Figure 6: Evaluation of clumped isotope derived temperature and
δ18Owater for locations with dual
materials. A. Clumped-isotope derived temperature reconstructions using
material specific calibrations (top row; black frame) and composite
freshwater calibration (top row; gray frame). Bottom panel shows
residuals for both the material specific calibrations and composite
calibrations (semi-transparent symbols), along with the average
difference in temperature between the two archives (black and gray
numbers and lines for material specific and composite calibration,
respectively). Clumped isotope derived temperatures are denoted as
TΔ47, while independently observed water temperatures
are denoted as Tobservations. Sites are labeled on plot:
D: Daija Co, C: Cuona Lake, W: Wulungu Lake, GSL NA: North Arm, Great
Salt Lake, GSL SA: South Arm, Great Salt Lake. We find a reduction of
temperature residuals in most cases using a material specific
calibration. We also find more realistic temperature estimates when
applying material specific calibrations, given each lake’s individual
setting. B. Reconstructed water δ18O values using
material specific (top row, black frame) and composite (top row, gray
frame) calibrations. Clumped isotope based reconstructions of water
δ18O and independently measured δ18O
values are denoted as water
δ18Oreconstructed and water
δ18Oobserved, respectively. The
residuals from measured values are displayed on the bottom plot, and
residuals between different archives are denoted by black and gray bars
and values for the material specific and composite calibrations,
respectively.
3.5 Applications to Paleoclimate Reconstructions
3.5.1 Paleoclimatology of Lake Surprise, CA
We applied the calibration relationships derived within this study to
reconstruct terrestrial paleo-hydroclimate variables, using samples
of tufa that are Last Glacial Maximum (LGM; 23,000-19,000 years ago) and
deglacial (19,000-11,000 years ago) in age from Lake Surprise,
California (Egger et al., 2018; Ibarra et al., 2014; Santi et al., 2019;
Santi et al., 2020). In the original study from Ibarra et al. (2014),
the authors used pollen data to estimate temperature changes during the
LGM and deglacial. These estimates were used within an isotope mass
balance model to derive evaporation and precipitation rates at Lake
Surprise. Santi et al. (2020) expanded on this work by providing further
constraints on water temperatures (and air temperatures, by use of a
transfer function) and δ18Owater for
the same sample set and used these updated values within an isotope mass
balance modeling framework to derive new estimates of evaporation and
precipitation rates. Here, we applied our biologically-mediated and
composite calibrations and the recently published calibration by
Anderson et al. (2021) to data from tufa samples for Lake Surprise from
Santi et al. (2020). Data used from Lake Surprise is identical to the
original publication and was reprojected into the I-CDES reference frame
following current best practices and standardization procedures
(Bernasconi et al., 2021; Upadhyay et al., 2021). Detailed methodology
can be found in Santi et al. (2020) and equations used for this analysis
can be found in the Supplementary Information.
New estimates of Δ47-temperatures, water
δ18O, evaporation, and precipitation rates are shown
in Supplementary Figure 1, and reported in Supplementary Table 5, 6, and
7 using the material-specific and composite calibrations derived in this
study, and the Anderson et al. (2021) calibration, respectively. Updated
temperature predictions using our biologically-mediated calibrations
generally result in cooler water temperatures throughout the LGM and
deglacial at Lake Surprise than the original publication (Table 4).
Water temperatures derived using the Anderson et al. (2021) calibration
are 1.2 and 1.7°C cooler than the material-specific calibration, during
the deglacial and LGM, respectively. Estimates derived using the
general calibration are warmer than the material specific calibration,
but slightly cooler than the original publication, with water
temperatures estimated to be 12.5 ± 1.1°C during the LGM and 11.4 ±
0.7°C during the deglacial. Average LGM water temperatures of 11.5 ±
1.4°C are estimated using the biologically-mediated calibration, which
results in temperatures that are 1.2°C cooler than the originally
reported value (Santi et al., 2020) of 12.8 ± 1.4°C. Using the same
water-to-air temperature transfer function (Hren & Sheldon, 2012) as
used in Santi et al. (2020) to translate water temperatures to LGM MAAT,
we find a 3.0°C difference between the material-specific values and
published values (1.8 ± 1.9°C and 4.8 ± 1.3°C, respectively). The
composite calibration estimates similar temperatures to the original
publication (3.2 ± 1.5°C), while the Anderson et al. (2021) calibration
estimates much cooler temperatures during the LGM (-0.5 ± 1.7°C). Our
deglacial MAAT estimates follow a similar trend, with estimates from
Santi et al. (2020) resulting in the highest MAAT estimates, followed by
our composite calibration, the biologically-mediated calibration, and
the Anderson et al. (2021) calibration (Table 4).
