Marine production in the Congo-influenced SE Atlantic over the past 30,000 years: A novel dinoflagellate-cyst based transfer function approach

The sediments from the Congo deep-sea fan contain valuable information about past environmental conditions of the east equatorial Atlantic during the transition from the Last Glacial Maximum (LGM) to present-day climatic conditions. The high-resolution marine (organic-walled dinoflagellate cysts = dinocysts) and terrestrial (pollen) palynological records from two cores off equatorial West Africa, covering the last 30,000 years, document three major phases in surface productivity. (1) During the LGM relative sea level was low, nutrient enrichment due to seasonal coastal upwelling prevailed off the Congo mouth and high aridity prevailed in the catchment area. (2) At around 15.2 cal ka BP, monsoonal precipitation strengthened over the Congo Basin, generating high river discharges, river-induced upwelling, and increased nutrient flux to the ocean. In parallel, erosion of shelf sediments during shelf transgression further enhanced nutrient flux. (3) At around 9–8 cal ka BP, rainforest vegetation inhibited soil erosion, depleted nutrient supply, and restricted marine productivity to its modern levels. The study presents the application of modern analogues and a dinocyst transfer function to the reconstruction of primary palaeoproductivity (PP) in a region of freshwater influence. A database of recent dinocyst assemblages comprising of 208 sites in the equatorial Atlantic enabled the reconstruction of sea-surface annual PP 1.3 times higher during the LGM and also during the deglaciation, between 15.2 and 13.2 cal ka BP. The reconstruction is independent of, and thus provides corroboration for, other biological and geochemical proxies.