¹³C-depleted sedimentary organic matter may be related to enhanced kinetic fractionation during photosynthetic assimilation by means of proton pumping in Characean algae. This interpretation is supported by a substantial offset between ¹³C of DIC as recorded by mollusc shells and ¹³C of fine-grained calcite.     

Sedimentary geochemical evidence for recent eutrophication of Lake Chenghai, Yunnan, China

Geochemical anomalies and stable isotope ratios (¹⁸O, ¹³C) in authigenic carbonates and organic matter (¹³C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO₃, and pigment concentrations, and lower ¹⁸O and ¹³C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of ¹⁸O and ¹³C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO₃ ^2–] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.     

Evidence of mid-Holocene climate instability from variations in carbon burial in Seneca Lake, New York

The amounts and types of carbon delivered to the sediments Seneca Lake, New York, have varied since the middle Holocene. Concentrations of CaCO₃ first fluctuate between 14 and 6% around 7 ka before decreasing erratically until about 5 ka and then remain 2% in younger sediments. Because the amount of calcite that precipitates in hard-water lakes is related to summertime thermal stratification, the carbonate fluctuations suggest that cyclic strengthening and weakening of seasonality at intervals of about three centuries accompanied the end of the Holocene Hypsithermal in northeast North America. Organic C/total N values record short, decade-long intervals of enhanced delivery of land-plant material during episodes of wetter climate that are independent of the temperature variations. Higher organic ¹³C values indicate that recent fertilization of lake waters from soil disturbance and land-derived runoff has increased aquatic productivity.     

Anthropogenic nitrogen deposition induces rapid ecological changes in alpine lakes of the Colorado Front Range (USA)

Recent sediments from two alpine lakes (> 3300 m asl) in the Colorado Front Range (USA) register marked and near-synchronous changes that are believed to represent ecological responses to enhanced atmospheric deposition of fixed nitrogen from anthropogenic sources. Directional shifts in sediment proxies include greater representations of mesotrophic diatoms and increasingly depleted ¹⁵N values. These trends are particularly pronounced since ~ 1950, and appear to chronicle lake responses to excess N derived from agricultural and industrial sources to the east. The rate and magnitude of recent ecological changes far exceed the context of natural variability, as inferred from comparative analyses of a long core capturingthe entire 14,000-year postglacial history of one of the lakes. Nitrogen deposition to these seemingly pristine natural areas has resulted in subtle but detectable limnological changes that likely represent the beginning of a stronger response to nitrogen enrichment.     

Influence of vegetation change on watershed hydrology: implications for paleoclimatic interpretation of lacustrine δ18O records

Stratigraphic shifts in the oxygen isotopic (¹⁸O) and trace element (Mg and Sr) composition of biogenic carbonate from tropical lake sediment cores are often interpreted as a proxy record of the changing relation between evaporation and precipitation (E/P). Holocene ¹⁸O and Mg and Sr records from Lakes Salpetén and Petén Itzá, Guatemala were apparently affected by drainage basin vegetation changes that influenced watershed hydrology, thereby confounding paleoclimatic interpretations. Oxygen isotope values and trace element concentrations in the two lowland lakes were greatest between ~ 9000 and 6800 ¹⁴C-yr BP, suggesting relatively high E/P, but pollen data indicate moist conditions and extensive forest cover in the early Holocene. The discrepancy between pollen- and geochemically-inferred climate conditions may be reconciled if the high early Holocene ¹⁸O and trace element values were controlled principally by low surface runoff and groundwater flow to the lake, rather than high E/P. Dense forest cover in the early Holocene would have increased evapotranspiration and soil moisture storage, thereby reducing delivery of meteoric water to the lakes. Carbonate ¹⁸O and Mg and Sr decreased between 7200 and 3500 ¹⁴C-yr BP in Lake Salpetén and between 6800 and 5000 ¹⁴C-yr BP in Lake Petén Itzá. This decline coincided with palynologically documented forest loss that may have led to increased surface and groundwater flow to the lakes. In Lake Salpetén, minimum ¹⁸O values (i.e., high lake levels) occurred between 3500 and 1800 ¹⁴C-yr BP. Relatively high lake levels were confirmed by ¹⁴C-dated aquatic gastropods from subaerial soil profiles ~ 1.0–7.5 m above present lake stage. High lake levels were a consequence of lower E/P and/or greater surface runoff and groundwater inflow caused by human-induced deforestation.     

