Collaborative Research: Field and Modeling Studies of the Magnitude and Variability of 234-Th Based Estimates of POC Export Flux and Remineralization in the Upper Ocean

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ABSTRACTOCE-0327721 / OCE-327644 / OCE-0327693The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. Over the past decade, 234Th has been increasingly used as a tracer to estimate POC export from the euphotic zone by multiplying the depth-integrated 234Th flux by the POC/234Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/234Th ratio and 234Th flux, yet the significance of this variability to estimates of POC export remains uncertain. The global distribution of 234Th-derived POC export fluxes can vary by 2-10 times or more due solely to variability in the POC/234Th ratio and procedures used to estimate the 234Th flux. Thus, despite over a decade of research, our understanding of the magnitude and variability of 234Th-derived POC export fluxes, and the dominant controlling mechanisms, is not well constrained.In this project field, researchers at the University of Rhode Island, Texas A & M University, and University of Georgia will conduct field and modeling studies to address the following major question: How accurate are 234Th-based estimates of POC export flux from the euphotic to the mesopelagic zone, what are the dominant mechanisms controlling 234Th scavenging and export, and how can this tracer technique be improved? They have two working hypotheses designed to address this central question. Hypothesis 1: Regional variability in the magnitude of 234Th-derived POC export is determined by differences in particle source/organic chemical composition and food web structure. Hypothesis 2: Particle-particle interactions and differential biogeochemical recycling of organic C and 234Th controls POC/234Th ratios and hence the magnitude of 234Th-derived POC export and remineralization. The research strategy is to combine new and existing measurements of sediment trap and particle size-fractionated 234Th, POC and specific organic compounds in the upper ocean (~0-500 m) with newly developed particle coagulation models to validate, test and further develop the proposed working hypotheses. Samples will be collected from three study sites characterized by distinct differences in food-web structure and particle export (Mediterranean, Arctic, NW Atlantic) and designed to provide maximum information on the mechanisms controlling POC/234Th ratios and POC export and remineralization. Model development and testing is a key aspect of the project and serves to guide and validate the field observations and provide information on mechanisms of 234Th estimates upper ocean POC export and remineralization.The intellectual merit of the proposed research bears on a number of national and international carbon cycle research programs (JGOFS, VERTIGO, DYFAMED, CARBOCEAN). Specifically, this proposal has broad intellectual implications for previous and currently NSF funded projects that use (or have used) 234Th to measure POC export fluxes from the upper ocean. For example, POC fluxes obtained during the JGOFS program may need to be revised, and future investigations of the mesopelagic, which will depend on accurate determination of the particle flux entering this region of the ocean, may need to take the results of this work into account. The results could also have implications for the SCOR/IGBP OCEANS initiative of improved understanding of the global ocean carbon cycle.