The need to correct for the Suess effect in the application of δ<Superscript>13</Superscript>C in sediment of autotrophic Lake Tanganyika,as a productivity proxy in the Anthropocene

The change in dissolved inorganic δ¹³C in the ocean resulting from the change in δ¹³C in atmospheric CO₂ owing to anthropogenic activities (the Suess effect) is well known. The need to correct for the Suess effect when applying δ¹³C in organic matter in lacustrine sediment deposited during the anthropocene as a productivity proxy, is widely although not universally acknowledged. This paper reviews conceptions about the Suess effect in lacustrine δ¹³C_org and methods to adjust for the Suess effect when δ¹³C_org is used to infer recent changes in aquatic productivity. Lake Tanganyika is used as an example to illustrate the necessity of the correction. When the Suess effect is not considered, interpretations of sediment core data can result that are opposite to those achieved with the correction applied, as is here shown in Lake Tanganyika and in other lakes. A new method to correct for the Suess effect is provided which has the advantage of being applicable to data for a larger period (1700–2000) than methods currently available. In addition, Lake Tanganyika is shown to be a net sink for CO₂.     

Assessment of nutrient sources and paleoproductivity during the past century in Longgan Lake,middle reaches of the Yangtze River,China

Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ¹³C_Org, δ¹⁵N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ¹³C_Org, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ¹⁵N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.     

Isotopic fingerprints on lacustrine organic matter from Laguna Potrok Aike (southern Patagonia, Argentina) reflect environmental changes during the last 16,000 years

A combination of carbon-to-nitrogen ratios (TOC/TN), Rock Eval-analyses, and stable isotope values of bulk nitrogen (δ¹⁵N) and organic carbon (δ¹³C_org) was used to characterize bulk organic matter (OM) of a piston core from the Patagonian maar lake Laguna Potrok Aike (Argentina) for the purpose of palaeoenvironmental reconstruction. Sedimentary data were compared with geochemical signatures of potential OM sources from Laguna Potrok Aike and its catchment area to identify the sources of sedimentary OM. Correlation patterns between isotopic data and TOC/TN ratios allowed differentiation of five distinct phases with different OM composition. Before 8470 calibrated ¹⁴C years before present (cal. yrs BP) and after 7400 cal. yrs BP, isotopic and organo-geochemical fingerprints indicate that the sediments of Laguna Potrok Aike consist predominantly of soil and diatom OM with varying admixtures of cyanobacterial and aquatic macrophyte OM. For a short phase of the early Holocene (ca. 8470–7400 cal. yrs BP), however, extremely high input of soil OM is implied by isotopic fingerprints. Previous seismic and geochronological results indicate a severe lake-level drop of 33 m below present-day shortly before 6590 cal. yrs BP. It is suggested that this lake level drop was accompanied by increased erosion of shore banks and channel incision enhancing soil OM deposition in the lake basin. Thus, isotopic data can be linked to hydrological variations at Laguna Potrok Aike and allow a more precise dating of this extremely low lake level. An isotopic mixing model was used including four different sources (soil, cyanobacteria, diatom and aquatic macrophyte OM) to model OM variations and the model results were compared with quantitative microfossil data.     

Environmental changes in northern New Zealand since the Middle Holocene inferred from stable isotope records (δ15N, δ13C) of Lake Pupuke

Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165?cal.?year BP. The origin of organic matter was established using total-organic?Ccarbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (??¹³C_OM) and nitrogen (??¹⁵N) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. >7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165?years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600?cal.?year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the ??¹⁵N and ??¹³C values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600?cal.?year BP.     

