An Estimate of Thorium 234 Partition Coefficients Through Global Inverse Modeling

Guillaume Le Gland,Olivier Aumont,Laurent Mémery

doi:10.1029/2018jc014668

Abstract

AbstractThorium‐234 (234Th), an insoluble radioisotope scavenged by marine particles, can be used as a proxy of the biological carbon pump. Thorium‐234 observations can constrain biogeochemical models, but a necessary first step is to estimate the poorly known partition coefficients between particulate and dissolved phases. In this study, the 234Th partition coefficients for five particle types, differing in size and chemical composition, are estimated by fitting a global 3‐D 234Th model based on the coupled ocean general circulation‐biogeochemistry model NEMO‐PISCES (at a resolution of 2°) to a global 234Th data set (including GEOTRACES data). Surface partition coefficients are estimated between 0.79 and 16.7×106. Biogenic silica has the smallest partition coefficients. Small particulate organic carbon and lithogenic dust have the largest. Thorium‐234 observations at depth cannot be recovered without allowing partition coefficients to increase by one order of magnitude from surface to 1,000 m deep. In our time‐dependent global 3‐D model, the biases introduced by three common assumptions made in biological carbon pump studies can be quantified. First, using the C:234Th ratio of large particles alone leads to an overestimation of carbon export at the base of the euphotic layer, by up to a factor 2. Furthermore, assuming steady state and neglecting transport by advection and diffusion can bias fluxes by as much as 50%, especially at high latitudes and in upwellings, with a sign and intensity depending on the season.

Highlights

The biogeochemical cycles of carbon and macronutrients are complex, depending on many sources and sinks, associated with biological, chemical, and physical processes
Lithogenic dust only accounts for 8.4% of the 234Th export and biogenic silica (bSi) for 12.1%. sPOC, bPOC, and cal are the largest contributors to 234Th export globally, with 30.2% for sPOC, 23.4% for bPOC, and 25.9% for cal. sPOC dominates in the oligotrophic gyres, whereas bPOC and cal dominate in productive regions such as the eastern boundary upwelling systems
Our model suggests that this assumption may not hold as the contribution from small materials, whose partition coefficients are much larger, may be far from small: For instance, in the subtropical gyres, small particles contribute to more than 90% to 234Th export

Summary

Introduction

The biogeochemical cycles of carbon and macronutrients (nitrogen, phosphorus, and silicon) are complex, depending on many sources and sinks, associated with biological (photosynthesis, respiration, ... ), chemical (dissolution, adsorption, ... ), and physical (sedimentation, aggregation, ... ) processes. The distribution of these elements cannot be directly related to specific sources or sinks. Some trace elements and isotopes (TEIs) are affected by a relatively small number of sources and sinks. This is why their distribution contains workable information on key oceanic processes. The international program GEOTRACES has been designed to improve our knowledge on TEIs cycles and the oceanic processes controlling their distribution, by means of observations, modeling, and laboratory experiments. Several TEIs are used as proxies because of their relatively simple behavior in the ocean. One trace isotope of particular interest is thorium-234 (234Th), a proxy of particle fluxes to the deep ocean. Thorium-234 has often been used to estimate a key flux in the global carbon cycle, the biological carbon pump (BCP)

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Conclusion