Award: OCE-1737024

We participated in an oceanographic research cruise ("GP15") in late 2018 going from the Aleutians to Tahiti along 152 W in the Pacific Ocean. The cruise was part of an international research effort known as GEOTRACES. The Pacific Ocean is a particularly useful and interesting realm for studying ocean processes such as export production and circulation: for instance, there is a large gradient in biological production moving southward from the subpolar region, plus the North Pacific is where one finds the oldest deep waters in the overturning circulation system. The GP15 cruise transect allowed for sampling of ocean waters in a wide variety of environments including the Aleutian margin (where there is significant input of continental materials), the subarctic North Pacific (where plant productivity may be limited by iron availability), deep waters of the North Pacific (which are the oldest oceanic deep waters) as well as oxygen minimum zones and equatorial waters. Approximately 900 water samples were collected and subsequently analyzed. We determined dissolved concentrations of barium (Ba), gallium (Ga), vanadium (V), rare earth elements (REEs), and methane. 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, REEs), development of paleoceanographic tracers (Ba, V, REEs), tracing sources of material (Ga, Ba, REEs, methane) including margin sources (Ba, REEs, methane), and understanding of conservative vs non-conservative changes in tracer distributions (Ba, REEs). Other researchers involved in the cruise are determining additional parameters including plant macro-nutrients (N, P, Si), iron (Fe), 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 ocean's biological productivity and its role in global climate: issues of greater societal relevance. The knowledge and experience gained from this project have been incorporated into the principal investigator's lectures in oceanography. Results of this work are also being broadly disseminated by publication in peer-reviewed journals, with data made publically available through established data repositories. A graduate student was trained as part of the project. Additionally, another graduate student and a postdoc participated in the cruise. Our research revealed a number of important and useful findings: 1. Our methane (Fig. 1) data provide one of the largest, full-depth oceanic sections of this greenhouse gas. Our data confirm other reports of the low flux of methane from the sea to the atmosphere. Comparisons with nutrients and trace elements (e.g., Ga, and REEs), although not definitive, constrain and support hypotheses regarding both generation of surface ocean methane and resulting indicators of increased methane consumption. 2. Export production, i.e., the export of biological material to the deep ocean, is a key factor not only in ocean biogeochemical cycling but ultimately for the control of atmospheric carbon dioxide and hence climate. Sedimentary barite (barium sulfate) has been used as a proxy of the record of export production through time, yet there is still a lack of understanding of how this comes about or what might be the limitations of this relationship. Our work on the oceanic barium distribution (Fig. 2) contributes to understanding the linkage between particulate barium burial and export production. Furthermore, our ongoing collaboration with colleagues at Woods Hole comparing our dissolved barium with their radium-226 data should help resolve differences in the distributions of these related elements. 3. There has been interest in using sedimentary vanadium as a paleoceanographic proxy. But, with controls on the ocean vanadium distribution uncertain, even to the issue of what controls the surface ocean vanadium depletion, paleoceanographic applications have been limited. Our work on the vanadium distribution allows a better understanding both on dissolved vanadium control processes as well as potential application of this to paleoceanography. 4. Our REE data (see neodymium [Nd] section, Fig. 3) support the notion of significant accumulation of material in the deep North Pacific due to the sluggish mixing along with bottom input. In shallow waters, comparative depletion of light REEs seems to correspond with regions of nutrient limitation, strengthening an overall linkage between nutrient limitation, methane generation, and REE distributions (which can be impacted by uptake during biological methane oxidation). In intermediate waters, the redox-sensitive cerium distribution shows the impact both of continental margin inputs as well as distal signatures of hydrothermal sources. 5. Our dissolved gallium (Fig. 4) distribution helps further establish the utility of this element both as a surface ocean dust tracer with greater spatial/temporal integration than other more reactive dust-derived elements (e.g., Al, Fe) as well as a complementary quasi-conservative tracer in intermediate waters. Indeed, this second aspect has led to a new collaboration with a Japanese GEOTRACES colleague. Last Modified: 02/27/2022 Submitted by: Alan M Shiller