西北干旱地区大气降水δ18O的特征及水汽来源

In order to reveal the characteristics and climatic controls on the stable isotopic composition of precipitation over Arid Northwestern China, eight stations have been selected from Chinese Network of Isotopes in Precipitation (CHNIP). During the year 2005 and 2006, monthly precipitation samples have been collected and analyzed for the composition of δD and δ18O. The established local meteoric water line δD=7.42 δ18O+1.38, based on the 95 ob-tained monthly composite samples, could be treated as isotopic input function across the region. The deviations of slope and intercept from the Global Meteoric Water Line indicated the specific regional meteorological conditions. The monthly δ18O values were characterized by a positive correlation with surface air temperature (δ18O (‰) =0.33 T (℃)-13.12). The amount effect visualized during summer period (δ18O (‰) = -0.04P (mm)-3.44) though not appeared at a whole yearly-scale. Spatial distributions of δ18O have properly portrayed the atmospheric circulation background in each month over Arid Northwestern China. The quan-titative simulation of δ18O, which involved a Rayleigh fractionation and a kinetic fractionation, demonstrated that the latter one was the dominating function of condensation of raindrops. Furthermore, the raindrop suffered a re-evaporation during falling processes, and the pre-cipitation vapor might have been mixed with a quantity of local recycled water vapor. Multiple linear regression equations and a δ18O-T relation have been gained by using meteorological parameters and δ18O data to evaluate physical controls on the long-term data. The estab-lished δ18O-T relation, which has been based on the present-day precipitation, could be considered as a first step of quantitatively reconstructing the historical environmental climate.     

Carbon and nitrogen isotopic composition of different strains ofArtemia sp

Stable carbon and nitrogen isotope ratios have been determined on 41 strains ofArtemia sp. from different geographic regions around the world. The δ¹³C and δ¹⁵N values ranged between −13.7 to−25.0 per mil and −0.7 to 21.2 per mil respectively.Artemia δ¹³C values from coastal environments are consistent with a marine origin for the food sourceArtemia from inland salt lakes have a range of carbon isotope values suggesting C₃, C₄ and CAM based organic matter could form the base of theArtemia food chain. These data indicate thatArtemia having a wide range of carbon and nitrogen isotope values are available for tropho-dynamic research studies that quantify the effect of respired CO₂ on tissue and CaCO₃ shell¹³C/¹²C ratios. Such stable isotope variation also suggests that stable isotope fingerprinting remains a viable technique for identifying specificArtemia collection sites.     

The δ18O in the Guliya shallow ice core during the prevailing summer monsoons and sea-air interactions

The correlations of the δ¹⁸O_max in the shallow ice core from the Guliya ice cap on the Tibetan Plateau with the global sea surface temperatures (SST) and height at the 500 hPa over the Northern Hemisphere were analyzed. The correlated regions on oceans that have a significant influence on the δ¹⁸O_max in the Guliya ice core are all located in ocean currents, or convergent regions of currents. They are the eastern Equatorial Pacific, the Northern Pacific Current, the Hot Pool in the eastern Indian Ocean, the Mozambique Current, the Northern Atlantic Current, the Canary Current and the Atlantic Equatorial Current. The δ¹⁸O_max in the Guliya ice core has negative correlations with the SST located in the lower latitudes, and positive correlations with the SST in the middle latitudes. The correlated areas at the 500 hPa that have a great impact on the δ¹⁸O_max are located in the subtropical highs over the mid-low-latitude oceans and the long-wave trough area over Balkhash Lake, where there are marked negative correlations between the heights in those areas and the δ¹⁸O_max. The influencing mechanism is displayed by the diversity of the vapor origins transported to the Guliya region. The strengths of the European ridge and the ridge over Baikal Lake have notable positive correlations with the δ¹⁸O_max. The two systems indirectly influence the vapor transportation towards the Guliya region by the adjustment of long-wave trough and ridge.     

