Figure 20: Possible ranges of precipitation factors and temperature depressions (with respect to modern) that modeled conditions necessary to sustain Lake Bonneville (this study), Lakes
Bonneville, Manly and Lahontan (Matusbara and Howard, 2009), Jakes Lake (Barth et al., 2016), and two Wasatch glacier staid during the LGM and Lateglacial period (Quirk et al., 2018). This figure has been adapted from Quirk et al. (2018).
Results from this study intersect with Wasatch Late glacial (15.5 ka +/- 0.8) results at -7.5°C and 0.9 times modern precipitation, and intersect with Wasatch LGM (20.2 ka +/- 1.0) results at -10°C and 0.8 times modern precipitation (Quirk et al., 2018). Inferring an intersection intermediate of these two results, a temperature depression of -8°C to -9°C and a precipitation factor of 0.85 would be required to sustain Lake Bonneville at its 18.5 ka maximum elevation.
Paleoclimate implications
Based on our analysis, the 18.5 ka highstand of Lake Bonneville was driven by reduced temperatures and subsequent reduced evaporation (~35% decrease), with little to no change in overall precipitation from modern values. The transgression of Lake Bonneville during the late LGM (~22 ka – 19 ka) is synchronous with maximum winter insolation and minimum summer insolation at 40° North (Laskar et al., 2004) along with decreased CO2 levels (Shakun et al., 2012), which supports a theory of colder temperatures and reduced summer evaporation enabling the rise of Bonneville to its 18.5 ka highstand. Ibarra et al. (2014) also report minimal
precipitation increases (10% relative to modern) and a high reduction in evaporation rate (~36% decrease) in their analysis of Lake Surprise, which experienced a late-LGM transgression coincident with the rise of Lake Bonneville. Our results support the work of Ibarra et al. (2014) in refuting the “long-standing paradigm of a ‘rainy LGM’”, and instead propose pluvial lake transgressions driven by decreased temperatures and subsequent reductions in evaporation during the late LGM.
Discussion of migrating westerlies model in relation to Lake Bonneville
Munroe and Laabs (2013) suggest a mean PJS primarily focused over southern latitudes during the late LGM, but that was able to progress northward for short intervals of time (Figure 5) (Munroe and Laabs, 2013). Under this model, Lake Bonneville would have reached its highstand during the eventual ‘permanent’ migration of the mean PJS to northern latitudes after the LGM (~ 18.5 ka), but smaller northern lakes, such as Lake Franklin or Lake Clover, reached their highstands during earlier ‘transient’ migrations of the PJS (~ 20 ka) (Munroe and Laabs, 2013). Munroe and Laabs (2013) also note that these smaller lakes transgressed to a second relative highstand around (or slightly after) 17 ka, synchronous with Heinrich Event 1 (H1). Coincident relative highstands of Jakes Lake (16.9 ka) and Lake Warring (16.8 ka, 16.9 ka) (Garcia and Stokes, 2006), Newark Lake (16.9 ka) (Kurth et al., 2011), Railroad Lake (16.3 ka) (Lillquist, 1994), and Lakes Franklin (16.1 ka, 16.8 ka) and Clover (17.3 ka) (Munroe and Laabs, 2013) all provide support for a theory of significant transgressions in small lakes during H1.
H1 may have driven lake transgressions through atmospheric teleconnections between the North Atlantic and Great Basin region. Melting of H1 ice suppressed Atlantic Meridional
Overturning Circulation (AMOC) (McManus et al., 2004), leading to cooling in the North Atlantic (Bard et al., 2000). Okumura et al. (2009) employ Global Circulation models and paleoclimate data to demonstrate that AMOC suppression corresponds to decreased North Pacific sea surface temperatures and winter enhancement of the Aleutian Low. Aleutian Low enhancement would steer the westerly storm track over the Great Basin, increasing effective moisture to levels capable of driving pluvial lake highstands throughout the region during H1.
Results from this study contradict the idea of a wetter late-LGM driven by a migrating PJS. Firstly, our model suggests that decreases in temperature and evaporation were responsible
for the transgression of Lake Bonneville from 22-19 ka, instead of increased moisture due to the presence of the PJS at Bonneville’s latitude. Secondly, we observed little to no spatial pattern in pluvial lake highstand ages during the suggested timeframe of “PJS retreat”. This northward migration of the PJS should lead to an observed pattern of sequential highstands at higher latitudes as the LIS retreated further north. However, no such pattern is observed.
Alternative hypothesis
An alternative hypothesis for the observed timing of pluvial lake highstands from 22-16 ka is proposed by this study. During Bonneville’s late-LGM transgression from 22-19 ka, lake level rise was driven by decreased evaporation due to decreased summer insolation and lower CO2 concentrations (Laskar et al., 2004; Shakun et al., 2012). Late-LGM highstands in surrounding lakes Franklin and Clover (Munroe and Laabs, 2013), along with observed transgressions due to reduced evaporation at Lake Surprise (Ibarra et al., 2014) downplay the importance of PJS-driven precipitation in raising lake levels during this time period. Instead, increased rainfall becomes a major factor during H1, causing re-transgressions in a number of Great Basin lakes (Lillquist, 1994; Garcia and Stokes, 2006; Kurth et al., 2011; Munroe and Laabs, 2013) after 17 ka, including the continued overflow of Lake Bonneville from the Provo shoreline during H1.
Chapter 5.1: Conclusions
The Condom et al. (2004) water-balance model was employed in this study to constrain paleoclimate conditions during the timing of the ~18.5 ka Lake Bonneville highstand. The Condom et al. (2004) evaporation equation was deemed the most accurate for use in the Bonneville Basin, following statistical analysis of three models with respect to observed
evaporation data at five sites throughout the basin. For the late LGM (~21-18 ka), the highstand of Lake Bonneville is well predicted by linear combinations of 6°C to 10°C decreases in
temperature and corresponding changes in precipitation from 1.1 to 0.9 times modern values. Exact paleo-precipitation is uniquely constrained by intersecting our results with the family of P- T solutions derived from Wasatch glacial reconstruction data (Quirk et al., 2018). This approach suggests a temperature depression of -8°C to -9°C and a precipitation factor of 0.85 are sufficient to grow paleo-lake Bonneville to its 18.5 ka highstand.
The results described above indicate that the rise of Lake Bonneville was driven by a 35% decrease in evaporation due to decreased summer insolation and lower CO2 concentrations, and did not require significantly wetter conditions, such as those associated with northward shifts in the westerlies. Bonneville’s transgression from 22-19 ka is synchronous with relative
highstands of surrounding pluvial lakes, and precedes increased relative moisture due to with Heinrich Event 1, which generated pluvial highstands after 17 ka.