Untitled Document

Diurnal origin of Mg/Ca banding in Orbulina universa and effects of cleaning on test composition

Eggins S.M. 1 , Kimoto K. 2 , De Deckker P. 3 , Sadekov A. 1 , and Maher W. 4

1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia
2 Japan Agency for Marine Science and Technology, Yokosuka , Japan .
3 Department of Earth and Marine Science, The Australian National University , Canberra , ACT 0200, Australia
4 Institute for Applied Ecology, The University of Canberra , ACT 2601

Culture experiments have been performed on the algal symbiont-bearing planktonic foraminifera Orbulina universa , using juvenile foraminifera collected by plankton net off the south coast of NSW . Individual foraminifera were grown at fixed temperature in a day-night lighting cycle (12 hour day + 12 hour night) through the full adult-stage of their life-cycle, which involved final spherical chamber formation, gametogenesis and death over a period of up to 10 days. The number and composition of Mg/Ca bands formed in the wall of the final spherical chamber of individual foraminifers was investigated using both electron microprobe and laser ablation ICPMS. The number of high- and low-Mg/Ca band pairs developed was found to accord with the number of diurnal light cycles over which the individual foraminifers calcified their final chamber. This result confirms the proposed diurnal origin for Mg/Ca banding in Orbulina universa proposed by Eggins et al. (2004), and points to a significant role for algal symbionts in determining the Mg/Ca composition of foraminiferal calcite, most likely through influence on [CO 3 2- ] concentration and calcite saturation state ( cf . Wolf-Gladrow et al., 1999).

The impact of a range of cleaning methods upon the Mg/Ca compositional banding developed in fossil and live-collected Orbulina universa has also been investigated, inclusive of the widely used oxidative and reductive cleaning protocols. Characterisation of test fragments taken from the same Orbulina universa tests were analysed by LA-ICPMS both prior to and following cleaning. Results reveal significant differences in the ability of the different procedures to remove Mg-rich outer surfaces and attached organic material (where present). The internal Mg/Ca banding resists chemical modification and is unchanged by any of the cleaning procedures.

References: Eggins S. M., Sadekov A., and De Deckker P. (2004). Modulation and banding of Mg/Ca in Orbulina universa tests by symbiont photosynthesis and respiration: A complication for seawater thermometry? Earth Planetary Science Letters 225 , 411-419.

Wolf-Gladrow D. A., Bijma J., and Zeebe R. E. (1999). Model simulation of the carbonate chemistry in the microenvironment of symbiont bearing foraminifera . Marine Chemistry 64 , 181-198.