Namibian Shelf Research Articles

Seasonal to interannual variability of water mass characteristics and currents on the Namibian shelf

We present long-term current meter records from the Benguela system together with salinity and temperature observations gathered by a mooring on the Namibian shelf across 13 years (2002–2015). From this unique data set a climatological mean state is estimated enabling us to investigate seasonal to interannual variations of these variables on the Namibian shelf. The present study highlights the importance of the alongshore advection for the water mass characteristics in the Benguela system on a seasonal time scale. The annual cycle of the alongshore transport is characterized by a biannual flow reversal. Poleward directed currents dominate from October to April, and from May to September equatorward currents prevail. In addition, we present observational evidence for a biannual intrusion of tropical waters into the Benguela system with maxima in October and February.Based on the in situ temperature data, several anomalous events are described that affect the whole water column. During the outstanding warm event in austral fall 2011 the monthly temperature anomaly exceeds one Kelvin for five consecutive months peaking in March (2.4 K) in the upper layer of the water column. Our study suggests, that the occurrence of such extreme temperature events in the Benguela upwelling system is closely related to the strength of the alongshore advection in austral summer.

Effects of current regimes and oxygenation on particulate matter preservation on the Namibian shelf: Insights from amino acid biogeochemistry

Many reconstructions of past biogeochemical states rely on proxies such as δ15N that in turn are affected by the preservation state of organic matter. N-turnover processes in the upwelling system on the Namibian shelf are difficult to reconstruct because lateral particle advection and differential degradation under oxic to anoxic conditions complicate particle tracing in suspended matter and surface sediments as well as the interpretation of sedimentary δ15N records. We analysed the amino acid composition of suspended matter, sinking particles, phytoplankton biomass and surface sediments in order to investigate the degradation pathway of organic matter from the sea surface to the surface sediment. A principal component analysis carried out with the amino acid data set results in a triangular-shaped model. Two branches evolve from phytoplankton biomass on the apex: The suspended matter branch is characterized by relatively enhanced contents of Glu, Ser and Leu, whereas the non-protein amino acids and Lys mark the sinking particle and sediment branch. The results suggest that both pools have taken different diagenetic pathways and that there is only limited exchange between them. The preservation state of organic matter in surface sediments within the oxygen minimum zone is generally better than on the oxic outer shelf, where organic matter preservation depends on water depth.A vertical increase in δ15N ratios from phytoplankton biomass to the surface sediment by ~2.5‰ is attributed to early diagenetic degradation in the water column and at the water/sediment interface. The δ15N ratios of sediments are mainly controlled by the N-isotope signature of nitrate on the outer shelf and upper continental slope. Nutrient-enriched deep water ascends at the shelf breaks due to the influence of internal waves and tides or curl-driven shelf break upwelling. The δ15N of upwelling nitrate at the shelf breaks is ~3‰ lower than that of nitrate from the inner shelf. This difference in nitrate sources is expressed in a band of decreasing δ15N ratios of surface sediments at the shelf breaks. The hydrodynamic upwelling regime at the shelf breaks flushes significant amounts of suspended matter from the mesopelagic ocean across the shelf break as indicated by amino acid monomer patterns of suspended matter.

