Award: OCE-1925503

We collaborated in an oceanographic research cruise (GP09) in April/June 2019 in the tropical Northwest Pacific Ocean roughly east of the Philippine Islands (Fig. 1). The cruise was part of an international research effort known as GEOTRACES and was led by scientists from Xiamen University, China. The cruise included a range of international investigators looking at trace elements and isotopes (TEIs) in dissolved, particulate, and aerosol phases as well as incubation experiments. GP09 afforded an opportunity to obtain trace element clean water samples from a region that is geologically complex as well as being a meeting place for various ocean currents and water masses. It sampled the southwest corner of the North Pacific subtropical gyre, one of the largest oligotrophic (low nutrient) ocean regions and arguably the largest continuous ecosystem on Earth. This region is likely impacted by dust input (including anthropogenic aerosols), nitrogen fixation, hydrothermal effects, and island effects associated with weathering of the Philippines and margin cycling. Understanding material sources, sinks, and cycling in this region is ultimately important for understanding controls on this important regions productivity. Over 500 water samples were collected and subsequently analyzed. We determined dissolved concentrations of barium (Ba), gallium (Ga), vanadium (V), manganese (Mn), cadmium (Cd), lead (Pb), copper (Cu), nickel (Ni) and rare earth elements (REEs). Our studies are pertinent to important issues including delivery of mineral dust and nutrient iron to the surface ocean (Ga), removal and internal cycling of trace elements (Ba, V, Cu, Cd, Ni, REEs), development of tracers of past ocean processes (i.e., paleooceanographic proxies; Ba, V, REEs), tracing sources of material (Ga, Ba, Mn, REEs, V), and understanding of conservative vs non-conservative changes in tracer distributions (Ba, REEs). Other researchers determined additional parameters including plant macro-nutrients (N, P, Si) as well as important micro-nutrients (iron) and source tracers such as aluminum (Al), and radium isotopes (Ra). Comparing our chemical distributions with those determined by others is key for all of the involved research groups to test hypothesized mechanisms of element input, removal, and cycling through the ocean. These mechanisms, in turn, are pertinent to understanding the oceans biological productivity and its role in global climate.issues of greater societal relevance. Results of this work are also being broadly disseminated by publication in peer-reviewed journals, with data made publicly available through established data repositories. A graduate student and postdoc were trained as part of the project. Additionally, this project established a number of international collaborations including with Chinese oceanographers who organized the cruise. We helped them verify their analytical methods and began working on joint publications. Also, we opportunistically took advantage of a 2021 French GEOTRACES cruise (GS02) which obtained samples from the Indian sector of the Southern Ocean, an important area of water exchanges and circulation. And, we established a collaboration with a Japanese scientist who provided us with samples from the Sea of Okhotsk, a marginal basin that appears to impact the Northwest Pacific Ocean. Among our important and useful findings: 1. From GP09, there is a modest hydrothermal plume stretching ~ 500 km westward from the Mariana Trough. Also, the North Equatorial Current (NEC) showed significantly higher surface water light REE (LREE) concentrations at 11 N than at stations at 17 N (e.g., Fig. 2). This is likely due to island weathering enrichment of the LREEs from erosion of the Philippines and recirculation of those coastal waters offshore to be entrained into the NEC. Concentrations of some elements (e.g., Cu, LREEs) also appear to be slightly higher in the deep Philippine Basin than further eastward in the open Pacific (Fig 3). This probably results from isolation of the deep Philippine Basin (i.e., longer residence time of those deep waters). 2. We examined the mass balance of dissolved gallium (Ga) in the North Pacific Intermediate Water (NPIW). The NPIW is a source of nutrients to some surface waters of the North Pacific. Thus, understanding what water sources contribute to NPIW is important. Our Ga data shows that another water mass, Central Mode Water, is a previously unrecognized source for the NPIW. Composition of NPIW was also examined using REEs and water mass analysis. We observed continental influences on NPIW REE concentrations. 3. For dust-derived tracers with short residence times (years, e.g., Fe, Al), surface ocean distributions necessarily reflect both input and removal processes. With a somewhat longer near-surface residence time (decades), dissolved Ga (Fig. 4) reflects the dust delivery of this element but with a broader spatial distribution than that of Al and Fe. That is, the Ga distribution reflects transport of dust-influenced waters. 4. Samples obtained from the French Southern Ocean cruise allowed us to refine our estimates of trace element concentrations in this important water mass formation region and speculate on mechanisms of element removal from surface waters. Last Modified: 05/03/2024 Submitted by: ChristopherTHayes