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

Van Santvoort, P.J.M., De Lange, G.J., Thomson, J., Colley, S., Meysman, F.J.R. and Slomp, C.P. (2002) Oxidation and origin of organic matter in surficial eastern Mediterranean hemipelagic sediments. Aquatic Geochemistry, 8 (3), 153-175. (doi:10.1023/A:1024271706896).

Abstract

Aerobic mineralisation of C_org 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 C_org reaching the sediments and is largely oxidised within the surficial 10 cm. A non-reactive C_org fraction (GNR) dominates at depths >10 cm, and makes up 20–40% of the total C_org flux to the sediments. First-order rate constants for decomposition of the reactive fraction calculated from the C_org 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 C cm^-2 y^-1 and thus are typical for oligotrophic, deep-sea environments. The low fluxes and rapid remineralisation of C_org are accompanied by ²¹⁰Pb_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 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 ²¹⁰Pb, may not be fully appropriate; calculations with higher D_b values improve the O₂ fits. The surficial sediment δ¹³C_org values of -22 become less negative with increasing depth and decreasing C_org concentrations. The major ¹³C change occurs in the top 3 to 4 cm and coincides with the interval where 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 δ¹³C_org than the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from the sediment 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 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).

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