The distribution and abundance of chironomids in high-latitude Eurasian lakes with respect to temperature and continentality: development and application of new chironomid-based climate-inference models in northern Russia

The large landmass of northern Russia has the potential to influence global climate through amplification of climate change. Reconstructing climate in this region over millennial timescales is crucial for understanding the processes that affect the global climate system. Chironomids, preserved in lake sediments, have the potential to produce high resolution, low error, quantitative summer air temperature reconstructions. Canonical correspondence analysis of modern surface sediments from high-latitude lakes, located in northern European Russia and central Siberia, suggests that mean July air temperature is the most significant variable explaining chironomid distribution and abundance. This strong relationship enabled the development of a chironomid-based mean July air temperature-inference model based on 81 lakes and 89 taxa which has a r_jack² = 0.92 and RMSEP = 0.89 °C. Comparison of taxon responses to July temperature between this Russian and existing Norwegian data-sets shows that the temperature optima of individual taxa were between 1 and 3 °C higher in the Russian data regardless of modelling technique. Reconstructions based on fossil assemblages from a Russian tundra lake core (VORK5) using a Norwegian chironomid-based inference model provide mean July air temperature estimates that are 1.0–2.7 °C colder than from the 81-lake Russian model and are also lower than the instrumental record from a nearby meteorological station. The Norwegian model also did not reconstruct decadal-scale fluctuations in temperature seen in the instrumental record. These observations suggest that chironomid-based inference models should only be applied to sediment cores which have similar climate regimes to the geographic area of the training set. In addition a 149 lake, 120 taxa chironomid-based continentality inference model was also developed from the modern Norwegian and Russian training sets. A 2-component WA-PLS model was the minimal adequate model with r_jack² = 0.73 and RMSEP = 9.9 using the Gorczynski continentality index. Comparison of reconstructed continentality indices from the tundra lake, VORK5, show close agreement with local instrumental records over the past 70 years and suggest that the model is reliable. Recent warming in the Arctic has been spatially and seasonally heterogeneous; in many areas warming is more pronounced in the spring and autumn leading to a lengthening of the summer, while summer temperatures have remained relatively stable. A continentality inference model has the potential to detect these seasonal changes in climate.     

Diatom-inferred total phosphorus from dystrophic Lake Arapisto,Finland, in relation to Holocene paleoclimate

A sediment core from Lake Arapisto, Finland, was examined for fossil diatom assemblages to reconstruct changes in Holocene nutrient availability. Our aim was to investigate the long-term relationship between lake trophic status and climate by comparing the diatom-based phosphorus reconstruction with paleoclimatic proxies. Our results showed that the cold early Holocene was characterized by elevated nutrient conditions concurrent with newly exposed fertile ground. As the climate rapidly warmed and ice sheet further retreated, the catchment vegetation developed, which resulted in decreased nutrient flux into the lake. The Holocene Thermal Maximum (HTM), between ~ 8000 and 4000 cal yr BP, was characterized by oligotrophic conditions, which may have been caused by low effective precipitation and stable watershed vegetation. After the HTM, the lake became more productive. There was no particular increase in the trophic state that could be connected to more recent human influence. Although lake productivity has been shown to be affected by temperature, our record indicated that the nutrient dynamics were driven by complex interactions between changes in temperature, precipitation, catchment, and in-lake processes. Understanding of long-term nutrient dynamics and the associated processes can help in resolving relationships between lake productivity and climate during past and present climate changes.     

