Abstract
Rapid drainage of glacial Lake Agassiz to the North Atlantic Ocean has been implicated as a triggering mechanism for the Younger Dryas (YD) cold event (13–11.6 ka cal). A key component to this hypothesis is the interpretation that the Ojata Beach of Lake Agassiz in North Dakota, USA, formed during a regression of the lake at the beginning of the YD. This paper reviews the chronological data for the low-water Moorhead Phase, presents new radiocarbon and optically stimulated luminescence (OSL) ages for the Ojata Beach, and utilizes plant and insect macrofossils to reconstruct the paleoenvironmental conditions during the Moorhead Phase in eastern North Dakota. The integrated analysis of the geochronologic data emphasizes the need to distinguish between in situ and reworked plant macrofossils. New ages obtained stratigraphically below the Ojata Beach sediments, and reinterpreted chronologic data for the Moorhead Phase, suggest that contrary to previous interpretations, the Ojata Beach is a record of transgression of the lake in the later part of the YD, and the oldest, in situ minimum age for the Moorhead Phase is 10.47±75 ka 14C BP. Paleoenvironmental analysis indicates that a spruce–sedge parkland was established at the new Ojata Beach site prior to inundation and gave way to a wetland/shoreline setting as glacial Lake Agassiz transgressed. Comparison of this data set with past paleoecological work 130 km to the south in Fargo, North Dakota, suggests that the ecotone between a cooler “spruce parkland” and a more temperate “deciduous parkland” vegetation during the Pleistocene–Holocene transition was somewhere between Grand Forks (47°58′N) and Fargo (46°51′N) at this time.
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