The oceanic thermostructure in the Late Cretaceous, known as a hothouse period in the Phanerozoic, has been reconstructed on the basis of oxygen isotopic analysis of microfossils recovered from DSDP and ODP cores. Although those of the Atlantic have been well conducted, only a few data in equatorial region are available in the Pacific. In this study, we present new isotopic data for the Late Cretaceous (Campanian) oceanic thermostructure in the northwestern Pacific. Furthermore, we discuss the mode of life and habitat of Late Cretaceous ammonoids by comparison of oxygen isotopic data for co-occurring plankton and benthos with nine species of ammonoids.

The Cretaceous Yezo Group, which was deposited in an epicontinental sea, is widely distributed in Hokkaido, Japan. The Campanian strata are composed of impermeable mudstone, yielding extraordinarily well preserved micro- and megafossils. Benthic foraminiferal assemblages and lithofacies indicate that the depositional environment of these strata corresponds to an outer shelf or upper slope (300--350 m in depth).

Oxygen isotopic compositions of calcareous remains of diagenetically unaltered planktonic foraminifers, benthic foraminifers, bivalve and gastropod were analyzed, and calcified temperatures were calculated. ƒÂ¹⁸O of planktonic and benthic foraminifera indicate that sea surface and sea bottom temperatures of the Campanian northwestern Pacific were about 26.2 and 18.8 °C, respectively. A time-series fluctuation of sea bottom water temperatures estimated from ontogenetically sequential analyses of bivalve and gastropod shells was about 5.5 °C. Although paleolatitude of Hokkaido was estimated about 40 °N, these thermoproperties correspond to those of off Taiwan (25 °N) in modern northwestern Pacific. Our data indicate that the Campanian northwestern mid-latitude Pacific was very warm, nearly subtropic condition.

Strikingly, the calculated temperatures of all the ammonoids analyzed, with different shell morphologies, showed no marked difference and are broadly similar to those of contemporaneous benthic organisms and completely different with those of planktonic foraminifers. Therefore it is concluded that these ammonoids species lived near the sea floor without significant vertical migration.