High temporal resolution paleoclimate reconstruction by the analysis of growth patterns and stable isotopes of fossil shells of the long-lived bivalve Mercenaria stimpsoni from MIS 5e, 7 and 9

Abstract

Paleoenvironments during past interglacial periods such as Marine Isotope Stages (MISs) 5e, 7 and 9 have been studied as possible analogues for current global warming. In central Japan, interglacial oceanic sediments of the Shimosa Group were deposited in the Paleo-Tokyo Bay between MIS 11 and MIS 5 when sea level was high. However, due to the scarcity of biogenic archives that recorded paleoenvironments over long intervals of time with high temporal resolution, seasonal variation of past seawater temperature in the bay during the Pleistocene warm periods remains unclear. Shells of the long-lived bivalve, Mercenaria stimpsoni (Stimpson's hard clam) can potentially provide such data from mid- to high-latitude northwestern Pacific coastal regions. Here we used growth patterns and oxygen isotope 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 based on sclerochronological analysis.

Our study demonstrates that fossil shells of this species lived over 100 years and grew most rapidly during the first ten years of life. During each studied interglacial, the highest seawater temperatures equaled approx. 20 °C, i.e., much lower than today in coastal regions of Chiba Prefecture (approx. 27 °C). 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 climatic conditions likely lasted at least for 100 years, i.e., during the lifespan of this species.