Lime mud records Toarcian ocean acidification
Micrite boron isotopes from Spain and Portugal show a transient pH drop at the Toarcian Oceanic Anoxic Event that brachiopod shells miss.
Source
Ocean acidification at the Toarcian Anoxic Event captured by boron isotopes in the lime mud record
Inspect source passages
Lime mud records Toarcian ocean acidification
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Atmospheric CO2 rose from about 260 ppm to as high as 1000 or even 1800 ppm.
This led to a rapid elevation of atmospheric carbon dioxide (CO 2 ) concentrations , , from ca. 260 ppm to as high as 1000 ppm or even 1800 ppm , then a return to pre-event values , .
The T-OAE interval lasted about 500,000 years.
This interval has been recently calibrated to last about 500,000 years consistent with the timescale for CO 2 removal by enhanced silicate weathering .
What they did
Authors measured boron, carbon, and oxygen isotopes in micrite, rhynchonellid brachiopods, and oyster bivalves from two NW Tethys carbonate sections, screened samples for diagenesis, and compared the records with a COPSE carbon-cycle model of ocean pH.
What they found
Micrite δ11B fell by about 4–6‰ at the negative carbon-isotope excursion, tracking modelled surface-ocean pH decline to just above 7.4, while brachiopod and bivalve δ11B stayed nearly flat. Faunal stress coincided with low pH, but community recovery did not require pH rebound.
The limits
What it doesn't show
There is no modern δ11B–pH calibration for micrite, so absolute pH values remain model-dependent. Brachiopod buffering and a data gap in the lower T-OAE limit a statistical kill-mechanism test, and Spain and Portugal recover on different timescales.
Key terms
- T-OAE
- Toarcian Oceanic Anoxic Event: Early Jurassic interval of widespread marine anoxia and a negative carbon-isotope excursion.
- δ11B
- Boron isotope ratio used as a proxy for seawater pH in carbonates.
- Micrite
- Fine-grained lime mud (grains up to ~4 μm) used here as a pH archive.
- CIE
- Carbon isotope excursion: a large shift in δ13C marking the T-OAE.
- COPSE
- Biogeochemical model of coupled carbon, oxygen, phosphorus, and sulfur cycles.
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Common questions
Why not just use brachiopod boron isotopes?
Rhynchonellids kept stable δ11B, likely buffering calcifying fluids, so they did not record the acidification pulse.
What caused the CO2 rise?
Volcanism and contact metamorphism of the Karoo–Ferrar large igneous provinces, possibly plus methane.
Did acidification kill the fauna?
It likely added stress at the extinction, but recovery of communities tracked cooling more than pH rebound.
How long was the T-OAE?
Chemostratigraphically about 500,000 years from CIE onset through δ13C recovery.
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