Trace elements and isotopes (TEIs) in the natural U-Th radionuclide series are central to the goals of the U.S. GEOTRACES during its funded North Atlantic phase over the next three years. The radionuclide 210Po-210Pb pair is designated as one of the priority TEIs by the U.S. GEOTRACES Zonal North Atlantic Survey Section Implementation Plan. The pair has seen application since GEOSECS for quantifying particulate scavenging and carbon flux within the ocean, but processes are still poorly understood at oceanic interfaces. The atmospheric source and half lives of the two isotopes (138 d and 22.3 y) present time frames uniquely suited to trace interface (air-water, bio-water, and sediment-water) processes in the North Atlantic sections.
Under this award, researchers at the University of Delaware, Wayne State University, and Queens College will participate in the U.S.GEOTRACES North Atlantic campaign to study the relationships between these two radioisotopes and the other trace elements and isotopic tracers that will be surveyed. Their work will center around a set of hypotheses. At the air-sea interface, they hypothesize that as the primary input of 210Pb into the surface ocean, levels will be source dependent on the continental input mixtures, in the western temperate and easterly sub-tropical sections. At the biotic-water interface, they hypothesize that different biogenic particle types encountered in the upper waters will affect the fractionation and remineralization depths of 210Po and 210Pb. At the particle-water interface, they hypothesize that interfaces between intermediate lithogenic nepheloid layers (INL) or hydrothermal plumes will be zones of enhanced 210Po and 210Pb scavenging from the surrounding waters.
To test these hypotheses, they will sample and analyze several hundred dissolved and particulate (large and small) samples for 210Po and 210Pb along the GEOTRACES North Atlantic section. About two thirds of the samples will be focused at the six designated "super stations", half above the main thermocline and the other half down across the BNL. The depths will be chosen according to regional atmospheric input, ecosystems, and coordinated TEI sampling. The other third will be detailed across INL detached plumes from coastal waters, across the BNL, and within hydrothermal plumes. The data will be synthesized according to interface scavenging models by particle types (e.g. fine/colloidal, lithogenic and biogenic). As such, the proposed work will be closely coordinated with GEOTRACES PIs already funded to for other particle-reactive (e.g. Th, Pa) or dissolved (e.g. Ra) radionuclide isotopes in the Atlantic Survey Section of GEOTRACES.
Broader Impacts: The broader impacts are closely linked to the GEOTRACES Program as a whole to enhance (1) research infrastructure by providing a broad array of 210Po and 210Pb data useful for biogeochemical scavenging models, (2) education by mentoring graduate and undergraduates, teaching by example from proposed research, (3) participation of under-represented students careers in the geosciences, (4) research training of graduates in marine radiochemistry, and 5) broad dissemination of results through publications, presentations, and on dedicated public UD websites (www.ocean.udel.edu) and at GEOTRACES (www.geotraces.org).
As part of the GEOTRACES project (www.geotraces.org) project, there was a research cruise across the North Atlantic during which scientists gathered samples to map the distribution of trace elements and isotopes in the world’s oceans. Sampling occurred from the sea surface to the sediments and were analyzed to determine the form (dissolved or particulate) and the chemistry (oxidized, inorganic, organic, etc.) of many of the trace elements on the periodic table. We were investigating the activity of the natural radionuclide pair 210Po ("polonium-210") and 210Pb ("lead-210") along the cruise track. These isotopes are part of the 238U ("uranium-238") decay chain. Because the atomic structure of 238U and other radionuclides is not stable, these elements release energy and matter as radioactive decay. The daughter products of 238U decay include thorium, radon, and radium isotopes. These isotopes, along with 210Po and 210Pb, each have unique half-lives (times until half the mass of the isotope has decayed) and chemical behaviors and have all been used to trace processes in the ocean. The half-lives of 210Pb (22 years) and 210Pb (138 days), as well as the fact that 210Pb is only physically and chemically active, while 210Po is also biologically active, make them a unique isotope pair. We know from laboratory experiments and high levels of 210Po in marine organisms and seafood that 210Po bioaccumulates in organisms, but we do not know how or why the 210Po is taken up into cells and tissues. The fact that 210Po behaves in a bio-reactive matter, while 210Pb does not, allows us to look at areas of the ocean where the two elements get fractionated, and infer that these differences are caused by biological activity. There have been fewer than 20 studies of these two isotopes in the ocean. Our results indicate that both 210Po and 210Pb is removed from the surface ocean by sticking to particles and sinking, but that the removal of 210Po is associated with organic, biogenic material. The activity of 210Pb was actually quite high in the surface ocean due to atmospheric input, but was removed from the deeper water column by sticking onto particles at depth. There is very little understanding of what happens to these isotopes in the deep ocean. This study was one of the first to measure the activities of 210Po and 210Pb all the way from the surface to the seafloor, and we found that they behave differently in the plume of the Mid-Atlantic Ridge and also in the suspended particle layer above the sediments. 210Pb was effectively transferred from the dissolved to the particulate phase in these regions, whereas 210Po was not. This is an initial observation and should be examined further, especially in the context of the other elements studied in these layers of the ocean with high lithogenic content. However, the largest surprise from the study was that 210Po is somehow removed from both the dissolved and particulate phase in the mid-depths of the ocean, at a rate that cannot be explained by our current understanding of particle sorption and sinking. Understanding the cycling of these isotopes is important for three reasons: 1) this radionuclide pair has application as a carbon cycle tracer, and the carbon cycle in the ocean has direct implications for Global Climate Change; 2) humans get the majority of their radioactive exposure from natural sources, including high levels of 210Po in seafood; and 3) understanding the rates of reactions and partitioning of elements in the surface ocean can help us understand the role of biology, physics, and chemistry in the biogeochemical cycles of important nutrients and trace elements which support life on the planet. This project supported the graduate education of a female masters student, as well as four (three under-represented minority) undergraduates and a minority female technician at Queens College, the most ethnically diverse college in the country. All the students participated in the analysis of samples and interpretation of results. Generally, women and minorities are not well represented in marine geochemistry, and particularly marine radiochemistry, so this project resulted in both training and increased diversity in this STEM field. The graduate student and PI presented this research at international conferences, while the undergraduates participated in local public presentations. The research from this project was incorporated into large undergraduate classes (500+ students) and was used as subject matter in talks with the public at Queens College events. The data has been submitted for publication in a peer-reviewed journal and should result in more manuscripts in the near future. All of the data can be found on password-protected websites for use by the other scientists involved in GEOTRACES. Normal 0 false false false EN-US X-NONE X-NONE
