Deglacial mobilization of pre-aged terrestrial carbon from degrading permafrost.

The mobilization of glacial permafrost carbon during the last glacial-interglacial transition has been suggested by indirect evidence to be an additional and significant source of greenhouse gases to the atmosphere, especially at times of rapid sea-level rise. Here we present the first direct evidence for the release of ancient carbon from degrading permafrost in East Asia during the last 17 kyrs, using biomarkers and radiocarbon dating of terrigenous material found in two sediment cores from the Okhotsk Sea. Upscaling our results to the whole Arctic shelf area, we show by carbon cycle simulations that deglacial permafrost-carbon release through sea-level rise likely contributed significantly to the changes in atmospheric CO2 around 14.6 and 11.5 kyrs BP.

Exceptional 20th century glaciological regime of a major SE Greenland outlet glacier.

The early 2000s accelerated ice-mass loss from large outlet glaciers in W and SE Greenland has been linked to warming of the subpolar North Atlantic. To investigate the uniqueness of this event, we extend the record of glacier and ocean changes back 1700 years by analyzing a sediment core from Sermilik Fjord near Helheim Glacier in SE Greenland. We show that multidecadal to centennial increases in alkenone-inferred Atlantic Water SSTs on the shelf occurred at times of reduced solar activity during the Little Ice Age, when the subpolar gyre weakened and shifted westward promoted by atmospheric blocking events. Helheim Glacier responded to many of these episodes with increased calving, but despite earlier multidecadal warming episodes matching the 20th century high SSTs in magnitude, the glacier behaved differently during the 20th century. We suggest the presence of a floating ice tongue since at least 300 AD lasting until 1900 AD followed by elevated 20th century glacier calving due to the loss of the tongue. We attribute this regime shift to 20th century unprecedented low sea-ice occurrence in the East Greenland Current and conclude that properties of this current are important for the stability of the present ice tongues in NE Greenland.

The long-term fate of permafrost peatlands under rapid climate warming.

Permafrost peatlands contain globally important amounts of soil organic carbon, owing to cold conditions which suppress anaerobic decomposition. However, climate warming and permafrost thaw threaten the stability of this carbon store. The ultimate fate of permafrost peatlands and their carbon stores is unclear because of complex feedbacks between peat accumulation, hydrology and vegetation. Field monitoring campaigns only span the last few decades and therefore provide an incomplete picture of permafrost peatland response to recent rapid warming. Here we use a high-resolution palaeoecological approach to understand the longer-term response of peatlands in contrasting states of permafrost degradation to recent rapid warming. At all sites we identify a drying trend until the late-twentieth century; however, two sites subsequently experienced a rapid shift to wetter conditions as permafrost thawed in response to climatic warming, culminating in collapse of the peat domes. Commonalities between study sites lead us to propose a five-phase model for permafrost peatland response to climatic warming. This model suggests a shared ecohydrological trajectory towards a common end point: inundated Arctic fen. Although carbon accumulation is rapid in such sites, saturated soil conditions are likely to cause elevated methane emissions that have implications for climate-feedback mechanisms.

Lead contamination of subarctic lakes and its response to reduced atmospheric fallout: can the recovery process be counteracted by the ongoing climate change?

Can a climate-triggered export of old contaminants from the soil alter the lead (Pb) contaminant burden of subarctic lakes? To address this question, we reconstructed the pollution history of three high latitude lakes situated in a region where a recent climatic shift has occurred. Dated sediment records were used as archives of past Pb inputs to the lakes, where the difference in the (206)Pb/(207)Pb ratio between atmospheric contaminants ((206)Pb/(207)Pb ratio <1.16) and geogenic Pb in the catchment soil ((206)Pb/(207)Pb ratio >1.22) were used to trace fluxes of Pb contaminants. Lead contaminants were found in sediments deposited since Roman times. A significant export of Pb from the soil contaminant pool is indicated in two of the lakes surrounded by near-shore permafrost soils. Here, levels of Pb contaminants and (206)Pb/(207)Pb ratios of sediments deposited after the 1970s appear not to have been strongly affected by the >or=90% reduction in atmospheric deposition rates and increasing (206)Pb/(207)Pb ratios of atmospheric Pb since the 1990s. We concluded that soil processes stimulated by the ongoing climate change at high latitudes might work counteractive to efforts to reduce contaminant levels in subarctic lakes.