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In this thesis early diagenetic processes in Angola Basin sediments are studied. The sediments discussed were recovered during the 1989 Angola Basin Cruise with the RIV Tyro. Pore water samples of box cores 8, 12, 17, 19,28, and 42 and of piston cores 17, 19, and 28 are presented. In addition, the solid phase of piston cores 17 and 19 was studied in detail. Chapter 2 deals with a controversial topic in the field of marine geochemistry: differences between (sub)oxic and anoxic decomposition of organic matter (OM). Pore water dissolved organic carbon (DOC) and fluorescence are used to demonstrate differences in decomposition pathways and external factors. In the oxic and suboxic redox zones low and constant contents of low molecular weight dissolved OM (LMW DOM) point to an efficient breakdown of high molecular weight (HMW) DOM into LMW components. In these redox zones, the activity of microbial grazers, the efficient respiratory chains of O2- and N03'-using bacteria, and the downcore decreases in the reactivity of OM result in the efficient breakdown of OM, with hydrolysis being the rate limiting step. In the anoxic redox zones HMW DOM concentrations increase linearly with depth suggesting a diffusion-controlled track. In the lower part of the methane-containing zone DOM becomes constant. Hypothetically, this may be due to OM decay rate limitation by fermentation. The HMW DOM is transported upwards to the suboxic/anoxic boundary. At this boundary HMW components are efficiently broken down to LMW DOM. In Chapter 3 pore water sulphate, sUlphide, and nutrients profiles are used to investigate the relative importance of sulphate reduction through OM decomposition versus that through anoxic methane oxidation. Anoxic methane oxidation was found to be the dominant sulphate-reducing process occurring in a narrow zone in Angola Basin sediments. Methane fluxes, calculated from the sulphate fluxes to the zone of anoxic methane oxidation range between 1.89 x 10'6 and 7.31 X 10'6 mol cm,2 yr'!. Nutrient fluxes indicate a deep source for methane. Several calculations show that this methane may be derived from microbial or thermic OM decomposition or tentatively from instable gas hydrates. Sulphate kinks occur between 3 and 10 m depth, not only in our cores but also in several other reported sediments. Four potential processes to explain the occurrence of these kinks are discussed: 1) bioturbation/bio-irrigation, 2) a non-steady state process caused by turbidites or erosion, 3) a non-steady state situation caused by variations in CH4-fluxes from below, and 4) pore water sulphide oxidation at the depth of the kink. At present, the best scenario available is a non-steady state response to variations in the methane flux from below. Chapter 4 explores the Fe and Mn chemistry in Angola Basin sediments. Relative amounts of solid phase Fe- and Mn-minerals were estimated using a sequential extraction scheme. Additionally, pore water data of Fe and Mn, and delta-34-Svalues of pore water SO/" HS-, and of pyrite were used to investigate suboxic diagenesis, pyritisation and authigenesis of Fe and Mn-minerals. Pyrite formation is the most important Fe-involved diagenetic process in the sediments discussed. The bulk of the pyrite in the upper parts of the sediments had been formed in the past, in an open system at one location and in a closed system at another. Present day pyritisation occurs in a closed system at much lower rates. Low values of acid volatile sulphur (AVS) compared to pyrite indicate an efficient transformation of FeS to FeS2 • The pyritisation occurs in three zones. In the upper and lower reaction zones pyrite formation is limited by the supply of HS- into these zones. In contrast, pyrite formation is Fe-limited in the HS--containing interval between these zones. Significant amounts of Mn appear to coprecipitate with pyrite, in a constant proportion to Fe. At the bottom of both cores sequential extraction results indicate authigenic carbonate formation. These carbonates contain Ca, Mg, Fe, and Mn, that may represent phases like ankerite, siderite, and dolomite. In Chapter 5 several controls on the bulk isotopic and elemental OM composition, such as depositional regime, climate, and diagenesis, are discussed. Sediments in the lower parts of the cores are turbiditic, whereas those in the the upper parts are mainly controlled by pelagic sedimentation. The OM in turbidites has distinctly higher (C/N) and more negative 613C values than the OM in pelagic samples, showing its more pronounced terrestrial origin. The effect of anoxic diagenesis on the amount and composition of OM, albeit subordinate to oxic decomposition, is significant. It results in a depletion of a Nand P-rich fraction, and a slight enrichment in 13Corg' However, qualitative mixing trends between marine and terrestrial OM components, which are climate-controlled, seem unaffected by OM decomposition. Variations in the marine OM fluxes are controlled by variations in the productivity in the surface ocean, the highest productivities being found during glacials. These variations appear to be larger than those in terrestrial OM fluxes, resulting in dilution of the terrestrial signals by the marine ones at both locations. Nonetheless, relatively N-, P-, and 12C-depleted terrestrial OM fluxes also show a glacial/interglacial pattern. This pattern is controlled by variations in vegetation of the drainage area of the river Congo, the position of the coastline, erosion, OM recycling, and biomass.

Marine sedimentary records may contain important information on environmental changes in the past. Modellers of global changes need such information to make a more accurate description of past climates, and to predict more precisely possible future climate changes. Signals in sediments can be divided in two groups: I. primary signals caused by variations in terrigenous and biological inputs, occurring before and during sediment deposition and 2. secondary signals, which are formed after deposition. These secondary signals form as a result of variation in primary signals and external changes, such as diagenetic changes or tectonic influences. This thesis deals with the cause of primary and secondary signals and their interaction, in marine sediments from the Mediterranean and North-Atlantic. In the Mediterranean, especially the eastern part, organic rich layers are found that are intercalated in hemipelagic "normal" marine sediment. These layers, "sapropels", have aroused tremendous interest in the scientific community because of their alleged similarities to Cretaceous Black Shales. In chapter 2, one of the primary signals in sapropels, 8180, is reassessed. Depletions in 81SO, coinciding with sapropels, are important evidence for the development of a low salinity surface layer. Such a layer is a prerequisite for a reversal of circulation in the Mediterranean, which has been suggested to be the cause for sapropel formation. We show that this is not necessarily true, because these depletions can as well be explained within the present-day circulation pattern of the Mediterranean.

Marine sedimentary records may contain important information on environmental changes in the past. Modellers of global changes need such information to make a more accurate description of past climates, and to predict more precisely possible future climate changes. Signals in sediments can be divided in two groups: I. primary signals caused by variations in terrigenous and biological inputs, occurring before and during sediment deposition and 2. secondary signals, which are formed after deposition. These secondary signals form as a result of variation in primary signals and external changes, such as diagenetic changes or tectonic influences. This thesis deals with the cause of primary and secondary signals and their interaction, in marine sediments from the Mediterranean and North-Atlantic. In the Mediterranean, especially the eastern part, organic rich layers are found that are intercalated in hemipelagic "normal" marine sediment. These layers, "sapropels", have aroused tremendous interest in the scientific community because of their alleged similarities to Cretaceous Black Shales. In chapter 2, one of the primary signals in sapropels, 8180, is reassessed. Depletions in 81SO, coinciding with sapropels, are important evidence for the development of a low salinity surface layer. Such a layer is a prerequisite for a reversal of circulation in the Mediterranean, which has been suggested to be the cause for sapropel formation. We show that this is not necessarily true, because these depletions can as well be explained within the present-day circulation pattern of the Mediterranean.

Anoxic hypersaline basins have been found in two different tectonic environments in the eastern Mediterranean. Within the Tyro area (the western Strabo Trench) there are three pull apart basins: the Tyro Basin, presently filled with anoxic hypersaline bottomwater, and the Poseidon and Kretheus Basins, thought to have been anoxic and filled with hypersaline water in the past. The Mediterranean Ridge shows a "cobblestone topography" (Camerlenghi, 1990). The Bannock Basin is the largest depression found within this area and is divided into nine sub-basins. Tectonic deformations of Messinian evaporites are thought to be the major processes involved in the formation of these anoxic hypersaline basins. The interstitial water chemistry of sediments from these basins have been studied. The fact that the Poseidon and Kretheus Basins have been filled with hypersaline bottomwater is reflected by an increase of the CI and Na concentration in the pore waters downcore. These brines are thought to have been similar to that of the Tyro Basin. A diffusionadvection model has been applied to the CI profiles. From time calculations it was possible to estimate the time elapsed since hypersaline conditions in the Poseidon and Kretheus Basins changed into normal saline conditions. A renewal time between 2000 - 3000 years was found. Organic matter reaching the sediments is decomposed by sulphate reducing bacteria. Despite the high salinity (260 %0) in the Tyro and Bannock Basins, sulphate reduction rates are higher than in the sediments from the Poseidon and Kretheus Basins. These higher sulphate reduction rates could be related to a better "preservation" of the organic matter in the anoxic brine. The release of inorganic metabolites, such as ammonium, alkalinity, phosphate and sulphide leads to the formation of authigenic minerals in these sediments such as dolomite, gypsum and sulphides. The sulphur chemistry in the anoxic hypersaline sediments from the Tyro and Bannock Basins have been studied in detail. Despite the difference in major element chemistry of the brines (De Lange et aI., 1990a) their sulphur chemistry in the sediments is very similar. The following sulphur species have been determined and quantified: elemental sulphur, Acid Volatile Sulphur (AVS), organic polysulphides, humic sulphur (0.5 M NaOH extractable) and pyritic sulphur. Pyritic sulphur was found to be the main phase of the inorganic reduced sulphur (50 - 80% of the total sulphur pool) and was at the same level (250 pmoles per gram dry weight) in both cores. Remarkably, humic sulphur was found to account for 17 to 28% of the total sulphur pool in the Tyro Basin and for 10 - 43 % in the Bannock Basin. Sulphur isotope data show negative 634S values for both pyritic sulphur (-16 to -40 %0) and humic sulphur (-16 to -30 %0). Pyritic and humic sulphur are thought to be mainly formed at the interface of oxic seawater and anoxic hypersaline bottom water. At this interface material is captured due to the large density difference. Higher amounts of particulate and dissolved organic carbon have been observed at these interfaces as well as sulphate reduction and sulphide oxidation processes. Turbiditic deposition transports pyritic and humic sulphur compounds to the sediments in the Tyro and Bannock Basins.