‘PALEOVAN’, International Continental Scientific Drilling Program (ICDP): site survey results and perspectives

Lake Van is the fourth largest terminal lake in the world (volume 607 km³, area 3570 km², maximum depth 460 m), extending for 130 km WSW–ENE on the Eastern Anatolian High Plateau, Turkey. The sedimentary record of Lake Van, partly laminated, has the potential to obtain a long and continuous continental sequence that covers several glacial–interglacial cycles (ca 500 kyr). Therefore, Lake Van is a key site within the International Continental Scientific Drilling Program (ICDP) for the investigation of the Quaternary climate evolution in the Near East (‘PALEOVAN’). As preparation for an ICDP drilling campaign, a site survey was carried out during the past years. We collected 50 seismic profiles with a total length of ～850 km to identify continuous undisturbed sedimentary sequences for potential ICDP locations. Based on the seismic results, we cored 10 different locations to water depths of up to 420 m. Multidisciplinary scientific work at positions of a proposed ICDP drill site included measurements of magnetic susceptibility, physical properties, stable isotopes, XRF scans, and pollen and spores. This core extends back to the Last Glacial Maximum (LGM), a more extended record than all the other Lake Van cores obtained to date. Both coring and seismic data do not show any indication that the deepest part of the lake (Tatvan Basin, Ahlat Ridge) was dry or almost dry during past times. These results show potential for obtaining a continuous undisturbed, long continental palaeoclimate record. In addition, this paper discusses the potential of ‘PALEOVAN’ to establish new results on the dynamics of lake level fluctuations, noble gas concentration in pore water of the lake sediment, history of volcanism and volcanic activities based on tephrostratigraphy, and paleoseismic and earthquake activities.     

Modeling spatial and temporal variations in temperature and salinity during stratification and overturn in Dexter Pit Lake,Tuscarora, Nevada,USA

This paper examines the seasonal cycling of temperature and salinity in Dexter pit lake in arid northern Nevada, and describes an approach for modeling the physical processes that operate in such systems. The pit lake contains about 596,200 m³ of dilute, near neutral (pHs 6.7–9) water. Profiles of temperature, conductivity, and selected element concentrations were measured almost monthly during 1999 and 2000. In winter (January–March), the pit lake was covered with ice and bottom water was warmer (5.3 °C) with higher total dissolved solids (0.298 g/L) than overlying water (3.96 °C and 0.241 g/L), suggesting inflow of warm (11.7 °C) groundwater with a higher conductivity than the lake (657 versus 126–383 μS/cm). Seasonal surface inflow due to spring snowmelt resulted in lower conductivity in the surface water (232–247 μS/cm) relative to deeper water (315–318 μS/cm). The pit lake was thermally stratified from late spring through early fall, and the water column turned over in late November (2000) or early December (1999). The pit lake is a mixture of inflowing surface water and groundwater that has subsequently been evapoconcentrated in the arid environment. Linear relationships between conductivity and major and some minor (B, Li, Sr, and U) ions indicate conservative mixing for these elements.Similar changes in the elevations of the pit lake surface and nearby groundwater wells during the year suggest that the pit lake is a flow-through system. This observation and geochemical information were used to configure an one-dimensional hydrodynamics model (Dynamic Reservoir Simulation Model or DYRESM) that predicts seasonal changes in temperature and salinity based on the interplay of physical processes, including heating and cooling (solar insolation, long and short wave radiation, latent, and sensible heat), hydrologic flow (inflow and outflow by surface and ground water, pumping, evaporation, and precipitation), and transfers of momentum (wind stirring, convective overturn, shear, and eddy diffusion). Inputs to the model include the size and shape of the lake, daily meteorological data (short wave radiation, long wave radiation or cloud cover, air temperature, vapor pressure, wind speed, and rainfall), rates for water inputs and outputs, the composition of inflowing water, and initial profiles of temperature and salinity. Predicted temperature profiles, which are influenced by seasonal changes in the magnitude of solar radiation, are in good agreement with observations and show the development of a strong thermocline in the summer, erosion of the thermocline during early fall, and turnover in late fall. Predicted salinity profiles are in reasonable agreement with observations and are affected by the hydrologic balance, particularly inflow of surface and groundwater and, to a lesser degree, evaporation. Defining the hydrodynamics model for Dexter pit lake is the first step in using a coupled physical – biogeochemical model (Dynamic Reservoir Simulation Model-Computational Aquatic Ecosystem Dynamics Model or DYRESM-CAEDYM) to predict the behavior of non-conservative elements (e.g., dissolved O₂, Mn, and Fe) and their effect on water quality in this system.     

