ALBERT

Beschreibung: Molybdenum (Mo) concentrations and isotope compositions in sediments and shales are commonly used as proxies for anoxic and sulfidic (i.e., euxinic) conditions in the water column of paleo-marine systems. A basic assumption underlying this practice is that the proxy signal extracted from the geological record is controlled by long-term (order of decades to millennia) Mo scavenging in the euxinic water column rather than Mo deposition during brief episodes or events (order of weeks to months). To test whether this assumption is viable we studied the biogeochemical cycling of Mo and its isotopes in sediments of the intermittently euxinic Gotland Deep in the central Baltic Sea. Here, multiannual to decadal periods of euxinia are occasionally interrupted by inflow events during which well‑oxygenated water from the North Sea penetrates into the basin. During these events manganese (Mn) (oxyhydr)oxide minerals are precipitated in the water column, which are known to scavenge Mo. We present sediment and pore water Mo and Mo isotope data for sediment cores which were taken before and after a series of inflow events between 2014 and 2016. After seawater inflow, pore water Mo concentrations in anoxic surface sediments exceed the salinity-normalized concentration by more than two orders of magnitude and coincide with transient peaks of dissolved Mn. A fraction of the Mo liberated into the pore water is transported by diffusion in a downward direction and sequestered by organic matter within the sulfidic zone of the sediment. Diffusive flux calculations as well as a mass balance that is based on the sedimentary Mo isotope composition suggest that about equal proportions of the Mo accumulating in the basin are delivered by Mn (oxyhydr)oxide minerals during inflow events and Mo scavenging with hydrogen sulfide during euxinic periods. Since the anoxic surface sediment where Mo is released from Mn (oxyhydr)oxides are separated by several centimeters from the deeper sulfidic layers where Mo is removed, the solid phase record of Mo concentration and isotope composition would be misinterpreted if steady state Mo accumulation was assumed. Based on our observations in the Gotland Deep, we argue that short-term redox fluctuations need to be considered when interpreting Mo-based paleo-records.

Beschreibung: Highlights • Sulphidic event on the shelf resulted in a temporal imbalance of the benthic N cycle. • Bacterial NOx storage is a major source of oxidative power during euxinia. • Peruvian shelf and upper slope sediments are strong recycling sites of fixed N. Abstract Oxygen minimum zones (OMZ) are key regions for fixed nitrogen loss in both the sediments and the water column. During this study, the benthic contribution to N cycling was investigated at ten sites along a depth transect (74–989 m) across the Peruvian OMZ at 12 °S. O2 levels were below detection limit down to ~ 500 m. Benthic fluxes of N2, NO3–, NO2–, NH4+, H2S and O2 were measured using benthic landers. Flux measurements on the shelf were made under extreme geochemical conditions consisting of a lack of O2, NO3– and NO2– in the bottom water and elevated seafloor sulphide release. These particular conditions were associated with a large imbalance in the benthic nitrogen cycle. The sediments on the shelf were densely covered by filamentous sulphur bacteria Thioploca, and were identified as major recycling sites for DIN releasing high amounts of NH4+up to 21.2 mmol m−2 d−1 that were far in excess of NH4+release by ammonification. This difference was attributed to dissimilatory nitrate (or nitrite) reduction to ammonium (DNRA) that was partly being sustained by NO3– stored within the sulphur oxidizing bacteria. Sediments within the core of the OMZ (ca. 200 to 400 m) also displayed an excess flux of N of 3.5 mmol m−2 d−1 mainly as N2. Benthic nitrogen and sulphur cycling in the Peruvian OMZ appears to be particularly susceptible to bottom water fluctuations in O2, NO3−and NO2−, and may accelerate the onset of pelagic euxinia when NO3−and NO2−become depleted.

