Source, transport and fate of soil organic matter inferred from microbial biomarker lipids on the East Siberian Arctic Shelf

Abstract

Abstract. The Siberian Arctic contains a globally significant pool of organic carbon (OC) vulnerable to enhanced warming and subsequent release by both fluvial and coastal erosion processes. However, the rate of release, its behaviour in the Arctic Ocean and vulnerability to remineralisation is poorly understood. Here we combine new measurements of microbial biohopanoids including adenosylhopane, a lipid associated with soil microbial communities, with published glycerol dialkyl glycerol tetraethers (GDGTs) and bulk δ13C measurements to improve knowledge of the fate of OC transported to the East Siberian Arctic Shelf (ESAS). The microbial hopanoid-based soil OC proxy R′soil ranges from 0.0 to 0.8 across the ESAS, with highest values nearshore and decreases offshore. Across the shelf R′soil displays a negative linear correlation with bulk δ13C measurements (r2 = −0.73, p = < 0.001). When compared to the GDGT-based OC proxy, the branched and isoprenoid tetraether (BIT) index, a decoupled (non-linear) behaviour on the shelf was observed, particularly in the Buor-Khaya Bay, where the R′soil shows limited variation, whereas the BIT index shows a rapid decline moving away from the Lena River outflow channels. This reflects a balance between delivery and removal of OC from different sources. The good correlation between the hopanoid and bulk terrestrial signal suggests a broad range of hopanoid sources, both fluvial and via coastal erosion, whilst GDGTs appear to be primarily sourced via fluvial transport. Analysis of ice complex deposits (ICDs) revealed an average R′soil of 0.5 for the Lena Delta, equivalent to that of the Buor-Khaya Bay sediments, whilst ICDs from further east showed higher values (0.6–0.85). Although R′soil correlates more closely with bulk OC than the BIT, our understanding of the endmembers of this system is clearly still incomplete, with variations between the different East Siberian Arctic regions potentially reflecting differences in environmental conditions (e.g. temperature, pH), but other physiological controls on microbial bacteriohopanepolyol (BHP) production under psychrophilic conditions are as yet unknown.

Quantifying Degradative Loss of Terrigenous Organic Carbon in Surface Sediments Across the Laptev and East Siberian Sea

“…They were highest close to the Lena River delta and northward (roughly along 130°E). This also agrees with the patterns reported for terrestrial biomarkers such as lignin phenols (e.g., Bröder, Tesi, Andersson, et al, 2016;Karlsson et al, 2014;Tesi et al, 2014) and solvent-extractable lipids (e.g., Bischoff et al, 2016;Bröder, Tesi, Andersson, et al, 2016;Doğrul Selver et al, 2015;Karlsson et al, 2011Karlsson et al, , 2014Sparkes et al, 2015). The comparison of the distribution of bulk OC with terrOC revealed that the high OC concentrations on the outer eastern East Siberian Sea shelf are probably to a large part caused by marine primary production, since terrOC concentrations in that area are rather low.…”

“…Earlier investigations of terrOC in Arctic margin surface sediments reported a strong decrease of terrigenous biomarker concentrations with increasing water depth/distance from the shore for the East Siberian Arctic Shelf (Bischoff et al, 2016;Bröder, Tesi, Salvadó, et al, 2016;Doğrul Selver et al, 2015;Sparkes et al, 2015Sparkes et al, , 2016Tesi et al, 2014;Vonk et al, 2010) and parts of the North American Arctic margin (Goni et al, 2013). The extent of terrOC degradation appears to be depending on its exposure to oxygen, which in turn is a function of the sediment transport time (e.g., Keil et al, 2004;Mollenhauer et al, 2007).…”

Sources, Sinks, and Subsidies: Terrestrial Carbon Storage in Mid‐latitude Fjords

“…The additional ~90% of missing OC terr could be removed from the system through multiple pathways (Figure ). The primary processes most likely to account for this missing OC terr are (i) some of the OC terr export may be in a dissolved rather than particulate form (Bauer et al, ; Bianchi, ); (ii) some of the eroded soils may not reach the fjord and may be deposited within the catchment (e.g., floodplains) (Wang et al, ); (iii) some of the OC terr may be degraded and lost to the atmosphere during transport in the fluvial system (Dinsmore et al, ; Leith et al, ), within the water column of the fjord (Burt et al, ) and within the sediment itself (Arndt et al, ; Glud et al, ); and (iv) some of the OC terr may be exported further offshore to the continental shelf and beyond (Bischoff et al, ; Haas et al, ; Painter et al, ) bypassing the coastal sediment. It is likely that other fjords may be more efficient traps of OC terr than these results suggest; we know that Loch Sunart does not suffer from periods of water column hypoxia (Gillibrand et al, ) and the absence of this OC preservation mechanism likely results in lower OC terr preservation in comparison to sites with hypoxic conditions (Middelburg & Levin, ; Woulds et al, ).…”

The spatial and interannual dynamics of the surface water carbonate system and air–sea CO<sub>2</sub> fluxes in the outer shelf and slope of the Eurasian Arctic Ocean

“…How climate change impacts the contemporary carbon cycle in the Eurasian Arctic seas, including its consequences for transformation and fluxes, has been the subject of intense interest during the last decade (Anderson et al, 2011;Bischoff et al, 2016;Bröder et al, 2016;Charkin et al, 2015;Gustafsson et al, 2011;Karlsson et al, 2016;Macdonald et al, 2008;Sánchez-García et al, 2014;Tesi et al, 2014;Vonk et al, 2014). Small changes in the largest marine carbon pool, the dissolved inorganic carbon (DIC) pool, can have profound impacts on the CO 2 flux between the ocean and the atmosphere and the feedback of this flux to climate.…”