Ages for the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz: Implications for deglaciation chronology

Abstract

Glacial Lake Agassiz has been implicated as the trigger for numerous episodes of abrupt climate change at the close of the last ice age, yet the beginning age of the lake has never been determined. Here we report the fi rst numerical age data on the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz. Organic remains from lakes, bogs, and channels distal to, and inset to, the Big Stone Moraine require that glacial activity at this moraine ceased prior to 12,000 14 C yr B.P. (13,950 cal [calendar] yr). A site near New Effi ng ton, South Dakota (United States), implies full glacial recession north of the topographic divide prior to 11,810 14 C yr B.P. (13,670 cal yr), synchronous with the beginning of glacial Lake Agassiz. Lake Agassiz shorelines inset to the moraine yield optically stimulated luminescence (OSL) ages from 14,200-12,600 yr cal. Lower strandlines are younger, but the similarity of ages suggests that initial lake lowering was faster than OSL ages can currently resolve. Nevertheless, the OSL ages represent the fi rst numerical age assignments for the Herman, Norcross, and Upham beach ridges, setting the stage for future numerical age assignments within the Lake Agassiz basin. These two dating methods yield strongly consistent results within stated uncertainties. The age of the Big Stone Moraine implies an interval of rapid retreat for the Des Moines lobe of the Laurentide Ice Sheet during the Bölling-Alleröd warm interval. The overlapping ages for the uppermost beach levels and abandonment of the highest Lake Agassiz spillway indicate a rapidly evolving lake until at least 13,500 yr cal.

Origin of the Herman–Norcross–Tintah sequence of Lake Agassiz beaches in Manitoba, Canada

“…The Cass phase is not relevant here because Lake Agassiz existed only south of the Canada-United States border. Most researchers have concluded that the H-N-T beaches were formed during the Lockhart Phase (e.g., Fenton et al, 1983;Fisher, 2005;Lepper et al, 2007).…”

“…The Lockhart Phase began when ice retreated from the Red River Valley, which occurred earlier in the southern region, 12.2-11.0 14 C kyr BP (~14.2-12.9 cal kyr BP) (see Clayton and Moran, 1982;Fenton et al 1983;Dyke, 2004;Lepper et al, 2007), and ended with the beginning of the Moorhead phase, when the level of Lake Agassiz dropped by tens of metres, due to the opening of a lower outlet before 10.7 14 C kyr BP (12.7 cal kyr BP) (Leverington et al, 2000;Fisher et al, 2008). There are no radiocarbon dates from sediments of the H-N-T series of beaches, so until recently assignment of ages mainly has been based on relationships to dated events in Lake Agassiz overflow channels, to dated ice-margin positions, and to sediments of the Moorhead low-water phase of the lake.…”

Long-term Land Use and Land Cover Changes Affected by the Conservation Reserve Program in the Minnesota River Valley

“…Possessing a total length of 534 km (355 mi) and a gradient of 24 cm/1.6 km (9.5 in/mi), the Minnesota River comprises a classic underfit stream. Formed by glacial River Warren, which breached a recessional moraine of the Des Moines Lobe bordering the southern end of glacial Lake Agassiz (a massive early Holocene freshwater lake encompassing over 100,000 mi 2 ) the Minnesota River Valley at Mankato measures about 2,470 m (9,000 ft) across and over 70 m (200 ft) deep (Ojakangas & Matsch 1982;Lepper et al 2007). Downstream, its annual discharge into the Mississippi averages 141 m 3 /sec (5,000 cfs), although it can range as high as 2,820 m 3 /sec or 100,000 cfs (Waters 1987).…”

Optical ages indicate the southwestern margin of the Green Bay Lobe in Wisconsin, USA, was at its maximum extent until about 18,500years ago

“…In recent years, optical dating has been applied in a number of settings in the upper Midwest in an attempt to better constrain the chronology of late glacial events by directly dating proglacial sediment (Hooyer, 2007;Lepper et al, 2007;Fisher et al, 2008;Rawling et al, 2008;Schaetzl and Forman, 2008). As important as these contributions have been to our understanding of the late Pleistocene in these areas, constraining the timing of ice-marginal positions has remained an elusive goal in the upper Midwest.…”