The 8200 yr BP cold event in stable isotope records from the North Atlantic region

An abrupt cold event ca. 8200 cal. yr BP, is believed to have been caused by the catastrophic release of ice-dammed meltwater from Lake Agassiz and associated disruption of the Atlantic Meridional Overturning Circulation (AMOC). Previous reviews have highlighted both the “ideal” nature of the 8200 yr event as a target for numerical model validation and the likely geographical restriction of the ensuing cold event to the circum-North Atlantic region but have cited a lack of sufficiently resolved palaeoclimatic records to test this hypothesis. We review the current set of high-resolution stable isotope records from multiple archives (lake, bog, marine and ice cores) in the North Atlantic region for the period 9200–7400 yr BP (present = AD 1950). The isotopic values of terrestrial records are closely linked to isotopic values of palaeoprecipitation. All sites provided evidence for at least one centennial-scale anomaly (beginning ~ 8500–8250 yr BP) that exceeded background variability. No evidence for spatial or temporal transgression of the isotope anomalies was identified, implying that a simultaneous climate signal was observed in the circum-North Atlantic region. Comparison with new simulations using the UK Hadley Centre model HadCM3, which was isotope-enabled to simulate changes in the stable isotope composition of precipitation and forced by freshwater input (“hosing”) of 5 Sverdrups (Sv) (0.005 km3/s), for 1 yr, indicated agreement with the observed decrease in the amplitude of the isotope anomaly with distance from the NW North Atlantic. The model-simulated duration of the event, however, was consistently shorter than that observed in palaeoclimatic records. A review of evidence for forcing additional to the catastrophic release of meltwater from Lake Agassiz (solar variability, sea-ice feedback and longer-term meltwater history) suggested that reduced solar output did not directly coincide with the 8200 yr event, but that a more complex history of meltwater discharges and sea-ice feedback may have conditioned the AMOC for sustained climatic impact.

Research highlights
► Stable isotope records from multiple archives (lake, bog, marine and ice cores) in the North Atlantic region each exhibit at least one anomaly which exceeds background variability for the period 9200–7400 yr BP (present = AD 1950).
► Alignment of those “primary” anomalies yields a regionally synchronous “event” that lasted 150 ± 30(1σ) years.
► The event probably occurred in response to a complex pattern of meltwater discharge associated with the final drainage of Lake Agassiz and subsequent prolonged re-routing of meltwater that created a salinity anomaly that was sufficient for weakening AMOC.
► New isotope-enabled GCM simulations reproduce spatial distribution but the duration of the event is too short.
► Proxy-model mismatch is a key challenge for future modelling experiments.