Sedimentary Signals of Freshwater Outflows Onto the Atlantic Canadian Shelf Over the Holocene: A Sortable Silt Perspective

Abstract

The sortable silt index (ss), defined as the average grain size of a sediment sample between 10 - 63 micrometers (μm), is a paleoceanographic proxy for estimating ocean current speeds at the time of sediment deposition. While the proxy was primarily developed for use in the deep sea, studies continue to both push ss into new environments and refine our understanding of ss interpretation. This thesis (a) contributes to this growing understanding of the strengths and limitations of ss and (b) generates new insights into the paleoceanographic conditions of the Labrador and Scotian Shelves (Northwest Atlantic) over the last 10,000 years, the Holocene. Chapter 1 provides a general review of ss and applications across the Northwest Atlantic. This includes several ways ss has been applied and interpreted to gain insight into local and global scale ocean change. To address the lack of standardized chemical sediment pretreatment methods across published studies, Chapter 2 examines the sensitivity of ss to various common pretreatment methods. My results indicate that ss is robust independent of pretreatment used. However, the results highlight the importance of replicates and consistent instrument operation. Chapter 3 applies ss to marine sediment cores from the Labrador Shelf. After demonstrating the suitability of ss, changes in Labrador Coastal Current flow over the Holocene and their potential drivers are discussed. My results suggest that changes in the strength of the Labrador Coastal Current were likely driven by the influence of warm and salty waters of Atlantic origin during an ‘establishment period’ 9,000 - 6,000 years before present (B.P.), and by cold, fresh waters from Hudson Strait during a ‘stabilization period’ from 6000 B.P. to the present. Chapter 4 expands the application of ss to the eastern Scotian Shelf. In this region, the results suggest an influence of the Labrador Current on the eastern Scotian Shelf, and also support ss as a proxy of environmental change, specifically the post glacial deepening of the Scotian Shelf, and the increase in wave and storm energy on the area. With this thesis, I highlight the robustness, limitations, and new applications for the sortable silt index.