Detailed organic geochemical investigations were performed on organic-carbon-rich sediments from Baffin Bay (ODP-Site 645) and the upwelling area off Northwest Africa (ODP-Site 658). Main objectives of this study were (1) to reconstruct the depositional environment of these sediments and (2) to point out factors controlling the formation of organic-carbon-rich sediments.

Sediments with high total organic carbon contents of up to more than 3% were deposited at both Site 645 and Site 658. Mass accumulation rates of total organic carbon are also similar, ranging from 0.05 to 0.4 gC cm^-2 ky^-1.

The types of organic matter, however, differ between Site 645 and Site 658, reflecting different depositional environments. In the narrow Baffin Bay, climate-controlled changes in terrigenous carbon supply have been the prime influence on organic-matter accumulation at Site 645 throughout the last 20 Ma, with maximum rates of input in the middle Miocene and the late Pliocene/Pleistocene. During the former period, additional quantities of marine organic carbon accumulated. These are thought to reflect a phase of higher productivity caused by the first inflow of cold Arctic water masses.

In the upwelling area off Northwest Africa, variations in marine organic carbon input, caused by changes in surface-water productivity, were the primary control on organic-carbon accumulation at Site 658. Nevertheless, terrigenous organic carbon may still represent a significant proportion of the total organic carbon. Estimated paleoproductivity values are in the same range as those measured in the area today. Between 3.6 and 3.1 Ma, the supply of terrigenous organic carbon reached its maximum, probably due to a phase of dominantly humid climatic conditions in Northwest Africa. The higher oceanic productivity also recorded for this interval may have been caused by increased fluvial nutrient supply.

End_Page 41-------------------------