Th(IV) and Pa(IV,V) isotopes are important proxies in oceanographic investigations, such as, for tracing particle dynamics and particulate organic matter (POC) fluxes out of the euphotic zone through the use of 234Th/POC ratios, and for studying boundary scavenging, paleoproductivity and ocean circulation through the use of 231Pa/230Th ratios. Even though almost routine, these approaches rely on often poorly constrained, empirically determined and variable isotope ratios or ratios to POC. Previously conducted laboratory and field investigations suggest that Th(IV) removal could be controlled through binding by exopolymeric acid polysaccharide (APS) rich biomolecules, potentially produced by both phytoplankton and bacteria. However, researchers at Texas A&M University - Galveston and the University of Alaska Fairbanks Campus believe that Pa(V) present in ocean water must first be reduced to Pa(IV) by organic biomolecules before efficient binding to solid phases can occur, and that the most efficient binding would occur to APS-rich biomolecules produced by phytoplankton species such as diatoms, prymnesiophytes and cyanobacteria.
In this study, the team of scientists at Texas A&M Research Foundation and the University of Alaska Fairbanks Campus will investigate the possible fractionation mechanisms between Pa(IV,V) and Th(IV) in the ocean. It is essential to understand such a mechanism, since the Pa/Th ratio is frequently used as a proxy in oceanographic applications. The proposed interdisciplinary experimental approaches will require instrumental approaches for characterization studies, in combination with controlled laboratory and field experimentation. Laboratory studies consist of uptake experiments to a number of substrates, including purified APS harvested from phytoplankton and bacterial cultures to be used in Th(IV) and Pa(IV,V) binding assessments. The most important analytical task will be to better characterize, both chemically, in terms of molecular composition, and physically, in terms of surface activity, the newly discovered strongly Th(IV) complexing APS of ~13 kDa molecular weight, found in particulate and colloidal material collected from the Gulf of Mexico, Atlantic and Pacific Ocean and the South China Sea. The field program in this study will include collection and extraction of diverse types of organic matter for use in laboratory studies, as well as the determination of temporal and spatial variations of radiochemical and biochemical parameters.
