Anna Ryan

B. Sc. Honours Thesis

The effect of a weakening Atlantic Meridional Overturning Circulation on ocean temperatures in the Gulf of St. Lawrence over the past 150 years

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Ocean circulation is essential for transporting heat, dissolved CO2, and nutrients all around the globe. The strength of the ocean circulation, therefore, has a major influence on climate and vice versa. Over time, ocean currents can change causing climate to vary on local, regional, and indeed global scales; in turn, changing climate can affect the strength and location of ocean currents. Recent studies suggest that freshwater addition to the western North Atlantic from melting Arctic and Greenland ice is causing the Atlantic Meridional Overturning Circulation (AMOC), which encompasses the northward flow of warm saline water of the Gulf Stream and the southward flow of colder deep water of the Labrador Current, to weaken. This results in surface ocean temperatures offshore of Atlantic Canada to rise faster than in the rest of the global ocean. Ocean warming, in turn, is detrimental to a wide variety of marine species, thus impacting both the environment and the local economy.

The objective of this study is to assess past changes in ocean currents in the western North Atlantic, based on sea surface temperature (SST) reconstructions from alkenones preserved in sediments on the Scotian Shelf. Alkenones are long-chain carbon molecules uniquely synthesized by a certain group of phytoplankton that change in composition depending on surface water temperatures. To improve the spatial and temporal resolution of previous studies investigating past SST variability in the open NW Atlantic, I analyzed a high-resolution sediment core from the Cabot Strait in the Gulf of the St. Lawrence that contains a detailed record of the last 150 years. From 1850 to present, the alkenone-derived SST estimates fluctuate between 6.5 – 7 °C with a local error of estimation of 1.4 °C. This absence of any significant trend in SST in the central Gulf of the St. Lawrence over the last 150 years implies that factors other than open ocean warming affect our study site. Three possible mechanisms are presented that may mask or offset any SST warming expected from the weakening AMOC: the hydrography of the Gulf of St. Lawrence, the distribution of water masses in the Cabot Strait, and the timing of the spring phytoplankton bloom. From such analyses, we can gain a better understanding of the changes we are seeing now and more reliably predict what is to come.

Key words: alkenones, sea surface temperature, Scotian Shelf, AMOC, currents
Pages: 40
Supervisor: Markus Kienast