Award: OCE-1130245

During the past 30 years, the development of sampling and analytical technologies has allowed chemical oceanographers to measure trace elements in the ocean that we previously could not examine. From this we have learned that iron plays a major role in determining where microscopic plants – phytoplankton - can grow in the ocean. We have also found that pollutants such as lead and mercury that come from our past use of gasoline additives (lead) and continued burning of fossil fuels have been globally dispersed and injected into the ocean. Using results for other trace elements and natural radioisotopes, we now know that major changes in ocean circulation have taken place during past climate changes, so we expect that ocean circulation will also vary as the climate changes in the future. However, these findings and observations have been based on relatively few datasets because of the time and efforts it takes to get them (imagine getting samples for iron on a rusty ship), and we really don?t know all the details. In order to simulate/model and predict how the circulating ocean and the life within it may change due to human perturbations to the planet?s surface and atmosphere, we need to have data on a global scale to define the processes that govern trace elements and radioisotopes in the ocean (see processes in Figure 1). The mission of the international GEOTRACES program, which begun its ocean-wide sampling program in 2010, is to identify processes and quantify the rates of transfer that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions. Specifically, we aim to determine global ocean distributions of selected trace elements and isotopes – including their concentrations, and chemical and physical forms – and to evaluate the sources, sinks, and internal cycling of these elements to characterize what regulates their distributions. As part of the international planning, United States oceanographers aimed to contribute 5 major "sections" (top to bottom measurements at ~25 "stations" along a ~4000 mile long ship?s track) during the 2010-2020 decade. The first US section was in the North Atlantic Ocean, and the second discussed here was in the tropical southeastern Pacific Ocean from Peru to Tahiti (Figure 2) in late 2013. Thirty four scientists set up shipboard sampling systems (Figure 3) and chemical laboratories on the Research Vessel Thompson based at the University of Washington, and then they and 22 crew members departed from Manta, Ecuador on October 25, 2014. We sailed southwards along the coast of South America. Our first major sampling station was in the Peru Trench that is over 6500 m deep, and from there we went into the shallow shelf waters near the capital city of Lima. After sampling shelf waters, we proceeded westward for over 3800 miles until we reached our last station northeast of Tahiti. We then ended the cruise in Papeete, Tahiti on December 20, 2013, where we offloaded all of our samples and scientific equipment to be shipped back to the United States. The work at sea went very well and the cruise was extremely productive. We collected more than 37,000 individual samples that were distributed to 32 laboratories for the measurement of more than 100 properties. Much of the shore-based work has been completed, but some laborious measurements are still underway as this as written. Much of the data was presented at a post-cruise workshop at the Wrigley Marine Science Center on Catalina Island, California, in 8-13 November 2015, and the participants found the results to be interesting and would change the way we view some oceanographic processes. In particular, in sampling the hydrothermal vent plume emanating from the East Pacific Ridge where new ocean plates are being formed (Station 18 in Figure 2), concentrations of dissolved iron and manganese were extremely elevated, similar to what has been found at with other hydrothermal vents. However, the surprising finding that will change our view of hydrothermal vents as sources of trace elements to the ocean was that we detected this iron "plume", albeit diluted, all the way to the end of the cruise (Station 36, more than 2000 miles away). Clearly, hydrothermal vents may be changing the chemistry of the entire ocean and not just within hundreds of miles. As the rest of the data are generated, compared, and synthesized, we are likely to learn much more about processes affecting trace elements and isotopes in the ocean (Figure 1). Last Modified: 08/19/2016 Submitted by: Gregory A Cutter