This article is part of the Fall 2015 issue of the Earth Science Matters Newsletter.
A 7,600-year record of climate and vegetation change from the northern Ruby Mountains, Nevada, USA
Recent drought episodes in the western United States highlight the need to understand their frequency, duration, and spatial extent. The characteristics of precipitation, including amount, type (snow vs. rain), and seasonality (winter vs. summer) have significant socioeconomic consequences. Winter precipitation across the western United States typically shows a pattern of wetter conditions in the Pacific Northwest and drier conditions in the Southwest. Historical and geological evidence indicates that this pattern can be enhanced during the cool phase of the El Niño-Southern Oscillation (ENSO), which is a major component of interannual and decadal climate variability. Warm ENSO events often result in a reversal of the precipitation pattern, with a wetter Southwest and a drier Pacific Northwest.
The northern Great Basin is located in the transition zone between these two regions. Continuous records of vegetation and climate change over the last 11,700 years (Holocene) are relatively rare in the northern Great Basin, particularly from high elevations where ecosystems are more sensitive to changes in temperature than lower elevations. As a result, past changes in precipitation patterns and vegetation in the northern Great Basin currently are not well understood.
Subalpine lakes in the Ruby Mountains, Nevada, preserve evidence of vegetation and climate change in their lake bottom sediments, and thus have the potential to produce much-needed high-elevation Holocene paleoenvironmental records for the northern Great Basin. In a recent study, U.S. Geological Survey scientists analyzed a 4.2 meter sediment core taken from Favre Lake, a small lake in the northern Ruby Mountains situated approximately 2,900 meters above sea level. The goal of this research was to assess changes in vegetation, fire history, lake level, and water conditions in order to better characterize regional climate during the Holocene. Scientists analyzed fossil pollen, charcoal, and diatoms, as well as the physical properties of the sediment. Ages in the core were determined using radiocarbon (14C) analysis. Additional age control came from the chemical analysis of a volcanic ash layer at the base of the core, which showed that it originated from Mount Mazama (Crater Lake) approximately 7,660 years ago.
Results from analysis of the Favre Lake sediment core show that for the past ~ 7,600 years climate conditions in the northern Great Basin have been quite variable with respect to temperature and precipitation. The data suggest that the primary controls on this variability are orbital forcing (relative strength of the sun in summer vs. winter) and, for the last few thousand years, changes in surface temperatures of the Pacific Ocean that are tied to ENSO activity. Additionally, the Favre Lake results indicate that, within the precipitation pattern, the Ruby Mountains have a climate affinity with the drier Southwest region as opposed to the wetter Pacific Northwest. The data from this study help to constrain where the transition zone between these two regions is located in the northern Great Basin and improve our understanding of ENSO dynamics in the area.
The timing of major changes in the Favre Lake data is similar to those recorded in other paleoclimate studies from the Great Basin. Taken together, these findings suggest that regional climatic controls in the western United States play an important role on local conditions. The conclusions drawn from these studies have the added benefit of providing long-term baseline information that can help resource managers to better plan for future change.
This paper, "Holocene environmental changes inferred from biological and sedimentological proxies in a high elevation Great Basin lake in the northern Ruby Mountains, Nevada, USA" was published in Quaternary International.