Paper No. 12
Presentation Time: 11:20 AM


DAVIS, P. Thompson, Department of Natural & Applied Sciences, Bentley University, 175 Forest St, Waltham, MA 02452-4705, MACHALETT, Bjoern, Institute of Geography, Humboldt-Universität zu Berlin, Unter den Linden 6, Berlin, 10099, Germany and GOSSE, John, Earth Sciences, Dalhousie University, Halifax, NS B3J 3J5, Canada,

The type Titcomb Basin (TTB) moraines lie between Lower and Upper Titcomb Lakes (LTL and UTL), about 3 km beyond, and 200 m lower than the modern glacier margin and Gannett Peak (Little Ice Age) moraines in Titcomb Basin. The TTB moraines are in a geomorphic position similar to several other outer cirque moraines throughout the western American Cordillera, proposed to be Younger Dryas (YD) as opposed to Neoglacial in age by Davis and Osborn (1987). Based on 10Be and 26Al exposure dating, Gosse et al. (1995) interpreted the TTB moraines to be YD in age. However, recent studies suggest that 10Be production rates are about 20% lower than previously thought, which could make the TTB moraine pre-YD in age. Many other outer cirque moraines in the Rockies dated by 10Be are pre-YD in age (Marcott, 2011).

Two AMS 14C ages from the 445-455-cm depths in LTL (about 2% loss on ignition, LOI) suggest that the lake basin may have been ice-free as early as 16.1-16.8 cal 14C kyr, consistent with 10Be and 26Al exposure ages from boulders and bedrock surfaces outside the TTB moraines. The 257-cm depth in the LTL core marks an abrupt transition from inorganic, sticky gray silt below (mostly <1% LOI) to more organic, less sticky, light brown silt above (4-10% LOI). Eight AMS 14C ages on bulk sediment and microfossils date the transition to about 11.6 cal 14C kyr. Thus, sampling resolution above the transition is about 22.6 yr and below the transition is about 12.6 yr, consistent with a decreased sediment accumulation rate in LTL when YD ice pulled back from the TTB moraines opening up UTL as a sediment depositional basin. However, this scenario conflicts with basal sediments from the UTL core that date 14.7 cal 14C kyr, although the dated material measured only about 1% LOI.

High-resolution grain-size records from the 520-cm long LTL core (n = 1040) and the 180-cm long UTL core (n=360) reveal amplitudes and other structural features similar to delta 18O records from Greenland ice cores. Increases in the 2-8 µm grain size fraction indicative of increased glacier rock flour production in both lakes appear to be roughly correlative with the YD-Alleröd-Bölling-Meiendorf-Heinrich 1 climate events recognized in proxy records elsewhere, but provide much higher resolution than many of those records from a climate-sensitive alpine region in North America.