Late Quaternary productivity changes in the western equatorial Atlantic: Evidence from 230Th-normalized carbonate and organic carbon accumulation rates

Abstract

Paleoproductivity changes in the western equatorial Atlantic have been estimated from carbonate and marine organic carbon accumulation for the last 180,000 yr from a CearáRise sediment core GeoB 1523-1. Accumulation rates were calculated by normalizing to excess 230Th activity. Paleoproductivity from carbonate accumulation was estimated by a correlation between carbonate and organic carbon fluxes derived from sediment traps deployed in oligotrophic waters. Results indicate minor productivity changes varying around 30 gC m −2 yr −1 for organic carbon-derived estimations and around 45 gC m −2 yr −1 for carbonate-derived estimations, suggesting that the study area remained a low productivity region throughout the time period examined. Maxima in western Atlantic paleoproductivity occurred during the warm substages of glacials and interglacials, in contrast to the eastern tropical Atlantic where maxima are recorded in the cold substages. This contrast might be caused by a deepening of the thermocline and nutricline in the west connected with a synchronous shallowing in the east. The carbonate preservation record was examined for the last 380,000 yr based on variations in percent planktonic foraminiferal fragments and percent coarse fraction. Only minor carbonate dissolution above 3300 m water depth is observed, except for three main dissolution events at isotope stages 4, 8.4 and 10. Considerable carbonate loss during these intervals is attributed to a decreased production of North Atlantic Deep Water which is associated with vertical expansion of Southern-Source Deep Water.