Comparison of North-American pollen-based temperature and global lake-status with CCCma AGCM2 output at 6 ka

We present new paleoclimate reconstructions for the time period of 6000 yr BP (6 ka) that permit a quantitative comparison to corresponding mapped patterns of the newest version of the Canadian CCCma AGCM2 climate model. July temperature reconstructions were estimated using the modern analog technique employing fossil pollen records from the North American Pollen Database. We present a new method of optimizing critical choices within the application of the MAT through modern calibration. The pattern of 6 ka temperature anomalies generated by the CCCma AGCM2 generally agree with those of the pollen-based reconstruction, particularly in the presence of a band of positive temperature anomalies across the continent from the Yukon through the Great Lakes region and along the St. Lawrence. This agreement suggests that both the model and data are reconstructing a contraction in the polar vortex over North America at 6 ka. Major areas of disagreement between the pollen-based reconstruction and model simulation are also areas where the data are sparse and edge effects produce less reliable reconstructions. The CCCma AGCM2 simulates drier conditions in a west–east band stretching from Alaska through the Canadian prairies and Midwest United States, in agreement with low lake-level observations at 6 ka. Simulated and reconstructed precipitation–evaporation are also in agreement over the southwest United States. With the exception of north central Africa, the CCCma AGCM2 fails to capture the northward and eastern extension of the Afro-Asian monsoon belt at 6 ka. General agreement between CCCma AGCM2 P-E anomalies and lake status change is found throughout Europe at 6 ka.     

Spatial variations in water composition at a northern Canadian lake impacted by mine drainage

Release of acid drainage from mine-waste disposal areas is a problem of international scale. Contaminated surface water, derived from mine wastes, orginates both as direct surface runoff and, indirectly, as subsurface groundwater flow. At Camp Lake, a small Canadian Shield lake that is in northern Manitoba and is ice-covered 6 months of the year, direct and indirect release of drainage from an adjacent sulfide-rich tailings impoundment has severely affected the quality of the lake water. Concentrations of the products from sulfide oxidation are extremely high in the pore waters of the tailings impoundment. Groundwater and surface water derived from the impoundment discharge into a semi-isolated shallow bay in Camp Lake. The incorporation of this aqueous effluent has altered the composition of the lake water, which in turn has modified the physical limnology of the lake. Geochemical profiles of the water column indicate that, despite its shallow depth (6 m), the bay is stratified throughout the year. The greatest accumulation of dissolved metals and SO₄ is in the lower portion of the water column, with concentrations up to 8500 mg L⁻¹ Fe, 20,000 mg L⁻¹ SO₄, 30 mg L⁻¹ Zn, 100 mg L⁻¹ Al, and elevated concentrations of Cu, Cd, Pb and Ni. Meromictic conditions and very high solute concentrations are limited to the bay. Outside the bay, solute concentrations are lower and some stratification of the water column exists. Identification of locations and composition of groundwater discharge relative to lake bathymetry is a fundamental aspect of understanding chemical evolution and physical stability of mine-impacted lakes.     

Holocene hydro-climatic change and effects on carbon accumulation inferred from a peat bog in the Attawapiskat River watershed,Hudson Bay Lowlands,Canada