Beschreibung: Many biological seep studies focused on the distribution, structure, nutrition and food web architecture of seep communities as well as on their interaction with the seep geochemistry. However, overall respiration at cold seeps received only little attention. We conducted in-situ oxygen flux measurements in combination with ex-situ oxygen micro-profiles, respiration measurements, as well as rate determinations of microbial methane and sulfate turnover to assess respiration pathways as well as carbon turnover at a seep habitat that was recently discovered alongside the Hikurangi Margin offshore northern New Zealand. This habitat is dominated by dense beds of tube-building, heterotrophic ampharetid polychaetes. Average total oxygen uptake (TOU) from this habitat was very high (83.7 mmol m− 2 day− 1). TOU at a non-seep reference site ranged between 2.7 and 5.8 mmol m− 2 day− 1. About 37% (30.8 mmol m− 2 day− 1) of the average TOU was consumed by ampharetids. Considering mean diffusive oxygen uptake (8.5 mmol m− 2 day− 1) the remaining fraction of ~ 53% of the TOU (44.4 mmol m− 2 day− 1) might be explained by respiration of epibenthic organisms as well as aerobic methane and sulfide oxidation at the sediment–water interface. The strongly negative carbon isotopic signatures (− 52.9 ± 5‰ VPDB) of the ampharetid tissues indicate a methane derived diet. However, carbon production via anaerobic oxidation of methane (AOM) was too low (0.1 mmol C m− 2 day− 1) to cover the mean carbon demand of the ampharetid communities (21 mmol C m− 2 day− 1). Likely, organic carbon generated via aerobic methane oxidation represents their major carbon source. This is in contrast to other seep habitats, where energy bound in methane is partly transferred to sulfide via AOM and finally consumed by sulfide-oxidizing chemoautotrophs providing carbon that subsequently enters the benthic food web.

Beschreibung: This study combines sediment geochemical analysis, in situ benthic lander deployments and numerical modeling to quantify the biogeochemical cycles of carbon and sulfur and the associated rates of Gibbs energy production at a novel methane seep. The benthic ecosystem is dominated by a dense population of tube-building ampharetid polychaetes and conspicuous microbial mats were unusually absent. A 1D numerical reaction-transport model, which allows for the explicit growth of sulfide and methane oxidizing microorganisms, was tuned to the geochemical data using a fluid advection velocity of 14 cm yr−1. The fluids provide a deep source of dissolved hydrogen sulfide and methane to the sediment with fluxes equal to 4.1 and 18.2 mmol m−2 d−1, respectively. Chemosynthetic biomass production in the subsurface sediment is estimated to be 2.8 mmol m−2 d−1 of C biomass. However, carbon and oxygen budgets indicate that chemosynthetic organisms living directly above or on the surface sediment have the potential to produce 12.3 mmol m−2 d−1 of C biomass. This autochthonous carbon source meets the ampharetid respiratory carbon demand of 23.2 mmol m−2 d−1 to within a factor of 2. By contrast, the contribution of photosynthetically-fixed carbon sources to ampharetid nutrition is minor (3.3 mmol m−2 d−1 of C). The data strongly suggest that mixing of labile autochthonous microbial detritus below the oxic layer sustains high measured rates of sulfate reduction in the uppermost 2 cm of the sulfidic sediment (100–200 nmol cm−3 d−1). Similar rates have been reported in the literature for other seeps, from which we conclude that autochthonous organic matter is an important substrate for sulfate reducing bacteria in these sediment layers. A system-scale energy budget based on the chemosynthetic reaction pathways reveals that up to 8.3 kJ m−2 d−1 or 96 mW m−2 of catabolic (Gibbs) energy is dissipated at the seep through oxidation reactions. The microorganisms mediating sulfide oxidation and anaerobic oxidation of methane (AOM) produce 95% and 2% of this energy flux, respectively. The low power output by AOM is due to strong bioenergetic constraints imposed on the reaction rate by the composition of the chemical environment. These constraints provide a high potential for dissolved methane efflux from the sediment (12.0 mmol m−2 d−1) and indicates a much lower efficiency of (dissolved) methane sequestration by AOM at seeps than considered previously. Nonetheless, AOM is able to consume a third of the ascending methane flux (5.9 mmol m−2 d−1 of CH4) with a high efficiency of energy expenditure (35 mmol CH4 kJ−1). It is further proposed that bioenergetic limitation of AOM provides an explanation for the non-zero sulfate concentrations below the AOM zone observed here and in other active and passive margin sediments.

