The fingerprint of climate variability on the surface ocean cycling of ironand its isotopes

The essential micronutrient iron (Fe) limits phytoplankton growth when dissolved Fe (dFe) concentrations are too low to meet biological demands. However, many of the processes that remove, supply, or transform Fe are poorly constrained, which limits our ability to predict how ocean productivity responds to ongoing and future changes in climate. In recent years, isotopic signatures (delta 56Fe) of Fe have increasingly been used to gain insight into the ocean Fe cycle, as distinct delta 56Fe endmembers of external Fe sources and delta 56Fe fractionation during processes such as Fe uptake by phytoplankton can leave a characteristic imprint on dFe signatures (delta 56Fediss). However, given the relative novelty of these measurements, the temporal scale of delta 56Fediss observations is limited. Thus, it is unclear how the changes in ocean physics and biogeochemistry associated with ongoing or future climate change will affect delta 56Fediss on interannual to decadal timescales. To explore the response of delta 56Fediss to such climate variability, we conducted a suite of experiments with a global ocean model with active delta 56Fe cycling under two climate scenarios. The first scenario is based on an atmospheric reanalysis and includes recent climate variability (1958-2021), whereas the second comes from a historical and high-emissions climate change simulation to 2100. We find that under recent climatic conditions (1975-2021), interannual delta 56Fediss variability is highest in the tropical Pacific due to circulation and productivity changes related to the El Nino-Southern Oscillation (ENSO), which alter both endmember and uptake fractionation effects on delta 56Fediss by redistributing dFe from different external sources and shifting nutrient limitation patterns. While the tropical Pacific will remain a hotspot of delta 56Fediss variability in the future, the most substantial end-of-century delta 56Fediss changes will occur in the Southern Hemisphere at middle to high latitudes. These arise from uptake fractionation effects due to shifts in nutrient limitation. Based on these strong responses to climate variability, ongoing measurements of delta 56Fediss may help diagnose changes in external Fe supply and ocean nutrient limitation.