Methane release in the Early Jurassic period

Arising from: D. B. Kemp, A. L. Coe, A. S. Cohen & L. Schwark Nature 437, 396–399 (2005); Kemp et al. reply Dramatic global warming, triggered by release of methane from clathrates, has been postulated to have occurred during the early Toarcian age in the Early Jurassic period1. Kemp et al.2 claim that this methane was released at three points, as recorded by three sharp excursions of δ13Corg of up to 3‰ magnitude. But they discount another explanation for the excursions: namely that some, perhaps all, of the rapid excursions could be a local signature of a euxinic basin caused by recycling of isotopically light carbon from the lower water column. This idea has been proposed previously (see ref. 3, for example) and is supported by the lack evidence for negative δ13C excursions in coeval belemnite rostra4. Kemp et al. dismiss this alternative, claiming that each abrupt shift would have required the recycling of about double the amount of organic carbon that is currently present in the modern ocean; however, their measurements are not from an ocean but from a restricted, epicontinental seaway and so would not require whole-ocean mixing to achieve the excursions.