Terrigenous organic matter in sediments of the eastern equatorial Atlantic - distribution, reactivity, and relation to Late Quaternary climate

Abstract

This study focuses on factors controlling temporal variations in quantity and quality of terrigenous organic matter (terrigenous OM) export from tropical Africa to the continental margin by two major rivers, the Congo and the Niger. High-resolution multi-proxy investigations were performed on late Quaternary sections of ODP Site 1075 (Congo deep-sea fan, 1200 - 9 ka) and core GeoB 4901 (Niger deep-sea fan, 245 - 0 ka) to characterize the bulk sedimentary OM and to reappraise the role of terrigenous OM. At both river fans, supply of terrigenous OM is closely connected to the dynamics of the African hydrological cycle (humidity/aridity, chemical weathering intensity, fluvial run-off, vegetation) that is primarily determined by cyclic fluctuations of insolation. Significant differences in OM quality and the dynamics of OM sedimentation and preservation however distinguish the two river systems. It is a fundamental result of this study that soil organic matter (SOM) is a major bulk OM component in the Congo and Niger deep-sea fan sediments. Different types of SOM (stable/reactive, nitrogen-rich/-poor, 12C-depleted/-enriched) exported during arid and humid climate stages determine the geochemical characteristics of the deep-sea fan sediments, especially C/N ratios and bulk d13Corg signatures. Due to low C/N ratios that may even reach values of marine OM and due to depletion of 12C, both resulting from intense humification processes, SOM may lead to massive underestimation of terrigenous OM proportions in marine sediments. Additionally, the supply of "heavy" terrigenous OM originating from C4 plants significantly shifts bulk d13Corg signatures towards "marine" values especially at the Niger fan where C4 grasses are an important element of the vegetation cover throughout the late Quaternary. The contribution of SOM and C4 plant remains thus rules out the conventional application of two end-member models for C/N and bulk d13Corg to assess marine and terrigenous OM amounts.