Links between ocean temperature and iceberg discharge during Heinrich events

Palaeoclimate records have revealed the presence of millennial-scale climate oscillations throughout the last glacial period1. Six periods of extreme cooling in the Northern Hemisphere—known as Heinrich events—were marked by an enhanced discharge of icebergs into the North Atlantic Ocean2,3, increasing the deposition of ice-rafted debris2. Increased sliding at the base of ice sheets as a result of basal warming has been proposed to explain the iceberg pulses4–6, but recent observations7,8 suggest that iceberg discharge is related to a strong coupling between ice sheets, ice shelves and ocean conditions. Here we use a conceptual numerical model to simulate the effect of ocean temperature on ice-shelf width, as well as the impact of the resultant changes in ice-shelf geometry on ice-stream velocities. Our results demonstrate that ocean temperature oscillations affect the basal melting of the ice shelf and will generate periodic pulses of iceberg discharge in an ice sheet with a fringing shelf. We also find that the irregular occurrence of Heinrich events seen in the palaeoclimate records can be simulated by periodic ocean forcing combined with varying accumulation rates of the ice sheet. Our model simulations support a link between millennial-scale ocean temperature variability and Heinrich events during the last glacial period. Marine and continental sediments as well as ice-core records reveal the existence of millennial climatic oscillations during the last glacial period, referred to as Heinrich2 and Dansgaard– Oeschger events1. The transitions between cold (stadial) and warm (interstadial) phases of the Dansgaard–Oeschger cycle have been simulated by forcing the ocean circulation with variable freshwater fluxes in models with a bi-stable regime of the thermohaline circulation in the glacial North Atlantic Ocean9. However, whether Dansgaard–Oeschger events are related to the internal ocean variability or to an external forcing is still an open debate10. ‘Heinrich events’ are defined by pulses of ice-rafted debris (IRD) in North Atlantic sediments. Six major pulses have been identified during the period 70–14 kyr bp. The materials composing the lithic layers have largely a Canadian11, but also European and Icelandic origin12. The transport of IRD across the Atlantic to lower latitudes, even down to the Portugal coast, can be explained only by enhanced discharge of icebergs and their subsequent melting1,3. The considerable sea surface cooling during these events has been associated with a collapse of the thermohaline circulation13. All six Heinrich events occurred within a stadial state at the end of Bond cycles, which have been defined as long-term cooling cycles including several progressively colder Dansgaard–Oeschger events14. These features suggest that Heinrich