Project: US GEOTRACES PMT: Shipboard determination of key dissolved trace elements

NSF Award Abstract:
This project will provide shipboard measurements of the dissolved trace elements aluminum (Al), iron (Fe), and manganese (Mn) during an Alaska to Tahiti cruise that will sail as part of the US GEOTRACES Pacific Meridional Transect (PMT). The PMT expedition is part of the international GEOTRACES program designed to improve understanding of global ocean biogeochemical cycles. The growth of microscopic oceanic plants in surface waters depends on the availability of certain trace elements (particularly dissolved Fe). In addition, the distribution of this and other trace elements record current and past chemical and biological processes in the ocean. The PMT cruise track includes multiple unique oceanic conditions and thus the results of the work will provide important insight into diverse global ocean processes. For broader scientific impact, results will be incorporated into models that address the current and future role of the ocean in global processes. Assessing the distribution of trace elements in the Clarion-Clipperton Fracture Zone, along the cruise track, will also provide an important baseline for this region where seabed mining of manganese nodules is planned. Educational outreach will take place with a mid-cruise stop in Hilo, Hawaii to engage local students with visits to the ship. Direct exposure to a working oceanographic research vessel together with a post-cruise follow up visit will provide students with a unique opportunity to discuss the findings from the expedition directly with the scientists engaged in the project.

To develop the data sets required for new understanding of the relative importance of trace element input processes in establishing oceanic chemical distributions, seawater samples from the entire water column will be collected using a custom designed sampling package at 41 locations during the 62-day cruise. The northern part of the cruise track originates in a region where very low concentrations of dissolved Fe in surface waters limit plant growth. Further to the south, increased atmospheric deposition of mineral dust blown from the continents will be traced by measuring the dissolved Al concentration in surface waters. The addition of this dust also adds dissolved Fe. Potential additional inputs of Fe from coastal regions will be identified by coincident increases in dissolved Mn concentrations. The project will compare and correlate variations in Al, Fe, and Mn to quantify and identify distinct addition processes along the cruise track. By using shipboard flow injection analysis to provide near real-time measurements for these trace elements, the potential exists to modify sampling strategies for maximum scientific return. The shipboard results will also help identify any sampling contamination problems that are a constant risk when collecting these low-level trace elements. Altogether, the shipboard analysis of these essential elements is an essential activity to ensure success of the GEOTRACES PMT.