Evidence of Anomalously Low δ13C of Marine Organic Matter in an Arctic Fjord

Accurate estimation of relative carbon deposition (marine vs. terrestrial) is required for understanding the global carbon budget, particularly in the Arctic region, which holds disproportionate importance with respect to global carbon cycling. Although the sedimentary organic matter (SOM) concentration and its isotopic composition are important tools for such calculations, uncertainties loom over estimates provided by organic-geochemical bulk parameters. We report carbon and nitrogen concentrations and isotopes (delta C-13 and delta N-15) of SOM at an Arctic fjord namely Kongsfjorden. We find that the bound inorganic nitrogen (ammonium attached to the clay minerals) forms a significant proportion of total nitrogen concentration (similar to 77% in the inner fjord to similar to 24% in the outer part). On removing the bound nitrogen, the C/N ratio shows that the SOM in the inner fjord is made up of terrestrial carbon while the outer fjord shows mixed marine- terrestrial signal. We further show that the marine organic matter is unusually more depleted in C-13 (similar to-24%) than the terrestrial organic matter (similar to-22.5 parts per thousand). This particular finding also helps explain high delta C-13 values of SOM as noted by earlier studies in central Arctic sediments despite a high terrestrial contribution.