A multi-element overview of upper ocean trace metal cycling in the Pacific Ocean: GEOTRACES GP15 PMT demi stations

The upper waters of the Pacific Ocean are an exciting place to study trace metal biogeochemical cycling because of the multitude of processes affecting metals in these waters, including dust and sediment fluxes, multiple water masses and circulation regimes, oxygen minimum zones, and gradients in biological productivity and community composition. Here, we present an overview of the dissolved Fe, Mn, Zn, Cu, Cd, Ni, and Pb concentrations across the demi stations (upper 1000 m) of the U.S. GEOTRACES GP15 section, which extends along 152°W from Alaska southward to Tahiti (20°S). We will make extensive use of metal:metal comparisons to trace dust and sediment inputs to Pacific waters. We will also calculate metal:macronutrient ratios and relationships with chlorophyll abundance across surface waters of the transect to assess nutrient limitation across the transect, especially in the context of prior North Pacific studies. We will also summarize which metals attain concentration minima in the subsurface deep chlorophyll maximum (DCM) and where in the transect this happens, as in the past these DCM concentration minima have been linked to both aggregative removal onto particles in dusty regions and biological removal in productive regions; the gradients in this dataset will allow those two processes to be distinguished. Finally, we will interpret all metal distributions in the context of water masses and circulation, especially in deeper waters where AAIW moves northward and meets NPIW. We hypothesize that these water masses carry unique metal:metal signatures indicative of biogeochemical processes native to their formation regions. We will use remineralization and scavenging tracers to track the relative roles of mixing, remineralization, and scavenging within these water masses as they age. Our goal is to bring a multi-element context to North Pacific trace metal biogeochemistry.