Award: OCE-1237011

Water samples collected in the ocean between Manta, Ecuador and Papeete, Tahiti (Figure 1; 2), show that underwater volcanoes and hydrothermal hot springs along the global mid-ocean ridge impact the chemistry of the ocean. It was discovered that iron and manganese from these volcanoes could travel great distances (Figure 3) carried by ocean currents. Models show that this iron can reach the surface Southern Ocean where it is a scarce but required nutrient for phytoplankton growth GEOTRACES is an international research program designed to determine the abundance of a broad range of trace elements -- chemical species that are present at concentrations less than one part per million -- in the world's oceans, and to understand the processes that control their abundances. Trace elements are important because some, such as iron, are essential for the growth of phytoplankton, which are the base of the oceanic food web and play a role in regulating the levels of the greenhouse gas carbon dioxide in the atmosphere, others like mercury, are toxic to oceanic biota and humans, while elements like thorium, can provide information on past oceanic conditions and how water moves through the ocean. These are needed if we are to understand how the ocean will change in the future. The abundance of trace elements in the ocean are affected by uptake by plankton, input from seafloor hotsprings, the transport of soil dust from land to the ocean, and through interactions with seafloor sediments and cold seeps on continental margins. The primary research activities of GEOTRACES have been a series cruises that cross entire oceanic basins, sampling the entire water column at regular intervals along a track. These activities will establish the present-day abundance of key elements across major ocean basins. After collection, measurements of elements are made both at sea and in laboratories at researchers' institutions. The US GEOTRACES program has completed four basin-scale studies including one in the eastern South Pacific. Researchers Joseph Resing (University of Washington) and Peter Sedwick (Old Dominion University) were awarded funding to undertake shipboard measurements of the trace elements iron, manganese and aluminum in seawater samples collected along the cruise track. Among the main scientific motivations for this cruise track was a crossing of the southern East Pacific Rise (EPR), a vast north-south chain of active submarine volcanoes known to host numerous seafloor hot springs that discharge super-heated (>300oC) water into the deep ocean. These hot, acidic waters dissolve and mobilize metals and other elements from Earth's solid crust and discharge them into the cold, alkaline waters of the deep sea. It had been widely thought that this dissolved iron and manganese would quickly form particles that would settle to the adjacent seafloor. However, a major and surprising finding of our research was that dissolved iron and manganese from the hot springs persisted in the water column, traveling at least 4,000 km to the west of the EPR (Figure 3). Our results also showed elevated levels of dissolved aluminum above the EPR plume, which was surprising because submarine hot spring fluids are not highly enriched in this trace element. These results suggest that mid-ocean ridge hot springs supply a substantial proportion of the dissolved iron to the deep ocean. This holds particular importance for the vast Southern Ocean, where the growth of phytoplankton in surface ocean is known to be limited by insufficient amounts of iron. When our cruise results were included in computer model simulations by UK collaborator Alessandro Tagliabue, it was revealed that a substantial portion of primary production (growth of phytoplankton) in the Southern Ocean is supported by iron supplied from hot springs along mid-ocean ridge submarine volcanoes. This indicates that this particular source of iron plays a role in sustaining a major oceanic ecosystem. The linkage between processes in Earth's interior, which drive submarine volcanic and hydrothermal activity, and biological processes in the surface ocean was unexpected. The information obtained in this project will facilitate the inclusion of iron, manganese and aluminum in computer models of ocean chemistry and biology, which will improve the ability to predict how the ocean will respond to and modulate future climate change. In addition, the award has supported graduate research by PhD students Susanna Michael and Pamela Barrett at the University of Washington. In addition to publishing scientific papers on our research, our project data have been made publicly available through the NSF Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/), and have contributed to the GEOTRACES Intermediate Data Product 2017 (http://www.geotraces.org/dp/idp2017). The results of this work have been presented at numerous national and international scientific forums by both the PIs and by the GEOTRACES community as a whole. Resing has also presented these results both for outreach at high schools and as science presentations at scientific institutions throughout the world. Last Modified: 01/14/2019 Submitted by: Joseph A Resing