Bivalve molluscs and Cenozoic paleoclimatic events in the northwestern Pacific Ocean

A new method for the calculation of Cenozoic paleotemperatures is proposed which is based on the statistical relation between the August (ta) and annual (ty) temperature of surface water and the ‘coefficient of thermotropy’ (CT). The coefficient presents the percentage ratio of the number of species of bivalve molluscs from temperate and warm-water genera to the total number of species in a definite region. For the northwestern Pacific, at 1.1≤CT≤92.1, the most adequate temperature estimations gave the equations:ty = 3.836·1.014CTand:ta = 10.998·1.009CT emperatures in the vicinity of western (Tochilinsky key section) and eastern (Korfa Gulf key section) Kamchatka, South Sakhalin (Makarovsky key section) and a number of areas of Hokkaido and Honshu (Japan) have been reconstructed using the proposed procedure. An early Middle Miocene (around 16 Ma) temperature maximum, which is of high climatic stratigraphic value, has been found to be present in all areas studied and to correspond to the level of the Kakertian Horizon of western Kamchatka, the Takinoue (‘Lower Togeshita’) Horizon of Hokkaido and the Nishikurosawa Formation of north Honshu. The average annual surface water temperatures at that time were 7.1°C, up to 9.6°C and 11.5°C, respectively. In close similarity to the northern areas of the Atlantic (Haq, 1982), a four-fold advancement of temperate faunistic complexes toward higher latitudes has been observed near western Kamchatka; specified as the Kuluvenian, Kakertian-Ilynian, early and late Etolonian temperature maxima. The paleoceanological situation near western Kamchatka during these maxima is assumed to be sufficiently close to that presently existing in Peter the Great Bay and in more southerly areas of the Sea of Japan. sic paleotemperature trends, which turn out to be similar for all regions studied and for the main Neogene North Pacific paleoclimatic events (Tsuchi, 1989, 1990, 1992), suggest the validity and suitability of the proposed method for the determination of paleotemperatures using the ‘coefficient of thermotropy’. This is also evidenced by the agreement between the paleotemperature estimates obtained here and the oxygen isotope values.