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Applications of U-decay series isotopes to studying the meridional overturning circulation and particle dynamics in the ocean Luo, Yiming


Two important questions in the fields of paleoceanography and marine biogeochemistry (the reconstruction of past changes in the strength and geometry of the ocean’s overturning circulation and the quantification of particle flux to the seafloor) are addressed using three isotopes from the U-decay series (²³⁴Th, ²³⁰Th and ²³¹Pa). Two-dimensional scavenging models of the Atlantic and Pacific Ocean were tuned to reproduce the ²³⁰Th and ²³¹Pa seawater activity profiles measured in these oceans and used to establish the distribution of sediment ²³¹Pa/²³⁰Th generated by simple meridional overturning circulation cells. The results indicate that circulation is the main factor controlling the distribution of sediment ²³¹Pa/²³⁰Th in the Atlantic and confirm the use of this proxy as a paleocirculation tracer. In the Pacific, both circulation and boundary scavenging are important in determining the distribution of sediment ²³¹Pa/²³⁰Th. Thorium-234 scavenging and moored sediment traps yield similar particle flux estimates in Saanich Inlet, on the coast of British Columbia. This study highlights the possibility of estimating the flux of organic carbon in coastal waters by simply measuring ²³⁴Th and POC on particles, which would provide a simple and rapid method for large scale monitoring. Measurements of ²³⁴Th and ²³⁰Th dissolved in seawater and adsorbed on three different size classes of particle were used to estimate particle flux in the epipelagic and mesopelagic zone of the ocean at station Papa. The results suggest that a significant fraction of the carbon flux can be associated with very large, rapidly-sinking particles with very low Th activities, and unaccounted for in Th-based flux estimates.

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