NISA Speleothems recording changes in past ocean circulation and ice sheet meltwater

In the subpolar North Atlantic, warm, northward flowing surface waters cool, become denser, and sink, flowing southward as cold deep water, forming a major artery of global heat transport known as the Atlantic Meridional Overturning Circulation (AMOC). Higher freshwater delivery to subpolar areas in coming centuries might lower the surface ocean density and weaken the AMOC, which would have global climate impacts such as cooler European temperatures and weaker Asian monsoon. Current climate models struggle to predict the scope of freshwater needed to disrupt AMOC. Dozens of times in the last 100,000 years, the AMOC has weakened abruptly, remained weak for centuries to millennia, and recovered. Yet, existing data is too limited to test whether these past events were even triggered by changes in freshwater in the North Atlantic, or if freshwater forcing ended to enable the recovery of AMOC.
To test the factors influencing the past episodes of weakening and recovery of AMOC, we are using dual geochemical indicators sensitive to both ocean freshening and the abrupt cooling that accompanies AMOC weakening. Measuring the indicators in annually layered stalagmites from coastal caves in a key area along the North Atlantic enables us to document when past freshwater pulses occurred and if they triggered or conditioned AMOC weakening. In assembling this perspective over a large number of events, we can evaluate if AMOC is more fragile or more resilient under certain climate states. These results also provide new information about the rates of ice sheet melting and feedbacks between climate and melting rates. Ultimately, these well documented templates of past AMOC behavior can be used to evaluate and improve the representation of AMOC in modern coupled ocean -atmosphere general circulation models.
The accurate interpretation of temperature and freshwater history encoded in stalagmites is grounded in a careful evaluation of the underlying system processes. Monitoring and cave process models are used to distinguish regional temperature signals from in-cave processes modifying carbon isotopic records. In collaboration with Dr. Ruza Ivanovic (University of Leeds), we also use tracers in coupled numerical ocean-atmosphere models to identify how freshwater delivered to the ocean is distributed in the moisture source for cave stalagmites.