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Additive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundary limestones

Journal Article · · Geochemistry, Geophysics, Geosystems
DOI:https://doi.org/10.1002/2016GC006724· OSTI ID:1398458
 [1];  [1];  [2];  [3];  [3];  [1]
  1. Stanford Univ., CA (United States). Dept. of Geological Sciences
  2. Utrecht Univ., Utrecht (Netherlands). Dept. of Earth Sciences
  3. Univ. of California, Berkeley, CA (United States). Dept. of Earth and Planetary Sciences; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Earth Science Division, Geochemistry Dept.

The end-Triassic mass extinction coincided with a negative δ13 C excursion, consistent with release of 13C-depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The co upled 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 (δ44/40 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 δ13 C, δ44/40 Ca decreases by ca. 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 δ44/40 Ca excursion is too large to be explained by changes to seawater δ44/40 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 δ44/40 Ca of carbonate sediments. Finally, 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.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Grant/Contract Number:
AC02-05CH11231
OSTI ID:
1398458
Journal Information:
Geochemistry, Geophysics, Geosystems, Journal Name: Geochemistry, Geophysics, Geosystems Journal Issue: 1 Vol. 18; ISSN 1525-2027
Publisher:
American Geophysical UnionCopyright Statement
Country of Publication:
United States
Language:
English

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Global perturbation of the marine calcium cycle during the Permian-Triassic transition journal January 2018

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