Using uranium- and thorium-series isotopes as tracers for trace-metal inputs to the oceans

This thesis explores the use of U-Th series isotopes as tracers to study ocean mixing and chemical inputs to the open ocean from atmospheric dust, rivers, continental shelves and ocean sediments.The use of Th isotopes as an improved tracer of dust input has been tested by measuring 232Th and 230Th concentrations of upper-ocean seawater at six stations along a meridional Atlantic section that spans the Saharan dust plume. The results of 232Th-derived dust fluxes show good agreements with dust model depositions, and suggest that thorium may be a more accurate tracer of dust input than Aluminium, which is frequently used for this purpose.A new technique has been developed for the precise measurement of 228Ra/226Ra ratios and 228Ra and 226Ra concentrations in seawater by multi-collector ICP mass spectrometry (MC-ICP-MS). This methods offers improved analytical precision and detection limits relative to previous decay-counting approaches, though is more labour intensive. The accuracy of this method has been proved in the GEOTRACES Ra intercalibration, and shows consistent results with decay counting measurements from other labs. This technique has been applied to water samples in Loch Etive, a Scottish fjord, and in the Cape Basin during the UK-GEOTRACES cruise (GA10E) to investigate ocean mixing and trace-metal fluxes in the oceans.In Loch Etive, Ra data provide an estimation of sedimentary 228Ra flux of 5.5 ± 0.3 (× 109 atoms m-2 yr-1) in the inner loch deep basin. Short-lived Ra isotope (223Raex) has also been used to estimate surface water transport rate, which is ~ 2.4 ±0.2 cm s-1 from the mouth to the head of the loch. In the Cape Basin, seawater 228Ra and 226Ra activities have been used to estimate vertical and horizontal mixing in the surface ocean. The horizontal mixing (Kx) from the continental shelf to the open ocean is 3.8 ± 0.8 (× 107 cm² s-1) and the vertical mixing (Kz) in the upper 400 m layer is 0.9 – 2.1 cm² s-1. These mixing rates enable the calculation of nutrient flux in the surface ocean. The calculated vertical nitrate fluxes in the Cape Basin are 0.4 – 0.5 mmol N m-2 d-1. This finding shed light on the nutrient inputs to the South Atlantic Subtropical Gyre.