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

Thesis

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 (K_x) from the continental shelf to the open ocean is 3.8 ± 0.8 (× 10⁷ cm² s^-1) and the vertical mixing (K_z) 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.

Authors: Hsieh, YT

• Publication date: 2012
• DOI: 10.5287/ora-7jxbd7bwg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: radium; thorium; uranium; geochemistry; dust; iron; nutrient; ocean mixing; isotope
• Subjects: Geochemistry; Earth Sciences; Oceanography; Uranium-Thorium-series Isotopes