A CALCIUM ISOTOPE TEST OF END-PERMIAN OCEAN ACIDIFICATION USING BIOGENIC APATITE

Submarine erosional truncation of uppermost Permian carbonate strata has been interpreted to reflect ocean acidification coincident with the end-Permian mass extinction. Although this scenario is consistent with carbon isotope and paleontological data, several alternative scenarios, such as ocean overturn or collapse of the biological pump, can also account for the carbon isotope and paleontological evidence.

Calcium isotopes provide a geochemical proxy to test between acidification and alternative scenarios. Specifically, a negative shift in the calcium isotope composition (δ⁴⁴/⁴⁰Ca) of seawater is predicted under the acidification scenario but not the alternatives. The δ⁴⁴/⁴⁰Ca of carbonate rocks from south China exhibits a negative excursion of approximately 0.3‰, but this shift could result from either a change in the δ^44/40Ca of seawater or a change in carbonate mineralogy because calcite and aragonite exhibit substantially different fractionation factors relative to seawater.

To test whether the negative shift in δ⁴⁴/⁴⁰Ca reflects seawater δ^44/40Ca or carbonate mineralogy, we measured the δ⁴⁴/⁴⁰Ca of conodont microfossils (calcium hydroxyapatite) from the global stratotype section for the Permian-Triassic boundary at Meishan, China. The conodont δ⁴⁴/⁴⁰Ca record shows a negative excursion similar in stratigraphic position and magnitude to that previously observed in carbonate rocks. Parallel negative excursions in the δ⁴⁴/⁴⁰Ca of carbonate rocks and conodont microfossils cannot be accounted for by a change in carbonate mineralogy but are consistent with a negative shift in the δ⁴⁴/⁴⁰Ca of seawater. These data add further support for the ocean acidification scenario, pointing toward strong similarities between the greatest catastrophe in the history of animal life and anticipated global change during the 21^st century.