Validating a new proxy for iron fertilization in the Southern Ocean


Dr Taryn Noble

Project Description:

The input and dissolution of continental material to high nutrient surface waters of the Southern Ocean plays an important role in biogeochemical cycling of carbon because it alleviates iron limitation and stimulates phytoplankton growth. In remote subantarctic regions dust is an important source of dissolved iron, but close to Antarctica, dissolved iron sources include melting sea ice, icebergs calved from glaciers, and upwelled deep waters.

The Antarctic Ice Sheet is experiencing rapid changes in response to anthropogenic climate warming. Melting ice shelves and glacier retreat will increase the input of freshwater and dissolved continental material to the surface ocean, but the biological response to these changes (and therefore impact on the global carbon cycle) is unknown. However, we can study the biological and chemical response to past changes in ice sheet retreat recorded in ocean sediments to improve our understanding of how the system might change in the future.

The aim of this Honours project is to test the reliability of a geochemical method (Robinson et al. 2008) for quantifying the dissolved iron flux along the continental margin of East Antarctica. This method uniquely targets the input of dissolved material, rather than the input of total continental material. The distinction is biologically important, because phytoplankton take up dissolved iron from seawater.

Chemical leaching experiments will be carried out on surface seafloor sediments, which preserve the most recent period of sediment deposition. Two isotopes of thorium will be measured in the leachates, Th-232 which only enters the ocean from continental inputs, and Th-230 which is produced mainly from the decay of U-234 dissolved in seawater. Thorium is very insoluble in seawater and as is dissolves from continental material it rapidly adsorbs to the surface of particles, making it an excellent tracer of dissolved components. 

The leaching is designed to recover only the Th that is adsorbed to the sediment particles, which should represent the 232Th/230Th ratio in the overlying seawater. The results of the leaching experiments will be compared to existing measurements of the dissolved 232Th/230Th in the overlying water column in order to assess the reliability of this method.

Authorised by the Executive Director, Institute for Marine and Antarctic Studies
November 26, 2020