Geology topics

Noriyuki Ikeya

Publications and source records attributed to Noriyuki Ikeya.

3 recordsLinked to original sources

USGS2010

Microfaunal evidence for elevated Pliocene temperatures in the Arctic Ocean

The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were ≥ 0 °C and > 3 °C, respectively, and that ice-free conditions existed for most or all of the Arctic. By at least 3.5–3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and Zoogeographic information on ostracode species from more than 800 modern “core top” samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 °C and 3 °C. This suggests ice-free summers, and probably, a perennially ice-free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Atlantic Ocean.

Paleoceanography and Paleoclimatology↗

USGS1994

Late Pliocene climate change 3.4-2.3 Ma: paleoceanographic record from the Yabuta Formation, Sea of Japan

Late Pliocene paleoceanographic changes in the Sea of Japan between 3.4 and2.3 Ma were investigated through study of molluscs, diatoms, and ostracodes from the Yabuta Formation in Toyama Prefecture. The period 3.4-2.7 Ma was characterized by relatively high sea level and cool water benthic faunas. A progressive paleoceanographic shift towards colder oceanic conditions and lower sea level occurred beginning near 2.7 Ma, intensifying about 2.5 Ma, when important changes in ostracode and molluscan faunas occurred. Between 2.7 and 2.3 Ma, eight glacial events can be inferred based on drops in sea level of 50-60 m and increasing proportions of cold, shallow water ostracode species whose modern ecology and zoogeography indicate colder winter water temperatures (3-4??C). The glacial events between 2.5 and 2.3 Ma were the most intense. Preliminary interpretation of the faunal and oceanographic events of the Yabuta Formation suggests that they correspond to Northern Hemispheric cooling also known from North Atlantic deep-sea oxygen isotope, IRD, and planktic foraminiferal records, North Pacific diatom and radiolarian record, and the Chinese loess sequences. The eight glacial events may record a 41,000-yr obliquity cycle which characterized other late Pliocene climate proxy records. Inferred sea level drops near 2.5-2.3 Ma of about 50-60 m provide direct evidence from an ocean margin setting that supports deep sea oxygen isotopic evidence indicating major changes in global ice volume changes.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS1993

Quantitative analysis of Ostracoda and water masses around Japan: Application to Pliocene and Pleistocene paleoceanography

An ostracode data base consisting of 273 samples from coretops and comprising 226 species was developed for the seas around the Japanese Islands to determine zoogeographic patterns and for application to Pliocene and Pleistocene paleoceanography in the area. Quantitative analyses of the 59 most common taxa between 0 and 300m water depth indicate that ostracode associations are controlled by the main oceanic water masses around Japan and that bottom water temperature is a key factor influencing species distributions. Ostracodes from the following water masses were studied: warm Kuroshio Current, Tsushima Current (Tsugaru Current and Soya Current), Japan Sea intermediate water, Japan Sea proper water and cold Oyashio Current. In order to apply the modem coretop data base to fossil ostracode assemblages, the modem analog technique (MAT) using a squared chord distance (SCD) measure of dissimilarity was tested as a means of comparing fossil and modem assemblages. SCD values of 0.25 or less adequately identify modem analogs from the coretop data set at the local ecological level (i.e. within the same modern bay), while values of 0.25-0.5 identify modem analogs at the level of the zoogeographic province. The MAT method was tested against 3 Pliocene and 11 Pleistocene formations in Japan to examine the use of the MAT in paleoceanographic reconstruction.

Micropaleontology↗