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 21.46ka 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 ^sup 13^C. (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 rate of accumulation during the Toarcian CIEs in the Yorkshire sections.
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"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"Please download to view full document