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Parts of the northwest Atlantic Ocean, including the Gulf of Maine, along the northeastern coast of the United States, are warming at a rate as much as four times faster than the global ocean, according to instrumental and satellite records. In addition to the longer-term temperature increases, the frequency and severity of marine heat waves have been increasing. Combined, the ecological impacts are numerous and concerning, yet our understanding of past climate in this region is incomplete due to sparse and short-duration instrumental records. Here we present annually resolved oxygen isotope records from the marine bivalve, Arctica islandica, from five locations ranging from Jonesport and Seguin Island in the Gulf of Maine to Long Island, New York, Cape May, New Jersey, and Ocean City, Maryland in the Mid-Atlantic Bight, a span of over 870 km along the Atlantic coast. Several of the isotope records span the last 100 years or more and all records show coherent, substantial warming since at least 1980 CE. The level of warming indicated in the shell oxygen isotopes is comparable to the 0.5 °C per decade (1980-2020 CE) warming also shown in the instrumental record of sea surface temperature from Boothbay Harbor along the central coast in the Gulf of Maine. These five spatially distant isotope records span different oceanographic conditions and dynamics, including water mass sources, yet they all indicate a substantial warming in recent decades, likely related to increased anthropogenic warming. Beyond reconstructing seawater temperature prior to instrumental records, a major goal of this work is to disentangle the global warming signal from these records to better understand the underlying ocean dynamics also influencing these records. 

Arctica islandica (ocean quahog), a commercially-important, long-lived bivalve species, is abundant on much of the northeastern United States continental shelf. Several recent studies have noted increases in growth rates of these clams over the last 200 years at some locations in the southern Mid-Atlantic Bight region whereas growth rates at sites farther north have remained constant through time. It has been suggested that these changes in growth rate are related to warming in the more southerly sites. However, a direct comparison between site-specific bottom-water temperatures and A. islandica growth rates has not been done. We present oxygen isotope data measured in Arctica islandica shells, a proxy for seawater temperature, paired with simulated temperature from high-resolution ocean model output to investigate the relationship between A. islandica shell growth rate and bottom water temperatures throughout the northeastern United States continental shelf. The relationship between oxygen isotopes and growth rate in A. islandica is assessed at several locations, including the continental shelf offshore New Jersey and Long Island, and the Georges Bank region. Bottom water temperature trends at these locations are further assessed using the VIKING20X ocean model, which uses JRA55-do (55-year Japanese Atmospheric Reanalysis for driving ocean-sea-ice models) atmospheric forcing from 1958 to present and nests a 1/20° Atlantic Ocean in a 1 ⁄ 4° global domain. The results of this work have implications for the ocean quahog fishery, in particular as water temperatures off the eastern coast of the United States are predicted to continue to increase in response to global climate change. Additionally, this research lends insights into the use of A. islandica growth as a paleoclimate proxy for bottom water temperature. 

The Gulf of Maine (GoM) is currently undergoing rapid environmental and ecological changes and this system is projected to become increasingly stressed in the coming decades. Thus understanding past spatial and temporal conditions of this region is key to understanding how future environmental changes and extreme events may impact fisheries and ecosystem dynamics in the GoM. Changes in the physical and chemical variations in the shells of mollusks can be used as a powerful proxy for studying past climates and environments. In this study, we used the growth and geochemistry signatures in the long-lived marine bivalve Arctica islandica collected from the Down East coastal region in the Gulf of Maine (Jonesport, ME) to evaluate past climatic and hydrographic variability in the northwestern North Atlantic Ocean. The recent collection of shells extends a previously developed master shell growth chronology by 11 years and now spans from 1954 to 2020 CE. Based on visual crossdating techniques, shell growth variability is highly coherent among the population indicating that environmental conditions are driving growth. Variability in annually resolved shell growth increments and stable oxygen isotope values are largely related to sea surface temperatures (SSTs) and water mass properties of the Eastern Maine Coastal Current. This master shell growth chronology and annually-resolved isotope series will fill data gaps prior to instrumental records and allow us to better understand the spatial oceanographic variability in the GoM.