Utilization of petrogenic organic carbon in Arctic Fjord sediments

Fjords have been identified as carbon burial hotspots, sequestering about 18 MtC annually, with most recent estimates suggesting 61±16% of sedimentary organic carbon (OC) to be labile. Towards higher latitudes and catchment glaciation the relative contribution of petrogenic OC increases in fjord sediments. Enhanced melting and mass loss due to anthropogenic climate change is expected to increase sedimentary runoff from glaciers and ice-sheets in the coming decades, including previously locked-up petrogenic OC. Since petrogenic OC has an accumulated history of pre-depositional degradation, sequestration, and diagenesis, it has commonly been regarded as non-bioavailable to today’s microbes and is typically not considered a carbon source in climate models. However, over the last two decades, several studies discussed microbial utilization of petrogenic OC. While glacially derived dissolved OC was identified as being highly bio-available in the water column, the bio-availability of its particulate counterpart is virtually unexplored, especially after redeposition. To investigate the bio-availability of sedimentary petrogenic OC and its carbon feedback potential to the atmosphere, we extracted and determined intact polar lipids (indicative of living microbes) and their radiocarbon signature down core in three sediment cores in a proximal-to-distal transect in Hornsund fjord, Svalbard. By applying an isotope mass balance, we were able to show that local subsurface bacteria use between 5 ± 2% and 55 ± 6% (average of 25 ± 16%) of petrogenic OC for their biosynthesis. Thus, providing evidence that particulate petrogenic OC is bio-available after redeposition and is used as an important substrate in the subsurface. This suggests a potential positive feedback of increased petrogenic OC supply to fjord sediments via its bacterial utilization and subsequent carbon release in a warming climate. Further, we observe increased bio-availability of petrogenic OC along both the distal-to-proximal transect and down core. We hypothesize that the spatial and temporal variability of petrogenic carbon utilization is related to the availability of recently synthesized marine OC as well as to the distance to glacial termini, thus to sediment accumulation rate and oxygen availability.