Multiple proxies from a 319-cm peat core collected from the Hudson Bay Lowlands, northern Ontario, Canada were analyzed to determine how carbon accumulation has varied as a function of paleohydrology and paleoclimate. Testate amoeba assemblages, analysis of peat composition and humification, and a pollen record from a nearby lake suggest that isostatic rebound and climate may have influenced peatland growth and carbon dynamics over the past 6700 cal yr BP. Long-term apparent rates of carbon accumulation ranged between 8.1 and 36.7 g C m^? 2 yr^? 1 (average = 18.9 g C m^? 2 yr^? 1). The highest carbon accumulation estimates were recorded prior to 5400 cal yr BP when a fen existed at this site, however following the fen-to-bog transition carbon accumulation stabilized. Carbon accumulation remained relatively constant through the Neoglacial period after 2400 cal yr BP when pollen-based paleoclimate reconstructions from a nearby lake (McAndrews et al., 1982) and reconstructions of the depth to the water table derived from testate amoeba data suggest a wetter climate. More carbon accumulated per unit time between 1000 and 600 cal yr BP, coinciding in part with the Medieval Climate Anomaly.     

Chironomids as a tool for inferring Holocene climate: an assessment based on six sites in southern Scandinavia

Chironomid subfossil assemblages from six low-alpine and sub-alpine Holocene stratigraphies are presented and compared. They are from five lakes in mid-southern and western Norway and one in central Sweden. When comparing the chironomid-inferred July air temperatures, there are many time segments with a poor among-lake fit in inferred temperatures. Possible environmental variables influencing the fossil chironomid assemblages are discussed using a modern Norwegian calibration data set to indicate taxon–environment relationships. These analyses indicate that local changes in pH, water chemistry, and productivity at times may have overridden the regional temperature signal. In addition, other causes of poor among-site temperature fit are discussed, in particular those related to chronological uncertainties. Holocene temperature inferences from single cores based on chironomids may not always be able to provide a reliable regional temperature signal, but can act as a guide from which hypotheses about past environmental conditions can be tested with the aid of chironomid-inferred temperatures from several sites and from other environmental proxies. We have obtained a regional picture of Holocene summer temperature change by developing a consensus reconstruction based on the overall temperature signal from all six sites. This consensus is developed by fitting a smoother through all 330 site-specific temperature-deviations from the Holocene mean. The consensus temperature deviations vary from −0.8 °C at 8800 cal years BP to +0.8 °C at 6500 cal years BP.     

A 11,000 yr palaeotemperature reconstruction from the southern boreal zone in Finland

The ecotone between the boreo-nemoral (hemiboreal) and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern Europe and is, to a large extent, controlled by climatic conditions. We present a quantitative annual mean temperature reconstruction from a high-resolution pollen stratigraphy in southern boreal Finland, using a pollen-climate calibration model with a cross-validated prediction error of 0.9°C. Our model reconstructs low but steadily rising annual mean temperature from 10,700 to 9000 cal yr BP. At 8000–4500 cal yr BP reconstructed annual mean temperature reaches a period of highest values (Holocene thermal maximum) with particularly high temperatures (2.0–1.5°C higher than at present) at 8000–5800 cal yr BP. From 4500 cal yr BP to the present-day, reconstructed annual mean temperature gradually decreases by ca 1.5°C. Comparison of present results with palaeotemperature records from the Greenland ice cores, notably with the NorthGRIP δ¹⁸O record, shows marked similarities, suggesting parallel large-scale Holocene temperature trends between the North Atlantic and North European regions. The verification of the occurrence, timing, and nature of the short-term temperature fluctuations during the Holocene in the southern boreal zone in Europe requires replicate, high-resolution climate reconstructions from the region.     

Altitudinal shift in stable hydrogen isotopes and microbial tetraether distribution in soils from the Southern Alps,NZ: Implications for paleoclimatology and paleoaltimetry