Beschreibung: Highlights • Review of sediment archives from the Peruvian margin since the LGM. • Focus on the evolutionary feature of the hiatus found in archives. • Modern analogue for current-dominated environments for paleo reconstructions. • New results for erosional potential of the non-linear internal waves (NLIWs). Abstract The Peruvian continental margin is characterized by the presence of one of the strongest and most distinct Oxygen Minimum Zones (OMZs) in today's oceans. Therefore, it has long been in the focus of oceanographic and geological investigations. Observations indicate that OMZs are expanding in relation with currently changing climate. To advance understanding of the temporal evolution of OMZs and climate change, complete paleoceanographic and palaeoclimatological reconstructions are needed. However, the development of paleoenvironmental scenarios for the period since the Last Glacial Maximum at this region was hampered by a ubiquitous hiatus and short-term interruptions of the stratigraphical record. In the present study, we combined the stratigraphical information from 31 sediment cores from the Peruvian margin located between 3 and 18°S and water depths of 90 to 1300 m within and below today's OMZ, in order to determine the extent of the hiatus and assess the responsible mechanisms. A widespread unconformity and related erosional features, omission surfaces and phosphorites, were observed in sediment cores from the area south of 7°S, depicting a prograding feature on the continental slope from south to north during the deglaciation. Combining recent oceanographic and sedimentological observations, it is inferred that, tide-topography interaction and resulting non-linear internal waves (NLIWs) shape the slope by erosion, carry sediments upslope or downslope and leave widespread phosphoritic lag sediments, while the Peru Chile Undercurrent (PCUC) transports the resuspended sediments southward causing non-deposition. This exceptional sedimentary regime makes the Peruvian margin a modern analogue for such environments. Overall, our compilation of downcore records showed that enhanced bottom currents due to tide-topography interaction were progressively evolving and affected a wider area with the onset of the last deglaciation. Elevated tidal amplitudes and variability of mid-depth water masses (i.e.; density changes) and hydrodynamics in relation with changing climate were potential reasons of this evolving feature of erosion and reworking. Additionally, erosion and non-deposition was observed widest and even was encountered on the continental shelf during the early Holocene, potentially indicating a strong phase of the PCUC mirroring today's El Niño-like conditions.

Beschreibung: Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 μmol Si cm−2 yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.

Beschreibung: Current de-oxygenation of the oceans is associated with severe habitat loss and distinct changes in the species composition of bentho-pelagic communities. We investigated the distributions of epibenthic megafauna across the Peruvian OMZ (11°S) at water depths ranging from ∼80 to 1000 m water depth using sea floor images. Likely controls of distributions were adressed by combining the abundances of major groups with geochemical parameters and sea-floor topography. In addition to bottom-water oxygen levels and organic-carbon availability, particular emphasis is laid on the effects of local hydrodynamics. Beside the occurrence of microbial mats at the shelf and upper slope, distinct zones of highly abundant megafauna, dominated by gastropods (900 ind. m−2), ophiuroids (140 ind. m−2), and pennatulaceans (20 ind. m−2), were observed at the lower boundary of the OMZ. Their distribution extended from 460 m water depth (O2 levels 〈 2 μM), where gastropods were abundant, to 680 m (O2 ∼6 μM) where epifaunal abundances declined sharply. Bottom water O2 represents a major factor that limits the ability of metazoans to invade deeply into the OMZ where they could have access to labile organic carbon. However, depending on their feeding mode, the distribution of organisms appeared to be related to local hydrodynamics caused by the energy dissipation of incipient internal M2 tides affecting the suspension, transport and deposition of food particles. This was particularly evident in certain sections of the investigated transect. At these potentially critical sites, energy dissipation of internal tides is associated with high bottom shear stress and high turbulences and coincides with elevated turbidity levels in the benthic boundary layer, increased Zr/Al-ratios, low sedimentation rates as well as a shift in the grain size towards coarser particles. In or near such areas, abundant suspension-feeding organisms, such as ophiuroids, pennatulaceans, and tunicates were present, whereas deposit-feeding gastropods were absent. The influence of local hydrodynamic conditions on the distribution of epibenthic organisms has been neglected in OMZ studies, although it has been considered in other settings.

