Calibration of stable isotope composition of Thoracosphaera heimii (dinoflagellate) calcite for reconstructing paleotemperatures in the intermediate photic zone

Abstract

In this study we investigate the temperature dependence of oxygen isotope ratios preserved in calcite formed by the dinoflagellate Thoracosphaera heimii, focusing primarily on the development of a geological proxy. Geochemical analysis of the calcite shells produced by this species represents a valuable proxy for reconstructing environmental conditions in the intermediate photic zone. Calibration is based on isotopic analysis from culture experiments performed in very dilute batch conditions, as well as from near-monospecific T. heimii assemblages separated from core top sediments. Results are similar for both approaches and indicate that T. heimii shells have oxygen isotope compositions close to equilibrium values predicted for inorganic calcite precipitation. This calibration of the isotopic composition of dinoflagellate calcite indicates that monospecific assemblages of T. heimii can be used to unravel paleotemperatures in the intermediate photic zone by applying isotopic transfer functions for equilibrium calcite. In culture, however, a δ 18 O offset of À1‰ is observed at temperatures <17°C, which falls below the natural temperature range of this species. Culture analyses also reveal a relationship between temperature and carbon isotope composition of calcite. The mechanisms behind this relationship remain to be explored, but their identification may provide a better understanding of carbon isotope systematics from both biogeochemical and geological perspectives. Comparison of the oxygen isotope composition of T. heimii shells with that of shallower dwelling organisms, such as the coccolithophores, represents a valuable proxy for determining temperature gradients within the photic zone and may enable reconstruction of the evolution of the depth of the thermocline.

Laboratory-grown coccoliths exhibit no vital effect in clumped isotope (Δ47) composition on a range of geologically relevant temperatures

“…Various proxies have been used to generate SST estimates, such as the oxygen isotope composition of calcareous fossils (d 18 O), the undersaturation index of coccolithophore-produced alkenones (U k' 37 ), TEX 86 , or elemental Mg/Ca ratios of microfossil shells (Epstein et al, 1951;Emiliani, 1966;Brassell et al, 1986;Prahl et al, 1988;Nürnberg et al, 1996;Kim et O'Neil, 1997;Conte et al, 1998;Lea et al, 1999;Elderfield et Ganssen, 2000;Minoletti et al, 2014). These approaches are, however, still confounded by a number of uncertainties inherent to all proxy reconstruction (Herbert, 2014;Lea, 2014;Zhang et al, 2016).…”

Vanishing coccolith vital effects with alleviated carbon limitation

“…Therefore, geochemical data from ancient biomineralising organisms must be corrected in order to derive the primary signals from palaeoseawater. In the case of the foraminifera, corals and coccoliths, the foremost carbonate producers in the marine realm, there has been a considerable number of studies during which living organisms were cultured in strictly controlled environmental conditions and their biominerals measured for a range of isotopic systems to generate empirical proxy calibrations (Erez and Luz, 1982;Dudley et al, 1986;Spero et al, 1997;Bemis et al, 1998;Ziveri et al, 2003;Tripati et al, 2010;Rickaby et al, 2010;Rollion-Bard et al, 2011;Grauel et al, 2013;Hermoso et al, 2014;Minoletti et al, 2014;Hermoso, 2015).…”

Isotopic record of Pleistocene glacial/interglacial cycles in pelagic carbonates: Revisiting historical data from the Caribbean Sea

“…Sixty years after Harold Urey and Cesare Emiliani demonstrated the potential of oxygen isotope composition of marine carbonate as a proxy for ocean temperatures (Urey, 1947;Emiliani, 1955), we are still working on refining this geochemical tool. Aside from the phenomenal gain of knowledge of the palaeoclimates that this proxy has permitted, it is laudable that we are continuously developing this isotopic tool to generate more reliable SST estimates (Zeebe et al, 2008;Arbuszewski et al, 2010;Watkins et al, 2013;Minoletti et al, 2014;Prentice et al, 2014;Reghellin et al, 2015;Tripati et al, 2015). It is now possible to isotopically measure individual foraminifera specimens thanks to analytical progress in mass spectrometry, but the interpretation of such high-precision results and how they can be translated into robust SST estimates remains a much larger source of uncertainty.…”