A 1000-YEAR VARVED SEDIMENT RECORD FROM THE BROOKS RANGE, ALASKA

Geomorphologic evidence of late Holocene glacial advances shows the central Brooks Range is sensitive to changes in temperature and moisture balance. Proxy records from the region detailing the mid to late Holocene are sparse. Blue Lake is a small (<0.5 km²), shallow (4.7 m) glacial-fed lake set on the crest of the Brooks Range (68º05.3 N, 150º27.8 W) in north-central Alaska at an altitude of 1265 m. The 4-km² watershed contains a small cirque glacier set on the north face of the 1890 m high headwall on the north side of the continental divide. Field observations and air photos indicate that melt-waters from the glacier contribute substantial quantities of fine-grained sediment to the lake. Sediment cores recovered in August 1999 contain mm-scale laminations comprised of thick, light colored silt and fine sand-sized laminae overlain by a thin, darker clay cap. Magnetic susceptibility, bulk density, grain-size, and organic matter content were measured and thin-sections prepared using a shock-freeze (sublimation) technique and embedded with low viscosity epoxy resin under vacuum. Laminae counts and thickness measurements were made using image analysis techniques. Scanning electron microscopy coupled with energy dispersive spectroscopy was used to characterize mineralogy and grain size. In thin-section, the laminae couplets appear as alternating light and dark bands. Light, coarser-grained laminae result from sedimentation during periods of high melt-water discharge during spring months. Dark, fine-grained laminae form when fine particles settle due to stagnant conditions during winter months. Laminae are interspersed with thick units of non-climatic events exhibiting a fining upward sequence suggesting deposition by turbidity current. These units are easily distinguished from the annual laminations. In addition to annual variability in varve thickness, long-term trends in thickness were observed and the trends are compared to the historical climate record. This study observes the climate signal as it is recorded in the sediments and presents a high-resolution record from a climate sensitive region.