Constraining calcium isotope fractionation (δ44/40Ca) in modern and fossil scleractinian coral skeleton

Abstract

The present study investigates the influence of environmental (temperature, salinity) shows calcium isotopic values increasing with salinity. The variability between samples cultured at a salinity of 40 is higher than those cultured at a salinity of 36 for this species.The present study reveals a strong biological control of the skeletal calcium isotope composition by the polyp and a weak influence of environmental factors, specifically temperature and salinity (except for S. pistillata). Vital effects have to be investigated in situ to better constrain their influence on the calcium isotopic signal. If vital effects could be extracted from the isotopic signal, the calcium isotopic composition of coral skeletons could provide reliable information on the calcium composition and budget in ocean.

Controlling factors of Ca isotope fractionation in scleractinian corals evaluated by temperature, pH and light controlled culture experiments

“…Subsequently, similar  44 Ca data were recently reported for different genera and varying environmental conditions (Pretet et al, 2013;Holmden et al, 2012). The overall mean value of  44 Ca was comparable within error: 0.81 ± 0.05 ‰ by Böhm et al (2006), yielding about + 0.5 ‰ offset from inorganic and sclerosponge aragonite, which was interpreted as the result of biological Ca isotope fractionation during Ca transmembrane transport (Böhm et al, 2006), similar to observations of biological fractionation on coccolithophores .…”

“…The overall mean value of  44 Ca was comparable within error: 0.81 ± 0.05 ‰ by Böhm et al (2006), yielding about + 0.5 ‰ offset from inorganic and sclerosponge aragonite, which was interpreted as the result of biological Ca isotope fractionation during Ca transmembrane transport (Böhm et al, 2006), similar to observations of biological fractionation on coccolithophores . No significant relationship between the rate of calcification and  44 Ca was found by Pretet et al (2013), corresponding to the results obtained by coccoliths of Emiliania huxleyi (Langer et al, 2007). A similar fractionation behavior is also shown by octocorallia composed of high magnesium calcite (Taubner et al, 2012), which have δ 44 Ca in the range of the aragonitic corals, an insignificant temperature dependence and a small negative dependence on pH, with no clear relation to growth rate.…”

“…and Montipora verrucosa) in the study of Pretet et al (2013) did not show any salinity dependence at least in the salinity range from 36 to 40.…”

“…As another controlling factor, Pretet et al (2013) proposed a possible dependence of Ca isotope fractionation of scleractinian coral (Stylophora pistillata) on salinity. But in this study, salinity is mostly stable and seasonal variation is usually less than 0.5 (Hayashi et al, 2013), and can therefore not contribute to the observed fractionation trends.…”

“…While coral δ 44 Ca-temperature calibration has been established based on culture experiments at variable temperatures and salinities (Böhm et al, 2006;Pretet et al, 2013), seasonal variability in Ca isotopes measured from fossil Porites corals from Tahiti does not follow temperatures reconstructed by Sr/Ca ratio and δ 18 O (Pretet et al, 2013). This indicates that possibly factors other than salinity and temperature might influence Ca isotope fractionation in coral skeleton.…”