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Lake El'gygytgyn in far northeastern Russia provides one of the few records that can improve understanding of terrestrial responses to climatic changes during the Early to Middle Pleistocene transition (EMPT; 1400–400 ka). Palynological data from MIS 38‐31 (~1260 to 1060 ka) help clarify the complex nature of that transition. In contrast to the evergreen conifer forest of the MIS 31 'super' interglaciation, Larix‐Betula‐Alnus forest with shrub tundra characterized MIS 37 and MIS 35. The Larix forest‐tundra of MIS 33 represents an even cooler interglaciation. The landscapes of MIS 38, MIS 36 and early MIS 34 were a tundra mosaic with areas of Larix‐forest tundra. Although marine records suggest that MIS 38 and MIS 36 were exceptionally harsh glaciations, the decrease in woody taxa (particularly Alnus and Pinus) indicate the coolest conditions at El'gygytgyn occurred during MIS 32 and late MIS 34. When combined with other palynological data from El'gygytgyn, the EMPT portion of the record indicates that Betula was important in the vegetation during both cool and warm periods prior to MIS 24. Following the build‐up of global ice volumes ~900 ka, the taxon was characteristic of warm intervals only, marking the establishment of the herb‐dominated tundra, which typifies the remainder of the Pleistocene glaciations. The more complex successional patterns in post‐900 ka vegetation suggest greater climate variability as ice volume increased. Pinus pumila became a significant component of interglacial vegetation following the shift from a 41 ka to a quasi‐100 ka orbital cycle, indicating an increase in snow cover and a lessening of aridification, at least in this part of the Arctic. [ABSTRACT FROM AUTHOR]

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Marine Isotope Stage 11 has been proposed as an analogue for the present interglacial interval; yet, terrestrial climate records from both this region and time interval are rare. The sediments deposited at Lake El'gygytgyn (67°30′N, 172°5′E) in Far East Russia contain a 3·56 Ma record of climate variability. This study presents a high-resolution record of sediment colour change from Marine Isotope Stage 8 to 12 ( ca 275 to 475 ka) and demonstrates the link between lake catchment processes and climate variability. The hue colour parameter, calculated from data collected via colour reflectance spectroscopy in the visible spectrum (380 to 720 nm), exhibits correspondence with global climate records. Determining the source of sediment colour changes was achieved through detailed mineralogical and sedimentological methods, and linked to colour changes through a series of colour sensitivity tests. Mineralogical data, measured by X-ray diffraction, reveal fluctuations in concentrations of clay minerals corresponding to colour changes. Further analyses of the clay mineral assemblages show no change in relative clay mineral abundances, yet demonstrate a lake catchment dominated by physical weathering processes. Using measured mineral abundances, reconstructions of sediment colour based on colour reflectance mineral standards link mineral and clay mineral content to overall sediment colour. Colour sensitivity tests demonstrate the ability of iron oxide minerals to stain sediments red. Additionally, colour sensitivity to organic matter content was tested, suggesting that organic content drives variability in the red portion of the spectrum and darkens the overall colour signal. Sediment colour is then ultimately linked to physical weathering of bedrock minerals, with small amounts of chemical weathering producing iron oxides during wet intervals. Fluctuations in the sediment colour reveal a high-resolution record of wet/dry cycles, and provide new information about wet periods for the Russian Arctic region not yet understood from other lake proxy records. [ABSTRACT FROM AUTHOR]

Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO2 is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15°C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO2 with radiocarbon (14C) ages ranging between∼21 and 24 ka bp was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q10 value of 1.9±0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions of Δ14CO2 values in respiration CO2 and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO2 in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5°C, about 1 Pg C will be initially released from labile C pools, followed by respiration of∼40 Pg C to the atmosphere over a period of four decades. [ABSTRACT FROM AUTHOR]

We measured δ13C of CO2, CH4, and acetate-methyl in profundal sediment of eutrophic Lake Dagow by incubation experiments in the presence and absence of methanogenic inhibitors chloroform, bromoethane sulfonate (BES), and methyl fluoride, which have different specificities. Methyl fluoride predominantly inhibits acetoclastic methanogenesis and affects hydrogenotrophic methanogenesis relatively little. Optimization of methyl fluoride concentrations resulted in complete inhibition of acetoclastic methanogenesis. Methane was then exclusively produced by hydrogenotrophic methanogenesis and thus allowed determination of the fractionation factors specific for this methanogenic pathway. Acetate, which was then no longer consumed, accumulated and allowed determination of the isotopic signatures of the fermentatively produced acetate. BES and chloroform also inhibited CH4 production and resulted in accumulation of acetate. The fractionation factor for hydrogenotrophic methanogenesis exhibited variability, e.g., it changed with sediment depth. The δ13C of the methyl group of the accumulated acetate was similar to the δ13C of sedimentary organic carbon, while that of the carboxyl group was by about 12% higher. However, the δ13C of the acetate was by about 5% lower in samples with uninhibited compared with inhibited acetoclastic methanogenesis, indicating unusual isotopic fractionation. The isotope data were used for calculation of the relative contribution of hydrogenotrophic vs. acetoclastic methanogenesis to total CH4 production. Contribution of hydrogenotrophic methanogenesis increased with sediment depth from about 35% to 60%, indicating that organic matter was only partially oxidized in deeper sediment layers. [ABSTRACT FROM AUTHOR]