Heinrich Event

Heinrich events were six major episodic glaciological and paleoceanographic upheavals during the Last Glacial Period (H1 through H6, between ~60,000 and ~16,000 years before present, ka BP, plus the Younger Dryas H0 event at ~12.8–11.7 ka BP) in which dynamic instabilities of the Laurentide Ice Sheet (and surrounding circum-Atlantic ice sheets) discharged vast armadas of icebergs through Hudson Strait into the subpolar North Atlantic Ocean. First discovered in 1988 by German marine geologist Hartmut Heinrich in deep-sea sediment cores from the Dreizack Seamount in the northeastern Atlantic, each event is preserved on the abyssal seafloor across the Ruddiman Ice-Rafted Debris (IRD) Belt (40°N–55°N, from Labrador to the coast of Ireland and Portugal) as a distinctive 5-to-50-cm-thick layer rich in coarse lithic gravel and detrital Paleozoic dolomitic carbonate and scarce in planktonic foraminifera shells. By releasing 0.05 to 0.15 Sverdrups (1 Sv = 10⁶ m³ s⁻¹) of glacial meltwater over 500 to 1,500 years, Heinrich events capped the North Atlantic with a buoyant freshwater lid that shut down Atlantic Meridional Overturning Circulation (AMOC) deep-water formation, plunging the Northern Hemisphere into extreme Heinrich Stadials while warming Antarctica via the bipolar seesaw.

In North Atlantic deep-sea sediment cores, normal glacial hemipelagic mud consists of fine clay mixed with calcitic tests of the polar planktonic foraminifer Neogloboquadrina pachyderma (sinistral / left-coiling). Within a Heinrich layer, however, lithic rock fragments coarser than 150 μm (ice-rafted debris, IRD) jump from <100 grains per gram of dry sediment to >2,000–10,000 grains per gram, while foraminiferal abundance drops to near zero due to cold, low-salinity meltwater surface conditions and rapid lithic sedimentation.

Radiogenic isotope fingerprinting (⁸⁷Sr/⁸⁶Sr, ε_Nd, and ⁴⁰Ar/³⁹Ar ages of hornblende and feldspar grains) by Sidney R. Hemming, Gérard C. Bond, and Gwenaël Grousset proved that the dominant source of four of the six classical layers (H1, H2, H4, and H5) was the Hudson Bay / Hudson Strait sector of the Laurentide Ice Sheet, which eroded Paleozoic limestone and dolomite bedrock and Archean Churchill/Superior Province crystalline basement in eastern Canada:

Why did the Laurentide Ice Sheet collapse through Hudson Strait every 7,000 to 10,000 years, and why did every Heinrich event occur strictly during an already-cold Dansgaard–Oeschger stadial rather than during a warm interstadial? Glaciologists and paleoceanographers combine two complementary mechanisms:

1. MacAyeal's (1993) "Binge–Purge" Basal Thermal Switch Oscillator

In 1993, glaciologist Douglas R. MacAyeal formulated the binge–purge free-oscillation model of the Laurentide Ice Sheet over Hudson Bay:

• The "Binge" (Cold-Based Growth Phase, ~7,000 years): Because the floor of Hudson Bay is carpeted by a soft, deformable layer of unconsolidated glaciomarine sediment and Paleozoic carbonate till, the ice sheet's dynamic behavior depends on the temperature of the basal sediment. During the slow binge phase, the base of the ice sheet is frozen solid (T_base < T_melt) to the underlying till. High basal shear stress (~100 kPa) restricts ice flow to slow internal creep, causing snow accumulation to thicken the ice dome over Hudson Bay to >3.0 to 3.5 kilometers.
• The "Purge" (Warm-Based Ice-Stream Surge, ~500–1,000 years): As the ice sheet thickens, the thick ice blanket insulates the bed from the −35 °C surface air, trapping upward geothermal heat flux (~40–50 mW m⁻²) and frictional strain heating until the basal temperature reaches the pressure-melting point (~−2 °C under 3 km of ice). Once liquid water saturates the basal carbonate till, pore-water pressure equals the ice overburden pressure, dropping basal yield stress from ~100 kPa to <5–10 kPa. The Hudson Strait ice stream accelerates by an order of magnitude (from ~50 m yr⁻¹ to 5–10 km yr⁻¹), draining 10% to 20% of the Laurentide Ice Sheet volume (~1–3 × 10⁶ km³ of ice, raising global sea level by 2 to 10 meters) as a 3,000-km-wide plume of tabular icebergs until the thinned ice dome conductive-cools and refreezes to the bed.

