The Chronology, Climate, and Confusion of the Moorhead Phase of Glacial Lake Agassiz: New Results from the Ojata Beach, North Dakota, USA
Timothy G. Fisher, Catherine H. Yansa, Thomas V. Lowell, Kenneth Lepper,
Irka Hajdas, and Allan
Ashworth
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 (1311.6 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 lowwater 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 10.47775 ka 14C BP. Paleoenvironmental
analysis indicates that a sprucesedge 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 130km 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.