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.

The Little Ice Age (LIA), a period from ~1400 CE to 1850 CE, was characterized by colder winters and more frequent extreme weather events, particularly in the Northern Hemisphere. While the exact causes of the LIA remain a topic of ongoing research, evidence suggests that changes in ocean circulation likely contributed to the observed global cooling, although the specific mechanisms and drivers of these changes are not yet fully understood. Here, we aim to generate new knowledge to help us better understand how ocean circulation changed before, during, and after this climatic event. More specifically, using marine cores collected at the head of the Laurentian Channel in the Lower Estuary of the St. Lawrence, we investigated variations in the temperature and seawater oxygen isotopic signal of western North Atlantic slope waters to assess the relative strength of the Labrador Current and the Gulf Stream. First, we established a Mg/Ca-temperature calibration curve for Globobulimina auriculata from the bottom water of the Lower St. Lawrence Estuary based on instrumental temperature data and geochemical analysis from a short, century-old sediment box core. Then, using a longer piston core, we produced new downcore measurements of Mg/Ca in Globobulimina auriculata for the LIA. We then coupled this new temperature reconstruction with existing oxygen isotope data (d18Ocalcite) to disentangle the influence of temperature on the d18Ocalcite. The resulting d18Oseawater record is interpreted in terms of changes in in their isotopic composition due to changes in freshwater input, and relative strength and position of the Labrador Current and the Gulf Stream. Our interpretation suggests dominance of fresh Labrador Sea-derived waters around 1500 CE, and during most of the LIA. In the later stage of the LIA, after 1800 CE, our data indicate a steep increase in the influence of Atlantic-derived waters, consistent with a northward shift of the Gulf Stream. The record of the ~1800-1950 CE interval exhibits high-amplitude variability, with a sudden freshening event at the end of the LIA. The upper part of the record, after 1950 CE, is characterized by a regional warming trend that has been widely documented in previous studies conducted in this area.