(Table 4) Estimated surface water phosphate concentrations for the past 200 kyr at the location of sediment core GeoB1016-3

We compared ocean atlas values of surface water [PO4]3- and [CO2(aq)] against the carbon isotopic fractionation (ep) of alkenones obtained from surface sediments of the South Atlantic and the central Pacific (Pacific data are from Pagani et al. 2002, doi:10.1029/2002PA000756). We observed a positive correlation between ep and 1/[CO2(aq)], which is opposite of what would be expected if the concentration of CO2(aq) were the major factor controlling the carbon isotopic fractionation of C37:2 alkenones. Instead, we found inverse relationships between ep and [PO4]3- for the two ocean basins (for the Atlantic, ep = -4.6*[PO4]3- + 15.1, R = 0.76; for the Pacific, ep = -4.1*[PO4]3- + 13.7, R = 0.64), suggesting that ep is predominantly controlled by growth rate, which in turn is related to nutrient concentration. The similarity of the slopes implies that a general relationship between both parameters may exist. Using the relationship obtained from the South Atlantic, we estimated surface water nutrient concentrations for the past 200,000 years from a deep-sea sediment core recovered off Angola. Low ep values, indicating high nutrient concentrations, coincide with high contents of total organic carbon and C37 alkenones, low surface water temperatures, and decreased bulk d15N values, suggesting an increased upwelling of nutrient-rich cool subsurface waters as the main cause for the observed ep decrease.

Pollen analysis of sediment cores from the mouth of the river Congo to Walvis Bay and Lüderitz

The distribution of pollen in marine sediments is used to record vegetation changes over the past 30,000 years on the adjacent continent. A transect of marine pollen sequences from the mouth of the river Congo (~5°S) to Walvis Bay and Lüderitz (~25°S) shows vegetation changes in Congo, Angola and Namibia from the last glacial period into the Holocene. The comparison of pollen records from different latitudes provides information about the latitudinal shift of open forest and savannahs (Poaceae pollen), the extension of lowland forest (rain forest pollen) and Afromontane forest (Podocarpus pollen), and the position of the desert fringe (pollen of Caryophyllaceae, Chenopodiaceae and Amaranthaceae). High Cyperaceae pollen percentages in sediments from the last glacial period off the mouth of the river Congo suggest the presence of open swamps rather than savannah vegetation in the Congo Basin. Pollen from Restionaceae in combination with Stoebe-type pollen (probably from Elytropappus) indicates a possible northwards extension of winter rain vegetation during the last glacial period. The record of Rhizophora (mangrove) pollen is linked to erosion of the continental shelf and sea-level rise. Pollen influx is highest off river mouths (10-2000 grains year**-1 cm**-2), close to the coast (300-6000 grains year**-1 cm**-2), but is an order of magnitude lower at sites situated far from the continent (<10 grains year**-1 cm**-2).

Sea-surface temperature reconstruction of ODP Hole 175-1078C

A prominent feature in the Southeast Atlantic is the Angola-Benguela Front (ABF), the convergence between warm tropical and cold subtropical upwelled waters. At present, the sea-surface temperature (SST) gradient across the ABF and its position are influenced by the strength of southeasterly (SE) trade winds. Here, we present a record of changes in the ABF SST gradient over the last 25 kyr. Variations in this SST contrast indicate that periods of strengthened SE trade-wind intensity occurred during the Last Glacial Maximum, the Younger Dryas, and the Mid to Late Holocene, while Heinrich Event 1, the early part of the Bølling-Allerød, and the Early Holocene were periods of weakened SE trade-winds.

Geological, geochemical and biogeochemical data on boundary zones of the Atlantic Ocean

Results of studies in two biogeochemically active zones of the Atlantic Ocean (the Benguela upwelling waters and the region influenced by the Congo River run-off) are reported in the book. A multidisciplinary approach included studies of the major elements of the ocean ecosystem: sea water, plankton, suspended matter, bottom sediments, interstitial waters, aerosols, as well as a wide complex of oceanographic studies carried out under a common program. Such an approach, as well as a use of new methodical solutions led to obtaining principally new information on different aspects of oceanology.

Siliceous phytoplankton in Late Quaternary sediments of ODP Site 175-1077

Late Quaternary fluctuations in the intensity of Congo River freshwater load were reconstructed using three different proxies (marine and freshwater diatoms, and the delta18O record of Globigerinoides ruber) preserved in the sediments of Ocean Drilling Program (ODP) Site 1077, located at the northern rim of the Congo River fan (5°10'S, 10°26'E). An abrupt change in the diatom assemblage is evident at Termination II: a two- to four-fold increase in (a) the relative abundance of a marine planktonic diatom tolerant of low salinity conditions (Cyclotella litoralis), and (b) in the concentration of freshwater diatoms. The microfossil data suggest a change in the environmental conditions surrounding Site 1077 from predominantly marine to mixed marine/brackish/fresh. The delta18O record of the planktic foraminifera G. ruber (pink) revealed negative deviations from the global oxygen isotope signal since Termination II which occurred during warm stage 1 and substages 3.2, 5.1, 5.3, and 5.5. Comparison of the isotopic signal of ODP Site 1077 with the record from a pelagic location (core GeoB1041 at 3°48'S, 7°05'W) confirms these results. The construction of an artificial delta18O curve using alkenone-derived sea surface temperature (SST) data from a nearby core (GeoB1008 at 6°S, 10°E) allowed us to estimate salinity and temperature effects on the ODP Site 1077 isotopic signal. Although increased SSTs may account for lighter delta18O values during warmer periods, they do not explain the extremely light values documented in the sediments of Site 1077. We used the oxygen isotope difference (Delta delta18O) between our site and GeoB1041 as a proxy for freshwater input. A general trend in the Delta delta18O was observed, with more negative values since Termination II. In addition, conspicuous Delta delta18O negative pulses coincided with periods of northern hemisphere summer insolation maxima over the African continent, suggesting an increase in the freshwater discharge from the Congo River due to enhanced precipitation on the hinterland. Here we propose that the abrupt change in environmental conditions at Site 1077 since Termination II is a consequence of a major reorganization in the depositional environment of the Congo River delta. This reorganization involved sustained equatorward displacement of the Angola-Benguela Front causing a northward deflection of the Congo River plume thus moving plume waters further north than normal and over Site 1077.

Organic geochemical comparison of Cretaceous green and black claystones at DSDP Hole 75-530A

Three pairs of Upper Cretaceous black shales and adjacent green claystones from Hole 530A were analyzed to compare types and amounts of organic matter and lipids and to seek information about their environments of deposition. The organic-carbon-rich black shales have C/N ratios nearly seven times those of the organic-carbon-lean green claystones. The lipid content of organic matter in the black shales is about ten times less than in adjacent green layers. Organic matter in both types of rocks is thermally immature, and distributions of alkanoic acids, alkanols, sterols, and alkanes contain large amounts of terrigenous components. Pristane/phytane ratios of less than one suggest that younger Turonian sediments were laid down under anoxic conditions, but ratios greater than one suggest that older Turonian Cenomanian deposits accumulated in a more oxic environment. Closely bedded green and black layers have very similar types of lipid distributions and differ primarily in concentrations, although black shales contain somewhat larger amounts of terrigenous lipid components. Geochemical and stratigraphic evidence suggests much of the organic matter in these samples originated on the African continental margin and was transported to the Angola Basin by turbidity flow. Rapid reburial of organic-carbon-rich sediments led to formation of the black shales.