MAAT estimates were used to calculate annual evaporation rate, using a
modified Penman equation (Linacre, 1993; Mering, 2015; Santi et al.,
2020). Therefore, shifts in temperature will dictate the direction and
magnitude of changes in estimated lake evaporation rate. When using
temperatures derived from our material-specific calibrations, we observe
a 91 mm difference in evaporation in our reanalyzed data (981 ± 60
mm/yr) and the original publication (1072 ± 56 mm/yr) during the
deglacial. Our estimates during the LGM are 50 mm/yr lower than
estimates from Santi et al. (2020), with our results suggesting 1103 ±
115 mm/yr and 1153 ± 100 mm/yr, respectively. Our derived weighted
evaporation rate, which takes into account free evaporation over the
lake surface and evapotranspiration over land, differs significantly
from Santi et al. (2020), with a 46 mm/yr decrease and 41 mm/yr increase
difference between the new and old estimates for the LGM and deglacial,
respectively (Table 4). Since we observe general decreases in
temperature using the Anderson calibration, overall, weighted and
unweighted evaporation values are lower than those derived in the
original publication, while the agreement in temperature from the
composite calibration derived evaporation estimates are similar to the
original publication (Supplementary Tables 6 and 7).
Evaporation estimates are used within the hydrological modeling
framework of Santi et al. (2020) to reconstruct precipitation that
incorporates clumped isotope derived MAAT and
δ18Owater and includes basin
hypsometry derived from modern topography and watershed delineations
(Broecker, 2010; Ibarra et al., 2014, 2018; Jones et al., 2007; McGee et
al., 2018). We also incorporate an ice-volume correction for
δ18Owater during the LGM and deglacial
based on sample age, following the approach used by Tripati et al.
(2014) and estimate anomalies with a revised value for modern annual
precipitation rates to account for snowfall. In Santi et al. (2020),
modern annual precipitation values used to calculate anomalies used a
snow water equivalent of 1:10 mm. However, snow densities vary depending
on temperature and location. To account for these changes in snow
density, we utilized 10 years of SNOTEL snow density data (USDA Natural
Resources Conservation Service, 2022) at two stations within the
Surprise basin (Cedar’s Pass and Dismal Creek) to assess typical modern
snow water equivalent in Surprise Valley. The average snow density for
the area was 33%, and we combined this value with average snowfall
amounts to integrate the contribution derived from snow into our modern
precipitation measurements, for an updated modern precipitation value of
630 mm/year (Desert Research Institute;
https://www.dri.edu/western-regional-climate-center/), a 102 mm/year
increase.
These updated calculations using the material-specific calibration
indicate that during the LGM, precipitation rates were 10% lower than
modern, and precipitation increased during the deglacial to values that
were 8% lower than today (Table 4). Estimates of precipitation rates
using all calibrations are similar, with estimates for the LGM ranging
from 551 to 565 mm/yr (Table 4). We observe a slight increase in
precipitation rate estimates when applying all calibrations during the
deglacial, when Lake Surprise reached its maximum extent, with values
using the original published calibration resulting in estimates of 567
mm/yr, which is 16, 1, and 8 mm/year lower than estimates derived from
our material-specific, composite, and the Anderson-derived values,
respectively. Overall, this supports the original findings of Ibarra et
al. (2014) and those of Santi et al. (2020) based on clumped isotopes
that temperature and evaporation were likely dominant controls on Lake
Surprise’s transgression and regression, and that increased evaporation
relative to precipitation contributed to the eventual disappearance of
Lake Surprise.