2. Subsurface Ocean Warming and Ice-Shelf Buttressing Collapse (Marcott et al., 2011; Bassis et al., 2017)

While the internal binge–purge mechanism explains the multi-millennial recharge time needed to regrow the Hudson Bay dome (explaining why not every D–O stadial produces a Heinrich event), it does not explain why Heinrich discharges were phase-locked to cold Greenland stadials (GS). High-resolution benthic foraminiferal Mg/Ca paleothermometry and glaciological modeling (Shaffer et al., 2004; Marcott et al., 2011; Alvarez-Solas & Ramstein, 2011; Bassis et al., 2017) resolved this paradox:

• When a normal Dansgaard–Oeschger stadial begins and surface AMOC deep-water formation weakens, a perennial sea-ice lid forms at the surface while intermediate-depth North Atlantic waters (at 300–1,000 m depth, the exact grounding-line depth of the Hudson Strait and Labrador marine-terminating ice shelves) warm by +2 to +4 °C due to downward heat diffusion and suppressed vertical convection.
• This subsurface ocean warming melts the floating buttressing ice shelf at the mouth of Hudson Strait from below, triggering Marine Ice Sheet Instability across the retrograde bedrock trough of Hudson Strait and unleashing the pent-up Laurentide "purge" into the North Atlantic.

When a Heinrich iceberg armada melted across the 40°N–55°N North Atlantic, it lowered surface ocean salinity by 1.5 to 3.5 practical salinity units (psu) (recorded by negative δ¹⁸O excursions in planktonic foraminifera after correcting for temperature). Radiochemical measurements of the sediment protactinium-to-thorium ratio (²³¹Pa/²³⁰Th, compiled by Jerry F. McManus and colleagues, 2004) in Bermuda Rise core OCE326-GGC5 confirm that during Heinrich Stadial 1 (H1), the deep-water ²³¹Pa/²³⁰Th ratio rose to the water-column production limit of 0.093, indicating a near-total shutdown of North Atlantic Deep Water export:

• Extreme Mediterranean and European Aridity: With North Atlantic SSTs plunging toward 0–2 °C off Portugal and Spain, evaporation and winter cyclone moisture dropped, replacing European open woodlands with cold Artemisia–chenopod steppe-tundra.
• Southward ITCZ Displacement and Tropical Megadroughts / Pluvials: Because the Northern Hemisphere cooled strongly while the Southern Hemisphere warmed via the bipolar seesaw, the global Intertropical Convergence Zone (ITCZ) shifted 5° to 8° of latitude southward:
  • Northern Tropics (Severe Drought): The Sahel, Lake Victoria / Lake Tanganyika, the Cariaco Basin (Venezuela), and the Indian and East Asian Summer Monsoons (Hulu Cave δ¹⁸O maxima) experienced their most severe millennial megadroughts of the Pleistocene.
  • Southern Tropics & Western US Great Basin (Pluvial Mega-Lakes): Simultaneously, northeast Brazil (Botuverá Cave speleothems) and the Bolivian Altiplano (paleolake Tauca) experienced intense wet pluvial intervals, while a southward-displaced Pacific jet stream steered winter storm tracks into the Great Basin of the western United States, filling Lake Bonneville (Utah) and Lake Lahontan (Nevada) to their highest Pleistocene highstands during H2 and H1 (Munroe & Laabs, 2013).
• Atmospheric CO₂ Rise via Southern Ocean Upwelling: During every major Heinrich Stadial (H4, H2, H1), Southern Hemisphere warming and poleward shift of the Southern Annular Mode westerly winds enhanced upwelling of carbon-rich Circumpolar Deep Water in the Southern Ocean, driving +15 to +25 ppmv jumps in atmospheric CO₂ recorded in Antarctic ice cores (Ahn & Brook, 2008).

1. Heinrich, H. (1988). Origin and consequences of cyclic ice rafting in the Northeast Atlantic Ocean during the past 130,000 years90057-9). Quaternary Research, 29(2), 142–152.
2. MacAyeal, D. R. (1993). Binge/purge oscillations of the Laurentide Ice Sheet as a cause of the North Atlantic's Heinrich events. Paleoceanography, 8(6), 775–784.
3. Hemming, S. R. (2004). Heinrich events: Massive late Pleistocene detritus layers of the North Atlantic and their global climate imprint. Reviews of Geophysics, 42(1), RG1005.
4. McManus, J. F., Francois, R., Gherardi, J.-M., Keigwin, L. D., & Brown-Leger, S. (2004). Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature, 428(6985), 834–837.