Northward Gulf Stream Migration During the Little Ice Age Revealed Using Monthly-Scale Geochemistry of a Bermudan Stalagmite

Abstract

The Gulf Stream (GS) forms part of the upper-ocean limb of the Atlantic Meridional Overturning Circulation (AMOC) and plays a vital role in redistributing heat northward, greatly influencing regional climates, and is likely why the Northern Hemisphere is approximately 1.4°C warmer than the Southern Hemisphere. However, anthropogenic warming is weakening the AMOC and as a result it is approaching collapse, which would have profound impacts on regional and global climate. Understanding GS path and strength variability on longer timescales is vital to contextualise its current day weakening and fully appreciate its sensitivity to forcing. This study provides evidence that the GS began migrating northward in the early 1700s. A robust chronology for a Bermuda stalagmite was developed by modelling annual geochemical cyclicity to the mean radiocarbon growth rate and yielded a magnesium concentration record spanning five centuries (1456–2013 CE). Temperature estimates were derived by calibrating the reconstruction to a published coralline sea surface temperature record. The resultant palaeo-oceanographic temperature reconstruction presented here indicates that the GS was likely further south during the Little Ice Age (LIA); however, the compounded effects of weaker westerlies during the pre-1700 extended negative North Atlantic Oscillation (NAO) phase obscure inferences about its exact southward positional extent. This study suggests that a combination of reduced GS transport, enhanced Labrador Current and Deep Western Boundary Current transport, and an extended negative NAO phase, caused the Gulf Stream to be at lower latitudes during the LIA, before migrating northward as the LIA abated. An earlier GS weakening could indicate that the system is more sensitive to additional forcing than previously thought, meaning a tipping event may occur earlier than forecasted. These results may help to guide and constrain predictions made about future Atlantic Ocean circulatory states as well as provide an insight into the climate during the LIA.