Description: A meaningful application of Mo as a paleo-redox proxy requires an understanding of Mo cycling in modern reducing environments. Stagnant euxinic basins such as the Black Sea are generally regarded as model systems for understanding euxinic systems during early Earth history. However, drawing direct parallels between the Black Sea and open-marine euxinic margins is somewhat complicated by differences in the seawater residence time between these two environments. We report sediment and pore water Mo, U, Mn and Fe data for a euxinic basin with a short seawater residence time; the weakly restricted Gotland Deep in the Baltic Sea. Here, prolonged periods of euxinia alternate with brief inflow events during which well-oxygenated, saline water penetrates into the basin. During these inflow events, dissolved Mn and Fe that has accumulated within the euxinic deep water can be oxidized and precipitated. Co-variations of Mo and U within the sediment suggest that these inflow and oxygenation events may favor Mo accumulation in the sediment through adsorption to freshly oxidized Mn and Fe solid phases. Once Mo is sequestered within the deeper euxinic water and sediments, Mo retention can be further facilitated by conversion to thiomolybdate species and interactions with organic matter and metal sulfides. By comparing our data with those from previous studies where a Mn and Fe “shuttle” for Mo has been demonstrated, we identify two prerequisites for the occurrence of this mechanism. First, there must be a water column oxic–anoxic redox-boundary; this provides a solubility contrast for Mn and Fe. Second, the residence time of seawater in the system has to be short (weeks to a few years). The latter criterion can be met through regular inflow in weakly restricted basins or upwelling in oxygen minimum zones at open-marine continental margins. Based on prior work, we suggest that similar conditions to those currently represented by the Gotland Deep may have prevailed at euxinic ocean margins during the Proterozoic. A boundary between euxinic and oxic water masses overlying the continental shelf may have resulted in accelerated Mo transport through the water column with Mn and Fe (oxyhydr)oxides. We propose that this mechanism, along with Mo isotope fractionation during adsorption, could contribute to the light Mo isotope composition observed in open-marine euxinic sediment facies of the Proterozoic.

Description: In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX ("In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies", www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of water-column oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts.

Description: Presently, oxygen minimum zones (OMZ) are incurring drastic changes from the combined impact of both rapidly declining O2 concentrations and increasing CO2 levels. The lower edge of OMZs, typically characterized by abundant benthic invertebrate communities and fish existing in already precarious conditions, are particularly susceptible to even the slightest O2 changes. Currently, there are very limited benthic O2 data from the upper- and lower-boundaries of OMZs. Using benthic landers and a sediment-water O2 micro-profiler, we resolved time series of O2 and temperature directly above the seafloor at the lower-boundary (depth range of 397 - 1015 m) of the Peruvian OMZ at a transect along 11°S. We observed an oscillating and persistent vertical movement of bottom water (BW) with displacement amplitudes exceeding 100 m. These vertical displacements have a ~ 12 hour period, and appear to be partially driven by tidal forcing. This regular BW vertical motion leads to distinct cyclic fluctuations of local O2 concentration and temperature. At ~ 1000 m water depth, O2 variability ranged from 23 to 44 µM. Cyclical benthic O2 fluctuations were observed with decreasing water depths until O2 concentrations 〈 3 µM were reached at ~ 400 m. The benthic environment immediately responds to the varying BW O2 levels. At ~ 1000 m, the diffusive oxygen uptake across the sediment water interface fluctuated between 0.5 to 1.4 mmol m-2 d-1, which is seemingly associated to variable O2 BW concentrations. Intermittent local benthic O2 levels affect the colonization and composition of both aerobic and anaerobic life at the lower boundary of the OMZ. The dominant communities have enormous consequences for the distribution and magnitude of benthic redox-sensitive biogeochemical processes. Similarly to investigations in other oxygen deficient environments, we found extremely high densities of epibenthic invertebrates that were sharply centered at ~ 650 m depth close to the oxic-anoxic interface. These organisms benefit substantially from the oscillating benthic oxygen levels. They may therefore inhabit regions deeper within the lower edge of the OMZ, thus exploiting increased availability of e.g. labile organic carbon, and perhaps taking a deep “breath” during high O2 conditions. These oxygen oscillations in association with intricate BW motions imply strong solute exchange between the benthos and the higher water column, which can have unforeseen consequences for the coupling of benthic and pelagic ecosystems in an increasingly oxygen deficient environment.