Composition of stable isotopes in geoduck (Panopea abrupta) shells: A preliminary assessment of annual and seasonal paleoceanographic changes in the northeast Pacific

We tested the suitability of using Panopea abrupta (geoduck) shells to evaluate changes in both annual and seasonal sea surface temperature. Geoducks are long-lived and deposit annual growth increments within their shell each winter, making them potentially excellent paleoceanographic archives. Shells were sampled by thin-section from the hinge plate region, where the first 20-30 years of growth provide relatively wide annual increments suitable for sampling for isotopic analysis. The geoducks used in this study were collected in AD 2000 from west of Protection Island, located at the eastern end of the Strait of Juan de Fuca. The isotopic record is compared to the historic instrumental record as well as the thickness of geoduck growth increment measurements, which were also correlated with water temperature [Strom, A., Francis, R.C., Mantua, N.J., Miles, E.L., Peterson, D.L., 2004. North Pacific climate recorded in growth rings of geoduck clams: a new tool for paleoenvironmental reconstruction. Geophysical Research Letters 31, p. LO6206. doi:10.1029/2004GL019440]. The annual average oxygen isotopic values from two geoduck clams, whose first years of growth correspond to the early historic instrumental record (∼AD 1920) appear to be recording trends in sea surface temperature. Intra-annual sampling of annual growth increments indicates that the shells also appear to be recording seasonal changes in sea-surface temperature. Preliminary analysis of carbon isotopic data on both the annual and seasonal timescales indicate the potential for assessing productivity changes in ambient seawater.