Project: Collaborative Research: U.S. GEOTRACES PMT: Dissolved trace metal distributions and size partitioning

NSF Award Abstract:
The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. Many trace metals, which are by definition present in very low amounts, are essential for life and thus considered nutrients for phytoplankton growth. Other elements can be useful for tracing other ocean processes, and some (such as lead) are important because they are pollutants. The primary goal of this project is to measure the concentrations of iron, manganese, zinc, copper, cadmium, nickel, lead, and scandium dissolved in seawater along a line of full-depth ocean stations extending south from Alaska to Tahiti in the Pacific Ocean. Specialized sampling and filtration techniques will also enable these investigators and their colleagues to determine the distribution of these metals on very small particles. A graduate student and several undergraduate students will take part in the project.

The U.S. GEOTRACES Pacific Meridional Transect, planned for the fall of 2018, aims to systematically and thoroughly determine the distribution of trace elements and isotopes on a cruise transect that spans zonal bands of sedimentary, atmospheric, and hydrothermal metal supplies; intersects several zonal biological production and oxygenation regimes; traverses the complex equatorial upwelling system; and encompasses the oldest known deep waters in the ocean. The primary goal of this proposal is to determine the concentrations of dissolved (<0.2 micron-filtered) micronutrient metals Fe, Mn, Zn, Cu, Cd, and Ni, as well as tracers Pb and Sc, on every water column and surface sample collected, using two well-established multi-element analytical methods that will be intercalibrated internally to maximize data quality. Many of these are "key elements" required for analysis on GEOTRACES cruises. Ultrafiltration will also be completed on all samples to determine the "truly soluble" (<0.003 micron) concentrations of these elements and to calculate, by difference, the colloidal (0.003-0.2 micron) fraction. The overarching goal of the proposed research is to understand the ocean fluxes and processes that control the distributions of micronutrient trace metals, which themselves modulate primary production and oceanic carbon dioxide uptake, in the Pacific Ocean. The proposed research will allow for an estimate of the meridional influence of each of these metal source fluxes and processes in the Pacific, which is poorly constrained to date due to the mostly zonal Pacific cruise transects in the past. Colloidal distributions are specifically targeted in order to derive additional information on the reactivity (with respect to both scavenging and bioavailability) of distinct dissolved metal pools, as well as to provide a basic constraint on metal physicochemical speciation, which is poorly defined for many of the eight metals.