USING BULK AND COMPOUND SPECIFIC STABLE ISOTOPES IN ARCTICA ISLANDICA SHELLS TO RECONSTRUCT WESTERN NORTH ATLANTIC OCEAN DYNAMICS

Despite the significant influence of the Atlantic Meridional Overturning Circulation (AMOC) on global climate, very few reconstructions of AMOC variability exist and those that do are low resolution or use terrestrial based proxy records. Modeling studies suggest that the Gulf of Maine is an ideal location for a high-resolution, marine based reconstruction of AMOC strength variability as water mass composition in the region appears to be related to AMOC strength.

Here we present two highly resolved proxies of water mass source from Arctica islandica shells collected in the western Gulf of Maine and dated back to 1762. Both the oxygen isotopic composition (δ¹⁸O) of the shell carbonate and the nitrogen isotopic composition (δ¹⁵N) of the shell periostracum indicate significant changes in proportion of different water masses within the Gulf of Maine over the last several centuries. Increasing δ¹⁸O values from the early 1800s until the turn of the twentieth century indicate more cold, Labrador Slope Water (LSW), which originates in the Labrador Sea, entering the Gulf of Maine whereas decreasing δ¹⁸O values in the most recent 100 years suggest increasing proportions of Warm Slope Water (WSW), which forms adjacent to the Gulf Stream.

These interpretations of the oxygen isotope data are supported by the δ¹⁵N record. Increasing bulk δ¹⁵N values in the latter part of the nineteenth century indicate an increasing proportion of LSW, which has a higher δ¹⁵N signature than WSW due to its surface water origin, entering the Gulf of Maine. Like the δ¹⁸O record, δ¹⁵N values have decreased since 1900, indicating increased proportions of WSW in the region more recently. Compound specific δ¹⁵N values measured in the periostracum samples confirm that the changes in bulk δ¹⁵N are not related to changes in the diet of the clam.

Because increased (decreased) AMOC strength leads to a southward (northward) shift of the Gulf Stream and therefore less (more) WSW entering the Gulf of Maine, the stable isotopes measured in A. islandica shells from the western Gulf of Maine indicate increasing AMOC strength during the nineteenth century followed by a weakening AMOC over the last 100 years. These findings are consistent with other studies that suggest recent AMOC weakening and put these oceanographic changes in the context of the last several centuries.