A non-traditional stable isotope perspective on coral calcification

Corals are unique marine organisms whose skeletons are a primary component of both tropical and cold water reefs. The ability of corals to convert aqueous seawater ions into the calcium carbonate (CaCO3) mineral aragonite at a tremendous rate in the tropics, and at all in the inhospitable deep ocean, is an impressive feat that has long been a topic of intrigue. The isotope and trace element composition of annually-banded coral skeletons are often used to generate multi-century reconstructions of past environmental variability with a near-absolute chronology. However, geochemical signals preserved in coral skeleton that cannot be attributed to environmental variables also provide insight into the biomineralization process. The fractionations of carbon (C) and oxygen (O) isotopes between coral skeleton and seawater have been particularly valuable in this regard due to the ease with which they can be compared with abiogenic aragonite precipitation experiments that hope to isolate the effects of specific processes. Such studies have prompted the development of a number of semi-quantitative coral biomineralization models, some of which indicate that bulk transport of seawater to the calcifying space, and subsequent biologically mediated modification of alkalinity and dissolved inorganic carbon, is a likely explanation for observed variability. A number of “non-traditional” isotope systems can now be measured with unprecedented precision due to recent advances in mass spectrometry, including boron (B), calcium (Ca), magnesium (Mg), and strontium (Sr), as well as the “clumping” of heavy carbon and oxygen isotopes in multiply substituted isotopologues. This chapter reviews the fractionation of these isotopes between coral and seawater, and interprets their trends in the context of biomineralization models based on C and O isotopes. Based on consistent differences between corals and abiogenic experiments, it can be generalized that coral skeletogenesis is an extremely rapid, biologically induced process that does not favor equilibrium isotope fractionation. Processes such as pH, precipitation rate and Rayleigh fractionation can explain much of the variability in coral non-traditional stable isotopes, although their relative importance varies among elements and coral species. For example, pH is the primary control on coral B isotope composition, but only influences clumped isotopes indirectly by affecting the rate of isotopic equilibration. Rayleigh fractionation may be of importance to Ca isotope fractionation, and perhaps that of Sr isotopes, but cannot explain Mg isotope fractionation, which may be more strongly impacted by precipitation rate. Despite many remaining uncertainties, this review demonstrates that geochemical processes influenced by, but not completely controlled by, physiology can explain the nontraditional stable isotope composition of corals.