Suchergebnisse

The Bay of Bengal hosts persistent, measurable, but sub-micromolar, concentrations of oxygen in its oxygen-minimum zone (OMZ). Such low-oxygen conditions are not necessarily rare in the global ocean and seem also to characterize the OMZ of the Pescadero Basin in the Gulf of California,
as well as the outer edges of otherwise anoxic OMZs, such as can be found, for example, in the Eastern Tropical North Pacific. We show here that biological controls on oxygen consumption are required to allow the semistable persistence of low-oxygen conditions in OMZ settings; otherwise, only
small changes in physical mixing or rates of primary production would drive the OMZ between anoxic and oxic states with potentially large swings in oxygen concentration. We propose that two controls are active: an oxygen-dependent control on oxygen respiration and an oxygen inhibition of denitrification.
These controls, working alone and together, can generate low-oxygen concentrations over a wide variability in ocean mixing parameters. More broadly, we discuss the oxygen regulation of organic matter cycling and N2 production in OMZ settings. Modern biogeochemical models of nitrogen
and oxygen cycling in OMZ settings do contain some of the parameterizations that we explore here. However, these models have not been applied to understanding the persistence of low, but measurable, concentrations of oxygen in settings like the Bay of Bengal, nor have they been applied to
understanding what biological/physical processes control the transition from a weakly oxygenated state to a "functionally" anoxic state with implications for nitrogen cycling. Therefore, we believe that the approach here illuminates the relationship between oxygen and the biogeochemical
cycling of carbon and nitrogen in settings like the Bay of Bengal. Furthermore, we believe that our results could further inform large-scale ocean models seeking to explore how global warming might influence the spread of low-oxygen waters, influencing the cycles of oxygen, carbon, and nitrogen
in OMZ settings.

ABSTRACT. Thriving benthic communities were observed in the oxygen minimum zones along the southwestern African margin. On the Namibian margin, fossil cold-water coral mounds were overgrown by sponges and bryozoans, while the Angolan margin was characterized by cold-water coral mounds covered by a living coral reef. To explore why benthic communities differ in both areas, present-day environmental conditions were assessed, using conductivity–temperature–depth (CTD) transects and bottom landers to investigate spatial and temporal variations of environmental properties. Near-bottom measurements recorded low dissolved oxygen concentrations on the Namibian margin of 0–0.15 mL L−1 (≜0 %–9 % saturation) and on the Angolan margin of 0.5–1.5 mL L−1 (≜7 %–18 % saturation), which were associated with relatively high temperatures (11.8–13.2 ∘C and 6.4–12.6 ∘C, respectively). Semidiurnal barotropic tides were found to interact with the margin topography producing internal waves. These tidal movements deliver water with more suitable characteristics to the benthic communities from below and above the zone of low oxygen. Concurrently, the delivery of a high quantity and quality of organic matter was observed, being an important food source for the benthic fauna. On the Namibian margin, organic matter originated directly from the surface productive zone, whereas on the Angolan margin the geochemical signature of organic matter suggested an additional mechanism of food supply. A nepheloid layer observed above the cold-water corals may constitute a reservoir of organic matter, facilitating a constant supply of food particles by tidal mixing. Our data suggest that the benthic fauna on the Namibian margin, as well as the cold-water coral communities on the Angolan margin, may compensate for unfavorable conditions of low oxygen levels and high temperatures with enhanced availability of food, while anoxic conditions on the Namibian margin are at present a limiting factor for cold-water coral growth. This study provides an example of ...