Phosphorite deposits on the Namibian shelf

Phosphorite deposits provide a source of fertilizer to feed an increasingly populated world and they provide a record of paleoceanographic changes in upwelling systems linked to climate. The Benguela Upwelling System (BUS) is among the most productive today and is associated with major phosphorite deposits exposed over an area of 24,700km2 on the Namibian shelf. Analyses of cores associated with recent offshore mineral exploration provide new insights into the age and origin of these phosphorite deposits. The deposit consists of coarsening upward muddy to gravelly pelletal phosphorite sand, up to several meters thick, on the middle to outer shelf (190 and 350m water depth) offshore of Lüderitz and Walvis Bay. Less extensive and less continuous deposits occur offshore of Walvis Bay as far north as the Kunene River mouth on the inner to middle shelf (50 and 250m water depth). Pelletal phosphorite sand (some concentrically banded) and concretionary phosphorite pebbles are the dominant grain types consisting of up to 90wt.% carbonate fluorapatite (francolite) cement and inclusions of organic matter, pyrite and terrigenous mud. Strontium isotope stratigraphy and foraminiferal biostratigraphy indicate that phosphogenesis was initiated in the latest Miocene but that most phosphorite formed in the Plio/Pleistocene during early burial diagenesis of organic-rich mud as it does today in the Holocene diatomaceous mud belt. The highly-condensed, coarsening-upward succession reflects increasingly high-amplitude Pleistocene sea-level fluctuations. Phosphorite formation correlates to terrestrial aridification as well as to marine proxies of intensified coastal upwelling in the 600m thick equivalent successions on the upper slope. Repeated phosphorite formation and reworking over Pleistocene glacial to interglacial cycles resulted in the economic concentration of phosphorite, with an estimated total resource of 7800milliontons of phosphate rock at an average grade of 19wt.% P2O5 on the Namibian shelf.

Reproductive tactics of male bearded goby (Sufflogobius bibarbatus) in anoxic and hypoxic waters

The bearded goby (Sufflogobius bibarbatus), a key species in the northern Benguela Upwelling Ecosystem, tolerates extremely low levels of oxygen. Yet little is known about how its reproduction is affected by these harsh living conditions. The distribution patterns of alternative reproductive tactics of male bearded goby across the continental shelf off Namibia were investigated. Histology and stereology were for the first time used to validate macroscopic maturity development by estimating volume fraction of the different stages of spermatogenesis using “Delesse principle”, an approach so far for teleosts barely used in studies on testes but applied in advanced oocyte estimation. The macroscopic scale appeared suitable for the purpose, and the prevalence of territorial and sneaker tactics could therefore be documented. The sneakers had relative large testes and small seminal vesicles (SV), with the opposite being the case for the territorials. A third, numerous category with intermediate sized testes and SV was also recognized with unclear underlying tactical rationale, although regression analyses indicated similar investment in testes weight in relation to somatic weight as for the territorials. Low oxygen levels were the most important factor limiting spawning activity in territorial males. Our data suggested the existence of a spawning site on the outer shelf of the central Namibian shelf where the bottom water is hypoxic (oxygen saturation of 6.2–6.7%) while the anoxic middle shelf area (oxygen saturation of 1.7–2.9%) appeared to show too low oxygen levels for spawning to take place. Hence, significant parts of this large shelf area appear unsuited for successful reproduction of the bearded goby, in particular for nest building by the territorials.

Benthic remineralisation rates in shelf and slope sediments of the northern Benguela upwelling margin

The Benguela Upwelling System off Namibia is a region of intensive plankton production. Remineralisation of this biomass frequently causes the formation of an oxygen minimum zone. A part of the organic matter is further deposited on the broad shelf in form of an extensive mudbelt with high TOC concentrations. During February 2011 we retrieved sediment samples from shelf and slope sediment along the Namibian coast to establish fluxes of nutrients, oxygen, and N2 on the basis of pore water concentrations. In mudbelt sediment, fluxes were estimated as high as 8mmol NH4+ m−2d−1 and 0.9mmol PO43− m−2d−1, which is probably attributable to the activity of large sulphur bacteria. Especially phosphate is mobilised from sediment overlain by oxygen deficient bottom water when and where bottom water oxygen concentrations fall below 50µmoll−1. In comparison to nutrient transport by Southern Atlantic Central Water flowing onto the Namibian shelf, benthic nutrient fluxes of the mudbelt contribute less than 5% to the nutrient budget of the shelf.

Ammonia-oxidizing Bacteria of the Nitrosospira cluster 1 dominate over ammonia-oxidizing Archaea in oligotrophic surface sediments near the South Atlantic Gyre.

