Organic matter accumulation and preservation controls in a deep sea modern environment: An example from Namibian slope sediments

The Lüderitz upwelling cell is presently the most productive area of the Benguela current system and abundant organic matter (OM) accumulates on the adjacent slope sediments even at great water depth. OM from two cores taken on the slope and covering the last 280 kyear was analysed in terms of "petroleum quality" (Rock-Eval), chemical features (FTIR, EDS) and petrographic composition (light microscopy and TEM). These data indicate that the OM is more oxidized at 3606 m water depth than on the upper slope sediments (1029 m) although the petroleum quality of the OM throughout the deep-water core remains surprisingly high for hemipelagic deep-sea sediments (HI=200-400 mg/g). The petroleum quality of OM accumulated on the upper slope is consistently high: HI averages 450 mg/g. Two petrographic types of OM are distinguishable from microscopic observation, each ascribed to distinctive preservation mechanisms: (1) 'Granular' amorphous OM, which dominates in the deep-water core, is formed by organo-mineral aggregates. Aggregation appears to be the primary preservation mode at this depth although is quantitatively limited (maximum TOC value of 4 wt.% of bulk sediment obtained through this process). The ultrastructure of the aggregates highlights an intimate association pattern between sedimentary OM and clays. (2) 'Gel-like' nannoscopically amorphous OM (NAOM) largely dominates at 1000 m water depth and contains sulfur. Thus, early diagenetic sulfurization was probably involved in the preservation of this OM, but a contribution from the classical degradation-recondensation pathway cannot be ruled out. Moreover, selective preservation occurred at both sites but represents an insignificant part of the OM. Organic fluxes mainly control the occurrence and extent of sulfurisation at both water depths by determining the redox conditions at the sea floor. Aggregate formation is limited by both organic and mineral fluxes at the lower slope whereas OM supply is never limiting on the upper slope. Although consistently operating through time at both depths, preservation by organo-mineral association is limited by mineral availability and thus accounts for a relatively minor portion of the OM accumulated on this organic-rich slope. In the case of large organic fluxes, sulfurisation and/or degradation-recondensation is required to obtain TOC contents above 4 wt.% of bulk sediment in the area.