Late-Holocene equatorial environments inferred from deposition processes,carbon isotopes of organic matter,and pollen in three shallow lakes of Gabon,west-central Africa

Seven vibro-cores were collected from three shallow lakes of the Gabon (Kamalété, Nguène, Maridor) along a 300-km west–east transect close to the Equator. These lakes are located in very distinct landscapes: coastal forest-savanna mosaic, rain forest and savanna with colonising forest, respectively. Core chronologies were established by radiocarbon dating. Study of these lacustrine archives (textural variables, clay minerals, organic matter components, δ¹³C, pollen) allowed comparison of late Holocene environmental changes recorded at each site and with results from other studies. Lake Kamalété indicates minor climatic deterioration (increased drying and greater seasonality) between 1,410 and 500 cal. years BP, which is also recognised in southern Cameroon and east-central Africa. Lake Nguène was surrounded by dense moist forest throughout the last 4,110 years, but shows significant deterioration from ~2,800 cal. years BP, a phenomenon seen at nearby sites. Lake Maridor shows a decline of forest initiated a little after 3,800 cal. years BP, which indicates timing that is distinct from the two other sites. This was probably a response to local conditions (i.e. outlet damming). Although the three lakes display generally parallel climatic trends perhaps linked to SST oscillations, there is not perfect coherence between these three sites. Differences among the three basins may be attributable to local factors like groundwater hydrology and slope instabilities of such shallow lake systems in this equatorial region.     

Elemental (C,N, H and P) and stable isotope (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) signatures in sediments from Zeekoevlei,South Africa: a record of human intervention in the lake

We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ¹³C_organic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ¹⁵N_organic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ¹⁵N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l⁻¹) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.     

青藏高原各拉丹冬冰芯记录的季节气温变化

A 70-year history of precipitation δ18O record has been retrieved using an ice core drilled from a plat portion of the firn area in the Guoqu Glacier (33o34′37.8″ N, 91o10′35.3″ E, 5720 m a.s.l.) on Mt. Geladaindong (the source region of Yangtze River) during October and November, 2005. Based on the seasonality of δ18O records and the significant positive relationships between monsoon/non-monsoon δ18O values and summer/spring air temperature from the nearby meteorological stations, the history of summer and spring air temperature have been reconstructed for the last 70 years. The results show that both summer and spring air temperature variations present similar trends during the last 70 years. Regression analysis indicates that the slope of the temperature-δ18O relationship is 1.3℃/‰ for non-monsoon δ18O values and spring air temperature, and 0.4℃/‰ for monsoon δ18O values and summer air temperature. Variation of air temperature recorded in the ice core is consistent with that in the Northern Hemisphere (NH), however, the warming trend in the Geladaindong region is more intense than that in the NH, reflecting a higher sensitivity to global warming in the high elevation regions. In addition, warming trend is greater in spring than in summer.     

Carbon stable isotopes ratios of pelagic and littoral communities in Alchichica crater-lake, Mexico

Carbon stable isotope ratios were determined in dominant biotic components of pelagic and littoral systems in Alchichica crater-lake. Results showed that carbon signatures were significantly different between both systems. The pelagic environment was more depleted (−26.15 to −15.14 per mille) than the littoral zone (−21.03 to −17.91 per mille). The potential source end-point in the simplified pelagic community was established to be diatomaceous phytoplankton; its predicted value was −21.7 per mille. There is a clear evidence thatNodularia does not sustain the pelagic food chain. In contrast, the highly diverse littoral community was sustained by epiphytes. No allochthonous sources seemed to influence this food web.¹³C enrichment was observed along the components of both systems with fractionations of 0.8 to 1.4 per mille. The contribution of the seagrassRuppia maritima is probably associated with the detritus pathway. Carbon source partitioning between both systems was not recorded. The δ¹³C in Alchichica crater-lake was more enriched than in other saline lakes and could be attributed to different salinity and CO₂ concentrations among lakes.     

Carbon and nitrogen stable isotope ratios in surface sediments from lakes of western Ireland: implications for inferring past lake productivity and nitrogen loading

We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ¹³C and δ¹⁵N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ¹³C and δ¹⁵N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ¹³C and δ¹⁵N measurements similar to values from catchment vegetation. δ¹³C and δ¹⁵N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ¹³C values from lake sediment samples in agricultural catchments were more negative than δ¹³C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ¹³C values than fractionation due to algal productivity. δ¹⁵N from lake sediment samples in agricultural catchments were more positive than δ¹⁵N in non-impacted bogland catchments. Lower δ¹⁵N values from non-impacted lake catchments reflected δ¹⁵N values of catchment vegetation, while higher δ¹⁵N values in agricultural catchments reflected the high δ¹⁵N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ¹⁵N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ¹⁵N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ¹³C and δ¹⁵N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ¹³C and δ¹⁵N in a study area.     

