"Phosphorus (P) is a key and often limiting nutrient for phytoplankton in the ocean. A strong positive feedback exists between marine P availability, primary production and ocean anoxia: increased production leads to ocean anoxia, which, in turn, decreases the burial efficiency of P in sediments and therefore increases the availability of P and production in the ocean. This feedback likely plays an important role in the present-day expansion of low-oxygen waters (“dead zones”) in coastal systems worldwide. Moreover, it contributed to the development of global scale anoxia in ancient oceans. Critically, however, the responsible mechanisms for the changes in P burial in anoxic sediments are poorly understood because of the lack of chemical tools to directly characterize sediment P. I propose to develop new methods to quantify and reconstruct P dynamics in low-oxygen marine systems and the link with carbon cycling in Earth’s present and past. These methods are based on the novel application of state-of-the-art geochemical analysis techniques to determine the burial forms of mineral-P within their spatial context in modern sediments. The new analysis techniques include nano-scale secondary ion mass spectrometry (nanoSIMS), synchotron-based scanning transmission X-ray microscopy (STXM) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). I will use the knowledge obtained for modern sediments to interpret sediment records of P for periods of rapid and extreme climate change in Earth’s history. Using various biogeochemical models developed in my research group, I will elucidate and quantify the role of variations in the marine P cycle in the development of low-oxygen conditions and climate change. This information is crucial for our ability to predict the consequences of anthropogenically-enhanced inputs of nutrients to the oceans combined with global warming."