A Proxy for Water

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Science  20 Jun 2003:
Vol. 300, Issue 5627, pp. 1849
DOI: 10.1126/science.300.5627.1849a

The geographic distributions of the size and morphology of modern foram assemblages show that these traits depend on environmental conditions, such as temperature, nutrient availability, and community structure. Because these conditions vary as climate fluctuates, it has been assumed that they would influence forams in a temporal sense as well as a geographic one.

Schmidt et al. report that the sizes of planktonic foraminifers change in response to environmental conditions during glacial-interglacial cycles, and that these changes mimic the geographic size variability seen for the Holocene. The amplitude of the changes is directly related to the amplitude of water temperature variations over these periods, and change in assemblage size is linked to species replacement or intraspecific variability, depending on the environmental setting. These results support the idea that the ecological preferences of individual species have not changed much during the past 300,000 years, and suggest that the similar size distributions seen in the last three interglacials imply that the environmental conditions experienced by the planktonic foraminifers were similar as well. On the downside, these findings may complicate the interpretation of studies that use variations in the foraminifera weight as a proxy for the carbonate ion concentration of the ocean across glacial-interglacial intervals.—HJS

Paleoceanography 18, 1039 (2003).

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