Mid- to late-Holocene climate variability and anthropogenic impacts: multi-proxy evidence from Lake Bliden,Denmark

We conducted multi-proxy geochemical analyses (including measurements of organic carbon, nitrogen and sulphur stable isotope composition, and carbonate carbon and oxygen isotope composition) on a 13.5 m sediment core from Lake Bliden, Denmark, which provide a record of shifting hydrological conditions for the past 6,700 years. The early part of the stratigraphic record (6,700–5,740 cal year BP) was wet, based on δ¹⁸O_carb and lithology, and corresponds to the Holocene Thermal Maximum. Shifts in primarily δ¹⁸O_carb indicate dry conditions prevailed from 5,740 to 2,800 cal year BP, although this was interrupted by very wet conditions from 5,300 to 5,150, 4,300 to 4,050 and 3,700 to 3,450 cal year BP. The timing of the latter two moist intervals is consistent with other Scandinavian paleoclimatic records. Dry conditions at Lake Bliden between 3,450 and 2,800 cal year BP is consistent with other paleolimnological records from southern Sweden but contrasts with records in central Sweden, possibly suggesting a more northerly trajectory of prevailing westerlies carrying moisture from the North Atlantic at this time. Overall, fluctuating moisture conditions at Lake Bliden appear to be strongly linked to changing sea surface temperatures in the Greenland, Iceland and Norwegian seas. After 2,800 cal year BP, sedimentology, magnetic susceptibility, δ¹³C_ORG, δ¹³C_carb and δ¹⁸O_carb indicate a major reduction on water level, which caused the depositional setting at the coring site to shift from the profundal to littoral zone. The Roman Warm Period (2,200–1,500 cal year BP) appears dry based on enriched δ¹⁸O_carb values. Possible effects of human disturbance in the watershed after 820 cal year BP complicate attempts to interpret the stratigraphic record although tentative interpretation of the δ¹⁸O_carb, magnetic susceptibility, δ¹³C_ORG, δ¹³C_carb and δ¹⁸O_carb records suggest that the Medieval Warm Period was dry and the Little Ice Age was wet.     

Sedimentary evidence for recent eutrophication in the northern basin of Lake Taihu,China: human impacts on a large shallow lake

Environmental change in Lake Taihu and its catchment since the early to middle part of the twentieth century has left a clear geochemical record in the lake sediments. The human activities in the lake and its catchment responsible for the change include agriculture, fishery, urbanisation, sewage and industrial waster disposal. Sediment cores were collected from Meilian Bay of northern Lake Taihu to investigate the record of anthropogenic impacts on the lake’s ecosystem and to assess its natural, pre-eutrophication baseline state. Two marked stratigraphic sediment units were identified on the basis of total phosphorus concentration (TP), pigments, total organic carbon (TOC)/total nitrogen (TN), δ¹³C and δ¹⁵N corresponding to stages in the lake history dominated by phytoplankton, and by aquatic macrophytes. Results show that as TP loading increased from the early 1950s the lake produced sediments with increasing amounts of organic matter derived from phytoplankton. In the early 1950s, the first evidence for eutrophication at the Meilian Bay site is recorded by an increase in C/N values and in sediment accumulation rate, but there is little change in phosphorus concentrations, pigments, δ¹³C and δ¹⁵N at this time. After 1990 a more rapid increase in trophic status took place indicated by increased levels of phosphorus, pigments, δ¹⁵N and by decreased δ¹³C and TOC/TN values in the lake sediments. The first increase in trophic status of the early 1950s results mainly from agricultural development in the catchment. In contrast, the acceleration from ca. 1990 originates from the recent development of fisheries and the urbanisation and industrialisation of the catchment.     