The Southern Alps are an ideal locality for studying patterns of isotopic fractionation associated with orographic precipitation. We have evaluated whether altitudinal change is reflected in the stable hydrogen isotopic composition (δ²H) of stream water, plant stem water and leaf wax lipids (n-alkanes) from living plants and soils, as well as in soil temperature. Samples were collected along an altitudinal transect from the windward side of the Southern Alps to Lake Hawea in the rain shadow. The results indicate that δ²H values of stem water overlap with stream water, demonstrating a gradual decrease with elevation that complied with modeled Rayleigh distillation, reflecting an isotopic lapse rate of −18.0 (± 1.1, 1σ)‰/km. Leaf and soil n-alkanes shared similar δ²H values and were ²H depleted relative to stem/stream waters. The values for soil n-alkanes indicated an isotopic lapse rate of −21.8 (± 2.0, 1σ)‰/km, consistent with precipitation data and long term observations. MBT/CBT derived soil temperature values based on the relative distribution of microbial tetraether lipids were similar to midsummer temperature observations, displaying an elevational decrease rate of −5.6 (± 1.5, 1σ) °C/km, consistent with regional and global observations.The results indicate that sedimentary lipid δ²H and microbial tetraether temperature estimates captured altitudinal trends in the isotopic composition of precipitation and mean temperature and further support their application in the reconstruction of past climate and surface uplift histories. However, notable differences in isotopic composition and temperature estimates between in situ soils and those with downslope transport of material emphasize the importance of facies analysis when interpreting past systems.     

Postglacial climates inferred from a lake at treeline,southwest Yukon Territory,Canada

Pollen, chironomid, and ostracode records from a lake located at alpine treeline provide regional paleoclimate reconstructions from the southwest Yukon Territory, Canada. The pollen spectra indicate herbaceous tundra existed on the landscape from 13.6–11 ka followed by birch shrub tundra until 10 ka. Although Picea pollen dominated the assemblages after 10 ka, low pollen accumulation rates and Picea percentages indicate minimal treeline movement through the Holocene. Chironomid accumulation rates provide evidence of millennial-scale climate variability, and the chironomid community responded to rapid climate changes. Ostracodes were found in the late glacial and early Holocene, but disappeared due to chemical changes of the lake associated with changes in vegetation on the landscape. Inferred mean July air temperature, total annual precipitation, and water depth indicate a long-term cooling with increasing moisture from the late glacial through the Holocene. During the Younger Dryas (12.9–11.2 ka), cold and dry conditions prevailed. The early and mid-Holocene were warm and dry, with cool, wet conditions after 4 ka, and warm, dry conditions since the end of the Little Ice Age.     

Oxygen isotope composition of diatoms from sediments of Lake Kotokel (Buryatia)

This is a summary of new oxygen isotope record of diatoms from Lake Kotokel sediments, with implications for responses of the lake system and its environment to global change over the past 46 kyr. Fossil diatoms in all samples are free from visible contamination signatures and contain no more than 2.5% Al₂O3, which ensures reliable reconstructions. The 5¹⁸O values in diatoms vary between + 23.7 and + 31.2%c over the record. The results present mainly diatom assemblages of summer blooming periods, except for the time span between 36 and 32 kyr, when the isotopic signal records mainly a shift from summer to spring blooming conditions. Possible water temperature changes only partly explain the changes in the isotopic record. The observed isotopic patterns are produced mainly by isotope changes in lake water in response to variations in air temperature, hydrology, and atmospheric circulation in the region. During Marine Isotope Stage (MIS) 2 (Last Glacial maximum), high 5¹⁸Od_iatom resulted from rapid evaporation and low fluvial inputs. The high 5¹⁸Od_iatom values of about + 29 to + 30%c during the first half of MIS 1 (Holocene interglacial) suggest an increased share of summer rainfalls associated with southern/southeastern air transport. The 5¹⁸Odi_atom decrease to + 24%c during the second half of MIS 1 is due to the overall hemispheric cooling and increased moisture supply to the area by the Atlantic air masses. The record of Lake Kotokel sediments provides an example of complex interplay among several climatic/environmental controls of 5¹⁸O_diatom during the Late Pleistocene and the Holocene.     

