Terrestrial Control on Organic Carbon Burial in Late Cretaceous Tropical Atlantic Black Shale (OAE 3) by Clay Mineral Surfaces

The dominant long-term sink of organic carbon occurs at the interface of the marine and terrestrial realms in marine continental margin sediments. While the locus of sediment preservation is marine, terrestrial processes play an important role in determining the total amount of carbon buried. Here we show the influence of detrital clay minerals formed under terrestrial conditions on the patterns of organic carbon burial in a Late Cretaceous (Coniacian “oceanic anoxic event 3”) black shale from the tropical Atlantic; an enrichment commonly assumed to result from oceanographic processes. Studies of deep ocean sediments recovered from ODP 959 from West Africa document a strongly systematic variation of TOC from ~2 to 16% coinciding with movement of the intertropical convergence zone (ITCZ) on precessional time scales (~22 kys) (Beckmann et al., 2005). We analyzed clay mineral composition in four of these cycles at cm scales and found a strong relation (r=0.81; n =105) between TOC and the concentration of 2:1 (smectite) clay minerals in the sediment. The close association of organic enrichment and clay minerals over these abrupt changes in TOC suggest a linked mechanism through mineral surface preservation of OM, capable of accounting for organic enrichment up to 20% of the non carbonate fraction. Since the detrital clay minerals in these sediments are a product of terrestrial weathering conditions, the mineral surface preservation provides a direct link between organic carbon preservation in marine sediments and changes in hydrology and soil formation on land. Changes in terrestrial soil forming conditions in tropical west Africa during the late Cretaceous thus affected a strong influence on organic carbon burial patterns in deep water Atlantic sediments.