Kyushu University [Tatsuya Hayashi (Lecturer) Faculty of Social and Cultural Studies, Department of Environmental Changes]

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Tatsuya Hayashi
Lecturer
Department of Environmental Changes
Department of Environmental Changes
Faculty of Social and Cultural Studies
Last modified date：2024.04.09

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Department of Integrated Sciences for Global Society
Graduate School of Integrated Sciences for Global Society

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1.	Tatsuya Hayashi, Toshiro Yamanaka, Yuki Hikasa, Masahiko Sato, Yoshihiro Kuwahara, Masao Ohno, Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation, Communications Earth & Environment, doi.org/10.1038/s43247-020-00023-4, 1, 1-10, 2020.09, [URL], The global climate has been dominated by glacial–interglacial variations since the intensification of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic meridional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning circulation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly increased after 2.7 million years ago and was actively maintained even in early stages of individual glacials, in contrast with late stages when it drastically decreased because of iceberg melting. Probably, the active Nordic Seas overturning during the early stages of glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations..
2.	Tatsuya Hayashi, William N. Krebs, Megumi Saito-Kato, Yoshihiro Tanimura, The turnover of continental planktonic diatoms near the middle/late Miocene boundary and their Cenozoic evolution, PLoS ONE, doi.org/10.1371/journal.pone.0198003, 13, 6, e0198003, 2018.06, [URL], Fossil evidence indicates that modern assemblages of temperate nonmarine planktonic diatoms began near the middle/late Miocene boundary when the genus Actinocyclus, an important constituent of lacustrine planktonic diatom assemblages during the early to middle Miocene, was replaced by genera of the family Stephanodiscaceae. This floral turnover has been confirmed in many regions of the world, except eastern Asia where taxonomic data about early and middle Miocene planktonic diatom assemblages have until recently been scarce. Our analysis of Lower and Middle Miocene lacustrine diatomaceous rocks in Japan confirms that species of nonmarine Actinocyclus were important constituents of lake phytoplankton there as well. The appearance of nonmarine Actinocyclus species near the beginning of the Miocene may have resulted from the introduction of euryhaline species into lacustrine environments during a highstand of sea level at that time. Similarly, it is possible that species of Stephanodiscaceae evolved from marine thalassiosiroid ancestors that invaded high latitude lacustrine environments during multiple Paleogene highstands, resulting in a polyphyletic origin of the family. The turnover from nonmarine Actinocyclus to Stephanodiscaceae genera near the middle/late Miocene boundary may be linked to a contemporaneous increase in silica concentrations in lakes caused by active volcanism, increased weathering of silicate rocks due to orogeny, and the expansion of C4 grasslands. This turnover may also have been influenced by enhanced seasonal environmental changes in the euphotic zone caused by the initiation of monsoon conditions and a worldwide increase in meridional temperature gradients during the late Miocene. Morphological characteristics of Stephanodiscaceae genera, such as strutted processes and small size, suggest their species were better adapted to seasonal environmental changes than nonmarine species of Actinocyclus because of their superiority in floating and drifting capabilities and possibly metabolism, intrinsic growth rate, and reproductivity. As climates deteriorated during the late Miocene, Stephanodiscaceae species may have spread from high latitudes to temperate lakes where they diversified, ultimately displacing Actinocyclus..
3.	Tatsuya Hayashi, Yoshihiro Tanimura, Morphological variability of Cyclostephanos ramosus sp. nov. from Pleistocene sediments of the Paleo-Kathmandu Lake, Nepal, Diatom, 31, 1-11, 2015.12, [URL], A new diatom species, Cyclostephanos ramosus sp. nov., is described from Pleistocene sediments of the Paleo-Kathmandu Lake, Nepal. Cyclostephanos ramosus is a polymorphic species characterized by three morphological variations, which are related to valve silicification, valve size and stratigraphic distribution. Heavily-silicified valves exhibit an annulus and network-like structures in the valve center and narrow secondary costae in the mantle area. Other valves have a variable number of valve face areolae, mantle areolae, costae, and mantle fultoportulae, and show differences in the shape of the mantle costae. The number of costae and mantle fultoportulae also shows variations related to stratigraphic distribution, which occur on a time scale of a few tens of thousands of years. On the basis of morphological characteristic features, C. ramosus is most similar to Cyclostephanos malawiensis, but differs in its domed cribra on the mantle, much deeper mantle and larger distance between spines and mantle fultoportulae. The three morphological variations have different implications. Valves showing variation in silicification reflect an eco-morphological response to available silica, while valves with size-related variations show morphological modification brought about by vegetative cell divisions. The stratigraphic variations are highly suggestive because they indicate that the number and shape of costae, which have been traditionally used to delineate Cyclostephanos species, can vary intraspecifically on long time scales. These facts suggest the careful examination of intraspecific morphological variations on various time scales is required for the taxonomy of Cyclostephanos..
4.	Tatsuya Hayashi, Monospecific planktonic diatom assemblages in the Paleo-Kathmandu Lake during the middle Brunhes Chron: implications for the paradox of the plankton, Palaeogeography, Palaeoclimatology, Palaeoecology, 10.1016/j.palaeo.2010.12.007, 300, 1-4, 46-58, 2011.02.
5.	Tatsuya Hayashi, Masao Ohno, Gary Acton, Yohan Guyodo, Helen F. Evans, Toshiya Kanamatsu, Fumiki Komatsu, Fumi Murakami, Millennial-scale iceberg surges after intensification of Northern Hemisphere glaciation, Geochemistry, Geophysics, Geosystems, 10.1029/2010GC003132, 11, 9, 2010.09.