Holocene lake-level trends in the Rocky Mountains, U.S.A.
Introduction
Studies of drought in the western U.S. during the past few centuries and millennia (e.g. Stahle et al., 2000, Woodhouse, 2003, Cook et al., 2004, MacDonald, 2007, Meko et al., 2007) provide insights on patterns of drought generated by “unforced” climate phenomena such as El Nino and decadal variability in Pacific climates (e.g., Swetnam and Betancourt, 1998, McCabe et al., 2007, McCabe et al., 2004, Stone and Fritz, 2006; Stevens and Dean, 2008). Future regional changes will, instead, be forced by rising greenhouse-gas levels (e.g., Stewart et al., 2004, Diffenbaugh et al., 2005) and may be analogous to large externally-forced changes in the past, even if such changes developed over long-time scales (Overpeck et al., 1991, Webb et al., 1993). Both past and future boundary-condition changes have the potential to push climatic conditions beyond the envelope of natural variability experienced during the past millennium (Diffenbaugh et al., 2005, Williams et al., 2007). Here, we examine such potential by reconstructing Rocky Mountain lake-level trends during the Holocene, including intervals when insolation and atmospheric composition differed significantly from today (Berger and Loutre, 1991, Monnin et al., 2001).
Climate model experiments have simulated large changes in precipitation and annual moisture balance across the western U.S. during the Holocene in response to orbital change (Bartlein et al., 1998, Harrison et al., 2003, Diffenbaugh et al., 2006, Shin et al., 2006). Likewise, pollen and macrofossil data from Colorado, Wyoming, and Montana reveal long-term trends in Rocky Mountain climates and vegetation (Maher, 1961, 1972; Baker, 1983; Thompson et al., 1993, Whitlock, 1993, Fall, 1997, Lynch, 1998, Mock and Brunelle-Daines, 1999); variation in fire activity is also tied to climatic changes (Millspaugh et al., 2000; Brunelle et al., 2005; Whitlock and Bartlein, 2004, Toney and Anderson, 2006, Power et al., 2008, Power et al., 2006, Marlon et al., 2006). Additionally, dunes in intermountain basins (Stokes and Gaylord, 1993, Langford, 2003; Forman et al., 2006) and immediately east of the Rocky Mountains (Forman et al., 2001, Forman et al., 2006; Miao et al., 2007) were active during the Holocene. However, zonal atmospheric flow has been widely cited to explain dry mid-Holocene conditions in the mid-continent (e.g., Bartlein et al., 1984, Bradbury and Dean, 1993, Yu et al., 1997), and might be expected to produce wet conditions in the Rocky Mountains because of enhanced orographic precipitation (Fig. 1).
Evidence of past lake levels can reveal the range of moisture variation over multiple time scales (Benson et al., 1990, Harrison and Digerfeldt, 1993, Stine, 1994), but few studies have documented Holocene lake-level changes in the Rocky Mountains (Fritz et al., 2000). A growing number of diatom, geochemical, and geomorphic records from the Rocky Mountains indicate both long- and short-term variability in lake levels (Cumming et al., 2002, Langford, 2003, Stone and Fritz, 2006, Stevens et al., 2006; Bracht et al., 2008; Stevens and Dean, 2008). Here we synthesize new and existing sedimentological evidence for long-term water-level trends, and compare the evidence for these trends with the limited sedimentary evidence of shorter-term variations. Three key questions motivate this study:
- (1)
What long-term hydrologic trends are evident in the Rocky Mountains during the Holocene?
- (2)
What spatial patterns characterize the long-term trends in moisture variation?
- (3)
How similar were long-term moisture variations in the Rocky Mountains and mid-continent?
