Early diagenetic processes in sediments of the Angola Basin, eastern South Atlantic

Abstract

1.5 In this thesis early diagenetic processes in Angolan Basin sediments are studied.The sediments discussed were recovered during the 1989 Angola Basin Cruise with the R/V 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 NO3- 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 concentrationsincrease 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 l0 to the power of -6 and 7.31 x l0 to the power of -6 mol cm-2 yr-1. 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 l0 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 d34S values of pore water SO42-, 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 upper parts are mainly controlled by pelagic sedimentation. The OM in turbidites has distinctly higher (C/N) and more negative d13C 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 N and 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