Physico-chemical parameters of sediment core PG1975-1 (09-Tik-05) from a Tundra Lake, Northeastern Siberia

The relationships between thermokarst activity, limnogeological processes and climate change in the Siberian Arctic are not well understood. The objective of this paper is to identify the factors controlling the patterns of deposition, using grain size distribution, organic content, elemental composition and mineralogical composition of a 137-cm long sediment core with a maximum age of ~10.9 cal. kyr BP from Lake El'gene-Kyuele in the tundra of northeastern Siberia. Eight fine sand layers are attributed to depositional events associated with thaw slump activity acting upon orthogonally oriented patterns of ice-wedge networks in the ice-rich permafrost on the NW margin of the lake catchment. Sr/Rb ratios, which correspond to the total feldspar and illite content, serve as high-resolution grain size proxies. The Br content relates to the total organic carbon content, and the Fe/Mn ratio reflects the degree of oxidisation. Our results indicate a relationship between repeated phases of fine sand input and retrogressive thaw slumping dependent on hydroclimate variability and orthogonally oriented ice-wedge networks within the catchment.

Magnesium/calcium ratio of the foraminifera Oridorsalis umbonatus in surface sediment samples off Namibia

A laser ablation system connected to an inductively coupled plasma mass spectrometer was used to determine Mg/Ca ratios of the benthic foraminifera Oridorsalis umbonatus. A set of modern core top samples collected along a depth transect on the continental slope off Namibia (320-2300 m water depth; 2.9° to 10.4°C) was used to calibrate the Mg/Ca ratio against bottom water temperature. The resulting Mg/Ca-bottom water temperature relationship of O. umbonatus is described by the exponential equation Mg/Ca = 1.528*e**0.09*BWT. The temperature sensitivity of this equation is similar to previously published calibrations based on Cibicidoides species, suggesting that the Mg/Ca ratio of O. umbonatus is a valuable proxy for thermocline and deep water temperature.

Hydrochemistry, water level, and discharge of water from Samoylov Island, Lena Delta, northeastern Siberia, in 2008

Although ponds make up roughly half of the total area of surface water in permafrost landscapes, their relevance to carbon dioxide emissions on a landscape scale has, to date, remained largely unknown. We have therefore investigated the inflows and outflows of dissolved organic and inorganic carbon from lakes, ponds, and outlets on Samoylov Island, in the Lena Delta of northeastern Siberia in September 2008, together with their carbon dioxide emissions. Outgassing of carbon dioxide (CO2) from these ponds and lakes, which cover 25% of Samoylov Island, was found to account for between 74 and 81% of the calculated net landscape-scale CO2 emissions of 0.2-1.1 g C/m**2/d during September 2008, of which 28-43% was from ponds and 27-46% from lakes. The lateral export of dissolved carbon was negligible compared to the gaseous emissions due to the small volumes of runoff. The concentrations of dissolved inorganic carbon in the ponds were found to triple during freezeback, highlighting their importance for temporary carbon storage between the time of carbon production and its emission as CO2. If ponds are ignored the total summer emissions of CO2-C from water bodies of the islands within the entire Lena Delta (0.7-1.3 Tg) are underestimated by between 35 and 62%.

Magnesium isotope ratios of pore-fluids and dolomites

Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid d26Mg values increase with depth by as much as 2 per mil. Because carbonates preferentially incorporate 24Mg (low d26Mg), the increase in pore-fluid d26Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid d26Mg values decrease with depth by up to 2 per mil. The decline in pore-fluid d26Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion-advection-reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured d26Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0-2.7 per mil depleted in d26Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in d26Mg by 0 per mil to +1.25 per mil relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.