Physical weathering intensity controls bioavailable primary iron(II) silicate content in major global dust sources

Abstract

The speciation of iron (Fe) reaching the ocean, for instance in wind‐blown dust and coastal sediments, impacts its bioavailability to phytoplankton and its impact on atmospheric carbon dioxide (CO2) and climate. For dust reaching the Southern Ocean, primary Fe(II) silicates that are physically weathered from bedrock are highly bioavailable compared to more chemically weathered, Fe(III)‐rich species, suggesting that weathering in dust source regions impacts the bioavailable Fe supply. However, this phenomenon has not been studied in other important terrestrial Fe sources, where weathering regimes and source geology vary. Here, we use Fe X‐ray absorption spectroscopy on marine sediment cores to show that major global dust and sediment sources impacted by high physical weathering contain abundant primary minerals and thus are overlooked as a source of highly bioavailable Fe globally. Thus, it is important to consider the role of physical versus chemical weathering in Fe fertilization and biotic CO2 cycling. Plain Language Summary Iron is an important trace metal nutrient that is necessary for all life. In many regions of the ocean, terrestrial dust and sediment are important iron sources to living things, especially photosynthetic organisms that can lower atmospheric CO2 and global temperatures over time. For an important dust source in the iron‐limited Southern Ocean, the chemical form of iron controls how well it is utilized by organisms; the iron contained in fresh bedrock minerals is more bioavailable than iron that has been exposed for longer periods on land and has turned to rust. Thus, the physical weathering of bedrock by glaciers has been shown to mobilize highly bioavailable iron that likely fertilizes phytoplankton, but we do not know whether physical weathering by other mechanisms in other parts of the globe influences iron bioavailability. Here we show that physical mobilization of primary minerals increases the bioavailable iron content of other major dust and sediment sources, and we confirm that regions dominated by chemical rather than physical weathering do not contribute sediments with significant amounts of highly bioavailable iron.