RECORDS OF LATE QUATERNARY ENVIRONMENTAL CHANGE FROM HIGH-ELEVATION LAKES IN NORTHEASTERN NEVADA

Sediment cores retrieved from three high-elevation lakes in the Ruby and East Humboldt Mountains of northeastern Nevada were analyzed for a suite of properties sensitive to environmental change. A 454-cm record that bottomed in a layer of Mazama ash was retrieved from 9.1 m of water in Angel Lake (AL) in 2007. A 470-cm record that penetrated through the Mazama was recovered from 16.5 m of water in Overland Lake (OL) in 2009. And a 415-cm record through the Mazama was recovered from 3 m of water in Soldier Lake (SL) in 2011. AMS ¹⁴C analyses reveal that these records cover very different time intervals, despite their similar physical lengths. The AL record covers ~8000 yrs, equating to a mean sedimentation rate of 0.56 mm/yr. The OL record extends back to ~14,000 yrs BP (0.33 mm/yr). Finally, the SL record reaches ~25,000 yrs BP (0.16 mm/yr). The sedimentary record in AL undoubtedly extends farther back in time, but we were not successful in retrieving sediment from below the Mazama ash layer. In contrast, both the OL and SL cores reached the point of refusal and their basal ages are consistent with their observed relationship to glacial moraines: OL is upslope from moraines corresponding to the last glacial maximum, whereas SL is downslope from LGM moraines, and upslope from moraines of the penultimate glaciation. Physical properties measured at 1-cm intervals in these cores include loss on ignition (LOI), bulk density (BD), magnetic susceptibility (MS), carbon-nitrogen ratio (CN), and grain size distribution (GS). GS in SL is dominated by very fine silt with high BD and MS values before 19 ka BP, recording rock flour deposition during the LGM. Values of LOI and CN begin to rise in SL and OL ca. 14 ka BP, and reach near-record highs between 11 and 9 ka BP, suggesting enhanced aquatic productivity associated with the summer insolation maximum. Values of LOI are also high in all three records after ~2 ka BP, but CN is low, signifying greater delivery of terrestrial organic matter to the lake basins in the late Holocene. Finally, values of LOI reach record highs, while CN reaches record lows, in the top few cm of each core, indicating that current levels of aquatic productivity are unprecedented.