Paleohydrology and paleochemistry of Lake Manitoba,Canada: the isotope and ostracode records

Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), ¹⁸O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative ¹⁸O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the ¹⁸O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.¹⁸O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The ¹⁸O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the ¹⁸O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The ¹³C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

Palaeoclimatic implications of isotopic data from modern and early Holocene shells of the freshwater snail Melanoides tuberculata, from lakes in the Ethiopian Rift Valley

Carbon and oxygen isotope ratios in the shells of the freshwater snail Melanoides tuberculata yield information on the isotopic composition of the water in which the shell was formed, which in turn relates to climatic conditions prevailing during the snails' life span. Melanoides is particularly important because it is widespread in Quaternary deposits throughout Africa and Asia and is ubiquitous in both fresh and highly evaporated lakes. Whole-shell and incremental growth data were collected from modern and fossil shells from two lakes in the Ethiopian Rift Valley. 18O values in the modern shells from Lake Awassa are in equilibrium with modern waters, while 18O values in subfossil shells from the margins of Lake Tilo indicate high rainfall during the early Holocene. Sequential analysis along the growth spiral of the shell provides information on seasonal or shorter-term variability of lake water during the lifetime of the organism.     

Evidence for Holocene environmental change from C/N ratios, and δ13C and δ15N values in Swan Lake sediments, western Sand Hills, Nebraska

Profiles of percent carbon and nitrogen, carbon/nitrogen (C/N) ratios and stable carbon (¹³C), and nitrogen (¹⁵N) isotopic ratios in organic matter from an 11.6 m core were used to reconstruct environments of deposition in the Swan Lake basin during the past 5300 YBP. The upper 6.5 m consisted of gyttja containing variable amounts of reddish brown-colored fine organic matter and calcium carbonate. It was followed by a 0.5 m sandy silt, which was followed by a 3.6 m reduced layer characterized by large quantities of black organic plant remains, sapropel, and then by another sapropel layer consisting mainly of well-sorted sapropelic sand with relatively low organic matter content. The C- and N-contents in the organic matter in the sediment profile ranged from 0.5 to 23% and from 0.02 to 2%, respectively. Carbon content were positively correlated to both N and clay content while carbon content was negatively correlated to sand content. Two major environmental phases in Swan Lake were apparent from large differences in the C and N data of the sediment organic matter. These include the sapropel (marsh) stage that stretched from approximately 5330 to 3930 YBP, and the following gyttja (open water stage). During the sapropel marsh plants identified in a previous pollen study as cattails and sedges proliferated and produced copious amounts of well-preserved organic matter. C/N ratios, ¹³C values, and ¹⁵N values in the sapropel were significantly different from those that characterized organic matter in the gyttja. During the gyttja ¹³C values indicated that deep primary producers have dominated lake biomass. By utilizing bicarbonate as their C-source, the accumulating biomass became relatively enriched ¹³C values. The presence of high sediment CaCO₃ contents indicated more alkaline and deeper water conditions prevailed during the gyttja. Further refinement of the data suggested that each major phase initially contained an identifiable transition stage. During the sapropelic (initial marsh stage) which occurred before 5330 YBP, sand content gradually decreased as organic matter increased. As reflected by high C/N ratios and slightly enriched ¹³C values, these sands appear to have contained sufficient permeability to promote partial mineralization of accumulated organic-N containing compounds. A short initial gyttja transition period from about 3930–3830 YBP occurred in which the sediment silt content was anomalously high relative that measured in the surrounding layers. The silt content suggests that this turbid transition layer can not be completely explained by sediment mixing via bioturbation. The silts appeared to have been associated with the sharp climate change that resulted in higher water-table conditions during the gyttja stage.     

Relationship of Mn-carbonates in varved lake-sediments to catchment vegetation in Big Watab Lake, MN, USA

Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals.Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High ¹⁸O and low ¹³C values are associated with abundant Mn-carbonates. Low ¹⁸O and high ¹³C values are associated with only minor concentrations of Mn-carbonates.The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.     

Lacustrine oxygen isotope record of 20th-century climate change in central EuropeL evaluation of climatic controls on oxygen isotopes in precipitation

We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (¹⁸O_p) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (¹⁸O_c). Comparison of the calcite oxygen isotope proxy for ¹⁸O_p in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This ¹⁸O_c/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite ¹⁸O values since the 1960s. Possible explanations for this recent ¹⁸O_c shift, as it is not related to mean annual air temperature, include changes in ¹⁸O_p due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in ¹⁸O_c data.