Organic geochemical record of environmental changes in Lake Dianchi,China

In order to investigate the natural ecosystem of Lake Dianchi and to assess its anthropogenic impacts, a stratigraphic study of bulk and molecular compositions of organic matter was conducted using a 63-cm long sediment core. The results show that two apparent environmental changes occurred during the evolution of Lake Dianchi: (1) the first change occurred in the 43–63 cm sediment depth, and was revealed by the amount and the composition of organic matter in the stage. Natural changes were possibly major factors responsible for triggering the environmental change, but the influence of human activities could not be excluded. Subsequently, the lake entered into a relatively stable and oligotrophic stage, which maintained until 20-cm sediment depth. (2) Eutrophication started in the upper 20 cm depth. Human activities became a major factor influencing environmental changes in this stage. Vertical profiles of various organic geochemical variables in the upper 20-cm sediments show evidence that primary productivity of the lake increased progressively and that the lake started eutrophic. Especially in the uppermost 10 cm, notable excursions to less negative δ¹³C_org and δ¹⁵N_total and high TOC concentrations have recorded an abrupt change in the lacustrine environment, suggesting that the lake entered a hypereutrophic stage. In addition, enhancement of αβ-hopanes reflects the contribution of fossil fuels to the lake sediments.     

玉龙雪山冰川稳定同位素分馏冬夏对比

1 Introduction Under the dominance of South Asian monsoon, China’s temperate glaciers are distributed on Hengduan Mountains (southeast of Tibetan Plateau), the central part and southern slope of the Himalaya Mountains as well as the central and eastern p…     

A multiple stable isotope record of Late Quaternary limnological changes and chironomid paleoecology from northeastern Iceland

We present results from multiple stable isotope analyses (δ¹⁸O of chironomid larval head capsules, chironomid adult thoraxes and other insect remains and δD, δ¹³C, δ¹⁵N of total organic matter—TOM) of a lake sediment core (04-SVID-03) taken from Stora Vidarvatn in northeastern Iceland to reconstruct past environmental, limnological and δ¹⁸O of past lake water changes during the Holocene. Core 04-SVID-03 represents a ∼12,000 cal. yrs BP to present record. Large magnitude changes in δ¹⁸O occurred during the Holocene at the site. Downcore shifts in δ¹⁸O of chironomids did not correlate with measurements of the δ¹³C and δ¹⁵N of chironomid head capsules, implying that the δ¹⁸O changes were not primarily driven by changes in chironomid diet during the Holocene. The δD of TOM provided a proxy of relative lake-water δD changes at the site and also showed large magnitude changes during the record. This approach was supported by analyses of a modern training set where δD_TOM analyses were conducted using surface sediments from a suite of freshwater lakes over a large latitudinal gradient. The magnitude of changes in both the δ¹⁸O and δD and the relatively negative δ¹⁸O values throughout much of the core suggest that the proxies represent more paleoenvironmental information than solely temperature. Additional possible influences on lake-water isotopic composition are discussed, including changes in the seasonality of precipitation, in the patterns of air masses supplying precipitation to Iceland and in the dominant mode of the North Atlantic Oscillation.     

Stable carbon isotope ratios in lake and swamp sediments as a proxy for prehistoric forest clearance and crop cultivation in the Neotropics

Close correspondence between stable carbon isotope ratios ( ¹³ C), pollen, and charcoal profiles in sediment cores from Laguna Zoncho and Machita swamp, Costa Rica, shows that prehistoric forest clearance and crop cultivation can be detected in the stable carbon isotope ratios of total organic carbon ( ¹3C _TOC ). Analyses of δ 13C _TOC complement evidence from pollen, charcoal, and phytoliths and provide a proxy that is sensitive to the intensity and/or proximity to core sites of prehistoric forest clearance and agriculture in watersheds. Stable carbon isotope analyses are particularly useful in situations in which other evidence of forest clearance and agriculture is limited.     

