Oxidation and Origin of Organic Matter in Surficial Eastern Mediterranean Hemipelagic Sediments

Aerobic mineralisation of C_org in surface sedimentsof the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailedbox core C_org concentration versus depth profiles and the modelling environment for early diageneticproblems MEDIA. The reactive fraction comprises 60–80% of the total C_org reachingthe sediments and is largely oxidised within the surficial 10 cm. A non-reactive C _orgfraction (G_NR) dominates at depths >10 cm, and makes up20–40% of the total C _org flux to the sediments. First-order rateconstants for decomposition of the reactive fraction calculated from theC _org profiles range from 5.4 × 10^-3 to8.0 × 10^-3 y^-1 to 8.0 × 10^-3 y^-1. Total mineralization rates in thesurface sediment are between 1.7 and 2.6 mol C cm^-2 y^-1 and thus are typical for oligotrophic, deep-seaenvironments. The low fluxes and rapid remineralisation of C _org are accompanied by²¹⁰Pb_excess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments.Model results indicate a mismatch between the C _org profiles and O₂ microprofileswhich were measured onboard ship. This can be attributed to a combination of decompression artefactsaffecting onboard measurement of the O₂ profiles or the leakage ofoxygen into the core during handling on deck. Furthermore, the used D_b values, based on ²¹⁰Pb, may not befully appropriate; calculations with higher D_b values improve the O₂ fits. The surficial sediment¹³C _org values of -22 become less negative with increasing depth and decreasing C _orgconcentrations. The major ¹³C change occurs in the top 3 to 4 cm and coincides with the interval weremost of the organic carbon oxidation takes place. This indicates that the reactive fractionof organic matter, commonly assumed to be marine, has a more negative ¹³C _orgthan the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from thesediment data by means of literature algorithms yield low surface productivities(12–88 gC m^-2 y^-1), which are in good agreement with field measurements of primary productivity in otherstudies. Such values are, however, significantly lower than those indicated by recent productivitymaps of the area derived from satellite imagery (>100 gC m^-2 y^-1).