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Paleontological Research
Abstract
Abstract.
Ocean acidification is now progressing, primarily due to the fact that the oceans have absorbed about 50% of the anthropogenic CO2 emitted since the industrial revolution. Many marine calcifying organisms, such as foraminifers and coccoliths, are known to build their shells using carbonate ions present in the seawaters surrounding them. Carbonate saturation state has a crucial influence on foraminiferal calcification, and foraminiferal shell production is known to be sensitive to increase in ocean pCO2. Moreover, ocean warming is also progressing along with acidification. Therefore, both environmental changes could affect foraminiferal shell formation. However, the relationship between foraminiferal shell parameters (i.e., size, weight, volume, and density) and ocean pCO2 or sea surface temperature (SST), or both, remains unclear. In this study, we used fossil planktic foraminifer Globigerinoides ruber (white) in a late Quaternary sediment core (MD98-2196) from the East China Sea to investigate a relationship between the shell parameters and oceanographic properties estimated based on the proxies from the same core. The foraminiferal shells were scanned using high-resolution micro-X-ray computed tomography (MXCT) to determine shell volume and density. The results showed that the size-normalized weight and the size-normalized volume of the shell had a negative correlation with the SST and atmospheric pCO2. The negative correlation between weight/volume and atmospheric pCO2 agrees with the previous laboratory experiments and geological record during the Pliocene. However, the correlation between weight/volume and SST should be interpreted with caution because it might be an artifact due to the correlation between SST and atmospheric pCO2. On the other hand, shell density is only weakly or insignificantly correlated with SST and pCO2, suggesting that these environmental parameters do not exert any impact on shell density. Thus, future ocean acidification will negatively affect the carbonate productivity of planktic foraminifers, even if it will not affect shell density. The temperature effect on the shell formation of the planktic foraminifers might be much less than ocean acidification considering controversial results of the temperature sensitivity in previous studies.
We are grateful for the constructive comments and suggestions given by Tamaki Sato, Takuya Sagawa, Hiroki Hayashi, and an anonymous reviewer to help us improve this manuscript. We are also grateful to Yuki Ota and Ayumi Maeda (Atmosphere and Ocean Research Institute, the University of Tokyo) for their help with sample pretreatments. We appreciate Shinsuke Yagyu for performing MXCT measurements at Kochi Core Center. Osamu Sasaki in Tohoku University also provided us valuable comments when we processed MXCT data at Tohoku University. We would like to thank Shuhei Nomura in National Museum of Nature and Science for his support in using SEM. We also appreciate Megumi Saito, Yoshimi Kubota, Harumasa Kano, Hiroshi Saito and Hisae Watanabe for their many suggestions. This research was supported by the Sasakawa Scientific Research Grant from The Japan Science Society (2019-4069) and Grants-in-Aid from the Japan Society for the Promotion of Science to H. K. (No. 15H02139) and A. K. (No. 19K04053) and “Super Warm Earth: Understanding Global Warming Processes from Paleoceanographic Big Data” project (Kochi University).
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