Efficient Burial of Labile Organic Carbon in Sediments of Oxygenated Icelandic Fjords

Abstract Fjords are hotspots of organic carbon (OC) burial, making them key components in the global carbon cycle. Recent research shows high marine OC burial even under oxic fjord water columns, broadening the scope of key drivers of OC burial therein. Here, we investigate the fate of marine OC in fjord sediments underlying oxic bottom waters in East Iceland. We estimate OC sources using stable isotopes, degradation using chemical biomarkers, and burial using 210 Pb and 137 Cs profiles. The average OC accumulation rates in East Iceland fjords were ~ 15 g OC m −2 year −1 , lower than the fjord global average of 54 g OC m −2 year −1 . Marine OC was the overwhelming source of OC to sediments and accounted for 91% of the OC buried in Iceland. The gradient in OC source from the head-to-mouth of the fjords was minimal, due to limited terrestrial vegetation and soil cover, low riverine inputs, and effective exchange with coastal waters. Lignin and total hydrolyzable amino acid biomarkers suggest negligible degradation of terrestrial OC and minor degradation of marine OC. Although a portion of marine OC was prone to remineralization in sediments, it represented only a small fraction of the total OC resulting in high OC transfer (burial) efficiencies of 68–96% within the top 25 cm of sediments. Overall, Icelandic fjords efficiently bury labile marine OC despite oxic water columns.