Sabine Kasten

DEEP SULFATE REDUCTION COMPLETELY MEDIATED BY ANAEROBIC METHANE OXIDATION IN SEDIMENTS OF THE UPWELLING AREA OFF NAMIBIA

Abstract Porewater concentrations of sulfate, methane, and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off Namibia which were recovered from the continental slope at water depths of 1300 and 2000 m. At all four stations a distinct sulfate-methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed. Nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones. Flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100% of deep sulfate reduction within the sulfate-methane transition zone and consumes the total net diffusive sulfate flux. A significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could, therefore, be excluded. We state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction.

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.

North Atlantic Deep Water export to the Southern Ocean over the past 14 Myr: Evidence from Nd and Pb isotopes in ferromanganese crusts

The intensity of North Atlantic Deep Water (NADW) production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate. Here we present a new approach to reconstruct the overall strength of NADW export from the North Atlantic to the Southern Ocean over the past 14 Myr applying the deep water Nd and Pb isotope composition as recorded by ferromanganese crusts and nodules. We present the first long-term Nd and Pb isotope time series for deep Southern Ocean water masses, which are compared with previously published time series for NADW from the NW Atlantic Ocean. These data suggest a continuous and strong export of NADW, or a precursor of it, into the Southern Ocean between 14 and 3 Ma. An increasing difference in Nd and Pb isotope compositions between the NW Atlantic and the Southern Ocean over the past 3 Myr gives evidence for a progressive overall reduction of NADW export since the onset of Northern Hemisphere glaciation (NHG). The Nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37% depending on location.

Simultaneous formation of iron-rich layers at different redox boundaries in sediments of the Amazon Deep-Sea Fan

High resolution examinations of the solid phase have been performed on a gravity core that shows the typical Amazon fan stratigraphy of the last 20,000 years. The sedimentary record is characterized by two pronounced enrichments of solid phase Fe. The first Fe peak, from 0.34 to 0.37 m sediment depth, consists of an indurated Fe oxyhydroxide crust, located close to the Pleistocene/Holocene boundary. This peak represents a widespread, nonsteady-state, diagenetic feature on the fan. The second Fe peak is a pronounced enrichment of authigenic Fe sulfides, located between 6.15 and 6.45 m in the sediment. The formation of this maximum is attributed to sulfate reduction that is mainly driven by anaerobic methane oxidation. Based on SO42− pore water profiles and calculations of the time of formation of the Fe sulfide enrichment, we conclude that both Fe-rich layers formed more or less simultaneously. The condition that caused both Fe enrichments is a period of nonsteady-state diagenesis, induced by a pronounced decrease in sedimentation and organic carbon accumulation rates between the Pleistocene and the Holocene. During this depositional change, the redox boundaries and reaction fronts were fixed at particular sediment levels for a prolonged time, thus producing higher concentrations of the authigenic minerals than possible under steady-state diagenetic conditions.

Global rates of marine sulfate reduction and implications for sub–sea-floor metabolic activities

Sulfate reduction is a globally important redox process in marine sediments, yet global rates are poorly quantified. We developed an artificial neural network trained with 199 sulfate profiles, constrained with geomorphological and geochemical maps to estimate global sulfate-reduction rate distributions. Globally, 11.3 teramoles of sulfate are reduced yearly (~15% of previous estimates), accounting for the oxidation of 12 to 29% of the organic carbon flux to the sea floor. Combined with global cell distributions in marine sediments, these results indicate a strong contrast in sub–sea-floor prokaryote habitats: In continental margins, global cell numbers in sulfate-depleted sediment exceed those in the overlying sulfate-bearing sediment by one order of magnitude, whereas in the abyss, most life occurs in oxic and/or sulfate-reducing sediments. Up to 89% of microbial cells in the sub–sea floor at continental margins are sustained by fermentation and methanogenesis Mapping sub–sea-floor communities The sea floor is teeming with microbes, whose sheer numbers produce a major effect on the global biogeochemical cycles of carbon, sulfur, and other important nutrients. Bowles et al. constructed a map showing how deeply sulfates penetrate marine sediments worldwide and how quickly that sulfate is chemically reduced by microbes in the sub–sea-floor. Globally, almost a third of the organic carbon that reaches the sea floor is consumed during sulfate reduction, and the vast majority of microbial cells in the sub–sea-floor at continental margins get their energy through the biochemical processes of fermentation and methanogenesis. Science, this issue p. 889.

