Carbon isotope stratigraphy of Torinosu-type limestone in the western Paleo-Pacific and its implication to paleoceanography in the Late Jurassic and earliest Cretaceous

摘要

Carbon isotope stratigraphy of the Late Jurassic and earliest Cretaceous was revealed from Torinosu-type limestone, which was deposited in a shallow-marine setting in the western Paleo-Pacific, in Japan. Two sections were examined; the Nakanosawa section of the late Kimmeridgian to early Tithonian age (Fukushima Prefecture, Northeast Japan), and the Furuichi section of the late Kimmeridgian to early Berriasian age (Ehime Prefecture, Southwest Japan). The age-model was established using Sr isotope ratio and fossil occurrence. The limestone samples have a low Mn/Sr ratio (mostly <0.5) and lack a distinct correlation between δ~(13)C and δ~(18)O, indicating a low degree of diagenetic alteration. Our composite δ~(13)C profile from the two limestone sections shows three stratigraphic correlation points that can be correlated with the profiles of relevant ages from the Alpine Tethyan region: a large-amplitude fluctuation (the lower upper Kimmeridgian, ～152 Ma), a positive anomaly (above the Kimmeridgian/Tithonian boundary, ～150 Ma), and a negative anomaly (the upper lower Tithonian, ～148 Ma). In addition, we found that δ~(13)C values of the Torinosu-type limestone are ～1‰lower than the Tethyan values in the late Kimmeridgian. This inter-regional difference in δ~(13)C values is likely to have resulted from a higher productivity and/or an organic burial in the Tethyan region. The difference gradually reduces and disappears in the late Tithonian, where the Tethyan and our δ~(13)C records show similar stable values of 1.5–2.0‰. This isotopic homogenization is probably due to changes in the continental distribution and the global ocean circulation, which propagated the ~(13)C-depleted signature from the larger Paleo-Pacific to the smaller Tethys Ocean during this time.