Sediment lithology and stable isotope composition of organic matter in a core from a cirque in the Krkonoše Mountains,Czech Republic

This study presents detailed lithostratigraphy and stable carbon and nitrogen isotopic variations in a 520-cm-long sediment core from a cirque basin in the Labsky důl Valley, Krkonoše Mountains, Czech Republic. Detailed study of the core reveals five major periods of sedimentation during the last 7600 years: silt and sand deposition during ~7.6–5.1 ka cal BP, Sphagnum peat accumulation during ~5.1–4.0 ka cal BP, sandy silt and sand during ~4.0–2.8 ka cal BP, raised peat bog during ~2.8–2.0 ka cal BP (Sphagnum peat), and sedimentation of sandy silt since ~2.0 ka cal BP. The δ¹³C values of the organic matter in the core vary in the range typical for C3 plants, from −24.35 to −27.68‰, whereas the δ¹⁵N values vary from −2.65 to +4.35‰. Core sections having ash contents ≥70% have δ¹⁵N > 1‰ and δ¹³C < −26‰, whereas those having ≤70% ash content have δ¹⁵N < 1‰ and δ¹³C > −26‰. Strong linear correlations are observed between δ¹³C and δ¹⁵N values as well as between C:N ratios and δ¹⁵N values in the horizons with ash content >10%, primarily for sand and silt horizons. On the other hand, poor correlations between δ¹³C and C:N ratio, as well as δ¹⁵N and C:N ratio, were observed in Sphagnum peat layers (45–125 and 185–265 cm). We conclude that the primary stable isotope variations are not preserved in the layers where significant correlation between δ¹⁵N and C:N ratio is observed. The relatively small δ¹³C variation in the uppermost Sphagnum peat layer suggests stable temperature during ~2.8–2.0 ka cal BP.     

Limnology of Two Antarctic Epishelf Lakes and their Potential to Record Periods of Ice Shelf Loss

George VI Ice Shelf is the largest ice shelf on the western side of the Antarctic Peninsula and its northern margin marks the southern most latitudinal limit of recent ice shelf retreat. As part of a project to reconstruct the long-term (Holocene) history of George VI Ice Shelf we studied two epishelf lakes impounded by the ice shelf at Ablation Point, on the east coast of Alexander Island. These lakes, Moutonnée and Ablation, are stratified water bodies with a lower marine layer and an upper freshwater layer. To determine if their sediment records could be used to detect past changes in the presence or absence of the ice shelf it was necessary to describe their present-day limnology and sedimentology. We measured water column chemistry and sampled the water column and sediments of the lakes along vertical and horizontal transects. We analysed these samples for diatoms, stable isotopes (δ¹⁸O, δ²H, δ¹³C_DIC, δ¹³C_org), geochemistry (TOC, TN, C/N ratios) and physical sedimentology (grain-size). This was supplemented by chemical and biological reference data from the catchments. Results showed that the water columns of both lakes are nutrient limited and deficient in phytoplankton. Benthic productivity is low and decreases with depth. Comparison of water column chemistry with an earlier survey shows a net increase in the thickness of the freshwater layer in Moutonnée Lake between 1973 and 2001, which could indicate that George VI Ice Shelf has thinned during this period. However, a similar trend was not observed in Ablation Lake (5 km to the north) and an alternative explanation is that the changes are a seasonal phenomena. Data from the surface sediment transects identified a number of proxies that respond to the present day stratification of the water column including diatom species composition, stable isotopes and geochemistry, particularly in Moutonnée Lake. Collectively these data have been used to develop a conceptual model for determining past ice shelf configuration in epishelf lakes. Specifically, periods of past ice shelf loss, and the removal of the ice dam, would see the present stratified epishelf lake replaced by a marine embayment. It is suggested that this change would leave a clear signature in the lake sediment record, notably the deposition of an exclusively marine biological assemblage, increased ice rafted debris and δ¹³C_org values that are indicative of marine derived organic matter. These authors contributed equally to this work