North Atlantic Freshening and Abrupt Cooling During the Last Glacial Maximum and Deglaciation recorded by Iberian Speleothem

The last deglaciation represents a valuable test case for understanding abrupt climate events as it triggers cascading feedback among Earth system components, particularly involving the ice sheets. Constraining the timing, magnitude, and order of these events within the critical North Atlantic realm remains challenging.

Here, we present a new U-Th-dated stalagmite from northwestern Iberia spanning the Last Glacial Maximum (LGM) and the entire last deglaciation (24-12 ka BP). Given its coastal and moisture-replete cave location, stable isotopes (δ¹⁸O, δ¹³C) capture both the influence of the Northern Hemisphere ice sheet’s meltwater inflow on surface ocean chemistry, and regional surface air temperature changes via their effects on local vegetation.  Since both stable isotopes are measured on the exact same samples, this allows a direct and high-resolution study of the temporal relationship between Northern Hemisphere meltwater ocean in-flux and temperature change in the North Atlantic realm, advancing speleothem applications in ocean and cryosphere studies.

We have compared our results to meltwater histories derived from the ice sheet model GLAC-1D, and our findings confirm/reveal gradual meltwater inflow during the LGM and early deglaciation (~20.8-18.2 ka BP), followed by a set of abrupt increases in meltwater starting at 18.04±0.16, 16.22±0.24 and 15.44±0.19 ka BP. In our record, abrupt cooling begins at 17.18±0.16 ka BP, indicating that the peak weakening of deep Atlantic convection lagged the first abrupt meltwater pulse by ~850 years. This suggests a non-linear connection between surface ocean freshening and the consequential disruption to the early deglacial Atlantic meridional overturning circulation. In contrast, a brief cooling phase, synchronous with a response in global archives, aligns with the meltwater pulse at 16.22 ka BP, whereas no cooling is associated with the 15.44 ka BP pulse. The transition into the Bølling-Allerød period, featuring two warm phases, is marked by rapid warming starting at 14.78±0.12 ka BP concurrent with a decline in meltwater anomalies, likely related to the re-strengthening of deep Atlantic convection. Remarkably, our record does not show a freshwater signal coincident with the classically cited onset of MWP 1a (~14.6 ka BP), suggesting that this event happened earlier or that the freshwater anomaly was rapidly advected out of the surface North Atlantic by a strong AMOC.