Constraints on the Cycling of Iron Isotopes From a Global Ocean Model. Issue 9 (16th September 2021)

• Record Type: Journal Article
• Authors: König, D.; Conway, T. M.; Ellwood, M. J.; Homoky, W. B.; Tagliabue, A.
• Is Part Of: Global biogeochemical cycles. Volume 35:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-16
• Subjects: biogeochemistry -- iron isotopes -- ocean -- model; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2021GB006968
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Although iron (Fe) is a key regulator of primary production over much of the ocean, many components of the marine iron cycle are poorly constrained, which undermines our understanding of climate change impacts. In recent years, a growing number of studies (often part of GEOTRACES) have used Fe isotopic signatures (δ 56 Fe) to disentangle different aspects of the marine Fe cycle. Characteristic δ 56 Fe endmembers of external sources and assumed isotopic fractionation during biological Fe uptake or recycling have been used to estimate relative source contributions and investigate internal transformations, respectively. However, different external sources and fractionation processes often overlap and act simultaneously, complicating the interpretation of oceanic Fe isotope observations. Here we investigate the driving forces behind the marine dissolved Fe isotopic signature (δ 56 Fediss ) distribution by incorporating Fe isotopes into the global ocean biogeochemical model PISCES. We find that distinct external source endmembers acting alongside fractionation during organic complexation and phytoplankton uptake are required to reproduce δ 56 Fediss observations along GEOTRACES transects. δ 56 Fediss distributions through the water column result from regional imbalances of remineralization and abiotic removal processes. They modify δ 56 Fediss directly and transfer surface ocean signals to the interior with opposing effects. Although attributing crustal compositions to sedimentary Fe sources in regions with low organic carbon fluxes improves our isotope model, δ 56 Fediss signals from hydrothermal or sediment sources cannot be reproduced accurately by simply adjusting δ 56 Fe endmember values. This highlights that additional processes must govern the exchange and/or speciation of Fe supplied by these sources to the ocean. Plain Language Summary: Iron is key in regulating biological activity over much of the ocean so that changes to iron availability affect the global climate. Despite this understanding, we have incomplete knowledge regarding the oceanic processes that cycle iron and the external supply mechanisms. In recent years, iron isotopes have been used to disentangle some of these aspects, thanks to characteristic isotopic endmembers of external sources and assumed isotopic fractionation during certain oceanic processes. However, the effects of external endmembers and oceanic fractionation often overlap, complicating the interpretation of iron isotope signatures in the water column. Here we investigate what drives the distribution of these signatures by incorporating iron isotopes into a global ocean biogeochemical model. We find to best reproduce observations, we needed to include distinct external source endmembers in our model as well as isotopic fractionation during two oceanic processes: iron uptake by phytoplankton and complexation by organic ligands. Other modeled processes affect the iron isotope distribution indirectly, namely remineralization of iron particles and abiotic removal of iron, which often have opposite effects. Poor model performance near some hydrothermal and sedimentary iron sources further indicates that our current understanding of these source processes, as reflected in our model representation, is too simple. Key Points: External source endmembers and fractionation during phytoplankton uptake and organic complexation shape oceanic Fe isotope distribution Imbalances in remineralization and abiotic removal rates produce regionally distinct dissolved Fe isotopic signatures Hydrothermal and sedimentary Fe supply processes have additional controls that are not yet accounted for in global ocean models