To address these three questions, we studied the lake-level history of a lake in western Montana and two lakes in Colorado, and compiled existing sedimentary evidence for shoreline shifts or desiccation events in an additional 14 lakes or wetlands (Figs. 1a and 2). The small size of our new study lakes (5 ha to 1.1 km2) ensures that past lake-level fluctuations capture local hydrologic conditions (Harrison and Digerfeldt, 1993). Each new study lake was chosen to represent a different region defined by Cayan (1996) for having broadly-coherent snowpack variation at interannual scales (Fig. 1a), because lake levels depend heavily on surplus moisture derived primarily from winter precipitation (Fig. 2), and long-term forcing may have generated broad-scale moisture anomalies similar to those at interannual time scales (Mock and Brunelle-Daines, 1999, Harrison et al., 2003, Whitlock and Bartlein, 2004, Shin et al., 2006). Indeed, some of Cayan's (1996) snowpack patterns (i.e., the “Idaho” pattern; Fig. 1a) are associated with atmospheric and oceanic conditions like those that have been simulated and inferred for the mid-Holocene, including a deep Aleutian low (e.g., Bartlein et al., 1998, Anderson et al., 2005) and a cool north Pacific (Kim et al., 2004). Likewise, Licciardi (2001) compared the histories of four lakes (Chewaucan, Lahontan, Bonneville, and Owens) in the Great Basin and showed important synoptic differences like those that exist at short time scales today.
We present detailed data for our three study sites, and then review all of the other direct evidence of lake desiccation or shoreline changes in the Rocky Mountains (Fig. 1a). We discuss the long-term patterns in these data, some evidence for millennial and shorter changes in climate, and comparisons among the lake-level data, other paleoenvironmental records, and climate model simulations.
Section snippets
Study sites
We targeted small lakes with small watersheds and minimal stream inputs to obtain records that (a) were minimally affected by local stream processes (e.g., down cutting of lake outlets; variable contributions of stream-derived sediments to sediment sequences), and (b) were controlled by groundwater levels, which integrate multiple years of precipitation. Such lakes have been shown to be climatically sensitive and to have regionally-coherent trends (Harrison and Digerfeldt, 1993, Shuman and
Methods
Our approach follows the methods of Digerfeldt (1986) and Shuman et al., 2001, Shuman et al., 2005, and uses transects of sediment cores within each lake, in combination with sub-surface profiles collected by seismic and ground-penetrating radar (GPR) geophysical techniques, to track shifts in the position of near-shore sediments. The method depends on the assertion that wave energy prohibits the accumulation of sediment in the shallowest areas of the lake. Littoral sediment types (e.g., sands)
CHIRP data
Seismic profiles from Foy Lake show widespread stratified sediments across deep areas of the lake (Fig. 3). The Mazama Ash (MZ) layer, dating to ca 7600 cal yr BP (Zdanowicz et al., 1999), appears evident as a dark band in the profiles. Biogenic gas obscures the lowermost sediments in many parts of the lake, but appears constrained below specific layers. Less gas is evident in areas where homogenized cones of sediment extend upward from the MZ as though these features formed during the release of
Orbital-scale trends
We find evidence that mid-Holocene lake levels were lower than today at a wide range of settings (Fig. 13). Differences among our records indicate that moisture trends were spatially patterned and modulated by other local factors (i.e., hydrology) and short-term variability, but important long-term trends exist throughout the region. The trends are consistent with the effects of orbital forcing, especially given that mid-latitude lake-level responses to the seasonal progression of orbital
Conclusions
Well-documented mid-Holocene aridity in the central U.S. extended into the headwater regions of the Rocky Mountains, and demonstrates a high sensitivity of key water supplies to global changes. The stratigraphies of Foy, Hidden, and Little Molas Lakes show consistent evidence of high lake levels during the past two millennia, and low-lake levels throughout the majority of the Holocene. Aside from these long-term trends, however, the records differ significantly with unique millennial-scale
Acknowledgements
Funding to B. Shuman was provided from the NSF Earth System History Program (ATM-0402308), a University of Minnesota Graduate School McKnight Land-Grant Professorship, and an NOAA Climate and Global Change Postdoctoral Fellowship. Additional funding was provided by NSF to S. Fritz, L. Stevens, and C. Whitlock (EAR-9905262; EAR-9906100), and by the USGS to S. Colman. We thank K. Westover and D. Guliver for assistance in coring, D. Weiss and D. Nichols for assistance in sub-surface data
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- 1
Present address: Department of Geological Sciences, California State University – Long Beach, Long Beach, CA 90840-3902, USA.
- 2
Present address: Utah Museum of Natural History, University of Utah, Salt Lake City, UT 84102, USA.
- 3
Present address: Department of Geography, University of Utah, Salt Lake City, UT 84102, USA.
- 4
Present address: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA.