Biological control of calcium isotopic abundances in the global calcium cycle

Measurements of Ca-44/Ca-40, expressed as delta(44)Ca, were made on igneous rocks and on shell and bone material from modern organisms to investigate the magnitude and origins of calcium isotopic fractionation in nature. The results document a span of 4 parts per thousand in delta(44)Ca, measured with the double spike technique to a precision of +/- 0.15 parts per thousand Volcanic rocks, including basalt and rhyolite, show little variability and cluster near delta(44)Ca = 0 +/- 0.2. Systematic analysis of biological samples indicates that biological processing of calcium discriminates against heavy isotopes, and that biological fractionation is the primary generator of calcium isotopic fractionation in nature. Preliminary data suggest that calcium becomes isotopically lighter as it moves through food chains. Calcium carbonate shells of marine microorganisms and deep-sea carbonate ooze have delta(44)Ca about 1.0 parts per thousand lower than seawater; this fractionation causes seawater to be enriched in heavy calcium (delta(44)Ca = +0.9) relative to igneous rocks. Marine organisms consequently are isotopically heavier than their terrestrial counterparts at similar trophic level. The calcium isotopic composition of living and fossil organisms may record information on diet and environment. Copyright (C) 1997 Elsevier Science Ltd.