Oxygen isotopes and MgCO3 in brachiopod calcite and a new paleotemperature equation

Abstract

Modern brachiopods and their ambient seawater, compiled from 34 localities, covering shallow-waters from the poles to the tropics were analyzed for their oxygen isotopic compositions, and SrCO3 and MgCO3 contents. If the routine seawater δ18O composition and the ‘MgCO3 effect’ on oxygen isotopes are considered in calculations, calcification temperatures are concordant with measured ambient seawater temperatures. The SrCO3 contents of modern brachiopods appear to be unrelated to ambient growth temperatures, while their MgCO3 contents exhibit some relationship with temperature and growth rate, controlled, in part, by ambient productivity or clearance rates or local environment, and by taxonomic affinity such as presence or absence of caeca.Linear least-squares regression analysis yields a new oxygen isotope paleotemperature equation for brachiopod calcite, which makes adjustment for shell MgCO3 contents, and covers habitats with water temperatures ranging from −2° to +32°C:T°C=16.192–3.468δc–δSW–MgcN=319,R2=0.98.The Mg-effect adjustment is based on the change in shell-calcite δ18O by +0.17‰ per mol% MgCO3 (Jiménez-López et al., 2004). This adjustment is critical to the determination of calcification temperatures for modern articulated brachiopods, especially the ones from cold and warm water habitats, and/or the ones with exceptionally low or high MgCO3 contents, which otherwise may be offset (lower or higher) by as much as 7°C. Furthermore, this adjustment may be critical for fossil brachiopods and other marine calcitic invertebrates with variable MgCO3 contents, and when using their δ18O and ∆47 for determining ancient seawater δ18O compositions and temperatures.