BIOLOGICAL CONTROL OF CALCIUM ISOTOPIC ABUNDANCES IN THE GLOBAL CALCIUM CYCLE

Abstract

Measurements of 44Ca/40Ca, expressed as δ44Ca, 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%% in δ44Ca, measured with the double spike technique to a precision of +/-0.15%%. Volcanic rocks, including basalt and rhyolite, show little variability and cluster near δ44Ca = 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 δ44Ca about 1.0%% lower than seawater; this fractionation causes seawater to be enriched in heavy calcium (δ44Ca = +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.