Evaluation of the microgrowth patterns of shells of long-lived bivalve, Mercenaria stimpsoni as a paleoenvironmental proxy

Mercenaria stimpsoni is a new paleoclimatic archive in the mid- to high-latitude western Pacific coastal area. This species is a cold-water bivalve with a long life span (>100 years old), and shell growth patterns and oxygen isotope ratios are useful tools as paleoenvironmental proxies. So far it is known that the shells of M. stimpsoni have distinct annual lines with microincrements between each annual line. However, the relationship between microgrowth patterns and marine environment is not understood. Fossil shells of this species are often found in interglacial marine sediments in Central Japan. Thus, understanding the relationship between microgrowth patterns and marine environment is key to reconstruct paleoclimate with high temporal resolution in this region.

The purpose of this study was to evaluate the usefulness of the microgrowth patterns in this species as a paleoenvironmental proxy. Sample shells were collected from the coasts of Hokkaido and Iwate Prefecture, Japan. Shells were then cut into thick sections along the maximum growth axis. The surfaces of the thick sections were polished. Photographs were taken with a Keyence VHX2000 at 300x to 1000x magnification. Photomosaics were created with Adobe Photoshop CC. Then, the number of microincrements and microincrement widths were measured with ImageJ. Then, 120 to 150 μg of carbonate powder was collected from the outer outer layer along the growth direction and provided for oxygen isotope analysis. Finally, we compared microgrowth patterns with marine environmental data. Growth line observations confirmed that approximately 100 microgrowth lines were formed per year in the shells, and that the micorogrowth patterns might reflect mainly seawater temperatures and planktonic blooms. In the poster presentation, we will report the relationship between microgrowth patterns and marine environment. By clarifying the relationship between them, the temporal resolution of paleoclimate reconstruction using this species can be improved to less than the annual scale.