What Caused the Younger Dryas? An Assessment of Existing Hypotheses

The Younger Dryas cold event (~12.9-11.7 ka) has long been viewed as the canonical abrupt climate event. It was originally attributed to northward retreat of the southern Laurentide Ice Sheet (LIS) and eastward routing of Lake Agassiz from the Mississippi River to the St. Lawrence River, with the attendant freshening of the North Atlantic causing a reduction in Atlantic meridional overturning circulation (AMOC) strength. This original hypothesis has now been questioned based on 1) LIS model simulations that suggest an abrupt Arctic discharge of freshwater as the trigger, and 2) new dates from the outlets of Lake Agassiz, which are interpreted as indicating that the lake had no outlet during most of the Younger Dryas. With regards to Arctic freshwater forcing, one LIS model simulation produced a 0.09 Sverdrup (Sv) pulse of freshwater to the Arctic Ocean at the start of the Younger Dryas forced by a linearly interpolated Greenland ice-core climate scheme. However, this pulse lasted <300 yrs, which is too short to explain the 1200-yr long event in the most advanced atmosphere-ocean general circulation models. The negligible sea-level rise that occurred during the Younger Dryas also rules out a significantly longer forcing from LIS melting. Finally, existing paleoceanographic records show no evidence of an increase in North American freshwater discharge to the Arctic Ocean at the start of the Younger Dryas, thus strongly questioning the Arctic-forcing hypothesis. Recent dating efforts on the Lake Agassiz outlets have mainly used minimum limiting radiocarbon dates, exclusively in the case of the eastern outlet where Lake Agassiz freshwater was supposedly routed during the Younger Dryas. A strict interpretation of these dates demonstrates, however, that they are not in conflict with the original routing hypothesis. The oldest minimum limiting macrofossil date constraining the southern outlet requires abandonment prior to ~12.8 ka, with the oldest macrofossil date from the eastern outlet indicating it was open prior to ~12.6 ka (potentially prior to ~13 ka based on the oldest bulk radiocarbon date). This chronology agrees with runoff proxies from the mouths of the Mississippi and St. Lawrence Rivers, where multiple planktonic oxygen-isotope records indicate the abandonment of the southern outlet and five independent geochemical runoff proxies record the routing of Lake Agassiz freshwater to the eastern outlet at the start of the Younger Dryas. Geochemical modeling of these latter proxies suggests freshwater discharge increases to the North Atlantic of 0.06-0.12 Sv for the duration of the Younger Dryas, which is sufficient forcing to explain this cold event. AMOC-sensitive proxy records show Younger Dryas-like events during earlier deglaciations that were forced by similar magnitude changes in boreal summer insolation as during the last deglaciation, arguing against a unique bolide forcing of the Younger Dryas.