Sapropelic deposits in a sediment from the Guinea Basin, South Atlantic

Abstract

Until recently it has been generally thought that a stagnant water column is a prerequisite to the formation of organic-rich deposits in deep-water marine sediments, a belief best exemplified by studies on the eastern Mediterranean sapropels. Many workers have assumed that, following the hydrological isolation of the deep waters, stagnation of the eastern Mediterranean basins and the concomitant development of anoxic conditions occurred before the deposition of the sapropelic layers1–10. Opposing arguments suggesting a role for increased marine productivity have, however, been put forward, both specifically with respect to the eastern Mediterranean sapropels11,12 and more generally regarding deep-water organic-rich deposits13–15. We report here the discovery of sapropelic layers (>0.5% organic carbon)16 in a recent sediment core from a deep-water site in the Guinea Basin. It is thought most unlikely that stagnant conditions could prevail in the water column at this site, and preliminary analyses of the sediments suggest that the organic-rich deposits have been a direct result of increased productivity in the overlying water column, the most recent occurring about 18,000 yr ago and coinciding with the last glacial maximum.