Abstract
Deposition of volcanic ash is thought to impact marine biogeochemical cycling by adding soluble iron (Fe) to the surface ocean. The magnitude of this input is a function of the amount of ash deposited, the total Fe content in the ash, and ash‐derived Fe's fractional solubility. However, the relative importance of chemical composition, acidic processing by the volcanic plume, and ash particle size in determining solubility is unclear. We paired an aerosol leach meant to provide an upper limit for fractional Fe solubility with chemical analyses of ash from the Cumbre Vieja (CV) and La Soufrière eruptions, which both impacted the North Atlantic in 2021. Fe in the ash samples is <6% soluble, but Fe fractional solubility in CV ash is approximately triple that of La Soufrière ash. Compared to La Soufrière, a larger proportion of the Fe in CV ash is in silicate rather than oxide minerals, which release more soluble Fe. Elevated levels of surficial fluorine (F) also suggest that CV ash was subjected to a more fluorine‐rich eruption plume and underwent more acidic processing. Particle size does not appear to be a primary control on Fe release. We estimate that the CV eruption had a much larger impact on dissolved Fe (DFe) concentration in the surface ocean than the La Soufrière eruption because of differences in soluble Fe content and particle deposition velocity. These differences may help explain why some eruptions elicit a biological response in the ocean while others do not.
Volcanic ash deposition into the surface ocean supplies seawater with iron (Fe), an important nutrient for marine microbial life. However, we analyzed ash from two recent eruptions and found that <6% of the total Fe in the ash is soluble and potentially available for microbial uptake. Our results suggest that Fe in ash is less soluble than Fe in mineral dust, a major aerosol input to the North Atlantic Ocean. We found that particle composition and processing by the volcanic plume, rather than ash particle size, are the dominant controls on soluble Fe release from volcanic ash. We conclude that some, but not all, eruptions likely add enough soluble Fe to the surface ocean to fuel marine biota.
All volcanic ash samples analyzed have fractional iron solubilities of <6%, but solubility varies Differences in iron fractional solubility for ash can be explained primarily by ash particle composition and plume processing