ALBERT

Description / Table of Contents: Starting from a more general discussion of mechanisms controlling organic carbon deposition in marine environments and indicators useful for paleoenvironmental reconstructions, this study concentrates on detailed organic-geochemical and sedimentological investigations of late Cenozoic deep-sea sediments from (1) the Baffin Bay and the Labrador Sea (ODP-Leg 105), (2) the upwelling area off Northwest Africa (ODP-Leg 108), and (3) the Sea of Japan (ODP-Leg 128). Of major interest are shortas well as long-term changes in organic carbon accumulation during the past 20 m.y. As shown in the data from ODP-Legs 105, 108, and 128, sediments characterized by similar high organic carbon contents can be deposited in very different environments. Thus, simple total organic carbon data do not allow (i) to distinguish between different factors controlling organic carbon enrichment and (ii) to reconstruct the depositional history of these sediments. Data on both quantity and composition of the organic matter, however, provide important informations about the depositional environment and allow detailed reconstructions of the evolution of paleoclimate, paleoceanic circulation, and paleoproductivity in these areas. The results have significant implications for quantitative models of the mechanisms of climatic change. Furthermore, the data may also help to explain the formation of fossil black shales, i.e., hydrocarbon source rocks. (1) BAFFIN BAY AND LABRADOR SEA The Miocene to Quaternary sediments at Baffin Bay Site 645 are characterized by relatively high organic carbon contents, most of which range from 0.5% to almost 3%. This organic carbon enrichment was mainly controlled by increased supply .of terrigenous organic matter throughout the entire time interval. Two distinct maxima were identified: (i) a middle Miocene maximum, possibly reflecting a dense vegetation cover and fluvial sediment supply from adjacent islands, that decreased during late Miocene and early Pliocene time because of expansion of tundra vegetation due to global climatic deterioration; (ii) a late Pliocene-Pleistocene maximum possibly caused by glacial erosion and meltwater outwash. Significant amounts of marine organic carbon were accumulated in western Baffin Bay during middle Miocene time, indicating higher surface-water productivity (up to about 150 gC m -2 y-l) resulted from the inflow of cold and nutrient-rich Arctic water masses. The decrease in average surface-water productivity to values similar to those of the modern Baffin Bay was recorded during the late Miocene and was probably caused by the development of a seasonal sea-ice cover. At Labrador Sea Sites 646 and 647, organic carbon contents are low varying between 0.10% and 0.75%; the origin of most of the organic matter probably is marine. A major increase in organic carbon accumulation at Site 646 at about 7.2 Ma may indicate increased surface-water productivity triggered by the onset of the cold East-Greeniand Current system. Near 2.4 Ma, i.e., parallel to the development of major Northern Hemisphere Glaciation, accumulation rates of both organic carbon and biogenic opal decreased, suggesting a reduced surface-water productivity because of the development of dosed seasonal sea-ice cover in the northern Labrador Sea. The influence of varying sea-ice cover on surface-water productivity is also documented in the short-term glacial/interglacial fluctuations in organic carbon deposition at Sites 646 and 647. (2) UPWELLING AREAS OFF NORTHWEST AFRICA The upper Pliocene-Quaternary sediments at coastal-upwelling Site 658 are characterized by high organic carbon contents of 4%; the organic matter is a mixture of marine and terrigenous material with a dominance of the marine proportion. The upper Miocene to Quaternary pelagic sediments from close-by non-upwelling Sites 657 and 659, on the other hand, display low organic carbon values of less than 0.5%. Only in turbidites and slumps occasionally intercalated at the latter two sites, high organic carbon values of up to 3% occur. The high accumulation rates of marine organic carbon recorded at Site 658 reflect the high-productivity upwelling environment. Paleoproductivity varies between 100 and 400 gC m "2 y-1 during the past 3.6 m.y. and is clearly triggered by changes in global climate. However, there is no simple relationship between climate and organic carbon supply, i.e., it is not possble to postulate that productivity was generally higher at Site 658 during glacials than during interglacials or vice versa. Changes in the relative importance between upwelling activity (which was increased during glacial intervals) and fluvial nutrient supply (which was increased during interglacial intervals) may have caused the complex productivity record at Site 658. Most of the maximum productivity values, for example, were recorded at peak interglacials and at terminations indicating the importance of local fluvial nutrient supply at Site 658. Near 0.5 Ma, a long-term decrease in paleoproductivity occurs, probably indicating a decrease in fluvial nutrient supply and/or a change in nutrient "content of the upwelled waters. The former explanation is supported by the contemporaneous decrease in terrigenous organic carbon and (river-borne) clay supply suggesting an increase in long-term aridity in the Central Sahara. At Site 660, underneath the Northern Equatorial Divergence Zone, (marine) organic carbon values of up to 1.5% were recorded in upper Pliocene-Quaternary sediments. During the last 2.5 Ma, the glacial sediments are carbonate-lean and enriched in organic carbon probably caused by the influence of a carbonate-dissolving and oxygen-poor deep-water mass. (3) SEA OF JAPAN Based on preliminary results of organic-geochemical investigations, the Miocene to Quaternary sediments from ODP-Sites 798 (Oki Ridge) and 799 (Kita-Yamato-Trough) are characterized by high organic carbon contents of up to 6%; the organic matter is a mixture between marine and terrigenous material. Dominant mechanisms controlling (marine) organic carbon enrichments are probably high-surface water productivity and increased preservations rates under anoxic deep-water conditions. In the lower Pliocene sediments at Site 798 and the Miocene to Quaternary sediments at Site 799, rapid burial of organic carbon in turbidites may have occurred episodically. Distinct cycles of dark laminated sediments with organic carbon values of more than 5% and light bioturbated to homogenous sediments with lower organic carbon contents indicate dramatic shortterm paleoceanographic variations. More detailed records of accumulation rates of marine and terrigenous organic carbon and biogenic opal as well as a detailed oxygen isotope stratigraphy are required for a more precise reconstruction of the environmental history of the Sea of Japan through late Cenozoic time.