SummarySediments across the Namibian continental margin feature a strong microbial activity gradient at their surface. This is reflected in ammonium concentrations of < 10 μM in oligotrophic abyssal plain sediments near the South Atlantic Gyre compared with ammonium concentrations of > 700 μM in upwelling areas near the coast. Here we address changes in apparent abundance and structure of ammonia‐oxidizing archaeal and bacterial communities (AOA and AOB) along a transect of seven sediment stations across the Namibian shelf by analysing their respective ammonia monooxygenase genes (amoA). The relative abundance of archaeal and bacterial amoA (g−1 DNA) decreased with increasing ammonium concentrations, and bacterial amoA frequently outnumbered archaeal amoA at the sediment–water interface [0–1 cm below seafloor (cmbsf)]. In contrast, AOA were apparently as abundant as AOB or dominated in several deeper (> 10 cmbsf), anoxic sediment layers. Phylogenetic analyses showed a change within the AOA community along the transect, from two clusters without cultured representatives at the gyre to N itrososphaera and N itrosopumilus clusters in the upwelling region. AOB almost exclusively belonged to the N itrosospira cluster 1. Our results suggest that this predominantly marine AOB lineage without cultured representatives can thrive at low ammonium concentrations and is active in the marine nitrogen cycle.

The geochemical similarity of Oligocene and recent phosphorites from the Chiatura deposit (Georgia) and the Namibian shelf

Detailed studies of the distribution of macro- and microelements in phosphorites associated with Oligocene manganese ores of the Chiatura deposit revealed their geochemical similarity to the Late Quaternary phosphorites on the recent shelf of Namibia in the Atlantic Ocean. This shows that the Chiatura nodular phosphorites were formed in the shallow-water bioproductive zone of the Oligocene basin by biogenic supply of phosphorus onto the bottom and its subsequent diagenetic concentrating similarly to the granular-nodular phosphorites on the shelf of the recent ocean. The Chiatura phosphorites were later subjected to the epigenetic impact of hydrogeological processes, which modified the initial composition of the contained rareearth elements.

Distribution of zooplankton biomass and potential metabolic activities across the northern Benguela upwelling system

The distribution of zooplankton biomass and potential metabolic rates, in terms of electron transport system (ETS) and glutamate dehydrogenase (GDH), were analyzed along a cross-shelf transect in waters off Namibia. The highly variable dynamics of upwelling filaments promoted short-term fluctuations in the zooplankton biomass and metabolism. Maximum values were characteristically found over the shelf-break, where zooplankton biomass as dry mass (DM) reached peaks of 64.5mgm−3 within the upper 200m in late August. Two weeks later, the zooplankton-DM decreased by more than a third (19mg DM m−3). Zooplankton potential respiration and NH4+ excretion averaged 234μmol O2 m−3 d−1 and 169μmol NH4+m−3 d−1 in the Namibian shelf, respectively. High protein-specific ETS activities even in the low-chlorophyll waters outside the filament suggested a shift into greater omnivory seaward. In this light, zooplankton elemental and isotopic compositions were used to investigate the pelagic food web interactions. They evidenced spatial changes in the carbon resource for zooplankton as well as changes in the form of nitrogen that fueled the biological production in aging advected waters. Overall, both aspects of zooplankton metabolism impacted the primary productivity at a level less than 10% under all the different oceanographic conditions.

