Table 4: Comparison of recalculated Δ47-based
proxy reconstructions from this study to published values. Top:
Comparison of water temperature, water δ18O,
precipitation, and weighted evaporation anomalies at Lake Surprise, CA
to published values from Santi et al. (2020). Modern MAAT is 9.2 ± 1.0°C
from Cedarville, CA and modern δ18O used for anomaly
calculations is -14.6 ± 0.6‰. Modern precipitation used to calculate the
anomaly is 630 mm/year, including contributions from rainfall and
snowmelt, and modern evapotranspiration rates are 528 mm/year. Bottom:
Comparison of clumped isotope derived water temperature, water
δ18O, and elevation estimates for samples run for
clumped isotope analysis from the Nangquian Basin to published values
from L. Li et al. (2019).
3.6.2. Origin of travertine and tufa deposits in Ainet, Austria
We apply our calibrations to study the formation of travertines and
tufas in a mountainous region of the Alps. To demonstrate this, we
examined a lateglacial sequence of travertine and calcareous tufa from
Ainet, Austria that was first described in Boch et al. (2005). It
represents the only aragonitic travertine sequence known in the Eastern
Alps. This sequence of ~2.7 m thickness formed over the
course of ~1,000 years, following the rapid initiation
of warming during the Bølling-Allerød (Figure 7). Carbonate deposition
within the travertine sequence alternated between aragonite and calcite
layers (on mm-scale), which was hypothesized to represent differences in
seasonality, with aragonite precipitation occurring during the warm
season and calcite precipitation occurring during cooler intervals.
Following the deposition of the compact and aragonite dominated
travertine (~2.5 m), this sequence was then capped by a
highly porous and calcareous tufa layer (~0.2 m)
consisting of calcite solely.
Here, we constrain formation temperature and
δ18Owater for these differentiated
calcium carbonates using our biologically-mediated and travertine
calibrations from this study. Temperature estimates for the tufa and
travertine sequence range from 9.0 to 15.1°C, with an average value of
12.7°C for the travertine terrace (Supplementary Table 8). Modern water
temperatures taken in May, July, and October range from 6.6 - 12.2°C,
similar to our Δ47-derived estimates. Although the
initial study suggested a seasonal control on aragonite and calcite
formation, with aragonite being precipitated in the warmer months and
calcite being precipitated in the cooler months, we do not resolve a
clear relationship between temperature and mineralogy within this
limited dataset (Figure 7).
Our Δ47-temperatures and
δ18Owater values support the
hypothesis that the travertine sequence did not have a hydrothermal
origin (thermal water discharge), but are consistent with being derived
from rapid CO2 degassing from groundwater discharge of
meteoric origin, with sufficient time for dissolved inorganic carbon
equilibration to occur. We observe consistency between modern
δ18Owater values measured from a
series of nearby streams (-11.5 to -12.1‰ VSMOW) and
δ18Owater estimates derived from
clumped isotope analysis (-11.4 to -12.2‰ VSMOW). Given this consistency
in δ18Owater, we suggest recharged
meteoric groundwater (seasonal rainfall, snowmelt) and eventually some
contribution from ice melting due to a rapid increase in temperatures
during the Bølling-Allerød is likely to have been the surface dominated
paleofluid source for carbonate precipitation here. The calcium
carbonate source of the freshwater carbonate precipitates is probably
manifested in local marble lenses within the prevalent metamorphic rocks
and additional CO2 from underground might have been
dissolved in the percolating groundwaters favored by deep-seated slope
displacements (as a consequence of glacial ice melting) and faults that
have been locally detected (Boch et al., 2005).
The analysis of this travertine sequence illustrates the importance of
an appropriate calibration selection. While application of our composite
freshwater carbonate calibration would yield temperature values 2.5 -
2.7°C higher than modern streamflow, use of the material-specific
travertine calibration yields formation temperatures more similar to
modern stream temperatures (Supplementary Table 8), which are more
likely to be correct for carbonates forming in an interval of distinct
relative warmth in the last glacial period. The
δ18Owater values reconstructed from
the material-specific calibration are within error of modern groundwater
values, measured from spring-fed streams, while reconstructed
δ18Owater values are 0.5 to 0.6‰
higher when using the composite calibration relative to the
material-specific calibration. We note the Anderson et al. (2021)
calibration estimates the formation temperature of the calcareous tufa
sample to be 9.1°C, 1.6°C colder than the temperatures calculated using
the biologically mediated calibration. The cooler temperature estimated
by Anderson et al. (2021) results in
δ18Owater that are 0.4‰ more depleted
relative to the biologically mediated calibration. Reconstructed
temperatures and δ18Owater values for
the travertine sequence using the Anderson et al. (2021) calibration
result in temperature values that are similar to our travertine
calibration (0 - 0.2°C higher) and identical
δ18Owater values. Given that the
travertine calibration is indistinguishable from the Anderson et al.
(2021) calibration according to an ANCOVA test (slope: p = 0.6320;
intercept: p = 0.4440), the similarities in temperatures and
δ18Owater values between the two
calibrations is unsurprising.