Research specialty :
Modern biogeochemical cycle, paleo-oceanography, paleo-climatology, hydrothermal activity
The following brief but accurate statement is the same as the GF's recommendation
Research summary of Prof. Hodaka Kawahata
Although he started of his research on hydrothermal fields on mid-ocean ridges, his major contribution for research and education has been in the study of biogeochemical cycles with special reference to carbon cycle and the development and use of new geochemical tools in the research fields of paleoenvironments and paleoclimate. The focus has been on biogeochemical fluxes in coastal and oceanic regions and their response under natural and anthropogenically altered conditions. Especially he developed wide range of sediment trap experiments in the central Pacific, new method for reconstruction of paleo-environments by understanding the geochemical signals of modern and sedimentary particles, water-rock geochemical interaction in the coral reefs, and culture experiments of calcifiers (corals and foraminifers) under precisely geochemically monitored environmental condition. By applying modern knowledge to the past, he finally conducted the paleo-environmental research of the relationship between climate change of the immigration of Homo sapience from Afrika through Eurasia to Japanese islands and the evolution of Japanese society for the last 200,000 years. These works were carried out under a collaboration with his students and his coworkers.
Summary of the research results on each topic
([# number] denotes the article number in the Reference)
1) Sediment trap experiments: The world's only and longest sediment trap observation line, which spans about 10,000 km in latitudinal direction, was deployed to investigate the quality and quantity of settling particles in the Pacific. The results showed that there is a clear seasonal variation in settling particle fluxes even in the equatorial Pacific, and that El Niño causes large fluctuations in biological production. The data also provided the possibility that foraminiferal production could be influenced by the phases of the moon. [#91, #89, #54]
2) Plankton-net study: A plankton-net study was conducted around Japanese islands with much improved bathymetric resolution. Textbooks often state that water temperature, light intensity, salinity, and nutrients vary coherently versus water depth, but, in fact, each parameter changes differently. This characteristic enables us to determine parameters controlling the production of different foraminifera species. [#112].
3) Material cycle studies in modern coral reefs: This work, as an expression of gratitude, owes much to the research activities of Dr. Atsushi Suzuki, in particular. Coral reef ecosystems work as a source of CO2 to the atmosphere due to calcification. In addition, the pollution of endocrine disrupters such as pesticides and bisphenol A have been progressing even in the beautiful island of Okinawa. [#115, #81, #77].
4) Precise culture experiments of calcifiers: This work, as an expression of gratitude, owes much to the research activities of Dr. Atsushi Suzuki, in particular. Precise culture experiments of calcifiers mainly dwelling on coral reefs were conducted to understand their response to ocean acidification caused increased carbon dioxide concentrations in the atmosphere. The study discovered calcifying organism species that resist ocean acidification. [#275, Kawahata et al. 2019, Progress in Earth and Planetary Science, 6:64, long review article on ocean acidification]
5) Study on paleoenvironments and paleoclimate (open ocean): The fluctuation of eolian dust supply from the deserts in the Far East to the central Pacific was reconstructed in the glacial – ineterglacial time scale. The influence of eolian dust to biological production in the open ocean was evaluated. [#68].
6) Study on paleoenvironments and paleoclimate (Homo sapiens): Temperatures in the Japanese islands were quantitatively reconstructed by the analysis of alkenone for the last 17,500 years. Ten large extreme cold periods corresponded to ten boundaries of social evolution. In addition, the sedimentary core showed that the “Leaving Africa” by a group of Homo sapiens leading to modern humans, was conducted in a wet period around 60,000 years ago. [#276, #256, Climate Change and the 200,000 Years of Japanese History, Iwanami Shoten]
7) Subseafloor hydrothermal study: He led the first Japanese scientific survey of the Oman Ophiolite. Analyzed Sr, Mg and B isotopes demonstrated seawater (hydrothermal water) penetrating down to the Moho. Alteration temperature increased and the water/rock ratio decreased versus depth below seafloor. [#292, #157, #80]