Distribution and paleoecology of molluscan skeletal remains along an upwelling tract: Benguela system, Namibian shelf

Abstract This study describes and categorizes the preservable, and thus predominantly molluscan, component of the macrofauna death assemblages of sub-environments within the northern Benguela upwelling system. This is especially important for assembling actualistic data that can later serve for identifying similar settings in the geological record. A total of 16 samples were examined. Contrary to stereotypes, benthic communities are present but highly variable across the high-productivity tract and are characterized by distinctive community structures. Benguela system document a decrease in body size of chemosymbiotic bivalves (lucinids) and percent of chemosymbionts and deposit-feeders, and an overall increase in species richness and epifaunal-infaunal ratio. This trend is in accordance with the upwelling facies-gradient from diatomaceous ooze in areas of maximum upwelling intensity outward to the aerated but food-abundant carbonate oozes and spatially coexisting phosphorite concentrations. Differences in seafloor oxygen concentration along with variable food availability played major roles in structuring the death assemblages under the Benguela upwelling system. The ecologic data provide ways to distinguish between facies of low and high shell production along an increasing oxygen gradient, and also between assemblages with moderate and higher species richness associated with time-averaging. The data support the existence of a wider range of time averaging in the formation of molluscan sand and gravel samples in the northern Benguela than previously appreciated. Given the scarcity of information on macrobenthic ecology from high-productivity settings, these death assemblages can represent a modern analog.

Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system

Abstract. On a global scale the ratio of fixed nitrogen (N) and phosphate (P) is characterized by a deficit of N with regard to the classical Redfield ratio of N : P = 16 : 1 reflecting the impact of N loss occurring in the oceanic oxygen minimum zones. The northern Benguela upwelling system (NBUS) is known for losses of N and the accumulation of P in sub- and anoxic bottom waters and sediments of the Namibian shelf resulting in low N : P ratios in the water column. To study the impact of the N : P anomalies on the regional carbon cycle and their consequences for the export of nutrients from the NBUS into the oligotrophic subtropical gyre of the South Atlantic, we measured dissolved inorganic carbon (CT), total alkalinity (AT), oxygen (O2) and nutrient concentrations in February 2011. The results indicate increased P concentrations over the Namibian shelf due to P efflux from sediments resulting in a C : N : P : -O2 ratio of 106 : 16 : 1.6 : 138. N reduction further increase C : N and reduce N : P ratios in those regions where O2 concentrations in bottom waters are &lt; 20 μmol kg−1. However, off the shelf along the continental margin, the mean C : N : P : -O2 ratio is again close to the Redfield stoichiometry. Additional nutrient data measured during two cruises in 2008 and 2009 imply that the amount of excess P, which is created in the bottom waters on the shelf, and its export into the subtropical gyre after upwelling varies through time. The results further reveal an inter-annual variability of excess N within the South Atlantic Central Water (SACW) that flows from the north into the NBUS, with highest N values observed in 2008. It is postulated that the N excess in SACW occurred due to the impact of remineralized organic matter produced by N2 fixation and that the magnitude of excess P formation and its export is governed by inputs of excess N along with SACW flowing into the NBUS. Factors controlling N2 fixation north of the BUS need to be addressed in future studies to better understand the role of the NBUS as a P source and N sink in the coupled C : N : P cycles.

Short-term patterns of vertical particle flux in northern Benguela: a comparison between sinking POC and respiratory carbon consumption

Short-term variability of vertical carbon flux (C-flux) was assessed by two approaches at two stations on the Namibian shelf upwelling system (20°S). The first approach was based on modeling water-column respiratory CO2 production (Fc) by integrating vertical profiles of the respiratory electron transport system. The second approach was based on automated sediment trap sampling daily. In both cases, temporal variability at the same station proved to be greater than the spatial variability between stations. Comparison of the two methods yielded higher C-flux values from the sediment traps in the presence of upwelling filaments (7.85 vs 3.67mmolCm−2d−1). Contrarily, in the post-filament stage of the upwelling system, the values from the Fc models were higher (1.43 vs 2.57mmolCm−2d−1). This difference was attributed to compositional changes in sinking particles and their settling velocities. In combination, both approaches served to increase current understanding of the short-term particle fluxes in this northern Benguela upwelling system. The Fc approach was further applied to zooplankton from the same area in order to quantify their importance in attenuating the particulate organic C-flux. This attenuation, as measured by fractional consumption of the primary productivity, ranged from 2% to 20%.

