New evidence for preservation of contemporary marine organic carbon by iron in Arctic shelf sediments

The protection of organic carbon through association with iron minerals (Fe_R) is an important factor in its stabilisation, long-term storage, and burial efficiency in marine sediments. However, large uncertainties still exist concerning the sources, lability, age, and composition of the organic matter associated with Fe_R in natural sediments. Therefore, the timing and environmental setting of the carbon-iron bonding process remain elusive. Here we use radiocarbon (Δ¹⁴C) and stable isotopes (δ¹³C) of downcore bulk sedimentary organic matter, benthic foraminifera and the organic carbon fraction bound to Fe_R to interrogate the source and age of the organic carbon pool associated with Fe_R in Arctic marine sediments. In the Barents Sea, we find that the organic carbon associated with Fe_R is younger overall than the bulk organic matter and is probably marine derived. The comparison to other investigations of OC-Fe_R origins reveals that in large parts of Arctic shelf regions Fe_R associated organic carbon is radiocarbon enriched and has a higher δ¹³C_org value compared to the bulk sediment, irrespective of sediment depth/age. Our findings suggest a rapid and preferential binding of fresh and marine organic matter with Fe_R. Hence, labile organic matter prone to decomposition is protected and stabilised, underlining the potential of the organic carbon-iron association as an efficient carbon burial mechanism.