Large contributions of aged organic carbon to Arctic fjord sediments in Svalbard

Svalbard fjords are hotspots of organic carbon (OC) burial because of their high sedimentation rates. To identify sedimentary OC sources in Arctic fjords, we investigated surface sediments collected from eight Svalbard fjords using bulk and molecular geochemical parameters. All fjord surface sediments investigated were depleted in △¹⁴C_org (–666.9±240.3‰, n=28), suggesting that more recently fixed terrestrial and marine biomass is not the only contribution to the sedimentary OC. However, the source could not be determined by the most commonly used bulk indicators (i.e., N_org/TOC ratio and δ¹³C_org) in the Arctic realm. Thus, we applied a three-endmember model based on △¹⁴C_org and lignin phenols to disentangle the relative contributions of petrogenic, subglacial, and marine OC to the sedimentary OC pool. The fjord sediments (n=48) comprised on average of 79.3±26.1% petrogenic OC, 17.7±26.2% subglacial OC, and 3.0±2.5% marine OC. This three-end-member approach highlights the substantial contribution of petrogenic and subglacial OC to the present-day sedimentary OC in Svalbard fjords. Accordingly, under predicted warming worldwide, accelerated contributions of petrogenic and subglacial OC to fjords can be expected as a consequence of rapid glacier retreat, which may play an important role in the active carbon cycle as a potential CO₂ source to the atmosphere.