Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 6
Presentation Time: 8:00 AM-12:05 PM


BIGL, Matthew F., Earth Sciences Department, Dartmouth College, HB 6105 Fairchild Hall, Hanover, NH 03755 and MUNROE, Jeffrey S., Geology Department, Middlebury College, Middlebury, VT 05753,

Overland Lake is a tarn at 2880 m asl in the Ruby Mountains of northeastern Nevada. A core was retrieved from the lake in July, 2009 in order to develop a record of post-glacial environmental change. The core extended from the sediment-water interface to a depth of 4.7 m. A layer of tephra encountered at a depth of 328 cm is believed to be Mazama based on the presence of similar ash in nearby Angel Lake for which radiocarbon age control is available. Several proxies were considered including: water content, loss on ignition (LOI), magnetic susceptibility (MS), reflected light color spectrophotometry (CSP), grain size distribution (GS), dry bulk density (BD), biogenic silica content (bSi), and carbon to nitrogen ratios (C/N). LOI and water content both rise rapidly in the basal sediment, but then fall and rise again through a long oscillation that spans ~1.5 m. The ash is located at the low-point of this interval, but the falling LOI values begin ~50 cm deeper, indicating that the ash alone is not responsible for the LOI decrease. Analysis of bSi and C/N will shed light on whether this oscillation represents a change in aquatic or productivity or in the inwashing of terrestrial organic matter. The luminosity component of the CSP (L*) is strongly correlated with LOI, indicating that the darkness of the sediment is primarily controlled by organic matter content. Mean clastic grain size exhibits notable cyclic variability throughout the record, with values oscillating between 10 and 40 μm. Given the setting of the lake in a cirque surrounded by steep slopes with two major fluvial inputs, these variations likely indicate changes in the amount of precipitation resulting in fluctuating clastic input. Values of MS are greatest in the tephra, but lower-amplitude variability exhibits a pattern similar to the trends in mean grain size, which is consistent with inwashing of iron-bearing minerals. Ongoing radiocarbon dating and geochemical fingerprinting of the tephra will support construction of a depth-age model allowing comparison of these time series with other dated paleoenvironmental records from the region.