N‐cycling and balancing of the N‐deficit generated in the oxygen minimum zone over the Namibian shelf—An isotope‐based approach

AbstractThe northern Benguela upwelling system is a nutrient‐replete region with high plankton biomass production and a seasonally changing oxygen minimum zone. Nitrate:phosphate ratios in fresh upwelling water are low due to denitrification in the near‐seafloor oxygen minimum zone and phosphate efflux from sediments. This makes the region a candidate for substantial dinitrogen fixation, for which evidence is scarce. Nutrient and oxygen data, N isotope data of nitrate, nitrogen isotope ratios of particulate matter, particulate organic carbon content, and suspended matter concentrations on a transect across the shelf and upper slope at 23°S illustrate N‐cycling processes and are the basis for estimating the contribution of N‐sources and N‐sinks to the reactive nitrogen pool. It appears that N‐removal due to denitrification exceeds N gain by N2 fixation and physical mixing processes by a factor of &gt;6, although inorganic N:P ratios again increase as surface water is advected offshore. Nitrate and ammonium regeneration, nutrient assimilation with N:P &lt; 16, shelf break mixing, atmospheric input, and N2 fixation all contribute to the restoration of inorganic N:P ratios back to Redfield conditions, but in seasonally changing proportions. The Benguela upwelling system thus is a nutrient source for the oceanic‐mixed layer where N‐sources and N‐sinks are not in balance and Redfield conditions can only re‐adjust by advection and mixing processes integrated over time.

Phosphorites of the Sea of Japan

The lithology and geochemistry of the Miocene concretional phosphorites recovered from four submarine rises in the Sea of Japan (the North and South Yamato, the East Korean, and the Krishtofovich) were examined by means of analytical electron microscopy and ICP-MS chemical and microchemical elemental analysis. The microstructural studies and the data on the distributions of the 57 major and trace elements in these phosphorites revealed their close similarities to the Late Quaternary nodular phosphorites from the Namibian shelf, as well as phosphorites from other parts of the world, thus suggesting a common genetic affinity. Unlike the Namibian phosphorites, our samples collected from the Sea of Japan display a number of geochemical signatures indicative of volcanic and hydrothermal activity. This is supported by the presence of positive Ce and Eu anomalies in some samples and the Ga enrichment in a sample from the Chentsov Volcano.

Comparison of elemental composition of recent and ancient carbonaceous sediments

The concentrations of 13 macroelements and 36 microelements are determined in calcareous deposits from recent basins, namely, the Black Sea (coccolith and sapropel oozes), the Namibian shelf (diatomaceous oozes), and the Peruvian shelf (diatomaceous-terrigenous oozes). The essential similarity of the composition of the microelements is established for all three types of sediments, including calcareous, terrigenous, and siliceous sediments. The comparison of these data with the average composition of the world shales reveals similar trends of microelement distribution, which supports the former hypothesis about the comparable environment of formation of both ancient and some modern basins.

Vertical shifts in the microbial community structure of organic-rich Namibian shelf sediments

This study investigates the diversity and abundance of bacteria in organic-rich Namibian shelf sediments from two sampling stations, using the 16S rRNA library approach and Catalyzed Reporter Deposition Fluorescent in situ Hybridization (CARD-FISH). Six clone libraries were constructed. Clone libraries were dominated by Delta-proteobacteria (up to 48%) and Gamma-proteobacteria (up to 98%). Bacteroidetes were dominant in the clone library of the top 6 cm (up to 17%), while actinobacteria dominated at a depth of 10 to 12 cm (up to 34%). Sequences that were related to bacteria with hydrolytic and fermenting abilities include members from the Gamma-proteobacteria, Bacteroidetes, Actinobacteria, and Acidobacteria. Cloned sequences within the Delta-proteobacteria affiliates to sulfate reducing bacteria, including Desulfarculaceae, Desulfobacteraceae, Desulfobulbaceae,and Desulfuromonadales and were detected throughout the sediment. The two sampling stations differed in microbial diversity with a higher diversity prevailing at the station with higher metabolic rates for organic matter decomposition. At both sampling stations a shift in microbial community composition with depth was observed and is explained by gradients in organic substrate availability within the sediment, which affects the life strategies adopted by bacteria, resulting in niche diversification and ultimately affects bacterial community composition and structure throughout the sediment depth. Key words: Namibia, upwelling, microbial, diversity, abundance.

