Collaborative Research: US GEOTRACES GP17-OCE: Size-Fractionated Particle Collection and Analysis from Ultra-Oligotrophic to Antarctic Waters

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GEOTRACES is an ongoing international effort to study and understand how low-abundance (“trace”) elements and isotopes are distributed in the world’s oceans. Trace elements include both naturally occurring and human-influenced chemical components of seawater. They have several important roles in the functioning of the Earth that make them a focus of study: 1) their rarity is known to limit the growth of microscopic plants which are the base of ocean food webs; 2) the distributions of trace elements in remote parts of the world show how Earth’s atmosphere, ocean, crust, and ecosystems work together on large, planetary scales; 3) knowledge of these trace components is difficult to acquire but is also an important component to improving computer models of Earth’s climate in a rapidly changing world. The Pacific Ocean covers nearly a third of the planet, and the rarely sampled South Pacific Ocean at the focus of this project includes approximately a quarter of the world’s ocean volume. As part of a GEOTRACES expedition to this remote region, this project has three main goals: 1) to provide the sampling equipment and personnel to collect large volumes of particles from the ocean, because many chemical components in seawater are in particle forms; 2) to document and distribute ocean particle samples to collaborators of many GEOTRACES laboratories; 3) to analyze ocean particles for abundances of approximately two dozen trace elements at laboratories in Savannah, GA and Santa Cruz, CA. A post-doctoral scholar and two graduate students will participate in both field and laboratory aspects of the research.The GP-17 OCE research expedition to the South Pacific Ocean is planned for two months in late 2022 into early 2023. It comprises two transects: a southward transect along approximately 150ºW between French Polynesia and the Southern Ocean which will sample large biological and geochemical gradients from the low-dust, low-biomass oligotrophic sub-tropical gyre, though more productive regions of complex regions of watermass formation and frontal mixing, and into the Southern Ocean. An eastward transect along 67ºS and into Chile will sample the high-opal and Antarctic-influenced Southern Ocean, deep-water hydrothermal influences, and across the South American continental margin. The investigators will use large-volume in-situ pumps to collect size-fractionated particles from the water column which will be distributed to multiple collaborators; and they will operate and maintain particle-sensitive optical equipment to be deployed on the expedition. The research aims to address three main hypotheses: 1) that the large biological gradients from pico-plankton dominated gyre through frontal regions into opal- and Phaeocystis-dominated polar waters will present large gradients in particle export, particle composition, scavenging of trace elements and isotopes (TEIs), and remineralization length-scales of both carbon and TEIs; 2) that particulate iron is dominated by lithogenic (crustally-derived) phases throughout most of the water column, with the exception of regions of strong hydrothermal influence, and that lithogenic particle origins will reflect inputs of external aerosol dust, sediments, hydrothermal sources, and continental inputs as evident via TEI ratios determined after total digestion; 3) that the optical ratio of turbidity to beam attenuation coefficient is sensitive to iron-oxyhydroxide abundances thus acting as a high-resolution indicator of iron-rich authigenic and lithogenic particle inputs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.