Model predictions are regularly made about the state of marine ecosystems and carbon cycle in a future high CO2 world. But while increased ocean stratification means lower future biological export production in some models, in others, higher temperatures inducing faster rates of biological activity drive completely the opposite response. Rates of nitrogen fixation may increase in a warmer ocean and support higher production, but at the same time, loss rates of nitrate will increase as oxygen minimum zones expand and intensify. The cycling of iron also plays a critical role in controlling marine productivity and may limit nitrogen fixation. Yet, while iron bio-availability will be affected by changes in temperature, acidity, and oxygenation, no current model accounts for how iron solubility will respond to future ocean geochemical environmental changes. How can we increase confidence in predicting the intertwined response of nutrient and carbon cycles in a high CO2 world? The geological record contains case studies into the range of possible Earth system behaviours, states, and strengths of feedbacks. Of particularly interest here are the Ocean Anoxic Events of the Mesozoic, which were associated with profound global-scale perturbations of climate and marine biogeochemical cycles, high plankton turnover rates and biological extinction, and the burial of economically important quantities of organic carbon in black shales. They have relevance to future marine biological consequences of warming and acidification, and the spread oxygen minimum zones. Here we propose the first model of marine iron cycle applicable to a high CO2 world, the first analysis of the dynamical interaction of all three nutrient cycles (P, N, Fe), CO2, and climate associated with the onset of Ocean Anoxic Event like conditions, and the first explicit test of the role of iron in triggering an Ocean Anoxic Event.
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