Marine hypoxia/anoxia as a source of CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O

Abstract. We review here the available information on methane (CH4) and nitrous oxide (N2O) from major marine, mostly coastal, oxygen (O2)-deficient zones formed both naturally and as a result of human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O2 levels to varying degrees. Organic matter supply to seafloor appears to be the primary factor controlling CH4 production in sediments and its supply to (and concentration in) overlying waters, with bottom-water O2-deficiency exerting only a modulating effect. High (micromolar level) CH4 accumulation occurs in anoxic (sulphidic) waters of silled basins, such as the Black Sea and Cariaco Basin, and over the highly productive Namibian shelf. In other regions experiencing various degrees of O2-deficiency (hypoxia to anoxia), CH4 concentrations vary from a few to hundreds of nanomolar levels. Since coastal O2-deficient zones are generally very productive and are sometimes located close to river mouths and submarine hydrocarbon seeps, it is difficult to differentiate any O2-deficiency-induced enhancement from in situ production of CH4 in the water column and its inputs through freshwater runoff or seepage from sediments. While the role of bottom-water O2-deficiency in CH4 formation appears to be secondary, even when CH4 accumulates in O2-deficient subsurface waters, methanotrophic activity severely restricts its diffusive efflux to the atmosphere. As a result, an intensification or expansion of coastal O2-deficient zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH4 budget. The situation is different for N2O, the production of which is greatly enhanced in low-O2 waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of enclosed anoxic basins. Most O2-deficient systems serve as strong net sources of N2O to the atmosphere. This is especially true for coastal upwelling regions with shallow O2-deficient zones where a dramatic increase in N2O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N2O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N2O as well as insufficient data from key coastal regions.

Use of environmental parameters to explain the variability in spawnerrecruitment relationships of Namibian sardine Sardinops sagax

This study attempts to explain the variability in recruitment of sardine in the northern Benguela and to develop potential models by including environmental information to predict recruitment. Two different recruitment and spawner number datasets were available: a VPA-developed dataset, for the period 1952-1987, and data from a simple age-structured model for 1992-2007. In all, four environmental indices were used: the degree of the intrusion of the warm Angola Current into northern Namibia, termed the Angola-Benguela front index; the extent of the upwelling area off central Namibia; average sea surface temperature (SST) over the northern and central Namibian shelf; and wind stress anomalies at Luderitz as an indicator of upwelling strength. Contrary to general belief, it was found that extremely high recruitment can happen at low spawner levels. This occurred in years in which a large upwelling area existed in association with the minimum southward intrusion of the Angola Current. These effects override the normal negative linear relationships with SST and the positive linear relationship with wind. However, when the area of upwelling is average or small, the effects of spawner biomass, SST and wind become important factors in the variability of recruitment. To estimate exceptional recruitment, the upwelling and front indices were included in the model. To measure medium and weak recruitment, spawner numbers and the SST and wind anomaly formed part of the model. These models can be used simultaneously to predict recruitment before annual acoustic surveys take place and thus aid management decisions.

