The Gulf of Maine is undergoing rapid environmental and ecological changes, yet our spatial and temporal understanding of the climatic and hydrographic variability in this region, including extreme events, is limited and biased to recent decades. In this study, we utilize a highly replicated, multi-century master shell growth chronology derived from the annual increments formed in the shells of the long-lived bivalve Arctica islandica collected in 38 m from the central coastal region in the Gulf of Maine. Our results indicate that shell growth is highly synchronous and inversely related to local seawater temperatures. Using composite analyses of extreme shell growth events from CE 1900 to 2013, we extend our understanding of the factors driving oceanic variability and shell growth in the Northwestern Atlantic back to CE 1761. We suggest that extreme shell growth events are primarily controlled by Gulf of Maine sea surface temperature (SST) and stratification conditions, which in turn appear to be largely influenced by SST patterns in the Pacific Ocean through their influence on mid-latitude atmospheric circulation patterns and the location of the eddy-driven jet. The large-scale jet dynamics during these extreme years manifest as precipitation and moisture transport anomalies and regional SST conditions in the Gulf of Maine that either enhance or inhibit shell growth. Pacific climate variability is thus an important, yet understudied, influence on Gulf of Maine ocean conditions.

In order to document relative changes in water mass contributions in the Gulf of Maine (GoM), we used the shell material of the long‐lived ocean quahog (Arctica islandica). A multicentury, crossdated master shell growth chronology facilitated the reconstruction of a radiocarbon Δ14C history prior to the radiocarbon bomb‐pulse of the 1950s. This reconstruction reveals a highly variable Δ14C series (mean = −56.6 ± 8.0‰ (1σ); N = 34) from CE 1685 to 1935. Δ14C values indicate a rapid shift ca. 1860 CE in source waters to the GoM. From CE 1685 to 1860, GoM waters were dominated by an admixture of Warm Slope Water primarily composed of tropical Atlantic surface waters/Gulf Stream Waters, and Scotian Shelf Water. This water regime was followed by a rapid Δ14C transition to a Labrador Slope Water endmember after CE 1860, with an apparent decrease in Scotian Shelf Water. Together, this shift is likely related to broader changes in the Arctic and the Labrador Sea, and a short‐term strengthening of the Atlantic meridional overturning circulation. Labrador Slope Water dominating GoM hydrography in the 1900s is verified by the similarities between this record and other coral‐ and shell‐derived Δ14C records influenced by waters with Labrador Sea origin. This suggests that GoM radiocarbon variability broadly reflects large‐scale ocean circulation processes in the Northwestern Atlantic. The lack of Δ14C values much below the Labrador Slope Water endmember suggests that the interior GoM gets very little to no Antarctic Intermediate Water as other studies had previously suggested.