SOIL ORGANIC MATTER AS AN IMPORTANT CONTRIBUTOR TO LATE QUATERNARY SEDIMENTS OF THE TROPICAL WEST-AFRICAN CONTINENTAL MARGIN

Soil organic matter (SOM) is a fraction of bulk organic matter (bulk OM) in continental margin sediments that is largely ignored in most paleoceanographic investigations. SOM, however, may considerably bias established bulk proxies to assess marine and terrigenous sedimentary OM proportions.

We present high-resolution geochemical records from the Congo fan (ODP 1075) and compare them to records from the Niger deep-sea fan (GeoB 4901) also incorporating inorganic data. Inorganic records document changes in chemical weathering intensity but also provide valuable information on soil erosion. The records from both fans are used to discuss the influence of climate-forcing on quality and quantity of terrigenous OM and mineral matter exported to the African continental margin and to reconsider the applicability of established bulk proxies in paleoceanography, specifically Corg/Ntot and the isotopic composition of bulk sedimentary OM (d13Corg). The Congo basin is characterized by the predominance of deeply weathered (old) ferallitic soils. SOM from these soils reveals elevated levels of humification resulting in significant nitrogen enrichment and 12C depletion as well as reduced reactivity and thus an excellent preservation potential. Pyrolysis identifies stable SOM most probably from the Congo basin to be a major component of bulk OM in the Congo fan sediments during humid climate stages producing narrow ranges of C/N and d13Corg in the marine record. In contrast, a much larger variability and a positive covariation in C/N and d13Corg is observed at the Niger fan. Comparable to inorganic records they document African aridity/humidity cycles and export of clay-rich (soil) material.

This study emphasizes that marine geochemical bulk proxies are mainly determined by varying contributions of SOM from different types of eroded soils rather than changes in marine productivity. Ignoring the presence of SOM may thus result in a severe underestimation of terrigenous OM in continental margin records potentially leading to erroneous paleoenvironmental interpretations. SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO2.