Silicon isotope biogeochemistry: Diatoms and oceans, present and past

Christina De La Rocha, Ph.D.
University of California, Berkeley

Friday, February 12, 1999
12:00 Noon—Pacific Forum
(Video Conference from UC Berkeley)

Our understanding of the biogeochemical cycling of silicon lags behind our knowledge of other biogeochemical cycles, such as those of carbon and nitrogen. This is due in part to the technical difficulty of making measurements of biogenic silica production and dissolution rates in the ocean. One alternative means of studying silicon cycling that has been proposed is the measurement of natural variations in the abundances of the stable isotopes of silicon. Use of such a tracer requires knowledge of the distribution and behavior of silicon isotopes in the oceans and on Earth.

Diatoms utilize the nutrient, silicic acid, to form opal frustules. We have shown that diatoms fractionate silicon isotopes during this biomineralization. The final isotopic signature of the diatom opal and the unused silicic acid depend partly upon the extent to which silicic acid is utilized. In this manner silicon isotopes track changes in the utilization of silicic acid by diatoms. Subsequently, we have used variations in silicon isotope natural abundances to infer a decrease in silicic acid utilization by Southern Ocean diatoms between the present interglacial and the last glacial maximum.

While relative changes in silicic acid utilization offer a tantalizing glimpse into oceans of the past, absolute values for nutrient utilization would be even more useful. Refining the use of silicon isotopes for such quantitative estimates requires an expansion of our understanding of the silicon isotopic budgets of both the marine and terrestrial systems. In a first attempt at characterizing the distribution and behavior of silicon isotopes in natural waters, we have made the first measurements of silicon isotopes in seawater, rivers, and estuaries. These data, in combination with what we know of the marine silica cycle and the biological fractionation of silicon isotopes, have been used to construct a preliminary silicon isotopic budget for the global ocean and to provide a roadmap for future investigations.

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