Hydrogen sulphide and methane emissions on the central Namibian shelf

Hydrogen sulphide occurs frequently in the waters of the inner shelf coastal upwelling area off central Namibia. The area affected coincides with hatching grounds of commercially important pelagic fish, whose recruitment may be severely affected by recurring toxic sulphidic episodes. Both episodic biogenic methane gas-driven advective and molecular diffusive flux of hydrogen sulphide have been implicated as transport mechanisms from the underlying organic-matter-rich diatomaceous mud. To test hypotheses on the controls of hydrogen sulphide transport from the sediments on the inner Namibian shelf, water column and sediment data were acquired from four stations between 27 and 72 m water depth over a 3 year long period. On 14 cruises, temperature, salinity, dissolved oxygen, nitrate, methane, and total dissolved sulphide were determined from water column samples, and pore water dissolved methane, total dissolved sulphide, biomass of benthic sulphide-oxidising bacteria Beggiatoa and Thiomargarita, and bacterial sulphate reduction rates were determined from sediment cores. Superimposed on a trend of synchronous changes in water column oxygen and nutrient concentrations controlled by regional hydrographic conditions were asynchronous small-scale variations at the in-shore stations that attest to localized controls on water column chemistry. Small temporal variations in sulphate reduction rates determined with 35S-labeled sulphate do not support the interpretation that variable emissions of sulphide and methane from sediments are driven by temporal changes in the degradation rates of freshly deposited organic matter. The large temporal changes in the concentrations of hydrogen sulphide and the co-occurrence of pore water sulphate and methane support an interpretation of episodic advection of methane and hydrogen sulphide from deeper sediment depths – possibly due to gas bubble transport. Effective fluxes of hydrogen sulphide and methane to the water column, and methane and sulphide concentrations in the bottom waters were decoupled, likely due to the activity of sulphide-oxidising bacteria. While the causal mechanism for the episodic fluctuations in methane and dissolved sulphide concentrations remains unclear, this data set points to the importance of alternating advective and diffusive transport of methane and hydrogen sulphide to the water column.

An integrated sulfur isotope model for Namibian shelf sediments

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 (H 2S) to sulfate ( SO 4 2 - ) using sea water nitrate ( NO 3 - ) as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between SO 4 2 - and H 2S 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 SO 4 2 - pool diffuses directly into the sediment from the sea water. A fraction of the SO 4 2 - 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 H 2S production is trapped as sedimentary sulfide, mainly pyrite (FeS 2) and organic sulfur (S org) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS 2 and S org 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 H 2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H 2 32S from the pore water. A fractionation of 20–30‰ was estimated for SOx (ε SOx) with the model, along with a maximum fractionation for SR (ε SR–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 ε SOx (5‰) and ε SR (78‰).

A natural exposure experiment on short-term species-selective aerobic degradation of dinoflagellate cysts

Although studies have shown that organic-walled dinoflagellate cysts (dinocysts) can undergo species-selective aerobic degradation, the alteration rate of this process is not known. Here we provide data on the decay rates of dinocyst from a degradation experiment in which sediment samples of Holocene age from (a) anoxic Namibian shelf and (b) anoxic part of the eastern Mediterranean S1 sapropel have been exposed to oxic and anoxic conditions in natural environments. The two types of sediment were stored in bags composed of a dialyse membrane that allows oxygen penetration but prevents bacterial exchange. Sediment bags were placed in open containers connected to sediment traps and moored for 15 months in anoxic brines and oxic intermediate waters of the Urania and Bannock Basins areas. Within the short experimental time (15 months), exposure to oxygenated waters resulted in a 24% to 57% reduction in concentration of cysts attributable to Brigantedinium spp. and Echinidinium granulatum. Other taxa such as Spiniferites spp ., Lingulodinium machaerophorum and Echinidinium spp. appear to be less sensitive. A slight increase in cyst concentration is observed for Nematosphaeropsis labyrinthus, Echinidinium aculeatum, Operculodinium israelianum, and Impagidinium aculeatum, indicating that these cyst species are more resistant to aerobic degradation. Exposure to anoxic conditions has not lead to detectable differences between initial and exposed composition and concentration. Our study is the first to document that species-selective degradation of dinocysts in oxygenated natural environments is a rapid process that changes considerably dinocyst concentrations and assemblages.