Trace metals (TMs) are essential micronutrients required for marine life. They are indispensable components of phytoplankton enzymes which catalyse important biological functions. Due to their scarcity in the ocean, TMs can (co-)limit primary productivity and thus affect the efficiency of the biological pump. Marine sediments are an important source and sink for TMs to the ocean, especially in low-oxygen environments. However, the key processes and parameters that lead to TM release from or to fixation and burial within the sediments are not fully understood for most TMs and the corresponding fluxes are not well quantified. As the oceans are losing oxygen, oxygen minimum zones serve as a present-day example to study how benthic TM cycles will respond to future ocean conditions. In order to investigate environmental controls on benthic TM exchange and pathways from or to the sediment, this study combines sediment, pore water, bottom water and benthic flux data. The main study site is the Peruvian margin, where one of the largest and most intense oxygen minimum zones is located. Additional data stems from a seasonally anoxic fjord in the Baltic Sea. In the first scientific chapter of this thesis, Chapter II, the benthic cycling of the two TMs iron (Fe) and cadmium (Cd), which have a contrasting sulphide solubility (Fe > Cd), is compared. Hydrogen sulphide concentrations exert an important control on the benthic fluxes of both TMs at the Peruvian margin. Temporal magnitude changes of diffusive Fe effluxes into near-bottom waters are related to Fe retention via sulphide precipitation in the sediment due to high hydrogen sulphide concentrations. Further, benthic chamber incubation data indicated that Fe accumulation in anoxic near-bottom waters coincided with the depletion of nitrate and nitrite preventing Fe oxidation and subsequent (oxyhydr)oxide precipitation. Cadmium has one of the lowest sulphide solubilities among TMs. The removal of Cd from near-bottom waters during benthic chamber incubations covaried with hydrogen sulphide concentrations in the surface sediment. This suggest that Cd accumulation in the sediment is mediated by precipitation of cadmium sulphide at the sediment-water interface or within the water column. Oxygen minimum zone sediments are a source for manganese (Mn) and cobalt (Co) and a sink for nickel (Ni), cupper (Cu), zinc (Zn) and Cd. Chapter III, deals with the different mechanisms and pathways which lead to the enrichment or depletion of TMs in sediments at the Peruvian margin. Even though Mn and Co are both depleted in Peru margin sediments, the results of this thesis suggest that their cycling is partly decoupled. At least half of the Mn depletion in shelf sediments can be attributed to benthic diffusive efflux. In contrast, Co dissolution chiefly takes place in the water column as benthic diffusive Co effluxes are much lower compared to the rate of Co loss from the sediments. The majority of Ni accumulation in Peruvian shelf sediments can be explained by direct phytoplankton uptake in the photic zone and delivery with organic matter. For Cu, Zn and Cd however, this transport mechanism is rather of minor importance. Therefore, a covariation in sediments of Cu with particulate organic carbon suggests that the Cu accumulation may be primarily caused by scavenging by downward sinking organic matter. In addition, similar to Cd, the Cu delivery with sulphide minerals precipitated from the water column or near-bottom water likely contributes to the accumulation. The enrichment of Zn is driven by diffusive benthic fluxes from the near-bottom water into the sediment pore water, which matched the excess Zn accumulation. This is likely followed by sulphide precipitation, causing Zn retention in the sediment. Chapter IV presents a novel device that was developed to sample dissolved and particulate TMs in the layer of water above the seafloor, the benthic boundary layer. So far this has not been able to conduct with conventional TM sampling methods. The new device overcomes the existing limitations. Successful testing demonstrated that it enables simultaneous, uncontaminating and oxygen-free sampling of suspended particles and near-bottom water in high-resolution within the first few meters above the seafloor. The novel device will be an important tool for future studies on dissolved-particulate interactions at the ocean's lower boundary. It will help to solve remaining questions on how benthic TM fluxes are modified in this reactive interface layer and on TM particle association. The results of this thesis demonstrate that TM behaviour in the ocean is very diverse and future ocean conditions, with declining oxygen and increasing hydrogen sulphide concentrations, may modify benthic TM fluxes individually. The differing TM fluxes at the seafloor may change TM stoichiometry in upwelling water masses and the future ocean, which can impact marine ecosystems in the surface ocean.

Studies of the impact of hypoxic or anoxic environments on both climate and ecosystems rely on a detailed characterization of the oxygen (O-2) distribution along the water column. The former trivial separation between oxic and anoxic conditions is now often redefined as a blurry concentration range in which both aerobic and anaerobic processes might coexist, both in situ and during experimental incubations. The O-2 concentrations during such incubations have often been assumed to be equal to in situ levels, but the concentration was rarely measured. In order to evaluate the actual oxygen concentration in samples collected from low-oxygen environments, a series of measurements were performed on samples collected in the Pacific oxygen minimum zones. Our results show a significant deviation from in situ anoxic conditions in samples collected by Niskin bottles where leakage from the bottle material resulted in O-2 concentrations of up to 1 mu M. Subsequent sampling further increased the O-2 contamination. Sampling and analysis by Winkler method resulted in variable apparent concentrations of 2-4 mu M. Two common procedures to avoid atmospheric contamination were also tested: allowing gentle overflow and keeping the sampling bottle submersed in a portion of the sampled water. Both procedures resulted in similar O-2 contamination with values of 0.5-1.5 mu M when bottles were immediately closed and measurements performed with optical sensors, and 3-4 mu M apparent concentration when analyzed by the Winkler method. Winkler titration is thus not suited for analysis of low-O-2 samples. It can be concluded that incubation under anoxic conditions requires deoxygenation after conventional sampling. ; We would like to thank Lars B. Pedersen at Aarhus University for the construction of STOX sensors. We are grateful to the cruise leaders Bess B. Ward and Frank Stewart for the invitation to participate in OMZ cruises. We also thank the captains and crews of the R/Vs L'Atalante, New Horizon, Oceanus and Sally Ride. We additionally thank A. Franco-Garcia, M. Giraud, J. Ledesma, F. Baurand, D. Lefevre, B. Dewitte, C. Maes, V. Garcon and the PACOP platform (Toulouse) for operational and experimental support during the AMOP cruise. This work was funded by the Poul Due Jensen Foundation and co-financed by the 2014-2020 ERDF Operational Programme and by the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia (to EGR, project reference FEDER-UCA18-107225).