THE ROLE OF TOPOGRAPHY AND SOIL DEVELOPMENT IN ORGANIC CARBON-RICH LACUSTRINE SEDIMENTS FROM EARLY CRETACEOUS CONGO BASIN, WEST AFRICA

The stratigraphic record from the Lower Cretaceous lacustrine sediments in Congo rift basin reveals a pattern of upward increasing total organic carbon (TOC) deposition, from an average of 2 to 3% TOC in the active rift sequence to more than 6% TOC in the lower part of the overlying late rift sequence. Sources of organic matter also evolved, from a mixture of Type I/III (algal / bacterial and vegetation sources) in the active rift sequence to pure Type I (algal / bacterial) in the lower part of the late rift sequence. Faulting had essentially ceased in the late rift stage when the most organic carbon-rich sediments were deposited, suggesting that topography had been significantly reduced by this time and implying that deep lakes were not a prerequisite for deposition of lacustrine source rocks.

Late rift sediments show a positive correlation between δ¹³C of organic matter and %TOC, interpreted as indicating a control on TOC content by organic productivity. δ¹³C in sedimentary carbonate decreases upward through the rift sequence, interpreted as a result of increasing contribution of isotopically light soil-derived carbon to the carbon pool in the rift lake. This suggests that the triggering nutrient for high organic productivity in the late rift sequence was related to enhanced soil development as topography degraded in late stages of the rift evolution. The data do not definitively identify the critical nutrient but the most likely candidate is nitrogen.

This proposed relationship between topography, soil evolution and carbon cycling is tested by applying the CENTURY Soil Organic Matter Model to hypothetical soils in the Early Cretaceous Congo Rift. The CENTURY model quantitatively simulates C, N, P, and S dynamics in soils over time scales of centuries and millennia. Topographies assumed for the active and late rift stages are based on slopes in the modern East African Rift; soil thickness are based on soils from modern tropical settings at appropriate slopes. Parent material for the soils in the model runs are based on lithologies surrounding the Congo rift basin. Model runs are used to compare nutrient fluxes from soils as topography degrades during rift evolution.