Reconstructing Northeastern Pacific Climate Variability from the Annual Growth Increments of Pacific Geoduck

Abstract

The long-term character and range of Northeast (NE) Pacific climate variability is largely unknown due to the short period of instrumental record and poor agreement among existing reconstructions. To address this issue, a multi-centennial record of NE Pacific climate is developed from a new archive, the Pacific geoduck, a long-lived marine bivalve known to form annual growth increments within its shell. The widths of these increments strongly covary with ambient water temperature, and calcium carbonate within these shells contains radiocarbon, precipitated from ambient seawater, and serves as an indicator of ocean circulation. This study describes the development of a multicentennial geoduck chronology, with some chronology segments extending 3000 years before present. The first portion of the study outlines the development of this chronology and the accompanying sea surface temperature (SST) reconstruction from growth-increment widths. The chronology is the first to include dead-collected material from the ocean floor and represents the longest chronology and associated annually resolved SST reconstruction yet developed from any marine organism in the region. In the second portion of the study, radiocarbon is sampled from the geoduck chronology at decadal resolution to quantify water mass variability and assess relationships with SST. This new decadal radiocarbon record is the only record of its kind in the NE Pacific and describes a relatively stable state that can be interrupted by regimes of cold, radiocarbon-old water. In the final portion of this study, the uncertainty of paleoclimate reconstructions, including the geoduck SST reconstruction, are analyzed to determine the most skillful and robust method of defining uncertainties from crossdated paleoclimate proxies. The results lend support to the Maximum Entropy Bootstrap approach recently introduced to dendroclimatology but not yet widely adopted. Thus, this body of work demonstrates that Pacific geoduck can be utilized as both an SST and a water mass proxy over multiple centuries and that synthetic chronology experiments can serve to test novel methods across the growing diversity of crossdated paleoclimate proxies.