Holocene water levels of Silver Lake, Montana, and the hydroclimate history of the Inland Northwest

Abstract

The wettest portion of the interior of western North America centers on the mountainous region spanning western Montana, Idaho, British Columbia, and Alberta. Inland ranges there capture the remnants of Pacific storms. Steep east–west hydroclimate gradients make the region sensitive to changes in inland-penetrating moisture that may have varied greatly during the Holocene. To investigate potential hydroclimate change, we produced a 7600-yr lake-level reconstruction from Silver Lake, located on the Montana–Idaho border. Ground-penetrating radar profiles and a transect of four shallow-water sediment cores that were dated using radiocarbon dating and tephrachronology revealed substantial changes in moisture through time. An organic-rich mud unit indicating wet and similar to modern conditions prior to 7000 cal yr BP is overlain by an erosional surface signifying drier than modern conditions from 7000–2800 cal yr BP. A subsequent time-transgressive increase in water levels from 2800–2300 cal yr BP is indicated by a layer of late Holocene muds, and is consistent with glacier expansion and increases in the abundance of mesic tree taxa in the region. Millennial-scale trends were likely driven by variations in orbital-scale forcing during the Holocene, but the regional outcomes probably depended upon factors such as the strength of the Aleutian Low, Pacific sea-surface temperature variability, and the frequency of atmospheric rivers over western North America.

Wildfire activity in northern Rocky Mountain subalpine forests still within millennial-scale range of variability

“…The highest rate of burning in the paleorecords occurred over c. 2300-2500 yr BP, with a maximum of 99% of sites burning during the century centered on 2410 yr BP, yielding an estimated FRP of 101 yr. The timing of maximum burning corresponds with a drop in lake level at Silver Lake, MT, one of our northernmost study sites (figure 4); although consistent with increased aridity, the timing of the drop in lake level is not precisely constrained [54]. The period with minimum burning spanned c. 100-250 yr BP, with an average of 25% of sites burning per century (FRP = 400 yr).…”

“…Summer drought information in both study regions was obtained from tree-ring-based PDSI reconstructions [53]. Additional paleoclimate information specific to the Northern Rockies was derived from a previously published lake-level reconstruction from one of our study sites [54], an δ 18 O-inferred snow water equivalent (SWE) reconstruction in northwest Montana [55], and a spatially-explicit summer temperature reconstruction [56]. To assess the strength of centennial-scale correlations between paleoclimate and fire activity, paleoclimate data were averaged over 100-yr moving windows, and time series were subset to one value every century and compared using Spearman rank correlations.…”

Fire‐regime variability and ecosystem resilience over four millennia in a Rocky Mountain subalpine watershed

“…Positive values of PC1 are associated with higher concentrations of minerogenic elements (see Figure 5). Palaeoclimate was inferred from a previously published lake‐level reconstruction from Silver Lake (Parish et al, 2022). The vertical dotted line delineates the pollen zone break, which also corresponds to a significant change in fire frequency.…”

“…Ages were estimated using the constant rate of supply model (Binford, 1990). Tephra layers were identified visually, and ages were inferred based on known tephra deposits at Silver Lake (Parish et al, 2022). Radiocarbon dating was conducted at the Lawrence Livermore National Laboratory's Center for Accelerator Mass Spectrometry, and 14 C ages were calibrated using IntCal20 (Reimer et al, 2020).…”

“…Palaeohydrology in the study region is well‐resolved by a lake‐level reconstruction from Silver Lake itself (Parish et al, 2022). The record provides evidence of increasing effective moisture c. 2800 calendar years before present (hereafter yr BP, with present defined as 1950 CE), with modern lake levels reached c. 1800 yr BP.…”