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'Images of Clay'
'Images of Clay'
EMU Notes in Mineralogy - volume 10
Ion partitioning in ambient-temperature aqueous systems
Chapter 11: Ion partitioning and the geochemistry of coral skeletons: solving the mystery of the vital effect
Anne L. Cohen and Glenn A. Gaetani
Much of our understanding of Earth’s climate history is based on interpretation of geochemical variability within the CaCO3 tests and skeletons of marine organisms. Geochemical climate proxies are typically cast in terms of equilibrium thermodynamics, but there are important differences between the compositions of carbonates accreted by living organisms and predictions for carbonate minerals in equilibrium with seawater. These differences are commonly attributed to ‘vital effects’ thought to be caused by biological modification of the calcifying environment and of crystal-growth kinetics. If this were true, then biologically modified crystal chemistry may be unpredictable or challenging to model mathematically, making it difficult to extract accurate climate information from biogenic carbonates with any degree of confidence. Our goal with this paper is to demonstrate a systematic approach to the identification, characterization and understanding of ‘vital effects’ using coral skeletons as an example. We show, through insights gained from abiogenic aragonites precipitated experimentally from seawater under controlled conditions, that many of the so-called ‘vital effects’ in coral skeletal geochemistry are actually characteristic of abiogenic aragonites and can be described mathematically in terms of predictable physicochemical processes. By comparing elemental ratios (Mg/Ca, Sr/Ca, Ba/Ca) of abiogenic aragonite with that of coral aragonite, we show that Rayleigh fractionation and crystal-growth rate exert the dominant controls on the elemental chemistry of corals, and that the contribution of temperature is relatively small. Building on these insights, we have developed a new approach to extracting temperature information from coral skeletons that is fundamentally different from conventional palaeo-thermometry, by-passing ‘vital effects’ through the simultaneous use of multiple element ratios to reliably extract that small component of skeletal variability that is driven solely by temperature.
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