Beschreibung: This study investigates the biogeochemical processes that control the benthic fluxes of dissolved nitrogen (N) species in Boknis Eck - a 28 m deep site in the Eckernförde Bay (southwestern Baltic Sea). Bottom water oxygen concentrations (O2-BW) fluctuate greatly over the year at Boknis Eck, being well-oxygenated in winter and experiencing severe bottom water hypoxia and even anoxia in late summer. The present communication addresses the winter situation (February 2010). Fluxes of ammonium (NH4+), nitrate (NO3-) and nitrite (NO2-) were simulated using a benthic model that accounted for transport andbiogeochemical reactions and constrained with ex situ flux measurements and sediment geochemical analysis. The sediments were a net sink for NO3- (-0.35 mmol m-2 d-1 of NO3-), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO3- reduction to NO2- by denitrifying microorganisms. NH4+ fluxes were high (1.74 mmol m-2d-1 of NH4+), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO2-fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen(DIN = NO3- + NO2- + NH4+) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH4+, thus reducing the capacity for nitrification of NH4+. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m-2 d-1 of N2, which is at the lower end of previously reported values. A systematic sensitivity analysis revealed that denitrification and anammox respond strongly and positively to the concentration of NO3- in the bottomwater (NO3-BW).Higher O2-BW decreases DNRA and denitrification but stimulates both anammox and the contribution ofanammox to total N2 production (%Ramx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %Ramx and water depth in the seminal study of Dalsgaard et al. (2005). Despite remaining uncertainties, the results provide a general mechanistic framework for interpreting the existing knowledge of N-turnover processes and fluxes in continental margin sediments, as well as predicting the types of environment where these reactions are expected to occur prominently.

Beschreibung: Benthic nitrogen (N) cycling was investigated at six stations along a transect traversing the Peruvian oxygen minimum zone (OMZ) at 11 °S. An extensive dataset including porewater concentration profiles and in situ benthic fluxes of nitrate (NO3–), nitrite (NO2–) and ammonium (NH4+) was used to constrain a 1–D reaction–transport model designed to simulate and interpret the measured data at each station. Simulated rates of nitrification, denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) by filamentous large sulfur bacteria (e.g. Beggiatoa and Thioploca) were highly variable throughout the OMZ yet clear trends were discernible. On the shelf and upper slope (80 – 260 m water depth) where extensive areas of bacterial mats were present, DNRA dominated total N turnover (less-than-or-equals, slant 2.9 mmol N m–2 d–1) and accounted for greater-or-equal, slanted 65 % of NO3– + NO2– uptake by the sediments from the bottom water. Nonetheless, these sediments did not represent a major sink for dissolved inorganic nitrogen (DIN = NO3– + NO2– + NH4+) since DNRA reduces NO3– and, potentially NO2–, to NH4+. Consequently, the shelf and upper slope sediments were recycling sites for DIN due to relatively low rates of denitrification and high rates of ammonium release from DNRA and ammonification of organic matter. This finding contrasts with the current opinion that sediments underlying OMZs are a strong sink for DIN. Only at greater water depths (300 – 1000 m) did the sediments become a net sink for DIN. Here, denitrification was the major process (less-than-or-equals, slant 2 mmol N m–2 d–1) and removed 55 – 73 % of NO3– and NO2– taken up by the sediments, with DNRA and anammox accounting for the remaining fraction. Anammox was of minor importance on the shelf and upper slope yet contributed up to 62 % to total N2 production at the 1000 m station. The results indicate that the partitioning of oxidized N (NO3–, NO2–) into DNRA or denitrification is a key factor determining the role of marine sediments as DIN sinks or recycling sites. Consequently, high measured benthic uptake rates of oxidized N within OMZs do not necessarily indicate a loss of fixed N from the marine environment.