Preserving low-frequency climate signals in growth records of geoduck clams ( Panopea abrupta)

Abstract

The capacity of geoduck clams ( Panopea abrupta) to preserve detailed records of climatic change in their shells is limited primarily by longevity. Unlike the multi-centennial growth records commonly available from trees, shells rarely yield more than 160 years of data; consequently low-frequency signals may be lost when band width series are processed. Here we investigate use of ‘Regional Curve Standardization’ (RCS) to preserve decadal to centennial-scale growth trends in the relatively short segment lengths available from individual clams. Advantages of this method over the more common ‘exponential’ treatment are first explored through simulation experiments using sine waves embedded in geoduck growth curves. Both standardization methods are then used to remove biological trends from growth records of geoduck shells sampled near Protection Island, in the Strait of Juan de Fuca. The resulting chronologies are compared in terms of their spectral properties, their correlations with historical air temperature records and their agreement with historical observations of the marine environment. Simulation experiments illustrated the potential benefits of using the RCS growth model to standardize shell growth series, but empirical results were inconclusive. The RCS chronology exhibited enhanced spectral power at periods exceeding the length of growth segments, and provided stronger correlation with coastal air-temperature records, especially during the earliest portion of the chronology. Over the full period of comparison, however, the correlation coefficients were essentially identical. Observational data from the mid to late 1800s appears to support the RCS results. Northward range expansions of several marine fish species coincide with a period of conspicuously high values in the RCS chronology, and suggest that the coastal ocean was unusually warm along western North America from the 1850s through the 1870s.