Oxidation and origin of organic matter in surficial eastern Mediterranean hemipelagic sediments

Abstract

Aerobic mineralisation of Corg in surface sediments of the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailed box core Corg concentration versus depth profiles and the modelling environment for early diagenetic problems MEDIA. The reactive fraction comprises 60-80% of the total Corg reaching the sediments and is largely oxidised within the surficial 10 cm. A non-reactive Corg fraction (GNR) dominates at depths >10 cm, and makes up 20-40% of the total Corg flux to the sediments. First-order rate constants for decomposition of the reactive fraction calculated from the Corg profiles range from 5.4 × 10 −3 to 8.0 × 10 −3 y −1 to 8.0 × 10 −3 y −1 . Total mineralization rates in the surface sediment are between 1.7 and 2.6 µmol Cc m −2 y −1 and thus are typical for oligotrophic, deep-sea environments. The low fluxes and rapid remineralisation of Corg are accompanied by 210 Pbexcess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments. Model results indicate a mis- match between the Corg profiles and O2 microprofiles which were measured onboard ship. This can be attributed to a combination of decompression artefacts affecting onboard measurement of the O2 profiles or the leakage of oxygen into the core during handling on deck. Furthermore, the used Db values, based on 210 Pb, may not be fully appropriate; calculations with higher Db values improve the O2 fits. The surficial sediment δ 13 Corg values of ∼− 22 become less negative with increasing depth and decreasing Corg concentrations. The major δ 13 C change occurs in the top 3 to 4 cm and coincides with the interval were most of the organic carbon oxidation takes place. This indicates that the reactive fraction of organic matter, commonly assumed to be marine, has a more negative δ 13 Corg than the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from the sediment data by means of literature algorithms yield low surface pro- ductivities (12-88 gC m −2 y −1 ), which are in good agreement with field measurements of primary productivity in other studies. Such values are, however, significantly lower than those indicated by recent productivity maps of the area derived from satellite imagery (>100 gC m −2 y −1 ).