[B-PT03] Biomineralization and Geochemistry of Proxies

[BPT03-P04] Three-dimensional parameters (volume and density) of planktonic foraminiferal fossils in the core as potential paleoenvironmental proxies

*Shunichi Kinoshita¹, Quan Wang², Azumi Kuroyanagi³, Masafumi MURAYAMA⁴, Yurika Ujiie⁵, Hodaka Kawahata⁶ (1.Department of Geology and Paleontology, National Museum of Nature and Science, 2.Atmosphere Ocean Research Institute, the University of Tokyo, 3.Tohoku University Museum, The Center for Academic Resources and Archives, Tohoku University, 4.Faculty of Agriculture and Marine Science, Kochi University, 5.Center for Advanced Marine Core Research, Kochi University, 6.School of Creative Science and Engineering, the Faculty of Science and Engineering, Waseda University)

Keywords:planktic foraminifers, paleoenvironmental proxy, micro X-ray computed tomography

Various marine organisms, such as corals, coccoliths, and foraminifers, use carbonate ions in the surrounding seawater for their skeleton and shell production. There is some evidence that carbonate saturation affects foraminiferal shells, and foraminiferal shell production is known to be sensitive to increases in pCO₂. Size-normalized weights of foraminiferal shells have been used to estimate surface carbonate systems. However, it is unclear which parameters of foraminiferal shells are reflected in the size-normalized weights. Whether it is due to shell thickness, shell density, or both is considered obscure. Therefore, the relationship between foraminiferal shell parameters and environmental parameters, especially with three-dimensional parameters such as shell volume (which may reflect mean shell thickness) and density, is still unclear.
In this study, we measured the shell size-normalized weight, shell volume, and shell density of the planktic foraminifera Globigerinoides ruber (white) in core MD98-2196 and discussed the relationship between these shell parameters and environmental parameters (pCO₂ and SST). Furthermore, it was also aimed to explore the possibility that each shell parameter could be a proxy for estimating paleoenvironmental conditions. By using high-resolution micro X-ray computed tomography, it was possible to precisely and quantitatively measuring the three-dimensional parameters. The results suggest that shell size-normalized weight and shell volume may be negatively correlated with marine environmental parameters (pCO₂and SST). It was also found that these two parameters might be more influenced by pCO₂ than by SST. On the other hand, shell density did not show any change from 3-100 ka, thus it is expected to maintain a constant value. Therefore, the variation in size-normalized weight likely reflects the mass of the shell, and no effect of quality (i.e., density) was observed. Furthermore, the results suggest that shell volume could be used as an environmental proxy in addition to the conventionally used size-normalized weight. On the other hand, shell density was controlled nearly constant, so future work is required to determine what environmental parameters are contributing to the response of foraminifers to changes in shell density during foraminiferal shell building.