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Pore-water profiles of CO2, pH, Ca2+ and O2 in situ concentrations were measured at two stations on the upper continental slope off Gabon. The present study evaluates these measurements concerning their implications for the calcium carbonate system in deep-sea sediments. The model CoTReM, which was used to simulate the dynamics of this complex geochemical system, revealed a strong dependence of calcite dissolution on oxic respiration at both sites. All simulated calcite dissolution kinetics reached a dynamic equilibrium with almost equal calcite dissolution rates, but different sub-saturation states and pH values. The latter are mainly dependent on boundary conditions and kinetic rate law parameters. Boundary conditions are of immense importance. They define which pH deviations between measured data and the simulated equilibration (instantaneous kinetics) have still to be fitted by kinetic restrictions per rate law for the equilibration. These pH deviations set up the range of possible values for rate constants in a given rate law to yield a well simulated pH. A failure in the implementation of these boundary conditions may lead to non-linearly flawed rate constants, which fit only one (usually the maximal) pH deviation well. The whole depth distribution of pH deviations has to be fitted very well by a kinetic rate law. Only this procedure secures that the used boundary conditions and rate laws/constants are acceptable. Nevertheless, several kinetic rate laws may be used for good pH fits, featuring clear differences in parameter values for rate constants and reaction orders. The connection between these rate laws with different parameters is examined and the dependence of the rate constant on the given form of the rate law is demonstrated. This study achieves rate constants of 0.038 and 18%/d for a dissolution rate law of 4.5th reaction order and sub-saturation dependence from the term (1−Ω). These results are well within the range of rate constants obtained during former field studies for the same form of the dissolution rate law.

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite δ13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate–methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, SO42− and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.

The aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity. For this purpose, 190 surface sediment samples from the South Atlantic Ocean were analysed for their barium and aluminium concentrations. Biogenic barium is estimated by subtracting the calculated terrigenous barium (obtained from the terrigenous Ba/Al ratio and the amount of Al in the sample) from the total Ba content in the sample. Based on the accumulation rates of biogenic barium, export production is estimated using three different algorithms proposed by [Paleoceanography 7 (1992) 163; Global Biogeochem. Cycles 9 (1995) 289; Geomar. Report 38 (1995) 105]. Primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters. Only the primary productions calculated on the basis of the algorithm of [Paleoceanography 7 (1992) 163] yield productivity values comparable to those existing in ocean surface waters. This study further reveals that it is not sufficient to use a constant, generally applicable organic carbon/biogenic barium ratio, as is postulated by [Global Biogeochem. Cycles 9 (1995) 289]. This ratio has to be assessed regionally. For the sediments of the Cape Basin in the eastern South Atlantic Ocean, a new algorithm is developed which gives plausible primary productivities for the overlying surface waters.

The mineralogy and geochemistry of a suite of nine manganese nodules from the South Atlantic have been determined. The Ce/La ratios of the nodules were investigated to see if they could be used as redox indicators to trace the oxygen content of the ambient water mass and the flow path of the Antarctic Bottom Water as has previously been successfully carried out in the Pacific Ocean. The Ce/La ratios of the nodules decrease in the sequence Lazarev Sea, Weddell Sea (10.4 and 9.7)>East Georgia Basin (6.5 and 7.1)>Argentine Basin (5.0), but then increase in the Brazil Basin (6.2) and Angola Basin (9.8 and 15.1). A further decrease was observed in the Cape Basin (7.6). An extremely high Ce/La ratio of 24.4 had already been determined for nodules sampled north of the Nares Abyssal Plain in the western North Atlantic. These data reflect the more complicated pattern of bottom water flow in the South Atlantic than in the South Pacific. The penetration of more oxygenated North Atlantic Deep Water into the South Atlantic accounts for the higher Ce/La ratios in the nodules from the Angola and Brazil basins. Based on this study, the flow path of the Antarctic Bottom Water could only be traced as far north as the Argentine Basin. The unique geochemistry of nodules from the central Angola Basin (high Mn/Fe and Ce/La ratios, high contents of Ni, Cu, Zn and Mo) appears to be a function of the nature of the overlying water mass and of the multiple diagenetic sources of metals to the nodules.

Pore water of marine sediments recovered from two stations of the continental slope off southwest Africa were investigated. We present computer simulations of in situ and laboratory concentration profiles of oxygen as well as laboratory concentration profiles of nitrate, calcium, pH and alkalinity. The simulations were carried out with help of a numerical model (CoTAM) to describe the transport and the reaction of dissolved species in sediments. CoTAM is based on an operator-splitting approach comprising the independent calculation of transport and chemical reaction. The consumption rates of oxygen and nitrate were determined by optimal fits to the measured pore water profiles of these species. It could be shown that measured concentrations of nitrate in pore water correspond to a decomposition of organic matter with C N ratios between 3 and 3.7. However, artificially increased subsurface nitrate concentrations due to core recovery cannot be excluded, but our results show much greater deviations from expected concentrations (assuming Redfield stoichiometry) than previously reported from comparative studies (i.e. [Martin and Sayles, 1996]). Oxygen consumption in situ was shown to be distinctively lower than measured in multicorer cores after recovery. Simulations with varying denitrification rates indicate reduced diffusive nitrate release into the bottom water by up to 50% compared to shipboard results. Effects of nitrification and denitrification on pore water pH, carbonate alkalinity and calcium concentrations were simulated by recalculating concentrations of these species with regard to calcite equilibrium. For these calculations we used the standard software PHREEQE as a subroutine of CoTAM. Calcium and carbonate alkalinity increase due to solid phase calcite dissolution. ΔpH was calculated to be an order of magnitude lower within the zone of oxygen depletion than indicated by shipboard results. This difference is mainly related to non-equilibrium conditions during pH measurements.

Two types of authigenic zeolites from Late Pleistocene, hemipelagic sediments of the South Atlantic were examined by XRD- and SEM-analysis. The XRD-characteristics and the thermal behaviour of the zeolites suggest that type 1 is composed of clinoptilolite. Type 2 was identified as mordenite, based on morphological features and qualitative elemental composition. Their authigenic nature is suggested from the shape of the crystals and their growth on and within foraminifera tests. Volcanic ash material, acting as precursor minerals to zeolite formation could not be found, nor were the zeolites directly related to siliceous remains. An authigenesis from pore waters must be assumed. Mineral stabilities for amorphous SiO2 and zeolites were calculated with the computer program solmineq88. Interstitial waters proved to be oversaturated throughout the core in respect to all zeolites considered. Biogenic opal represents the limiting solid phase for Si-concentrations in pore waters. Therefore, the dissolution of opal is regarded as the main source of Si for interstitial waters. Below 9 m core depth, where opal is only present in traces, detrital feldspars and the zeolites themselves are affected by dissolution. This may indicate the metastable nature of the zeolites, also corroborated by the presence of two, almost isochemical members of the same zeolite group.