Discussion on Oceanic Plateau Formation: a Cause of Mass Extinction and Black Shale Deposition Around the Cenomanian–Turonian Boundary
Total Page:16
File Type:pdf, Size:1020Kb
Journal of the Geological Society, London, Vol. 156, 1999, pp. 208. Printed in Great Britain. Discussion on oceanic plateau formation: a cause of mass extinction and black shale deposition around the Cenomanian–Turonian boundary Journal, Vol. 155, 1998, pp. 619–626 Anthony Hallam writes: I was intrigued by Kerr’s relating of a bottom waters, because we consider that anoxia associated marine mass extinction event and associated sea-level rise, with marine transgression was the most significant cause of the oceanic anoxia and global warming to an episode of oceanic marine extinctions. (Contrary to the widespread view of major volcanism on massive scale. What was especially interesting late Permian regression, Wignall and I find no evidence in our was his invocation of a possible runaway greenhouse climate extensive stratigraphic and facies research for such an event.) developing rapidly as a consequence of the solubility of CO2 in Kerr’s invocation of the rise of major oceanic plateaus in the seawater decreasing by 4% for every 1 C temperature rise, so mid-Cretaceous (cf. Larsen’s (1991) superplume) provides a that the warmer the oceans get, the less CO2 will dissolve in ready mechanism for a global sea-level rise without a preceding them. The pattern of change envisaged by Kerr for the regression. Unfortunately, unlike for the Cretaceous, any Cenomanian–Turonian boundary corresponds remarkably oceanic crust so produced at the end of the Permian has been well with what Paul Wignall and I envisage for the biggest lost by subsequent subduction and/or obduction/continental mass extinction event of all, at the Permian–Triassic boundary accretion. Accordingly, I can only propose here as a plausible (Hallam & Wignall 1997). While the most fully documented speculation that a significant episode of global warming at extinctions were in the marine realm it is evident that there the end of the Permian was caused by a marked increase in were also extensive extinctions on land, which are most atmospheric CO2 as a consequence of both subaerial and reasonably explained by an episode of global warming. The submarine volcanism on a massive scale. Since, like CO2, the best evidence comes from the floras. The cold-adapted glos- solubility of oxygen in seawater diminishes with increased sopterids went extinct abruptly, to be replaced by warm temperature, a tendency towards marine anoxia would be a temperate floras; cold temperature and polar-type floras are notable by-product of such volcanism. completely unknown at this time. Data from both oxygen and strontium isotopes are consistent with the inference of global warming. References We have speculated that eruption at this time of the Siberian H,A.&W, P.B. 1997. Mass Extinctions and Their Aftermath. Traps, the largest known continental flood basalt province, Oxford University Press, Oxford. had a primary role in generating a significant increase in K , A.C. 1998. Oceanic plateau formation: a cause of mass extinction and black shale deposition around the Cenomanian–Turonian boundary. atmospheric CO2. However, we also need to invoke a Journal of the Geological Society, London. 155, 619–626. mechanism for producing a strikingly rapid and pronounced L, R.L. 1991. Geological consequences of superplumes. Geology, 19, eustatic sea-level rise, with the associated spread of anoxic 963–966. A. H, School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK (email: a.hallam@bham.ac.uk) 208.