Discussion on the Late Palaeocene–Early Eocene and Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales, associated environmental change, causes and consequences Journal , Vol. 164, 2007, 1093–1108

Abstract

R. J. Bailey and D. G. Smith write: Cohen et al . (2007) review the major changes in ocean chemistry in the Early Jurassic (Toarcian) and Late Palaeocene–Early Eocene, the latter corresponding to the Palaeocene–Eocene Temperature Maximum (PETM). These changes involved global carbon isotope excursions (CIEs), temperature increases, marine anoxia and biospheric crises. Their preferred explanation is the massive thermal dissociation of methane hydrates, releasing isotopically light carbon. What is problematical is their view that the isotopic data series from the Toarcian and Palaeocene–Eocene intervals (fig. 1a and b) record not only the amounts of change in the carbon isotopes, but also the rates of change. In practice, this claim is reliable only if the samples analysed derive from intervals characterized by unbroken accumulation at an unchanging rate. For the Toarcian, its justification lies in an earlier paper (Kemp et al . 2005) and involves: (1) radiometric dates that place the Toarcian CIE within a c . 2.2 Ma interval; (2) the assumption that this interval was characterized by continuous accumulation at a constant rate; (3) spectral analysis of lithological data series suggesting 81 cm cycles in carbon isotope and biogenic carbonate abundance in the laminated, unbioturbated, sediments accumulated during the CIE; (4) use of the estimated 2.2 Ma steady rate of accumulation to calibrate the cycles, and the duration of the CIE, in terms of a 21.46 ka periodicity; (5) identification of this periodicity with precession-related, c . 21 ka, M-orbital forcing of terrestrial insolation, which is presumed to have triggered the abrupt shifts in δ13C. (The origin of this solution of the quasi-period of Early Jurassic precession is not indicated.) The concordance between the time calibration of the spectrally determined cycles and the estimated period of Early Jurassic orbital precession, somewhat circularly, supports the assumed constant …