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).
Trace element cycling in the ocean has an important control on marine ecosystem functioning and the carbon cycle. Pb-210 and Po-210 are daugter products in the U-238-decay chain and has contrasting biogeochemical behavior in marine system. Po-210 preferentially bioaccumulates within organic tissues, in addition to the removal by terrigenous and biogenic particulate matter. Pb-210 is predominantly removed by terrigenous particulate matter. Their overall geochemical behavior in marine system is relevant to export of particulate organic carbon from the upper ~200 m, export fluxes of sinking particulate matter, sediment mixing and deposition rates. As a part of our group (Tom Church, University of Delaware - dissolved phase; Gillian Stewart, Queens College - >51 um particulate matter and Mark Baskaran, Wayne State University - 0.8-51 um), we measure particulate and dissolved activities of Po-210 and Pb-210. The primary objective is to improve our understanding of processes that control the distribution of Po-210 and Pb-210 nuclide cycles in the North Atlantic and its interfaces. We collected and analyzed seawater samples from 7 super stations and include: a well-developed bottom nepheloid layer from 2900 m depth to bottom, a station with oxygen minimum zone and at the Mid-Atlantic ridge where the Trans-Atlantic Geotraverse (TAG) hydrothermal plume was observed. Based on our results, we report the following: i) There is no disequilibrium between Po-210 and Pb-210 in oligotrophic station; ii) Remineralization of particulate matter could redistribute Po-210 and Pb-210 in the whole water column; iii) What we find at the interfaces (upper 300 m, bottom 300 m) seem to reflect the partitioning of nuclides in the overall water column; and iv) Interfaces provides clue on the extent of redistribution of Po-210 and Pb-210 - mechanistic linkages between interfaces and the whole water column needs to be reassessed.
