Episodic iceberg-discharge events from the Hudson Strait Ice Stream (HSIS) of the Laurentide Ice Sheet, referred to as Heinrich events, are commonly attributed to internal ice-sheet instabilities, but their systematic occurrence at the culmination of a large reduction in the Atlantic meridional overturning circulation (AMOC) indicates a climate control. We report Mg∕Ca data on benthic foraminifera from an intermediate-depth site in the northwest Atlantic and results from a climate-model simulation that reveal basin-wide subsurface warming at the same time as large reductions in the AMOC, with temperature increasing by approximately 2°C over a 1-2 kyr interval prior to a Heinrich event. In simulations with an ocean model coupled to a thermodynamically active ice shelf, the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the HSIS by a factor of approximately 6. By analogy with recent observations in Antarctica, the resulting ice-shelf loss and attendant HSIS acceleration would produce a Heinrich event. (2), their occurrence at the culmination of a large reduction in the Atlantic meridional ocean circulation (AMOC) suggests a possible trigger by climate (3, 4). Models suggest that ocean responses to an AMOC reduction might destabilize the HSIS grounding line and trigger Heinrich events either through dynamic and steric sea-level rise or warming of intermediate-depth (hereafter subsurface) waters causing destabilization of ice shelves and attendant HSIS surging (4-6). Grounding lines, however, are thought to be stable to the decimeter-scale sea-level rise associated with a reduced AMOC (7). Moreover, evidence for subsurface warming remains widely debated (8-10), and the relationship between ocean temperature and total iceshelf mass loss from basal melting is sensitive to the geometry and ocean setting of the specific ice shelf being considered (11).Our study is based on core EW9302-2JPC (1,251 m, 48°47.70′ N, 45°05.09′W) which, according to climate-model simulations, is at a depth and latitude that is ideal for monitoring subsurface warming associated with a reduction in the AMOC (Fig. 1) (4, 12). Previous work on this core identified ice-rafted detrital carbonate layers that represent Heinrich events ( Fig. 2A), with associated changes in benthic faunas and the δ 18 O of their carbonate tests that suggested intrusions of a relatively warm water mass coincident with the events (8). However, because the temperature transfer function for the benthic faunas is unknown, and ice-volume and hydrographic changes can mask the temperature signal in the δ 18 O of calcite, the inferred temperature changes remain poorly constrained.To further evaluate variability in bottom water temperature (BWT) at this site, we measured Mg∕Ca in benthic foraminiferal calcite associated with the four Heinrich events (H1, H3, H5a, and H6) for which sufficient numbers of foraminifera existed in this core. Considering analytical and calibration uncertainties, we calculate an error of 1.3°C for our Mg∕Ca-...
