EVALUATING THE COMPOSITION OF TERRESTRIAL NUTRIENTS DELIVERED TO THE SUBARCTIC PACIFIC OCEAN

Wind-blown sediment and volcanic ash deposited in high nutrient, low chlorophyll (HNLC) regions can fertilize phytoplankton by providing iron and other trace nutrients, in turn impacting ecosystems and the global carbon and nitrogen cycles. Twenty to thirty percent of the global ocean is considered HNLC and the NE subarctic Pacific is the second-largest HNLC region in the world. Sources of iron-bearing aerosols to this region include desert dust from East Asia, glaciogenic dust from Alaska and NW Canada, and volcanic ash from Pacific Rim volcanoes. The geochemical attributes that make iron biologically available are poorly understood; however, previous work shows that the solubility of iron likely plays a role in iron uptake from dust and ash deposition, and that reduced iron tends to be bioavailable. While chemically weathered desert dust from Asia dominates the dust deposition signal in the NE Pacific, we expect that dust mechanically weathered by glaciers plays a disproportionate role in supplying bioavailable iron and other bioactive metals to offshore waters. Similarly, volcanic ash may contain more soluble iron than dust from either source, due to the highly soluble iron-bearing salts formed during volcanic eruptions. To address these issues, we performed a comprehensive geochemical assessment of glacial flour, loess, and ash samples from Alaska and NW Canada, which we compare to available data on dust from Asia. We evaluated 20 dust samples, two pristine ash samples, and six environmentally aged ash samples spanning the mid-to-late Holocene. We performed complete acid digestions to find total elemental composition, Milli-Q leaches to assess solubility, and sequential iron extractions to determine iron partitioning, with analysis via ICP-OES and HR-ICP-MS. We also determined mineralogy using an XRD and particle size distributions using a Coulter Counter. Using this broad suite of data, we provide new perspectives on the geochemistry of terrestrial aerosols delivered to the NE Pacific and their potential impact on phytoplankton productivity in this iron-limited region.