Award: OCE-1737167

Intellectual Merit Outcomes Certain trace elements in seawater – including iron, zinc, copper, nickel, and manganese – serve as micronutrients for the photosynthesizing plankton that live in the surface ocean; these plankton are climatically important because they sequester carbon to the deep sea via what is known as the biological pump. The goal of this project was to collect seawater samples from the Central Pacific’s GEOTRACES GP15 cruise and analyze them for their dissolved trace metal micronutrient concentrations. The Central Pacific is a particularly interesting area to study micronutrients because it is so remote from land and thus must receive its inputs from hydrothermal sources or via transport from sedimentary and/or atmospheric sources closer to land. The Equatorial and Subarctic Pacific are also two regions where photosynthesis rates can be limited by the micronutrient iron, and so understanding the controls on metals fluxes to these important regions is particularly important. This award funded the successful collection, (ultra)filtration, and analysis of seawater samples for trace metal concentrations from the US GEOTRACES GP15 Pacific Meridional Transect cruise, which sailed from Alaska to Tahiti in November-December 2018. From these data, we developed high-resolution sections of the bioactive dissolved trace metals iron, zinc, copper, cadmium, nickel, and manganese. We also collected data on the size partitioning of these dissolved metals into soluble (truly dissolved) vs. colloidal (nanoparticulate) fractions, which has implications on the ultimate fate and bioavailability of these metals in the ocean. We also developed a section of the contaminant metal dissolved lead and worked with collaborators to measure the dissolved lead isotope ratios in seawater, which reveals the source of lead along the section. We collaborated with several other groups studying dissolved trace metals along GP15, including Seth John (University of Southern California), Tim Conway (University of South Florida), Edward Boyle (MIT), and Mak Saito (Woods Hole Oceanographic). We intercalibrated our dissolved metal concentrations with their laboratories and co-published nine scientific publications with their teams. Conclusions led by the team on this project included (but are not limited to): 1) Intraplate (hotspot) hydrothermal vents, such as Kama’ehuakanaloa (formerly Lo’ihi) Seamount near Hawai’I, can produce hydrothermal plumes carrying significant dissolved iron and manganese fluxes. At the end of thermohaline circulation, Kama’ehuakanaloa iron is rich in Fe(II) that precipitates slowly within the distal hydrothermal plume and thus persists longer than in other plumes of similar size. However, modeling estimates reveal that this hydrothermal iron does not make it to the surface ocean to facilitate carbon drawdown. 2) Biological uptake appears to fractionate dissolved iron isotopes using a similar 1 permil fractionation factor along the entire ~8000 km length of the GP15 section. 3) Dissolved iron scavenging onto sinking particles within North and Equatorial Pacific particle veils appears to remove ~0.15 nmol/kg of dissolved iron but does not fractionate dissolved iron isotopes during this process. 4) Dissolved lead isotopes reveal the influence of anthropogenic lead in the surface ocean. Surprisingly, this anthropogenic lead penetrates the ocean’s oldest waters in the abyssal Pacific, an area that has ages preceding the Industrial Revolution, within the two particle veils of the North and Equatorial Pacific Ocean. These waters should not contain anthropogenic signatures, yet this lead reaches those waters via reversible scavenging on the backs of sinking particles. Broader Impacts Outcomes The project provided significant professional development and training opportunities for participating personnel. It also provided support for the training of three PhD students. One of these students received mentorship training on this project, while the other received fieldwork training. The third PhD student’s dissertation was based entirely on this project; this student successfully defended his PhD and is continuing his research in a postdoc and adjunct teaching position. This project also provided support for the training of four undergraduate students (three female), two of whom are now graduate students studying marine biogeochemistry. Finally, this project supported a young lab technician, who continued on to a PhD in heavy metal contaminant following this project. This project also funded the ultrafiltration of samples for several community users during the sample collection expedition. It also funded the analysis of samples from an experiment of opportunity to investigate the effects of releasing deep-sea mining effluent into the oxygen minimum zones overlying the Clarion-Clipperton Zone, an area of interest for the extraction of polymetallic nodules from the seafloor near the GEOTRACES GP15 transect. Finally, this work addresses questions of international interest within the scope of the International GEOTRACES Program. All data will be archived through the GEOTRACES Data Assembly Center and will be made available to the public through the GEOTRACES Data Products and electronic atlases (www.egeotraces.org). Last Modified: 07/05/2023 Submitted by: Jessica N Fitzsimmons