EVALUATING THE TRACE METAL FERTILIZATION POTENTIAL OF TERRESTRIAL DUST AND VOLCANIC ASH SOURCES TO THE GULF OF ALASKA

The subarctic Pacific Ocean and Bering Sea comprise the second-largest high nitrate, low chlorophyll region in the world, where primary production is limited by the availability of iron (Fe). To estimate the potential impact of different terrestrial aerosol Fe sources such as glacially-sourced dust and volcanic ash on marine ecosystems, we performed a comprehensive suite of laboratory assessments following established protocols. These include: 1) leaching with Milli-Q ultrapure water and filtered seawater from the Gulf of Alaska to assess instantaneous trace metal solubility, 2) sequential extractions to determine Fe speciation, 3) complete acid digestions to measure total metal concentrations, 4) X-ray diffraction to determine mineralogy, and 5) X-ray absorption near-edge spectroscopy (XANES) to infer Fe oxidation state. Measurements were performed on a set of glacially-sourced silt and volcanic ash samples from Alaska, as well as one desert dust sample collected from the Tengger desert in the Tibetan Plateau. Volcanic ash samples represent a significant expansion from prior work (Koffman et al., 2021, Global Biogeochemical Cycles), including a total of 15 eruptions from six different volcanoes: Augustine, Katmai/Novarupta, Kasatochi, Redoubt, Pavlof, and Veniaminof. We build on prior results, which showed that the glacier-derived sediments have five times higher easily-reducible Fe (median 2.3 ± 0.6 wt. %) than desert-derived samples (0.49 ± 0.1 wt. %) and fourteen times higher easily-reducible Fe than fresh ash (0.16 ± 0.1 wt. %), as well as ten times higher fractional Fe solubility (median cumulative 0.31 ± 0.11% Fe_S) than volcanic ash (0.04 ± 0.02 % Fe_S). Glacial sediments also contained higher concentrations of other bioactive metals including Co, Ni, Cu, Zn, Mo, Cd, and Pb. Detailed geochemical analyses coupled with incubation experiments using natural phytoplankton communities from the Gulf of Alaska allow us to determine the geochemical properties of dust and volcanic ash that have the greatest impact on phytoplankton growth (e.g., easily-reducible Fe, proportion of Fe(II)). In turn, the results can be used to estimate the iron fertilization potential of different nutrient sources and their relative impact on the Gulf of Alaska ecosystem and across the subarctic Pacific.