Past seawater temperature in Paleo-Tokyo Bay during MIS 9 reconstructed from oxygen isotopes of fossil shells of Mercenaria stimpsoni (Bivalvia)

Paleo-Tokyo Bay, which existed in Central Japan during MIS 9 when climate was globally warm and sea level was high, potentially provides an analogue to the contemporary global warming trends. Although a large number of well-preserved shells is available from marine deposits of Paleo-Tokyo Bay (Yabu Formation, Shimosa Group), seawater temperature in Paleo-Tokyo Bay has so far largely remained unknown. The majority of shells found in respective strata belong to the cold-water bivalve, Mercenaria stimpsoni, with a lifespan exceeding 100 years. This is interesting as the global climate during MIS 9 was warm. Recently, Miki et al. (2024) quantified past seawater temperature using fossil shells of this species, but they used only one individual of MIS 9. Hence, a detailed understanding of seawater temperature and its influence on shell growth is still missing.

Here, we aimed to further substantiate seawater temperature conditions of Paleo-Tokyo Bay during MIS 9 using oxygen isotope data of five additional M. stimpsoni shells collected from two outcrops (Semata and Takakura). We also assessed how the paleoenvironmental conditions affected the growth rate of the bivalves.

Samples were cleaned with tap water and then cut along the maximum growth axis to obtain two slices of shells. One slice was polished and stained with Mutvei’s solution for growth pattern analysis. The other slice was used for isotope analysis. Shells were sampled with a computer-controlled micromilling system at low drill speed to produce CaCO₃ powder for oxygen isotope analysis. Isotope analysis was performed by means of CF-IRMS (continuous flow – isotope ratio mass spectrometry; Thermo Fisher MAT 253; reaction temperature: 72 °C). The analytical precision was better than ±0.10 ‰. 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 δ¹⁸O_seawater value was assumed to be 0 ‰ in all calculations.

The seasonal changes in seawater temperature during consecutive five to seven years were reconstructed from five specimens. All shells recorded maximum seawater temperatures above 18 or 20 °C (first value using the equation by Grossman & Ku, 1986; second value Kim et al., 2007). Such temperatures were probably highly stressful for specimens of this cold-water species resulting in growth cessation and formation of dark bands. Seasonal shell growth started at a temperature of 12 to 16 °C. Those values were clearly higher than those of modern shells (approximately 10 °C). Fossil M. stimpsoni seem to have been adapted to warmer seawater than modern specimens. In conclusion, fossil shells from Paleo-Tokyo Bay (Shimosa Group) can provide snapshots of a warmer world and a key insight into its influence on the bivalves.