Using the growth and geochemical signatures of Arctica islandica from Jonesport, Maine, USA to document environmental and oceanic variability of the North Atlantic

Earth’s current climate is changing and warming at a rapid rate across most of the planet. In order to place the recent warming into context, paleoclimate records are needed to demonstrate past climatic variability throughout geologic history. Paleoclimate records are essential when examining past climatic variability because there is lack of instrumental data prior to the 1900s, and these data are spatially biased. In order to robustly reconstruct global climate, both terrestrial and marine environments must be examined. Although there are numerous terrestrial proxy reconstructions available, there are a limited number of high-resolution (annual to decadal) marine paleoreconstructions. Particularly, marine paleoreconstructions in the northwestern North Atlantic Ocean are of interest because it is the home to the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is an important part of global ocean circulation, and is responsible for transporting heat, salt, energy, and necessary nutrients to sustain ocean life throughout the North Atlantic Ocean. A region thought to be impacted by the AMOC is the Gulf of Maine (GoM), a semi-enclosed sea, located at the intersection of the northward flowing Gulf Stream and the southward flowing Labrador Current. Situated in the northwestern North Atlantic Ocean, the GoM is home for several commercially important fisheries and it is an essential ecosystem for diverse flora and fauna. However, recent studies have shown that the GoM is warming faster than 99% of the global ocean due to shifts in water masses, Gulf Stream dynamics, and increasing levels of anthropogenic CO2 in the atmosphere. Because of these impacts, the GoM is becoming an increasingly stressed ecosystem that may be impacted by ocean acidification and more warming. Because of this, the GoM is a prime location for developing records of past climatic and oceanic variability. Importantly, understanding this region’s past spatial and temporal oceanographic and climatic conditions is key to estimating how future environmental changes may impact fisheries and ecosystem dynamics in the GoM. In order to document past climate variability prior to instrumental records, proxy archives or climate models must be considered. For example, changes of the physical and chemical variations in the shells of mollusks (a field known as sclerochronology) can be used as a proxy for studying past climates and environments. The research discussed in this thesis, uses the growth and geochemical signatures of the long-lived marine bivalve Arctica islandica collected from the Down East coastal region in the GoM (Jonesport, ME). These mollusk archives are used 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 (MSC) 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 shell growth. The Jonesport stable oxygen isotope series spans from 1956-2020 CE and demonstrates a decrease in δ18Oshell values over time indicating an increase in seawater temperature and a decrease in salinity over the last ~70 years. Fluctuations 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 region. This MSC and annually-resolved oxygen isotope series will fill spatial data gaps prior to instrumental records and allow us to better understand the spatial oceanographic variability in the GoM and the northwestern North Atlantic Ocean. This thesis aims to provide a marine paleoclimate reconstruction to document oceanic and climatic variability in the GoM since the mid 1900s and to evaluate the mechanisms that are responsible for the noted changes.

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