Passage:
During Marine Isotope Stage 3 (MIS 3), spanning roughly 57,000 to 29,000 years ago, the North Atlantic climate was punctuated by episodic, millennial-scale oscillations known as Dansgaard-Oeschger (D-O) cycles and Heinrich events. While D-O cycles represent rapid transitions between cold stadial and abruptly warm interstadial phases, Heinrich events designate massive catastrophic discharges of icebergs from the Laurentide Ice Sheet through the Hudson Strait into the open ocean. These icebergs deposited distinct layers of lithic material—termed ice-rafted debris (IRD)—composed primarily of detrital carbonate across a vast swath of the subpolar North Atlantic known as the Ruddiman IRD belt.
Sedimentological analyses of deep-sea marine cores reveal that Heinrich events (designated H1 through H6) were characterized not merely by physical IRD deposition, but by profound perturbations in ocean circulation. The melting of massive armadas of icebergs introduced enormous fluxes of low-density freshwater into the surface waters of the subpolar gyre. This freshwater capping suppressed the convective overturning that drives Atlantic Meridional Overturning Circulation (AMOC), effectively halting the formation of North Atlantic Deep Water (NADW). Consequently, northward heat transport across the equatorial Atlantic diminished sharply, plunging the high northern latitudes into severe cooling while simultaneously causing anomalous warming in the Southern Hemisphere via the 'bipolar seesaw' mechanism.
Crucially, geochemical analyses utilizing neodymium isotopes and protactinium-to-thorium ratios in benthic foraminifera have refined the classical oceanographic model. While earlier hypotheses posited that AMOC shut down completely during every stadial phase, isotope data demonstrate that deep water ventilation was only entirely collapsed during Heinrich stadials, whereas non-Heinrich stadials exhibited a shoaled, intermediate-depth AMOC cell termed Glacial North Atlantic Intermediate Water (GNAIW). Furthermore, contrary to initial assumptions that Heinrich events were triggered by external solar forcing or internal ice-sheet thermal instabilities alone, high-resolution core data indicate that subtle ocean warming at intermediate depths undermined the floating ice shelves lining the Labrador Sea prior to iceberg surging. Thus, subsurface oceanic thermal priming acted as an essential precursor to ice-sheet destabilization.
Statement:
According to the passage, geochemical isotope data from benthic foraminifera demonstrate that deep water ventilation collapsed entirely during every stadial phase occurring throughout Marine Isotope Stage 3.
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