Marine‐Calibrated Chronology of Southern Laurentide Ice Sheet Advance and Retreat: ∼2,000‐Year Cycles Paced by Meltwater–Climate Feedback.

Climatic warming following the Last Glacial Maximum caused the southern Laurentide Ice Sheet (LIS) to begin ∼2,000‐year cycles of retreat and readvance whose cause remains ambiguous. By developing a marine‐calibrated chronology of southern LIS position, we counterintuitively demonstrate that between 17.6 and 11.3 ka, ice advanced during times of northern‐hemisphere warming and retreated during times of northern‐hemisphere cooling. Here we propose a cyclical feedback: Meltwater from ice retreat cooled the northern hemisphere by weakening the Atlantic Meridional Overturning Circulation (AMOC). This eventually lead to ice‐sheet readvance, which reduced and rerouted meltwater discharge, and thereby allowed the AMOC to strengthen and the northern hemisphere to warm. Our data suggest that this antiphased ice–climate interaction, paced by ice‐sheet response time, was initiated by synchronous warming and ice retreat ∼18.7–17.6 ka (corresponding to the Erie "Interstade") and reached its apex during the Younger Dryas. Plain Language Summary: Twenty thousand years ago, a colder climate allowed glacial ice to flow from northern and eastern Canada into the Great Lakes region. But when climate began to warm, instead of simply melting from south to north, the margin of this ice front repeatedly retreated and re‐advanced in 2,000‐year cycles. First, we track these advances and retreats using records of ancient Gulf of Mexico water, stored in plankton shells retrieved from a sediment cores. Ice advance redirected water bearing "light" oxygen, with fewer neutrons, from Canada and the Great Lakes into the Mississippi River. When ice retreated, these waters found other courses to the sea, shifting Gulf of Mexico chemistry toward heavier oxygen (carrying more neutrons). Next, we ask why these ice‐margin cycles occurred during a time of monotonically increasing solar radiation to the northern hemisphere. We propose that ice‐sheet melt (during retreat) slowed Atlantic Ocean circulation, causing the northern hemisphere to cool and the ice sheet to gradually regrow. However, this same regrowth reduced meltwater flow to the ocean, causing the northern hemisphere to re‐warm and eventually leading the ice margin to retreat once more. This cycle persisted from approximately 17,600 to 11,300 years ago, until North American ice became too small to sustain this feedback. Key Points: δ18O records from the Gulf of Mexico refine the chronology of Laurentide Ice Sheet advance and retreat through the Great LakesCounterintuitively, from 17.6 to 11.3 ka, ice advanced during times of warming and retreated during times of coolingA feedback between meltwater, Atlantic Meridional Overturning Circulation strength, and temperature—paced by ice‐sheet response time—may have set this anticorrelation [ABSTRACT FROM AUTHOR]