Sulfate Reduction in Marine Sediments

The present chapter deals with the biogeochemical transformations of sulfur within marine sediments during early diagenesis. The term ‘early diagenesis’ refers to the whole range of postdepositional processes that take place in aquatic sediments coupled either directly or indirectly to the degradation of organic matter. We focus on the processes that drive sulfate reduction together with the manifold associated biotic and abiotic reactions that make up the sedimentary sulfur cycle. Furthermore, we will give an overview of the quantitative significance of microbial sulfate reduction in the remineralization of organic matter and oxidation of methane in different depositional environments and discuss the often observed discrepancy between sulfate reduction rates deduced from radiotracer methods and those calculated from pore-water concentration profiles and/or solid-phase sulfur data.

Barium peaks at glacial terminations in sediments of the equatorial Atlantic Ocean—relicts of deglacial productivity pulses?

In sediment cores recovered from oligotrophic settings of the western and eastern north-equatorial Atlantic Ocean, Ceara Rise and Sierra Leone Rise, distinct peaks of excess barium were detected at glacial/interglacial transitions. These Ba maxima are unrelated to any other potential productivity proxy, e.g. organic carbon, carbonate or opal. Moreover, they coincide with minima in organic carbon contents. Despite the lack of correlation between excess barium and organic carbon, we ascribe these Ba spikes to pulses of productivity that occurred during glacial/interglacial transitions. The discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward-progressing oxidation fronts during deglacial nonsteady-state depositional conditions. The oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced NADW production during interglacial periods. The fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short-time productivity events. This assumption is supported by the depth arrangement of the solid phase peaks of Ba, Fe, and Mn along the 6/5 oxygen isotope stage boundary in sediments of the Ceara Rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern Mediterranean sediments. It remains unclear whether productivity was proportional to the magnitude of the Baexcess maxima. Possibly, higher dissolved Ba concentrations in intermediate and deep waters during glacial/interglacial transitions caused the precipitation of barite to increase out of proportion of the (postulated) deglacial productivity pulses.

Biogenic Barium as a Proxy for Paleoproductivity: Methods and Limitations of Application

The obvious link between sedimentary barium and past and present ocean productivity has propagated numerous studies on this tracer in the past years. In this paper a summary is given on what is known of the generation of the barium signal and its link to the flux of organic carbon. We describe procedures to assess barium contents, distinguish biogenic from detrital barium and calculate barium accumulation rates. Two approaches to estimate absolute paleoproductivity rates from barium contents are described and compared. A considerable part of the paper is dedicated to synsedimentary and early diagenetic constraints involved in the application of the barium method. It is demonstrated that the most significant diagenetic overprinting of the barium signal occurs in sulfate depleted sediments by dissolution of barite. Examples from selected sites in the Atlantic Ocean show that barium may be a reliable proxy in this part of the world ocean to assess changes in ocean productivity, whereas other proxies are restricted to certain sedimentary environments. Recent studies suggest that for the oligotrophic areas of the Atlantic Ocean barium might be a more reliable tracer for past productivity than organic carbon. This is attributed to the action of oxidation fronts which resulted in a very efficient degradation of the organic matter initially present within the sediment. Nevertheless, quantitative paleoproductivity reconstructions are based on regional studies and future investigations must be carried out in a range of environments before a worldwide application can be obtained.

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.

A novel, multi-layered methanotrophic microbial mat system growing on the sediment of the Black Sea

A novel microbially diverse type of 1- to 5-cm-thick mat performing anaerobic oxidation of methane (AOM) and covering several square metres of the seafloor was discovered in the Black Sea at 180 m water depth. Contrary to other AOM-mat systems of the Black Sea these floating mats are not associated to free gas and are not stabilized by authigenic carbonates. However, supply of methane is ensured by the horizontal orientation of the mats acting as a cover of methane enriched fluids ascending from the underlying sediments. Thorough investigation of their community composition by molecular microbiology and lipid biomarkers, metabolic activities and elemental composition showed that the mats provide a clearly structured system with extracellular polymeric substances (EPS) building the framework of the mats. The top black zone, showing high rates of AOM (15 mumol g(dw) (-1) day(-1)), was dominated by ANME-2, while the following equally active pink layer was dominated by ANME-1 Archaea. The lowest AOM activity (2 mumol g(dw) (-1) day(-1)) and cell numbers were found in the greyish middle part delimited towards the sediment by a second pink, ANME-1-dominated and sometimes a black outer layer (ANME-2). Our work clearly shows that the different microbial populations are established along defined chemical gradients such as methane, sulfate or sulfide.