Unexpectedly low organic matter content in Cariaco Basin sediments during the Younger Dryas: Origin and implications

The Cariaco Basin (offshore Venezuela), the second largest anoxic basin after the Black Sea, knew intense upwelling activity and maximum primary production during the Younger Dryas (12.9-11.5 ka cal. BP). Relatively low total organic carbon (TOC) contents are, however, observed in the sediment deposited during this interval. Previous studies have attributed these low TOC values to dilution by inorganic phases during a time of high sediment accumulation rate. The present study demonstrates that the low TOC values result primarily from a relatively reduced flux of organic matter (OM) to the sediment and minor dilution by carbonate. Excellent preservation of the OM during this interval excludes OM degradation as a cause for low TOC values. Consistent with palynological observation and biomarker analyses, the lower relative flux of OM to the sediment is related to changes in the primary producers, with a significant decrease in the contribution of organic-walled organisms during the Younger Dryas. These results, that highlight the need of caution when using TOC values as paleoproductivity or paleoenvironmental indicator, underline the important role of organic-walled primary producers in the organic enrichment of sediments. (C) 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.