Hodaka KAWAHATA
Professor
Speciality:
Marine Geology, Marine Geochemistry, Paleoceanography, Paleoclimatology

Evolutionary changes in the biogeochemical cycles of the solid and surface Earth environments

Different levels of greenhouse effect and CO2 gas make significant difference in surface environments of the Earth and Venus, where mean temperatures at the equator are around 30C and >350C, respectively. Although the total amount of CO2 in the atmosphere of the Venus is comparable to that in the upper 120 km crust and upper mantle of the Earth, >70% of carbon is stored as carbonate in the Earth. Therefore, carbonate is a key chemical compound to provide fairly good environmental conditions for life on the earth surface. Especially, hydrothermal circulation system played an important role in reducing PCO2 in the Earth's atmosphere by precipitating carbonate during the initial stages of evolution of the Earth. This also provided suitable environment for the origin of life. In order to test this hypothesis, we conducted experiments by reacting carbonaceous sediments with artificial seawater in an air-tight vessel at elevated temperatures (100-300@C) and estimated the thermal stability of AAs (amino acids), which is the most important compounds in our body. The results show that the AAs cannot be synthesized or survive in hydrothermal waters at temperatures higher than 250@C unless solid phase such as clay minerals protect AAs significantly. This result suggests that primitive organisms were probably produced after the earth surface was cooled to <200@C .

Under the modern condition on the Earth, calcification is  mainly conducted by biological activity of coral, foraminifera, coccolithophores and others. We cultivated corals in the laboratory, and studied stable isotopic composition of their skeleton. It became evident that the oxygen isotope ratios in the skeleton had a surprisingly large intercolony variability (~1o/oo) at each temperature setting. This means that the formation of aragonite crystal is too rapid to reach isotopic equilibrium condition. In addition, biogenic carbonate has been a very useful material in reconstructing paleo-environments. Especially  G. ruber is the most important foraminiferal species in the warm ocean, including the equatorial region. Currently many scientists think that G. ruber is dwelling at the surface ocean. However, we conducted the plankton net study around the Japanese Islands and found that G. ruber s.s. is dwelling in the surface while G. ruber s.l. is appreciably dwelling at the 100 m water depth. We conducted DNA analysis of both morphotypes and found that the two morphotypes may belong to different species of foraminifera.

I would like to extend these studies as components of a study on "Evolutionary changes in the biogeochemical cycles on the earth surface". During mid-Cretaceous times, large amount of oil was produced in association with the formation of black shale under high PCO2 in the atmosphere. Therefore large amount of carbon should have been supplied from the earth's interior to the surface environments during that time. Currently many scientists, especially physical and chemical oceanographers, believe that the earth's surface environment is not so well related to the earth's interior. However, evolution of the earth's surface environment, including biological evolution, has been closely linked to the changes in the earth's interior (WHOLE Earth). I would like to provide definite pieces of evidence for this contention in near future.


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