Award: OCE-1756126

Understanding marine ecosystems requires a detailed understanding of the supply of certain nutrients.   This supply drives the growth of phytoplankton, the base of the marine food web, which, in turn, generate food for marine life, and sustain marine fisheries.  The trace element iron (Fe) is one of these essential nutrients.  Indeed, Fe is the key nutrient that limits the growth of phytoplankton in much of the subarctic NE Pacific Ocean.  Past work by Crusius et al (2011) suggested that glacier-derived dust was a source of iron, generated by unusual, northerly winds, from the southern coastline of Alaska. 

      This project sought to evaluate how far this glacier-sourced dust travelled, and thus where in the NE Pacific this glacier-derived dust supplied an important source of iron.  Our results suggests that this Alaskan, glacial dust is a source of Fe throughout much of the subarctic NE Pacific, travelling as far as Ocean Station Papa (50.1°N, 144.9°W), more than 1100 km from the glacial source.  This evidence comes directly from previously published seasonal measurements of iron from Ocean Station Papa (Nishioka et al, 2001) that reveal a significant autumn source of dissolved Fe to the upper water column, spanning two successive years (Crusius, JGR Biogeosciences, 2021).  This Fe is likely supplied by the glacial dust, as other possible sources, including eddies, upwelling, fossil fuel combustion, ocean current from coastal areas, and more, are largely ruled out.  This dust is caused by unusual, very strong wind from the north, channeled simultaneously through several different narrow mountain gaps, spanning ~1000 km of coastline.   This glacial dust  is not predicted by current atmospheric model simulations because typical global models use a spatial resolution that is 10-100x too coarse to resolve the processes driving winds through the narrow mountain gaps (1-2 km wide, in places).  Additional evidence for transport of this glacial dust more than 1000 km from the dust source emerged from measurements of rare earth elements, and the element thorium, on Alaskan glacial dust as well as previously published data from North Pacific sediment (Crusius et al, GRL, 2024).  Because the concentrations of these elements differ among the various source materials, we were able to quantify how much Alaskan glacial dust, dust from deserts in Asia, and volcanic materials were delivered to these locations. This approach confirmed that the Alaskan glacial dust was transported and deposited at least ~1000 km from its source.  The glacial dust contributes a larger portion of the annual supply of Fe, compared to dust from Asian and volcanic materials, both because of the annual nature of the dust transport events and because of evidence that fine, physically weathered glacial dust releases larger amounts of Fe than do many other lithogenic sources (Schoenfelt et al, 2017; 2018; 2019).

      This work also measured the ratios of the isotopes of lead (Pb) on six different size fractions of collected Alaskan glacial dust, as an additional tool to allow future researchers to identify the geochemical impacts of this dust source at a distance, from measurements in ocean sediments or the water column. This work revealed a consistent distinctive Pb isotope signature of the dust from the larger size fractions (>3 um).  The measurements also showed that a large proportion (up to 50%) of the Pb from the smallest size fractions (<1.5 um) was fossil fuel-derived Pb.    The fossil fuel-derived Pb in the smaller size fractions could be transported in the exhaust of small planes, which still use tetraethyl Pb as an additive.

      This project served as a research focus for a postdoctoral fellow, an M.Sc. student, and a research technician, each for about one year.  The technician has since started working on an M.Sc. degree in the field.

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Last Modified: 06/03/2025
Modified by: James W Murray