Quantification of last glacial-Holocene net primary productivity and upwelling activity in the equatorial eastern Atlantic with a revised modern dinocyst database

Abstract

In order to better explore quantitative reconstructions of net primary productivity (NPP) conditions using a dinoflagellate cyst (dinocyst)-based transfer function method, we revised the tropical Atlantic modern dinocyst database n = 208 (Marret et al., 2008). Modern assemblages from the worldwide dinocyst atlas (Zonneveld et al., 2013) were added to provide a better geographical coverage of the South Atlantic Ocean. The environmental dataset was updated using the World Ocean Atlas 2013 for sea-surface temperature (SST) and sea-surface salinity (SSS) data, as well as for 1997–2017 mean NPP values recorded during SeaWifs and MODIS spatial programs. New environmental datasets were tested using anomalies regarding mean latitudinal SST as a potential index to track past upwelling activity. Finally, preindustrial NPP values, simulated with the IPSL-CM5A-LR model developed at the Institut Pierre Simon Laplace (IPSL) have been added to the dinocyst environmental database to provide NPP values consistent with the mean age of “modern” dinocyst assemblages. The transfer function method using the updated modern database was then applied to fossil dinocyst assemblages of core KZAI-01, retrieved off the Congo River mouth and covering the last 43.2 kyr. Our results indicate a dominant orbital forcing, with higher primary productivity values reconstructed during precession minima (MIS 3 and the last deglaciation) and enhanced by a maximum of obliquity during the last deglaciation and the Holocene. The synchronicity between high upwelling intensity and high terrigenous inputs during high NPP period (e.g. during the last Deglaciation) highlighted the prevalent role of river-induced upwelling activity, which is specific to the Congo River. These results have improved our understanding of the environmental forcing leading to major trophic changes in the intertropical area.