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

Abstract

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 CO 2 in the ocean-atmosphere system through continental weathering, although unambiguous, high-resolution data supporting this hypothesis have hitherto not been available. Organic-rich mudrocks 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 187 Os/ 188 Os ratio of contemporaneous seawater, from ϳ0.4 to ϳ1.0. The most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of ϳ400%-800%. The Os isotope excursion coincided with a well-documented global ␦ 13 C 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 CO 2 and the high temperatures to preexcursion levels.Keywords: osmium, weathering, Toarcian, oceanic anoxic event, strontium, methane hydrate. of this manuscript, and the Natural Environment Research Council and the Open University for financial support. We are grateful for journal reviews provided by S.P. Hesselbo and an anonymous reviewer.

Basinal restriction, black shales, Re‐Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event

“…The date obtained was imprecise because the initial 187 Os/ 188 Os for their suite of samples was not constant. In the exaratum Subzone initial 187 Os/ 188 Os values are high and variable (0.8 to 1.0, Figure 6) [Cohen et al, 1999[Cohen et al, , 2004], values well above the 0.4 derived for higher stratigraphic levels ( Figure 6). …”

“…The value of 0.4 for initial 187 Os/ 188 Os is that for organic-rich sediments above the exaratum Subzone (Figure 6) [Cohen et al, 2004]; even that value may have been affected by the restriction. From Figure 10, it can be seen that to drive 187 Os/ 188 Os to the value of 0.8 seen in the exaratum Subzone from 0.4 required only 4% dilution of seawater by freshwater, i.e., a lowering of salinity by 1.3 psu, if the seawater Os had been drawn down to 5% of its original value by restriction and rainout to sediments.…”

“…[22] In Figure 2 we profile through the composite section data for TOC, Mo, Mo/TOC (ppm/%; note, not rsMo/TOC), TS, DOP-T, a proxy for degree of pyritization, the d 13 C of organic matter (d 13 C (org) ; data of Cohen et al [2004]), and the 87 Sr/ 86 Sr and d 13 C of belemnite calcite (d 13 C (bel) ) from McArthur et al [2000]. In Figure 3 we profile through the exaratum Subzone and upper part of the semicelatum Subzone our data for d 13 C of belemnite calcite, and the data of Pearce et al [2008] for Re/Mo, d 98 Mo, Mo/TOC, d 13 C (org) , Re/TOC, TOC, and initial 187 Os/ 188 Os.…”

“…The concretions typically occur in discrete horizons. Upsection from the exaratum Subzone, concretion horizons (org) ) (data from Cohen et al [2004]), through the Upper Pliensbachian and Lower Toarcian strata of Yorkshire, United Kingdom. In the lithological crossplot, bed numbers are from Howarth [1955Howarth [ , 1962Howarth [ , 1973, and the thick horizontal lines represent the more prominent carbonate concretions and cemented horizons.…”

“…[59] The anomaly was interpreted by Cohen et al [2004] as reflecting a 400-800% increase in continental weathering, communicated to the whole ocean. This explanation is unlikely to be correct and not only for the reason given here: the isotopic anomalies are recorded in a section that is condensed [McArthur et al, 2000;McArthur and Wignall, 2007], rather than expanded, and the increased sediment flux that increased weathering would entail have not been identified.…”