Kerstin Pfeifer

Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments

Abstract Gravity driven mass-flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western Argentine Basin. In this study we present geochemical data from a total of 23 gravity cores. Pore-water SO 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane (AOM). The different shapes of SO 4 profiles (concave, kink- and s-type) can be consistently explained by sedimentary slides possibly in combination with changes in the CH 4 flux from below, thus, mostly representing transient pore-water conditions. Since slides may keep their original sedimentary signature, a combined analysis and numerical modeling of geochemical, physical properties, and hydro acoustic data could be applied in order to reconstruct the sedimentary history. We present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day. The results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep-sea, depositing high amounts of reactive compounds. The high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by AOM. For the total region we estimate a SO 4 (or CH 4 ) flux of 6.6 × 10 10 moles per year into the zone of AOM. Projected to the global continental slope and rise area, this may sum up to about 2.6 × 10 12 moles per year. Provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1.2 × 10 12 moles per year as previously estimated. Thus, AOM obviously contributes very significantly to the regulation of global sulfur reservoirs, which is hitherto not sufficiently recognized. This finding may have implications for global geochemical models, as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time.

Biogenic barium and the detrital Ba/Al ratio: a comparison of their direct and indirect determination

Biogenic barium concentrations obtained from direct determination by a three-step sequential extraction procedure are compared to those obtained from the widely used indirect normative calculation based on total digestion. A comparison of the biogenic barium from the direct/sequential extraction and the indirect/normative calculation clearly shows that the detrital Ba/Al ratio is the critical factor in the normative approach, and that erroneous assumptions based on this ratio may introduce significant errors to the calculated biogenic barium. Overall, the crustal average Ba/Al ratio of 0.005–0.01 is much higher than our directly determined global average of 0.0037. This would result in an underestimation of the biogenic barium and thus of the primary productivity calculated using the Ba-flux obtained from the normatively calculated Ba record. Using our (Ba/Al)det ratio of ∼0.0037 leads to normatively calculated biogenic barium results that are in reasonable agreement with the biogenic barium from sequential extraction for samples of the Atlantic, Pacific and Indian Ocean. Our directly determined ‘regional’ (Ba/Al)det ratios deviate from those calculated or assumed from hinterland ratios and our global average (Ba/Al)det is lower than the one commonly reported. Therefore, in sediments with a significant terrigenous fraction, the sequential extraction technique is always required.

Reconstruction of primary productivity from the barium contents in surface sediments of the South Atlantic Ocean

Abstract 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.

Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Abstract Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3−, NH4+, Fe2+, and Mn2+ and SO42− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, NO3−, Fe(OH)3 and SO42−, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm−2 yr−1, of which 77% were due to O2, 17% to NO3− and 3% to Fe(OH)3 and 3% to SO42−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d−1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m−2 yr−1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m−2 yr−1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.

Modeling of calcite dissolution by oxic respiration in supralysoclinal deep-sea sediments

Abstract 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.

Fluxes at the Benthic Boundary Layer — A Global View from the South Atlantic

Fluxes between the ocean waters and the sediments are key regulation processes for the marine biogeochemical cycles and, thus, their quantification is of crucial importance. At this transition it is ultimately determined how much of a primary particulate signal is preserved or mineralized and hence recycled. Our review summarizes two major approaches how to use spatial information obtained from surface sediments: (1) In the first part we summarize the state-of-the-art regarding the use of biogenic barium as a proxy for primary productivity. We discuss the possibilities and limitations of this approach mainly based on the results of a recent study in the South Atlantic. The general outcome of this study was that the spatial pattern of primary productivity can well be traced back by calculating (sub-)recent accumulation rates of biogenic barium and applying available and newly formulated empirical equations. Most of those equations, however, fail to give the really observed magnitude of today’s productivity values. The main reasons for this are mostly the uncertainty of the Corg/Babio depth relation, which differs between distinct ocean regions, dynamic sedimentary processes at ocean margins combined with badly constrained values of terrigeneous barium input, and the effect of barite dissolution due to subsequent anoxic diagenesis. To improve the quality of prognoses for past productivity multi proxy approaches are recommended to bypass the uncertainty in predictions from a single proxy. (2) The more extensive second part is based on the large amount of studies that aimed at the quantification of benthic fluxes of nutrients and oxygen, which are good measures for the amount of reactive particulate material being mineralized at the seafloor and thus returned into the marine cycle. Those results enabled us to give profound calculations of the benthic oxygen consumption and the release of nitrate, phosphate, and silicate at the seafloor of the South Atlantic and give upscaled estimates for the global area of the sea floor. Additionally, we discuss more detailed studies focusing on control parameters for benthic fluxes like primary production and lateral advection along the ocean margins off Southwest Africa and Argentina. A very conspicuous result was obtained by calculating mass balances for biogenic opal in those regions indicating a dramatic underestimation of accumulation fluxes of opal by “conservative” methods, which is believed to be of global significance. The last section mainly focuses on the effect of benthic mineralization on the dissolution of calcium carbonate even above the chemical lysocline. This process is in discussion since more than two decades. A number of studies have been performed, mainly using in situ devices, to determine CaCO3 dissolution. We summarize and discuss the results obtained from the South Atlantic and use a recently developed empirical algorithm to show the worldwide distribution of supralysoclinal CaCO3 dissolution fluxes in marine surface sediments and give an estimate of their total amount. Finally, a table for benthic fluxes of major constituents is provided on ocean wide and global scales.