Ge-Si ratio in siliceous organisms: A proxy for oceanic circulation?

Jill Sutton¹

¹ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

This research project will utilise both the Si isotope signature and the germanium (Ge) to Si concentration ratio (Ge/Si) of diatom frustules and sponge spicules to reconstruct historical Si concentrations in surface and deep ocean waters, thereby allowing concentration depth profiles to be recreated. In addition, a radically new model for interpreting isotope and elemental fractionation within siliceous organisms will be utilised.  Further, new Si isotope and Ge/Si records will be generated to link changes in the Si cycle to the biological pump and the extent to which carbon is sequestered away from the atmosphere-equilibrated surface waters to the ocean's interior.

This research aims to lay the groundwork to understand the oceanic cycling of Si during the past, with a particular focus on the Southern Ocean, using a multi-proxy and multi-organism approach. The main objectives of this research are to: (1) examine Si isotope and Ge/Si fractionation of diatoms and sponges grown under controlled conditions to understand processes that lead to fractionation; (2) relate culture results for isotope and elemental fractionation within diatoms and sponges to the Si isotope and Ge/Si distribution patterns in the modern ocean; (3) measure the Si isotope and Ge/Si signatures of fossil organisms, with the aim of reconstructing the distribution of Si in the ocean during the last ice age.

Method development and construction of experimental culture facilities for this project began earlier this year.  The main objectives of this research are currently being investigated with a particular emphasis on the development of sponge culture.  A few experimental sponge culture studies (figure 1) have been successful and chemical analysis of the experiments is currently underway. 

A research cruise is planned from Hobart to Antarctica on the Aurora Australis (Figure 2) at the end of 2007.  On this cruise biological and water samples from the Southern Ocean will be collected and analysed for distribution patterns germanium and Si isotopes in the Southern ocean.

Figure 1. Experimental sponge culture experiment	Figure 2.  Aurora Australis