Iron (Fe) supply is the limiting factor for primary productivity over vast areas of the modern ocean. In the classical view, river runoff and wind-blown terrestrially derived dust are considered the main sources of bio-available Fe to the ocean. Another, yet poorly constrained source is the reductive recycling of Fe from marine sediments in oxygen-depleted ocean regions, the so-called oxygen minimum zones (OMZs). Anthropogenically induced eutrophication and climate change have caused a significant expansion of OMZs over the last decades. This continuing process of ocean de-oxygenation has the potential to modulate ocean fertility through time, as it enhances the recycling efficiency of bio-available Fe at the seafloor. The exploration of this largely disregarded link in the global Fe cycle is the aim of the present proposal entitled “BICYCLE – Benthic Iron Cycling in Oxygen Minimum Zones and Implications for Ocean Biogeochemistry”.Iron concentrations and Fe isotopes, a novel tracer for Fe source signatures in natural systems, will be analysed in sediments and pore waters across the Peru upwelling area, one of the most extended and dynamic OMZs of the modern ocean. The joint evaluation of reactive Fe contents and Fe isotope signatures in sedimentary archives will yield a quantitative estimate of the various Fe fluxes into and out of the OMZ since the last glacial maximum. Comparing Fe proxies with other tracers for marine conditions will help to explain how benthic Fe cycling may interact with marine productivity, continental weathering and redox conditions over contemporary and paleo timescales. The empirical concepts gained from that will be tested through application of numerical models simulating benthic Fe cycling and Fe export as function of changing environmental conditions. This project directly contributes to the European Union’s endeavour to predict how human-induced environmental change will affect the earth and ocean system in the future.