Sudden freshening and cooling of western North Atlantic slope water at the onset of the Little Ice Age based on Magnesium-to-Calcium ratio and oxygen stable isotope record.

Wai Ching Rachel Chu, Benoit Thibodeau

The Little Ice Age (LIA, ~1400–1850 CE) was characterized by colder winters and more frequent extreme weather events in the Northern Hemisphere. While changes in ocean circulation likely contributed to global cooling, the specific mechanisms remain poorly understood. Here, we investigate how ocean circulation changed before, during, and after the LIA using marine sediment cores from the Laurentian Channel in the Lower St. Lawrence Estuary. We first established a Mg/Ca-temperature calibration for Globobulimina auriculata using instrumental temperature data and a century-old box core. Applying this calibration to a longer piston core, we reconstructed bottom-water temperatures during the LIA. Coupling these results with existing δ¹⁸O calcite data allowed us to isolate the δ¹⁸O seawater signal, which reflects changes in the relative contributions of the Labrador Current and Gulf Stream. Our results indicate an increase of fresh and cold Labrador Sea-derived waters around 1500 CE. Throughout most of the LIA we observed a slow and steady warming of the bottom water associated with a gradual increase in the proportion of Atlantic-derived waters until ~1850 CE.  The ~1800–1950 CE interval shows high-amplitude variability, including a sudden freshening event at the LIA's end. After 1950 CE, regional warming dominates, consistent with previous studies documenting increased Atlantic influence over the Canadian shelf.