Additive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundary limestones. (20th January 2017)

• Record Type: Journal Article
• Authors: Jost, Adam B.; Bachan, Aviv; van de Schootbrugge, Bas; Brown, Shaun T.; DePaolo, Donald J.; Payne, Jonathan L.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 18:Number 1(2017)
• Page Start: 113
• Page End: 124
• Publication Date: 2017-01-20
• Subjects: calcium isotopes -- Triassic -- Jurassic -- carbon isotopes -- extinction -- aragonite; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2016GC006724
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The end‐Triassic mass extinction coincided with a negative δ 13 C excursion, consistent with release of 13 C‐depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high‐resolution calcium isotope ( δ 44/40 Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ 13 C, δ 44/40 Ca decreases by ∼0.8‰ in 20 m of section and then recovers to preexcursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ 44/40 Ca excursion is too large to be explained by changes to seawater δ 44/40 Ca alone, regardless of CO2 injection volume and duration. Less than 20% of the δ 44/40 Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ 44/40 Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end‐Triassic mass extinction, as well as for interpreting other environmental events in the geologic past. Key Points: We observe a large negative calcium isotope excursion above the Triassic/Jurassic boundary. Numerical modeling indicates the excursion is too large to be attributed to ocean acidification alone. Much of the excursion is due to higher proportions of aragonite in our samples, possibly favored during recovery from acidification.