A 900-year record of effective moisture in the Laurentian Great Lakes region

Abstract

The (Laurentian) Great Lakes region (GLR) of North America contains ∼20% of the world's freshwater by total area. Yet, the water quantity and quality of lakes in this region are threatened by water level fluctuations and excessive algal growth associated with climate warming. To understand the mechanisms that drive these changes, we have established a 900-year history of environmental change using sediments from a small kettle lake, Barry Lake, located within the GLR. To understand how water quantity changed over time, we used stable isotopes of oxygen and carbon in marl to reconstruct effective moisture. Our reconstruction for Barry Lake shows that the Little Ice Age (LIA) was wetter than present, whereas the Medieval Climate Anomaly (MCA) was drier than present. There is considerable spatiotemporal variation in effective moisture across the GLR and northeastern (NE) USA. This variation may reflect changes in the sign of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), which can alter moisture delivery by modifying the strength and position of the polar and subtropical jet streams. To identify changes in water quality, we reconstructed primary production using sedimentary chlorophyll a (Chl-a(s)) contents and stable isotopes of carbon and nitrogen in organic matter. There was relatively little change until the last 150 years, revealing that shifts in effective moisture did not noticeably influence primary production. Rather, primary production was impacted by anthropogenic pressures such as land use change and associated increases in nutrient delivery. Our research contextualizes recent threats to water quantity and quality in the GLR against a backdrop of environmental change, while also synthesizing hydroclimate records from across this region and the NE USA.