An inference model for mean summer air temperatures in the Southern Alps,New Zealand,using subfossil chironomids

Quantitative paleoclimate reconstructions from Southern Hemisphere mid-latitude sites are critical for identifying spatial and temporal patterns of abrupt climate change and for testing hypotheses about possible causal mechanisms. The analysis of chironomid distributions and environmental variables from 61 lakes was undertaken to develop a robust inference model for quantitative temperature reconstruction from sites in South Island, New Zealand. Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) indicated that mean summer air temperature (SmT), chlorophyll a, conductivity, organic content of sediment as determined by loss-on-ignition, and mean annual precipitation account for a significant portion of variance in chironomid taxa distribution. SmT was the dominant variable. The strongest performing inference model, based on weighted averaging-partial least squares (WA-PLS), was developed from 60 lakes for SmT, having a root mean square error of prediction (RMSEP_jack) of 1.27 °C and a coefficient of determination of 0.80 (r²_jack). These results confirm that New Zealand chironomids may be used to provide robust estimates of mean summer air temperature and are a reliable method for quantitative paleoenvironmental reconstruction.     

Glacial to Holocene climate changes in the SE Pacific. The Raraku Lake sedimentary record (Easter Island, 27°S)

Easter Island (SE Pacific, 27°S) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Although the general climate evolution of the south Pacific since the Last Glacial Maximum (LGM) is coherent with terrestrial records in southern South America and Polynesia, the details of the dynamics of the shifting Westerlies, the South Pacific Convergence Zone and the South Pacific Anticyclone during the glacial–interglacial transition and the Holocene, and the large scale controls on precipitation in tropical and extratropical regions remain elusive. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34 000 cal yrs BP based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 and 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest and a cold and relatively humid climate during the Glacial period. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The coherent climate patterns in subtropical and mid latitudes of Chile and Eastern Island for the LGM (more humid conditions) suggest stronger influence of the Antarctic circumpolar current and an enhancement of the Westerlies. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major flood events and erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery, inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low-latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca 9.5 cal kyr BP) and swamp and shallow lake conditions dominated till mid Holocene, partially favored by the infilling of the lacustrine basin. During the mid- to late-Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The presence of this dry mid Holocene phase, also identified in low Andean latitudes and in Patagonian mid latitudes, suggests that the shift of storm tracks caused by changes in the austral summer insolation or forced by “El Niño-like” dominant conditions have occurred at a regional scale. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2–0.8 cal kyr BP sedimentary gap. Our paleoclimatic data provides insights about the climate scenarios that could favor the arrival of the Polynesian people to the island. If it occurred at ca AD 800 it coincided with the warmer conditions of the Medieval Climate Anomaly, whereas if it took place at ca AD 1300 it was favored by enhanced westerlies at the onset of the Little Ice Age. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake.     

Chironomid record of Late Quaternary climatic and environmental changes from two sites in Central Asia (Tuva Republic,Russia)—local,regional or global causes?

Sediment cores from two mountain lakes (Lake Grusha at 2413 m a.s.l. and Ak-Khol at 2204 m a.s.l.) situated in the Tuva Republic (southern Siberia, Russia), just north of Mongolia, were studied for chironomid fossils in order to infer post-glacial climatic changes and to investigate responses of the lake ecosystems to these changes. The results show that chironomids are responding both to temperature and to changing lake depth, which is regarded as a sensitive proxy of regional effective moisture. The post-glacial history of this mountain region in Central Asia can be divided into seven successive climatic phases: the progressive warming during the last glacial–interglacial transition (ca 15.8–14.6 cal kyr BP), the warm and moist Bølling-Allerød-like interval (ca 14.6–13.1 cal kyr BP), the cool and dry Younger Dryas-like event (ca 13.1–12.1 cal kyr BP), warmer and wetter conditions during ca 12.1–8.5 cal kyr BP, a warm and dry phase ca 8.5–5.9 cal kyr BP, cold and wet conditions during ca 5.9–1.8 cal kyr BP, as well as cold and dry climate within the last 1800 years. The chironomid records reveal patterns of climatic variability during the Late-glacial and Holocene, which can be correlated with abrupt climatic events in the North Atlantic and the Asian monsoon-dominated regimes. Apparently, the water balance of the studied lakes is controlled by the interrelation between the dominant westerly system and the changing influence of the summer monsoon, as well as the influence of alpine glacier meltwater supply. It is possible that monsoon tracks could have reached the southwest Tuva, resulting in an increase in precipitation at ca 14.6–13.1 and ca 12.1–8.5 cal kyr BP, whereas cyclonic westerlies from the North Atlantic were likely responsible for considerable moisture transport accompanying the global Neoglacial cooling at ca 5.9–1.8 cal kyr BP. These events suggest the changes of the regional pattern of atmospheric circulation, which could be in turn induced by the global climatic shifts. Some discrepancies compared with other reconstructions from Central Asia may be associated with regional (spatial) differences between the changing predominant circulation mechanisms and with local differences in uplift and descent of air masses within the complicated mountain landscape. In this paper, we also discuss the possibilities and perspectives for using chironomids in reconstructions of past temperatures and climate-induced changes in water depth of lakes in Central Asia.     

