Description: Report on effects of changing predation pressure on benthic and pelagic species

Description: In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (32S, 34S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargarita namibiensis which oxidize sulfide (H2S) to sulfate () using sea water nitrate () as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between and H2S driven by coupled SR and SOx at rates exceeding 6.0 mol S m−2 y−1. More than 96% of the SR is supported by SOx, and only 2–3% of the pool diffuses directly into the sediment from the sea water. A fraction of the produced by Thiomargarita is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H2S production is trapped as sedimentary sulfide, mainly pyrite (FeS2) and organic sulfur (Sorg) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (〈1%). Yet, despite intense SR, FeS2 and Sorg show an isotope composition of ∼5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H232S from the pore water. A fractionation of 20–30‰ was estimated for SOx (εSOx) with the model, along with a maximum fractionation for SR (εSR–max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for εSOx (5‰) and εSR (78‰).

Description: Three sediment stations in Himmerfjärden estuary (Baltic Sea, Sweden) were sampled in May 2009 and June 2010 to test how low salinity (5–7 ‰), high primary productivity partially induced by nutrient input from an upstream waste water treatment plant, and high overall sedimentation rates impact the sedimentary cycling of methane and sulfur. Rates of sediment accumulation determined using 210Pbexcess and 137Cs were very high (0.65–0.95 cm year−1), as were the corresponding rates of organic matter accumulation (8.9–9.5 mol C m−2 year−1) at all three sites. Dissolved sulfate penetrated 〈20 cm below the sediment surface. Although measured rates of bicarbonate methanogenesis integrated over 1 m depth were low (0.96–1.09 mol m−2 year−1), methane concentrations increased to 〉2 mmol L−1 below the sulfate–methane transition. A steep gradient of methane through the entire sulfate zone led to upward (diffusive and bio-irrigative) fluxes of 0.32 to 0.78 mol m−2 year−1 methane to the sediment–water interface. Areal rates of sulfate reduction (1.46–1.92 mol m−2 year−1) integrated over the upper 0–14 cm of sediment appeared to be limited by the restricted diffusive supply of sulfate, low bio-irrigation (α = 2.8–3.1 year−1), and limited residence time of the sedimentary organic carbon in the sulfate zone. A large fraction of reduced sulfur as pyrite and organic-bound sulfur was buried and thus escaped reoxidation in the surface sediment. The presence of ferrous iron in the pore water (with concentrations up to 110 μM) suggests that iron reduction plays an important role in surface sediments, as well as in sediment layers deep below the sulfate–methane transition. We conclude that high rates of sediment accumulation and shallow sulfate penetration are the master variables for biogeochemistry of methane and sulfur cycling; in particular, they may significantly allow for release of methane into the water column in the Himmerfjärden estuary.

Description: During Ocean Drilling Program Leg 175, 40 holes were drilled at 13 sites, and 8003 m were recovered with the aim of reconstructing the late Neogene history of the Angola-Benguela upwelling system on the southwest African margin. This system is one of the great upwelling regions of the world and is characterized by organic-rich sediments that provide an excellent record of productivity back to the middle Miocene. Understanding the sedimentology and stratigraphy of these sediments will provide important new information on the paleoceanography of this region, including the complex role the ocean plays in global carbon cycling and climate change. This contribution summarizes the sedimentologic and stratigraphic data compiled on board the JOIDES Resolution between 12 August and 10 October 1997. The 13 drill sites can be divided into four regions based on sediment type and composition: (1) the Lower Congo Basin, (2) the Angola Basin, (3) the Walvis Ridge and Basin, and (4) the Cape Basin. The stratigraphy of each region is distinct and records the competing influences of current regime, shelf topography, and proximity to major river systems (both ancient and modern) and upwelling centers. The Lower Congo Basin is a hemipelagic environment containing finegrained sediments derived from the Congo River. Sedimentation within the Angola Basin is dominated by rain-out of hemipelagic silts and clays derived from coastal erosion and from the Kunene River to the south. Sediments within the Walvis Ridge and Basin consist of carbonate oozes and organic-rich clays, which record a strong upwelling signal from the Benguela Current. Deposition in the Cape Basin is dominated by pelagic settling of biogenic debris at the most southern tip of the Benguela Current upwelling center.

Description: Published