Organic geochemical and palynological evidence for terrigenous origin of the organic matter in Aegean Sea sapropel S1

Organic geochemistry and micropaleontology are used to determine the origin of sapropel S1 in the Aegean Sea. Low-molecular-weight (C-15, C-17 and C-19) n-alkane data show that net primary productivity (NPP) increased from similar to 14,000 to 10,000 yr BP at the glacial interglacial transition, but the onset of S1 at 9600 yr BP marks a sharp decline in NPP, which remained low until similar to 8200 yr BP. The start of sapropel deposition is marked by increased total organic carbon (TOC) and pollen-spore concentrations, together with increased high-molecular-weight (C-27, C-29, C-31 and C-33) n-alkanes. Pollen assemblages show large influx of tree pollen from central-northern European forests. Increases in high-molecular-weight n-alkanes suggest greater influx of fresh vascular plant material at the start of S1, although the amount is small compared to other insoluble organic matter. Palynological studies showed that most of this insoluble organic matter are flocks of dark-brown amorphous kerogen, typical of terrigenous humic compounds. From similar to 8200 yr BP to the top of S1 at similar to 6400 yr BP, there is a decline in high-molecular-weight n-alkanes and terrigenous kerogen, and an increase in low-molecular-weight n-alkanes, suggesting that NPP recovered during the later deposition of S1 in the Aegean Sea. The increase in low-molecular-weight n-alkanes coincides with the recovery of coccolithophores and dinoflagellates, suggesting that these phytoplankton are primarily responsible for the low-molecular-weight n-alkane variations. These data from the Aegean Sea support the model for sapropel deposition resulting from increased influx of TOC during times of stagnant bottom water, but disagree with Mediterranean models prescribing a large increase in marine productivity. (C) 1999 Elsevier Science B.V. All rights reserved.