Southeastern United States Hydroclimate During Holocene Abrupt Climate Events: Evidence From New Stalagmite Isotopic Records From Alabama

We present new high-resolution absolute-dated stalagmite delta O-18 and delta C-13 records from Alabama, southeastern United States (SE US), spanning the last 12 thousand years (ka). A local relationship between annual rainfall amount and its amount-weighed delta O-18 composition exists on interannual timescales, driven mostly by an amount effect during summer and spring seasons. Based on a novel quantitative interpretation of modern rainfall isotopic data, stalagmite delta O-18 variability is interpreted to reflect the relative contribution of summer and spring precipitation combined, relative to combined fall and winter precipitation. Precipitation amount in the SE US increases during an interval of 500 yr coeval with the Younger Dryas and during the 8.2 ka and Little Ice Age abrupt cooling events. High precipitation during these events reflects enhancement of spring and summer precipitation by 90% combined, relative to today's conditions, while the contribution of fall and winter rainfall remained unchanged or decreased slightly. Results from this study support model simulation results that suggest increased precipitation in the SE US during Atlantic Meridional Overturning Circulation (AMOC) slowdown/shutdown. In association with Northern Hemisphere mid-latitude cooling from the Early to mid-Holocene, annual precipitation in the SE US decreases, a pattern distinctive from that observed during abrupt cooling events related to AMOC shifts. Long-term hydroclimate change in the SE US is likely sensitive to summer insolation reduction as inferred to have occurred in other tropical and subtropical regions. The dynamical link between AMOC shutdown and precipitation variability in the SE US remains to be examined.