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 δ44/40Ca and δ88/86Sr 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 δ44/40Ca and δ88/86Sr show negative isotope excursions across the suggested acidification interval as would be expected had acidification occurred. A profile of δ11B, 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.

Values of δ88/86Sr are independent of temperature or Sr/Ca in our belemnites. For brachiopods, too few data are available to determine whether such a dependence exists. Values of δ44/40Ca show a weak temperature control of magnitude +0.020± 0.004‰/°C (2 s.d.). In belemnites, δ44/40Ca also correlates positively with Mg/Ca and Sr/Ca.

Li Q., McArthur J. M., Thirlwall M. F., Turchyn A. V., Page K., Bradbury H. J., Weis R. & Lowry D., in press. Testing for ocean acidification during the early Toarcian using δ44/40Ca and δ88/86Sr. Chemical Geology. Article.

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