Assessing the ages of the Moorhead and Emerson phases of glacial Lake Agassiz and their temporal connection to the Younger Dryas cold reversal

Abstract

The low-water Moorhead Phase of glacial Lake Agassiz has been attributed to rapid drawdown in lake level contemporaneous with the onset of the Younger Dryas cold reversal. We examine the radiocarbon and optically stimulated luminescence dates associated with the Moorhead and subsequent high-water Emerson Phases in the Lake Agassiz basin. We apply manual and statistical filters to vet the chronological dataset (n = 116), resulting in the exclusion of about one fourth of existing dates. A Bayesian model applied to the filtered dataset (n = 87) yields a modelled 2σ age range for the onset of the Moorhead Phase of 12,410–12,060 cal yr BP, post-dating the onset of the Younger Dryas. The modelled 2σ ages for onset and termination of the Emerson Phase are 11,570–11,290 and 10,690–10,340 cal yr BP, respectively. The dataset and model do not necessarily preclude freshwater flux from Lake Agassiz drawdown during the Moorhead Phase as the trigger for the Younger Dryas cold reversal, but they do indicate that this freshwater drainage interpretation relies on only a few low-precision radiocarbon dates that are identified statistically as outliers. The lingering uncertainty in the radiocarbon chronology for Lake Agassiz, despite rigorous filtering and application of a Bayesian calibration model, strongly suggests that additional high-quality radiocarbon dates from clear depositional settings in the Lake Agassiz basin are required to further constrain lake level histories and linkages to abrupt deglacial climate change.