- 地球化学 (ISSN:03864073)
- vol.54, no.2, pp.61-78, 2020-06-25 (Released:2020-06-25)
Biogenic calcium carbonate (coral skeleton, foraminifera shell, bivalve shell, otolith, etc.) is one of the best archives of the past changes of aquatic environment that includes ocean, river, and lake. I have dedicated myself to the development and application of the environmental proxies such as trace element, stable isotope, and radiogenic isotope in calcium carbonate of marine organisms to study paleoclimate, paleoceanography, marine carbon cycle, and calcification processes of them. In this manuscript, I focus on boron isotope proxy of skeleton of marine calcifiers, coral and foraminifera, that are often used in paleoclimatology. Boron isotopes can record pH of the seawater, from which atmospheric carbon dioxide concentration can be estimated, thus it has been recognized as a holy grail in marine carbon cycle study. First, I review the mechanism of boron isotope as seawater pH proxy, history in technical development in the isotope measurement, issues with regard to fractionation factor between boric acid and borate ion in seawater, reconsideration of the boron incorporation mechanism into calcium carbonate skeleton, and recent findings. Then I overview my recent marine carbon cycle studies using boron isotope proxy in different timescales, since the Industrial Revolution (since 1750 C.E.) and since the last glacial period (since ~20,000 years ago).The boron isotopic composition was determined in high precision and accuracy by using thermal ionization mass spectrometry and multi-collector inductively-coupled plasma mass spectrometry in Japan Agency for Marine–Earth Science and Technology.