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Silicon and Boron isotopic signatures in marine sponges

Martin Wille1, Jill Sutton1, Andrea DeLeon1, Michael Ellwood1, Steve Eggins1 and William Maher2

1 Research School of Earth Sciences, Building 47, Daley Road, Australian National University, ACT 0200, Australia
2 Ecochemistry Laboratory, Institute for Applied Ecology, Faculty of Applied Science, University of Canberra, Canberra, ACT 2601, Australia


The Southern Ocean is widely regarded as playing a key role in influencing atmospheric pCO2 over glacial-interglacial timescales; the mechanism by which this influence is exerted is, however, poorly constrained. Diatom production constitutes about 40% of the total oceanic primary production, making them key players in the modulation of atmospheric CO2 concentrations and global climate. Additionally better understanding of changes in the surface and deep water carbonate system (eg pH, alkalinity) would provide invaluable insight into the driver(s) of millennial scale climate change. Here, the boron and silicon isotope composition of biogenic silica (siliceous sponges), is being investigated as a potential seawater pH and paleo-productivity proxy.

Relationships were investigated between coupled sponge and seawater samples for Si isotope fractionation compared with equivalent Si concentrations from the water column. Our new results and the subsequent new model offers a new perspective on what controls biological fractionation in biogenic opal and in turn, will yield a novel interpretation of the paleo-oceanic distribution of silicon.

Boron analysis of siliceous sponges aims to examine the nexus between seawater pH, biogenic silica and boron, and thus the utility of boron in siliceous organisms (namely sponges and diatoms) as a pH proxy.  First order Comparison between boron and silicon signatures obtained from the same sponge material suggest a coupling between Silicon and Boron system in marine sponges.