Evidence for an Early Neoglacial Glacier Advance from Rock Glaciers and Lake Sediments in the Sierra Nevada, California, U.S.A.

Abstract

Lichenometric ages of active glacigenic rock glaciers and Matthes (Little Ice Age) moraines, together with sediment cores from basins below these landforms, provide evidence for a glacier advance ~3000 yr ago in the Sierra Nevada, California. Diameters of the largest lichens (70–108 mm) on the Mendel rock glacier indicate that it began forming well before the onset of the Little Ice Age (LIA) (~700 yr BP). Smaller lichens (30–45 mm) on the Matthes moraines of Darwin and Conness glaciers suggest that they formed during the LIA. The apparent lack of pre-LIA Holocene moraines below “clean” glaciers in the Sierra Nevada demonstrates that the pre-LIA Neoglacial advance must have been less extensive than the Matthes advance. Sediment cores recovered below Matthes glacial deposits provide additional support for a pre-LIA Neoglacial advance. Three cores from a meadow directly below the Powell rock glacier record grus deposition (suggesting nonglacial, summer-dominant mass wasting) between about 5300 and 3600 14C yr BP (~6000–3900 cal yr BP), and alternating gyttja and silt (suggesting cooler, winter-dominated precipitation) after about 3600 14C yr BP (3900 cal yr BP). A lake core from below Conness Glacier shows a transition from higher-organic sediments (10–16% total organic carbon) to lower-organic sediments (2–4% total organic carbon) at approximately 3200 14C yr BP (3400 cal yr BP), suggesting formation and growth of the glacier in the cirque at that time.