Testing for ocean acidification during the Early Toarcian using δ44/40Ca and δ88/86Sr

During the Early Toarcian, volcanic gases released by the Karoo-Ferrar large igneous province are widely believed to have caused severe environmental disturbances, including ocean acidification. Here we show records of delta 44/40Ca and delta 88/86Sr through the Early Toarcian, as recorded in three groups of biogenic calcite: Megateuthididae belemnites, Passaloteuthididae belemnites, and brachiopods of the species Soaresirhynchia bouchardi. We evaluate the data to eliminate the influence on isotopic composition of varying temperature, calcification rate, and salinity, through the section that may mask the environmental signals. Neither delta 44/40Ca nor delta 88/86Sr show negative isotope excursions across the suggested acidification interval as would be expected had acidification occurred. A profile of delta 11B, re-interpreted from a published study, shows no variation through the interval. Taken together, these data provide little support for ocean acidification at this time. In our belemnites, values of delta 88/86Sr are independent of temperature or Sr/Ca. For brachiopods, too few data are available to determine whether such dependences exist. Values of delta 44/40Ca show a weak temperature control of magnitude +0.020 +/- 0.004 parts per thousand/C (2 s.d.). In belemnites, delta 44/40Ca also correlates positively with Mg/ Ca and Sr/Ca.

Automated summary

This study shows that during the Early Toarcian, volcanic gases released by the Karoo-Ferrar large igneous province did not cause ocean acidification. Isotope data of calcium, strontium, and boron indicate a lack of significant acidification signals, while calcium isotope values are weakly influenced by temperature.