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Osmium isotope evidence for the regulation of atmospheric CO₂ by continental weathering

¹ Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK
² Geologisches Institut, Universität zu Köln, Zülpicher Strasse 49a, D-50674 Köln, Germany

The long-term stability of Earth's climate throughout the Phanerozoic stands in marked contrast to the dramatic fluctuations that have taken place on time scales as short as a few years, reflecting the high efficiency of longer-term climate regulation through negative feedbacks. A fundamental mechanism is thought to involve control of CO₂ in the ocean- atmosphere system through continental weathering, although unambiguous, high-resolution data supporting this hypothesis have hitherto not been available. Organic-rich mud rocks from Yorkshire, England, which were deposited during the Toarcian oceanic anoxic event (ca. 181 Ma, Early Jurassic), contain evidence of an exceptionally large excursion in the ¹⁸⁷Os/¹⁸⁸Os ratio of contemporaneous seawater, from 0.4 to 1.0. The most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of 400%–800%. The Os isotope excursion coincided with a well-documented global ¹³C excursion of –6 that affected all the major biospheric reservoirs of the time. Higher mean global temperatures caused global chemical weathering rates to increase substantially, while, in turn, chemical weathering was very effective in reducing the elevated levels of atmospheric CO₂ and the high temperatures to preexcursion levels.

Key Words: osmium • weathering • Toarcian • oceanic anoxic event • strontium • methane hydrate

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