Question

For decades, paleoclimatologists analyzing the sudden temperature drop of the Younger Dryas period—approximately 12,900 years ago—pointed to the catastrophic draining of Lake Agassiz as the primary instigator. According to this traditional model, the sudden influx of freshwater disrupted the Atlantic Meridional Overturning Circulation (AMOC) by reducing the salinity and density of surface waters in the North Atlantic, which in turn halted the northward transport of heat. However, recent sediment core analyses have revealed that this freshwater pulse actually preceded the oceanic slowdown by nearly three centuries. Instead, the immediate catalyst for the AMOC disruption appears to have been a prolonged period of intense atmospheric cooling driven by volcanic aerosols, which cooled the surface waters to the point of freezing. The resulting sea-ice expansion acted as a thermal barrier, preventing wind-driven mixing and thereby directly precipitating the stagnation of the deep-water circulation. The draining of Lake Agassiz, rather than initiating the cooling event, was actually triggered later when the advancing Laurentide Ice Sheet blocked alternative northern drainage routes and forced the lake to overflow its southern margins, creating a feedback loop that merely prolonged the pre-existing cold period.

According to the passage, the stagnation of deep-water circulation in the North Atlantic was directly precipitated by which of the following?