Late-Quaternary supply of terrigenous organic matter to the Congo deep-seafan (ODP site 1075): implications for equatorial African paleoclimate

Late-Quaternary sections (about I Ma) from the Congo deep-sea fan (ODP Leg 175, site 1075) were used to reconstruct the terrigenous organic matter sup ply to the easternmost equatorial Atlantic Ocean. Variations in quantity an d quality of the riverine organic matter reflect the interaction between th e paleoclimatic development within the continental catchment area and the p aleoceanographic conditions in the Congo river plume. To characterize the d elivery of organic matter from terrigenous and marine sources, we used elem ental and bulk carbon isotopic analyses, Rock-Eval pyrolysis, lignin chemis try, and organic petrology. High-amplitude fluctuations occurring about eve ry 15-25 ka reveal a mainly precessional control on organic sedimentation. Results from Rock-Eval pyrolysis indicate a mixed kerogen type III/II, as w ould be anticipated in front of a major river. Fluctuations in Tn,, from Ro ck-Evat pyrolysis demonstrate pronounced cyclic changes in the delivery of low- and high-mature organic matter. Contribution of the low-mature organic fraction was strongest during warm climates supporting enhanced marine pro duction offshore of the Congo. Organic petrological observations confirm th e existence of abundant terrigenous plant tissues, both non-oxidized (vitri nite) and oxidized (inertinite). Charcoal-like organic matter (fusinite) is attributed to periods of increased bush fires in the continental hinterlan d, and implies more and climatic conditions. Results from ratios of specifi c phenolic lignin components suggest that terrigenous organic matter in Lat e-Quaternary sections of site 1075 mainly derives from non-woody angiosperm tissue, i.e., grasses and leaves. Correlation between the amount of specif ic lignin phenols and the bulk delta C-13(org) signature fosters the conclu sion that an appreciable amount of the terrigenous organic fraction derives from C4 plant matter. This may cause an underestimation of the terrigenous proportion of bulk organic matter when assessments are based on bulk carbo n isotopic signatures alone.