Oceanic and atmospheric forcing of early Holocene ice shelf retreat,George VI Ice Shelf,Antarctica Peninsula

We use lake sediment records from an epishelf lake on Alexander Island to provide a detailed picture of the Holocene history of George VI Ice Shelf (GVI-IS). Core analyses included; micropaleontology (diatoms/foraminifera), stable isotope (δ¹⁸O, δ¹³C), geochemistry (total organic carbon (TOC), total nitrogen (TN), C/N ratios) and grain-size analyses. These data provide robust evidence for one period of past ice shelf absence during the early Holocene. The timing of this period has been constrained by 10 AMS ¹⁴C dates performed on mono-specific foraminifera samples. These dates suggest that GVI-IS was absent between c 9600 cal yr BP and c 7730 cal yr BP. This early Holocene collapse immediately followed a period of maximum Holocene warmth that is recorded in some Antarctic ice cores and coincides with an influx of warmer ocean water onto the western Antarctic Peninsula (AP) shelf at c 9000 cal yr BP. The absence of a currently extant ice shelf during this time interval suggests that early Holocene ocean-atmosphere variability in the AP was greater than that measured in recent decades.     

Distribution of glycerol dialkyl glycerol tetraether lipids in the water column of Lake Tanganyika

We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX₈₆ proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX₈₆ and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.     

Geochemistry,water balance,and stable isotopes of a “clean” pit lake at an abandoned tungsten mine,Montana, USA

The Calvert Mine is a small tungsten-rich (scheelite) skarn deposit in a remote, mountainous region of southwest Montana, USA. The open-pit mine closed in the 1970s and subsequently flooded to form a pit lake that is roughly conical in shape, 30 m deep and 120 m in diameter, with no surface inlet or outlet. The lake is holomictic with a groundwater flow-through hydrology and an estimated residence time of 2.5–5 y. Water isotopes show that the lake is at an approximate steady state with respect to water balance and has experienced 30% evaporation. The lake has a near-neutral pH, exceptional clarity, and extremely low concentrations of nutrients, sulfate, and most metals, including tungsten. Manganese concentrations are slightly elevated and increase with depth towards the sediment–water interface. Despite seasonally anoxic conditions in the deep water, dissolved Fe concentrations are orders of magnitude lower than Mn, suggesting that insufficient organic carbon is present in the sediment of this oligotrophic lake to drive bacterial Fe reduction. Based on stable isotope fingerprinting, diffuse seepage that enters a nearby headwater stream at the base of a large waste-rock pile can be directly linked to the partially evaporated pit lake. However, this seepage has neutral pH and low metal concentrations, and poses no threats to the environment. Stable isotopes of dissolved inorganic carbon (DIC) and dissolved oxygen (DO) are used to track the relative importance of photosynthesis and respiration with depth. In summer, a zone of high productivity exists near the base of the chemocline, releasing DO that is isotopically light. Respiration exceeds photosynthesis below the Secchi depth, which causes DO concentrations to approach zero towards the bottom of the lake. In winter, thick ice and snow cover prohibits photosynthesis. However, because of the low nutrient content, most of the volume of the lake remains oxic through the winter. Overall, the Calvert Lake is a good example of a pit lake formed from metal mining that has excellent water quality, which is a result of its favorable geology (paucity of sulfide minerals) and hydrology (flow-through lake with short residence time in a temperate climate).     

