Project: US GEOTRACES PMT: Rare earth elements, gallium, barium, and methane as indicators of internal cycling and input processes

NSF Award Abstract:
This project involves participation in an oceanographic research cruise scheduled for mid-2018 and going from Tahiti to Alaska along 152° W in the Pacific Ocean. This cruise transect will allow for sampling of ocean waters in a wide variety of environments. These environments include 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 deep waters of the ocean), as well as oxygen minimum zones, hydrothermal plumes, and equatorial waters subject to upwelling. The investigators will determine dissolved concentrations of barium (Ba), gallium (Ga), rare earth elements (REEs), and methane. These studies are pertinent to important oceanic issues including delivery of mineral dust and nutrient iron to the surface ocean (Ga), removal and internal cycling of trace elements (Ba, REEs), development of tracers of past ocean processes (Ba), and tracing sources of material (Ga, Ba, REEs, methane) including margin sources (Ba, REEs, methane). Other researchers involved in the cruise will determine additional elements and isotopes including iron (Fe), aluminum (Al), and radium isotopes (Ra). Comparing these chemical distributions 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. The knowledge and experience gained from this project will be incorporated into the principle investigator's courses in oceanography. A graduate student will also be supported and trained as part of this project.

A researcher from the University of Southern Mississippi will participate in the 2018 US GEOTRACES Pacific Meridional Transect (PMT) going from Tahiti to the Aleutians along 152° W. During the cruise, samples will be collected from regions exhibiting strong margin fluxes, the subarctic HNLC waters, the oldest deep water in the world's oceans, the distal ends of hydrothermal plumes from the Juan de Fuca Ridge and East Pacific Rise as well as oxygen minimum zones, equatorial upwelling, and some of the most oligotrophic waters in the world's oceans in the South Pacific gyre at 20°S. The samples will be analyzed for dissolved gallium (Ga), barium (Ba), rare earth elements (REEs) along with dissolved methane. These 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, REEs), development of paleoceanographic tracers (Ba), 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). Overall, the gradients in dust delivery, productivity, age of deep waters, and extent of oxygen minimum zones in the PMT provide opportunities to compare how trace element distributions are affected by these gradients and hence inform the interpretation of the distributions. The PMT will also provide the opportunity to examine evolution of chemical signals in deep and bottom waters in a basin with fewer water masses and a longer timescale of basin mixing than the Atlantic. As such, this data may provide an opportunity to tease apart conservative mixing from non-conservative biogeochemistry and will include using water mass deconvolution to estimate the conservative component of trace element distributions, element-AOU plots, and distributions of the deviations from global element-nutrient correlations. The cruise also allows extensive collaboration with other investigators. Thus, the dissolved Ga data will be compared with data obtained by colleagues on distributions of other lithogenic, rapidly-scavenged elements like aluminum (Al) and thorium-232; the dissolved Ba data will be shared with those determining radium and Ba isotopes; and, the REE data will be made available to those examining neodymium (Nd) isotopes as well as compared with other scavenging tracers such as scandium (Sc). 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.