[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-P09] High-temperature and -pressure experiments toward understanding the effect of burial diagenesis on brachiopod and bivalve shells

*Komei Yamagishi¹, Hideko Takayanagi^1,2, Katsuhiko Suzuki³, Hiroyuki Kikuta³, Hiroaki Ohfuji¹, Saburo Sakai³, Koshi Yamamoto⁴, Yasufumi Iryu^1,2 (1.Tohoku Univ., 2.WPI-AIMEC, 3.JAMSTEC, 4.Nagoya Univ.)

Keywords:experimental diagenesis, brachiopod, bivalve, SEM observation, carbon and oxygen isotope composition, Raman spectroscopic analysis

Fossil marine calcifiers serve as a powerful tool for reconstructing palaeoceanographic and paleoenvironmental conditions because stable isotopic (e.g., δ¹³C and δ¹⁸O) and trace element (e.g., Sr/Ca, Mg/Ca) compositions in their calcite and aragonite shells are reliable proxies for ambient water. However, primary environmental information recorded can often be overprinted by diagenetic alterations, such as meteoric and burial diagenesis. Therefore, previous studies have assessed the extent of diagenetic alterations by microstructural observation and geochemical analysis, in which well-preserved fossil specimens have been exclusively used for paleoenvironmental reconstruction. However, extraordinarily high seawater temperatures have been reconstructed based on clumped isotopes and oxygen isotope composition of well-preserved Paleozoic and Mesozoic fossil brachiopod shells, implying that influences of diagenesis (primarily by burial diagenesis) may not have been eliminated using conventional methods. While the processes, mechanisms, and effects of burial diagenetic alterations have not been well understood due to the limited availability of optimal materials and research techniques. In this study, we conducted 30-day experimental burial diagenesis (125℃ and 75 MPa) on modern brachiopod (Terebratulina crossei) and bivalve (Meretrix lusoria) shells to compare microstructural, crystallographic, and geochemical data obtained from original and altered shells to improve understanding of how burial diagenesis alters carbonate fossils.
The scanning electron microscope (SEM) observation confirms the partial dissolution and precipitation of calcite/aragonite crystals in studied brachiopod and bivalve shells. The Raman shift and δ¹⁸O from the bivalve shell shifted uniformly higher and lower values throughout the shell than the primary shells. In contrast, those of the brachiopod shells indicated heterogeneous alterations. This suggests that processes and mechanisms of isotope exchange with the surrounding experimental fluid and effects of decomposed organic matter are different between brachiopod and bivalve shells. Since our new artificial diagenesis experiment can independently control several conditions (e.g., temperature, pressure, burial fluids, sediments, and water/sediment ratio), further research will elucidate the processes, mechanisms, and effects of natural burial diagenesis on carbonate rocks and fossils.