A Calcium Isotope Record across the Permian-Triassic Boundary from An Isolated Carbonate Platform in South China

We measured the calcium isotope composition (δ⁴⁴Ca) of marine carbonate sediments spanning the Permian-Triassic boundary on an isolated carbonate platform in the Nanpanjiang Basin of southern China. The calcium isotope composition (δ⁴⁴Ca) of the sediments exhibits a transient negative excursion of 0.2‰ to 0.3‰ across the end-Permian extinction horizon. Isotopically light values persist through the basal Triassic Hindeodus parvus conodont zone. Strata within the overlying Isarcicella isarcica zone exhibit heavier values, similar to those observed in pre-extinction strata. The excursion could reflect either a change in the local fractionation between seawater Ca and carbonate minerals or a shift in the δ⁴⁴Ca composition of seawater. Because the dominant mode of carbonate deposition shifted from skeletal to microbial across the boundary, a local change in fractionation is difficult to rule out. However, δ⁴⁴Ca values return to pre-extinction values within strata still lacking any significant skeletal contribution, suggesting that the isotope excursion may instead record a global shift in δ⁴⁴Ca of seawater. If the values measured do reflect an excursion in the isotope composition of calcium in the oceans, they imply an increase of approximately 20% in the marine calcium concentration over a few hundred thousand years or less. Such an increase could result from ocean acidification via the addition of CO₂ (and possibly SO₂) to the ocean and atmosphere. Such a scenario could also account for the coeval negative excursion in δ¹³C and the preferential extinction of heavily calcified marine invertebrates.