Holocene fluctuations of Lake Ngami,Middle Kalahari: chronology and responses to climatic change

Lake Ngami, a 3000 km² basin at the distal end of the Okavango Delta, Botswana, has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl, linked to topographic thresholds and inflows, current and fossil, in the Okavango system. The lake has been ephemeral throughout the 20th century and completely dry since 1982, yet held a substantial body of water up to a level of 930 m asl when visited by European travellers in the mid-nineteenth century. The historical decline in the lake has been well documented and used as evidence for regional desiccation, but the Holocene record, based on archaeological studies and radiocarbon dating of inorganic carbonates, is more speculative.A study of a 30 km² diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the Holocene, linked to a now abandoned inflow of the Kunyere River. A programme of 18 luminescence age determinations of the peripheral Dautsa and Magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90–100 ka or earlier, and have been modified by Holocene fluctuations in lake level and subsequent aeolian reworking. The water level peaked at 936 m asl between 4 and 3 ka, returned to 934 m at 2.8–2 ka, ponding water into the Phatane Gap, and fell rapidly thereafter to 932 m and thence to historical levels. The 936 m level can be correlated with increased throughflow in the Okavango Delta, hence increased rainfall in the catchment. The subsequent decline can be attributed to episodic closure and rerouting of Okavango distributaries, in particular the Thaoghe, perhaps aided by anthropogenic activities. As such the desiccation of the lake is not indicative of regional climatic change.     

Late Pleistocene–Holocene rise and collapse of Lake Suguta,northern Kenya Rift

The Late Pleistocene to Middle Holocene African Humid Period (AHP) was characterized by dramatic hydrologic fluctuations in the tropics. A better knowledge of the timing, spatial extent, and magnitude of these hydrological fluctuations is essential to decipher the climate-forcing mechanisms that controlled them. The Suguta Valley (2°N, northern Kenya Rift) has recorded extreme environmental changes during the AHP. Extensive outcrops of lacustrine sediments, ubiquitous wave-cut notches, shorelines, and broad terrace treads along the valley margins are the vestiges of Lake Suguta, which once filled an 80 km long and 20 km wide volcano–tectonic depression. Lake Suguta was deep between 16.5 and 8.5 cal ka BP. During its maximum highstand, it attained a water depth of ca 300 m, a surface area of ca 2150 km², and a volume of ca 390 km³. The spatial distribution of lake sediments, the elevation of palaeo-shorelines, and other geomorphic evidences suggest that palaeo-Lake Suguta had an overflow towards the Turkana basin to the north. After 8.5 cal ka BP, Lake Suguta abruptly disappeared. A comparison of the Lake Suguta water-level curve with other reconstructed water levels from the northern part of the East African Rift System shows that local insolation, which is dominated by precessional cycles, may have controlled the timing of lake highstands in this region. Our data show that changes of lake levels close to the Equator seem to be driven by fluctuations of spring insolation, while fluctuations north of the Equator are apparently related to variations in summer insolation. However, since these inferred timings of lake-level changes are mostly based on the radiocarbon dating of carbonate shells, which may have been affected by a local age reservoir, alternative dating methods are needed to support this regional synthesis. Between 12.7 and 11.8 cal ka BP, approximately during the Northern Hemisphere high-latitude Younger Dryas, the water level of Lake Suguta fell by ca 50 m, suggesting that remote influences also affected local hydrology.