The Gulf of Maine, a semi-enclosed sea on the east coast of North America, has been warming faster than most of the rest of the world's oceans over the last three decades. Since the installment of the Boothbay Harbor (Maine) sea surface temperature (SST) record in 1905, surface waters have warmed by more than 2°C, with more rapid warming seen recently. However, it is unclear when this pronounced warming in the Gulf of Maine began or whether the warming in recent decades is the result of natural variability, anthropogenic causes, or a combination of both. Such considerations are not only of grave concern for Gulf of Maine ecosystems and the many economically important fisheries in the region but also for other regions of the world's oceans that are predicted to warm in the future with anthropogenic climate change.

This dissertation uses stable and radiogenic isotopes, in both the modern waters of the Gulf of Maine and preserved Arctica islandica shells collected in the western Gulf of Maine, in order to reconstruct hydrographic variability in the region from several centuries before the instrumental record began up until present day. Isotopic investigations (d18Owater, d15NNO3-, d18ONO3-) of modern water samples collected at varying depths throughout the Gulf of Maine enable a better understanding of factors, including ocean circulation and nitrogen cycling processes, which dictate isotopic systems in today's waters. Such investigations lend both insight into modern processes, such as the presence of nitrification in the upper water column, as well as aid in the interpretation of isotopic variability preserved in the geologic record.

Furthermore, d15N in periostracum (the brown, protein rich layer on the outside of shells) of A. islandica shells is investigated as a new proxy for d15N and therefore water mass source variability. Using compound specific nitrogen isotopes of amino acid, we show that this proxy can be used in place of carbonate d15N to track changes in d15N of the clam diet and therefore d15N of nitrogen substrates in the water. Such findings enable a much higher resolution record of d15N in the Gulf of Maine through time than would be possible using d15N measured in carbonate. These analyses also suggest that the clams have not changed trophic level significantly over the last 300 years.

Finally, this dissertation presents a 300-year reconstruction of hydrographic variability in the Gulf of Maine from crossdated A. islandica shells. Oxygen isotopes (d18O) and nitrogen isotopes (d15N) were measured in these shells and combined with radiocarbon (D14C) data from an earlier study. These latter two isotopic systems have been shown by others to vary with the major water masses that compose Gulf of Maine waters, with Warm Slope Water (WSW; originating from south of the Gulf of Maine) having lower d15N and higher D14C than Labrador Slope Water (LSW; originating in the Labrador Sea). Thus, these water mass tracers serve to distinguish among the different water masses entering the Gulf of Maine through time. Additionally, d18O varies with temperature, assuming a constant d18O of the water, and therefore is likely to also vary between warmer WSW and colder LSW.

The d18O data, which have statistically significant correlations with the nearby Boothbay SST record that extends back to 1905, suggest that the Gulf of Maine has been warming since the 1870s, following a cooling of the region coming out of the Little Ice Age. SST output from a Community Earth System Model-Last Millennial Ensemble (CESM-LME) multi-ensemble mean for the region shows similar trends and suggests external forcing mechanisms for these changes. Likewise, the d15N and d14C reconstructions record comparable changes to the d18O record after the 1840s and therefore suggest, assuming isotopic signatures of water mass endmembers are constant, that the warming seen in the Gulf of Maine since the 1870s is in part a result of changes in the proportion of water masses entering the Gulf of Maine, with increasing northward flowing WSW entering the region over the last century. Such findings, coupled with statistically significant correlations between the d18O record and instrumental records of Florida Current strength and the Atlantic Meridional Overturning Circulation (AMOC) instrumental array at 26°N, suggest that recent changes in Gulf of Maine hydrographic variability can largely be attributed to changes in AMOC strength, which has been shown by several other paleoceanographic reconstructions to be weakening since the late 1800s due to increased freshwater flux into the northern North Atlantic. The records presented here suggest that this weakening of the AMOC followed a strengthening out of the Little Ice Age in the mid-to-late 1800s.