The impact of volcanic ash deposition and the Storegga Slide on organic carbon preservation processes in marine sediments near Iceland

The burial of organic carbon (OC) in marine sediments is a considerable sink for carbon, removing OC from the active ocean-atmosphere system. Both the total OC buried, and the proportion of OC retained in sediments after burial, varies by location, with some areas of the ocean floor known to be ‘hotspots’ of OC sequestration. Two potential such hotspots may be sediments containing high proportions of tephra (the unconsolidated products of explosive volcanism), and locations of turbidite deposition, but knowledge of specific burial regimes in such locations remains poorly constrained. To fully investigate these processes, we performed a holistic (organic and inorganic) geochemical analysis of samples from the Aegir Ridge, which contain both tephra layers and material from the Storegga Slide, a large turbidite. We show sediments found between the Storegga Slide and the tephra are a location of high OC preservation, linked to reducing conditions caused by the rapidly deposited slide layer sealing the sediments from overlying water column O2. We see little evidence for tephra positively affecting OC preservation at our site, but this is likely a feature of specific burial conditions, with the responsible mechanisms depending highly on the nature of the tephra. Our findings demonstrate how even in locations proposed as OC burial hotpots, the processes controlling this burial are highly complex, and that levels of sedimentary OC burial must be assessed on a case-by-case basis.