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Final Report Summary - BICYCLE (Benthic Iron Cycling in Oxygen Minimum Zones and Implications for Ocean Biogeochemistry)

Iron (Fe) is a key element in marine biogeochemical cycles as it serves as an essential micro-nutrient in many biological processes. A poorly studied link in the oceanic Fe cycle is the reductive release of Fe from sediments in oxygen depleted ocean regions, the so-called oxygen minimum zones (OMZs). In the course of the ongoing trend of global environmental change, OMZs have experienced a significant expansion over the last decades. This continuing process of ocean de-oxygenation has the potential to modulate ocean fertility, as it may enhance the recycling efficiency of bio-available Fe from the seafloor. The goal of the BICYCLE project was to better characterize the response of iron (Fe) release from seafloor sediments to past and present ocean deoxygenation. To address this goal, three overarching research Objectives were formulated.
The first Objective was to establish a mass balance for reactive Fe and Fe isotopes across the Peruvian OMZ, which is one of the most pronounced OMZs of the modern ocean. Surface sediments and sediment pore waters from an offshore transect across the oxygen gradient where analysed for Fe concentrations and their isotope composition. Mass balance and flux calculations revealed that Peru margin sediments represent an important source of bioavailable Fe to the local water column. However, most of the Fe released is re-precipitated and deposited close to the source rather than transported offshore into the adjacent open-ocean regions, where primary productivity is strongly Fe-limited.
The second Objective was to track Fe release from seafloor sediments on the Peruvian margin over the last glacial-interglacial cycles. To this end, an age model was established for a 15 m long sediment core from the present-day center of Fe release. This core covers the last 140,000 years. Paleo-proxies for sedimentary Fe release, which were developed in the framework of Objective 1, were applied and compared with complimentary proxies for ocean circulation, oxygenation and productivity to characterize how seafloor Fe release interacted with global climate change and particularly deoxygenation in the past. The results reveal, that Fe release was more intense during periods of slightly enhanced oxygenation (e.g., under peak-glacial conditions). Enhanced Fe release under more oxic conditions is attributed to lower concentrations of hydrogen sulfide in the surface sediment, which decreases the retention of Fe as Fe sulfide minerals. Generally, the findings suggest that iron release from continental margin sediments is most effective in a redox window where neither oxygen nor sulfide is present. Following this rationale, ocean de-oxygenation is unlikely to ease iron limitation in the pronounced (and thus temporarily sulfidic) OMZs of the eastern equatorial Pacific. However, in the weaker OMZs of the subarctic Pacific and southeast Atlantic partial deoxygenation may enhance the iron supply.
The third Objective was to establish a general model of how ocean deoxygenation will affect Fe release from seafloor sediment in the future. To address this Objective, a data base of seafloor Fe fluxes was compiled. The data base was used to identify general dependencies between the seafloor Fe flux, organic carbon rain rate and bottom water oxygenation. A numerical transport-reaction model was used to derive a transfer function which can be used to incorporate the seafloor Fe source in global biogeochemical models. Based on this transfer function a global map of seafloor Fe fluxes was generated and reconciled with a modelled distribution of dissolved Fe in the surface ocean.
An additional Objective which evolved from the research findings of Objective 1 to 3 was to better characterize dissolved-particulate interactions in the water column which determine the extent to which sediment-derived Fe is exported out of anoxic ocean regions. To this end water column particulate matter was collected on a research cruise to the Peruvian continental margin and the distribution and mineralogy of Fe in the particulate matter was characterized by synchrotron radiation-based x-ray fluorescence and x-ray absorption spectroscopy techniques. The results were combined with dissolved and solid phase Fe concentrations as well as molecular genetic data. It could be shown that most of the Fe in the water column of anoxic oxygen minimum zones is oxidized (and thus demobilized) by nitrate rather than oxygen as the terminal electron acceptor.
The findings of the BICYCLE project provide new insights into the fate of sediment-derived Fe in oxygen-deficient ocean regions and how sedimentary Fe release was affected by environmental change in the past. The conceptual and quantitative models generated within the BICYCLE project contribute to the European Union’s endeavour to predict how human‐induced environmental change will affect the earth and ocean system in the future.

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