Award: OCE-1658318

Over the last 40 years, the development of specialized sampling and analytical methods has allowed chemical oceanographers to measure trace elements in the ocean that we previously could not examine due to contamination from the ship and concentrations that were too low. However, with these new methods we have learned that iron acts as a nutrient for microscopic marine plants, phytoplankton, and allows them to grow. We have also found that pollutants such as lead and mercury 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 it will also vary as the climate changes in the near future. However, these findings and observations are 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 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 control 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 will determine global ocean distributions of selected trace elements and isotopes, including their concentrations, and chemical and physical forms, to evaluate the inputs, outputs, and internal cycling of these elements to characterize what regulates their distributions. United States oceanographers aimed to contribute 6 major "sections" (top to bottom measurements at ~25 "stations" along a ~4000 mile long ship's track) during the 2010-2020 decade. The fourth section discussed here was a north-south transect in the Pacific Ocean from Alaska to Tahiti. Thirty-five scientists set up shipboard sampling systems (Figure 2) and chemical laboratories on the Research Vessel Revelle that is run by the Scripps Institution of Oceanography. These scientists and 22 crew members departed from Seattle, Washington on September 18, 2018, sailed north to Alaska and started sampling on the shallow shelf near Kodiak Island (Figure 3). Our first major sampling station was in the Aleutian Trench that is over 5500 m deep, and from there we proceeded due south sampling 17 stations over 2000 miles until we stopped in Hilo, Hawaii to refuel and resupply. We then continued sampling due south at 19 stations until we reached our last southwest of Tahiti. We ended the expedition in Papeete, Tahiti on November 24, 2018, where we shipped all of our samples and scientific equipment to the United States. We collected more than 40,000 individual samples that were distributed to 51 laboratories for the measurement of more than 100 properties. Interpretations of these sample results are still underway due to delays from COVID-19 shutdowns. Data were reviewed in a series of online Zoom sessions from 2020-2021 and in March 2022 we had an in-person data synthesis meeting at Old Dominion University with 48 scientists. Among the major findings were that in the northern Pacific Ocean there is a region of high particle production from biological activity that effectively removes some trace metals like lead from the water column, while near Hawaii the active underwater volcano Kamaʻehuakanaloa (formerly called Loihi) is a large source of the biologically-essential trace metal iron. We tracked this plume of iron-rich water eastward at about 3000 feet depth where it appears to travel all the way to Mexico. Another major event for this expedition was sampling waters above the Clarion-Clipperton Fracture Zone, a region of ocean bottom between 5-15 deg N where there are abundant manganese nodules that contain high concentrations of metals such as nickel and cobalt needed for electronic components and batteries. Deep sea mining of these valuable nodules is set to begin soon, but inputs of trace metals from the disturbance of the ancient sea bottom and debris in the water column could have extreme effects on the biota of this region. Our samples represent some of the first baseline data for trace metals in the water column before mining begins. As our vast amounts of data are compared and synthesized, we are likely to learn much more about processes affecting trace elements and isotopes in the ocean (Figure 1). Last Modified: 01/01/2024 Submitted by: GregoryACutter