Organic Carbon Sources in the Arctic Ocean

Constraints on continental organic carbon sources in the North American Arctic Ocean from lead isotopes

Student : Oliver Kost

Supervisor: Dr. Jörg Rickli, Prof. Dr. Derek Vance, ETH Zurich, Institute of Geochemistry and Petrology

Ongoing climate change and local warming destabilizes permafrost in high latitudes. This potentially increases riverine sediment transport to the Arctic Ocean via several rivers (Asahara et al., 2012). Land-derived particles contain organic carbon (Corg) and nutrients, which affect the biogeochemistry and ecosystems of the Arctic Ocean (Schaefer et al., 2011). Changing environmental conditions could influence the biological pump and the ecological system (Carmack et al., 2006). Hence the provenance, magnitude and the dispersal processes of continental inputs is of great interest.

In this study, the silicate fraction of core-top sediments from the North American Arctic Ocean (Bering Sea, Chukchi Sea, Barrow Canyon, Beaufort Shelf and Northwind Ridge) is measured for its Pb isotope composition to improve current constraints on the origin of continental Corg in the North American Arctic Ocean. The marine sediment samples were chemically treated to remove authigenic phases, including carbonates and iron oxides, prior to the digestion of the continent-derived silicate fraction. Lead was isolated by ion chromatography and subsequently measured by MC-ICP-MS using standard sample bracketing. This project complements and extends a previous Master Thesis, which reported Sr and Nd isotopes for the same and further Arctic sediment samples (Schwab, 2015).

Investigations on 20 sediment samples show that Pb is not a straightforward tracer of continental sediment sources in the Arctic environment. In comparison with potential source regions a systematic shift to more radiogenic values is observed. Hence, Pb isotopes are fractionated from source to sink due to weathering and sedimentary sorting processes. Two hypotheses are considered to explain the difference between source areas and marine sediments: (1) radiogenic heavy minerals enriched in Pb, but present only at accessory levels and / or (2) secondary clay minerals enriched in radiogenic Pb in the fine fraction of the sediments could lead to a shift to higher isotopic values. Although, there is no unambiguous evidence to argue that only one of these explanations is valid, secondary clays seem a more feasible explanation. Clays are relatively abundant in the sediments (~ 40%, Schwab, 2015) and expected to be rich in Pb (Garçon et al., 2014) and could therefore be significant for the Pb budget. Heavy minerals, on the other hand, are usually enriched in coarse sediment of rivers or proximal marine settings and should be underrepresented in the studied samples compared to the source. A large influence for clays is indicated by a special feature in Barrow Canyon sediments, where the isotopic Pb composition is more radiogenic for finer sediments further offshore.