•Professor (of Tohoku University Museum) Reishi Takashima,
specializing in Geology and Stratigraphy
•Associate Professor (of Tohoku University Museum) Osamu Sasaki,
specializing in Paleobiology, and Microfossil morphology
•Assistant Professor (of Tohoku University Museum) Azumi Kuroyanagi,

One of the warmest periods in Earth's history was during the Cretaceous Period. Short-term (<1 m.y.) rapid global warming events occurred frequently, especially during the mid-Cretaceous. Such warming events caused the expansion of anoxic conditions in the global ocean, so are called oceanic anoxic events (OAEs). The major OAEs during the Cretaceous include OAE1a (120 Ma), OAE1b (ca. 112 Ma), and OAE2 (94 Ma), the latter of which had the highest extinction rate. In fact, ichthyosaurs disappeared during this event. In order to clarify the causes and the processes of each OAE’s development and termination, we conduct chemical analyses of marine sediments and fossil assemblages accumulated during the OAE2 in Japan, France, the west coast of the United States, and Italy, and from deep-sea cores.

Geological formation of OAE1a in Provence, France

In order to reconstruct the detailed environmental changes of the warm Cretaceous, an accurate Cretaceous standard timescale must be established. The Cretaceous is subdivided into 12 "ages", ranging from the Berriasian to the Maastrichtian, but the numerical ages at the boundaries of each age are not always precisely determined or agreed upon. For example, the age of the boundary between the Aptian and the Barremian differs by as much as 6 m.y., with some suggesting 126 Ma and others 120 Ma. Our group, in collaboration with American and European universities, is conducting research to determine the numerical ages of the age boundaries as well as the OAEs of the Cretaceous.

Field research on the Cretaceous system in the Ashibetsu Mountains, Hokkaido, Japan

Apatite is common in felsic to intermediate volcanic and volcaniclastic deposits, and its trace-element composition is known to change with crystallization in response to various magmatic conditions. Apatite is also highly resistant to burial diagenesis and to welding and weathering processes. These characteristics are useful for correlating pre-Quaternary tephras or ash-flow tuffs. Our research group has established the Cretaceous–Pliocene tephrochronology in Japan and, based on apatite trace-element compositions, has identified the source caldera of the Miocene and Pliocene ash-flow tuffs distributed in NE Japan and Kii Penninsula.

Hirosegawa ash-flow tuff exposed widely in Sendai

In order to investigate historical marine environments we can study foraminifera, which are single-celled organisms with carbonate shells. Foraminifera live on the ocean surface and in sediment, and their shells record the temperature and pH of the ocean during the time they were alive. Our group is conducting analyses and culturing experiments of foraminifera to more precisely and accurately estimate marine environmental conditions in the past and future. Ocean acidification caused by recent anthropogenic increases in atmospheric carbon dioxide is considered to have a significant impact on calcifying organisms in the ocean, such as foraminifera and corals. Therefore, we are also investigating the effects of ocean acidification on the calcification of foraminifera.

Culturing experiments of planktic foraminifera