15:00 〜 15:15
[U02-06] 長寿二枚貝ビノスガイの化石を用いた間氷期の高時間解像度古気候変動復元
★招待講演
キーワード：貝殻、化石、酸素同位体、スクレロクロノロジー、間氷期
Paleoclimates of Paleo-Tokyo Bay during past interglacial periods MISs 5e, 7, and 9 have been reconstructed as possible analogues for ongoing global warming. To do so, fossil shells of Mercenaria Stimpsoni (Stimpson's hard clam), collected from Shimosa Group (marine deposits), were sclerochronologically studied. Interestingly, M. stimpsoni is a cold-water species while the global climate during interglacials was warm. Although a large number of well-preserved shells is available from Paleo-Tokyo Bay, seasonal variation of seawater temperature in this area has largely remained unknown.
Shells of the long-lived bivalve, M. stimpsoni can potentially provide paleoclimatological data from mid- to high-latitude northwestern Pacific coastal regions (Kubota et al., 2017; Tanabe et al., 2017; Shirai et al. 2018; Kubota et al., 2021). Here we analyzed growth patterns and δ18O values of fossil shells of M. stimpsoni from the Shimosa Group to reconstruct the seasonal variation in seawater temperature of Paleo-Tokyo Bay during MIS 5e, 7, and 9. We also assessed how the paleoenvironmental conditions affected the growth of the bivalves.
Articulated shell specimens were collected from respective strata of Shimosa Group, Chiba Prefecture, central Japan (MIS 5e: n=3; MIS 7: n=2; MIS 9: n= 6). A single valve of each specimen was cut along the maximum growth axis to obtain two thick-sections. One section was polished for growth pattern analysis, while the other was used for isotope analysis. In order to evaluate preservation condition of the shells, SEM observation and µXRD analysis were performed on polished shell sections. Shell carbonate samples were obtained with a dental drill or computer-controlled micromilling system to produce CaCO3 powder for oxygen isotope analysis. The latter was performed by means of CF-IRMS (reaction temperature: 72 °C). The analytical precision was better than ±0.11‰. δ18O-derived temperature was computed using two different paleothermometry equations, i.e., expression by (i) Grossman and Ku (1986, Chem. Geol. Isot. Geosci. Sect.) with the scale correction by Gonfiantini et al. (1995, IAEA Rep.) and (ii) Kim et al. (2007, Geochim. Cosmochim. Acta). For the latter, the oxygen isotope data were adjusted for different acid fractionation factors of aragonite (shell) and calcite (reference materials) (Kim et al., 2007, Chem. Geol.). The δ18Oseawater value was assumed to be 0‰ in all calculations.
Results of growth pattern analysis showed that fossil specimens [brs1] lived over 100 years and grew most rapidly during the first ten years of life. SEM and μXRD analysis revealed that all fossil specimens preserved their primary microstructures and mineral species. Therefore, these specimens were considered suitable for isotope analysis and reliable paleotemperature estimates.
Finally, the δ18O-derived seawater temperatures were obtained with seasonal resolution. During each studied interglacial, the highest seawater temperatures were much lower (approximately -5°C) than today in coastal regions of Chiba Prefecture. Our results suggest that during interglacial periods when the sea level was high, cold-water masses of the Oyashio Current reached the Paleo-Tokyo Bay. The habitat of cold-water species including M. stimpsoni thus expanded southward following the distribution of this cold-water mass. Such environmental conditions likely lasted at least for 100 years, i.e., during the lifespan of this species. Under such paleoenvironmental conditions, fossil M. stimpsoni seemed to have had a different growth behavior than those of modern specimens of this species. In conclusion, fossil shells from Paleo-Tokyo Bay (Shimosa Group) provide key insights into a paleoclimate in a warmer world and its influence on marine organisms.
Shells of the long-lived bivalve, M. stimpsoni can potentially provide paleoclimatological data from mid- to high-latitude northwestern Pacific coastal regions (Kubota et al., 2017; Tanabe et al., 2017; Shirai et al. 2018; Kubota et al., 2021). Here we analyzed growth patterns and δ18O values of fossil shells of M. stimpsoni from the Shimosa Group to reconstruct the seasonal variation in seawater temperature of Paleo-Tokyo Bay during MIS 5e, 7, and 9. We also assessed how the paleoenvironmental conditions affected the growth of the bivalves.
Articulated shell specimens were collected from respective strata of Shimosa Group, Chiba Prefecture, central Japan (MIS 5e: n=3; MIS 7: n=2; MIS 9: n= 6). A single valve of each specimen was cut along the maximum growth axis to obtain two thick-sections. One section was polished for growth pattern analysis, while the other was used for isotope analysis. In order to evaluate preservation condition of the shells, SEM observation and µXRD analysis were performed on polished shell sections. Shell carbonate samples were obtained with a dental drill or computer-controlled micromilling system to produce CaCO3 powder for oxygen isotope analysis. The latter was performed by means of CF-IRMS (reaction temperature: 72 °C). The analytical precision was better than ±0.11‰. δ18O-derived temperature was computed using two different paleothermometry equations, i.e., expression by (i) Grossman and Ku (1986, Chem. Geol. Isot. Geosci. Sect.) with the scale correction by Gonfiantini et al. (1995, IAEA Rep.) and (ii) Kim et al. (2007, Geochim. Cosmochim. Acta). For the latter, the oxygen isotope data were adjusted for different acid fractionation factors of aragonite (shell) and calcite (reference materials) (Kim et al., 2007, Chem. Geol.). The δ18Oseawater value was assumed to be 0‰ in all calculations.
Results of growth pattern analysis showed that fossil specimens [brs1] lived over 100 years and grew most rapidly during the first ten years of life. SEM and μXRD analysis revealed that all fossil specimens preserved their primary microstructures and mineral species. Therefore, these specimens were considered suitable for isotope analysis and reliable paleotemperature estimates.
Finally, the δ18O-derived seawater temperatures were obtained with seasonal resolution. During each studied interglacial, the highest seawater temperatures were much lower (approximately -5°C) than today in coastal regions of Chiba Prefecture. Our results suggest that during interglacial periods when the sea level was high, cold-water masses of the Oyashio Current reached the Paleo-Tokyo Bay. The habitat of cold-water species including M. stimpsoni thus expanded southward following the distribution of this cold-water mass. Such environmental conditions likely lasted at least for 100 years, i.e., during the lifespan of this species. Under such paleoenvironmental conditions, fossil M. stimpsoni seemed to have had a different growth behavior than those of modern specimens of this species. In conclusion, fossil shells from Paleo-Tokyo Bay (Shimosa Group) provide key insights into a paleoclimate in a warmer world and its influence on marine organisms.