Figure 7: Clumped isotope derived estimates of temperature and
δ18Owater for a travertine terrace in
Ainet, Austria. Solid symbols represent estimates derived from
material-specific calibrations, while lighter symbols represent
estimates derived from our composite freshwater calibration. Gray bands
represent the range of modern stream values measured in May, July, and
October (Boch et al., 2005) and data for GISP2 ice core record is from
Stuiver and Grootes (2000). Values are broadly consistent with modern
temperatures and δ18Owater of nearby
streams. Seasonally enhanced percolation of meteoric waters and melting
of the last glacial Alpine glaciers during initiation of the pronounced
Bølling-Allerød warm period are hypothesized to have recharged
groundwater via new flow routes (slope failure/graben and faults)
opening during the melting phase and promoting the relatively short
lived discharge and calcium carbonate precipitation event.
3.6.3 Application to Paleoaltimetry
Following on stable isotope and leaf margin-based paleoaltimetry
(Blisniuk & Stern, 2005; Gregory-Wodzicki, 2000; McElwain, 2004; Rowley
& Garzione, 2007; Wolfe et al., 1998), clumped isotope analyses have
been used to constrain the tectonic history of a region (Ghosh,
Garzione, et al., 2006; Huntington & Lechler, 2015; L. Li et al., 2019;
Quade et al., 2013). This proxy relies on the premise that lake water
temperature is directly related to air temperature; therefore, as basins
undergo surface uplift as a result of large-scale tectonic processes
(e.g., crustal shortening and thickening, convective removal of lower
lithosphere, etc.), the ambient air and water temperature should
decrease, as governed by the local lapse rate (Ghosh, Garzione, et al.,
2006; Huntington & Lechler, 2015; L. Li et al., 2019; Quade et al.,
2013). Reconstructed δ18Owater can
provide additional constraints on paleoelevation, because the stable
isotope compositions of meteoric and surface waters decreases as
altitude increases (Chamberlain & Poage, 2000; Poage & Chamberlain,
2001; Rowley & Garzione, 2007). To assess the impact of our
calibrations on a published paleoelevation reconstruction for the
Tibetan Plateau, we recalculate temperatures and
δ18Owater estimates from L. Li et al.
(2019) using the micrite, composite, and Anderson et al. (2021)
calibration for lacustrine samples in the Nangqian Basin. We used our
clumped-isotope derived results to compare estimates of elevation
changes during the late Eocene following the methodology used in the
original publication.
Overall, the micrite calibration equation derives similar temperatures
for unaltered carbonates to those derived in the original publication
using the Kelson calibration (Supplementary Table 9). Mean temperatures
for Units 1, 3, and 4 using the new calibration derived in this study
are 25.1 ± 3.0, 39.0 ± 3.7, and 29.0 ± 3.5°C. These values are 0.2°C
higher, 1.9°C lower, and 0.7°C lower than published results,
respectively. The general calibration estimates slightly cooler values
than our material-specific derived results (1.4, 0.2, and 1.1°C lower
for Units 1, 3, and 4, respectively). The Anderson calibration projects
even cooler values, with Units 1, 3, and 4 estimates being 3.4, 1.2, and
3.4°C lower, respectively.
In the new calibration, samples from Unit 2 yield high temperatures
between 44.2 and 67.8°C, which agree with previous observation and
interpretation of deep burial diagenetic alteration of the Unit 2
samples (L. Li et al., 2019). We also note that the previous
interpretation for the new temperatures of Unit 1 still holds, such that
the lower temperatures in the Late Cretaceous represent either less
heating of lake water or possibly higher surface elevation in the
Nangqian Basin.
Late Eocene (38-37 Ma) carbonates analyzed from Unit 4 were suggested to
be deposited near-surface and their Δ47-temperature
values were used to reconstruct
δ18Owater values and paleoelevation
(L. Li et al., 2019). We calculate similar values using our calibration
for late Eocene highland water δ18O values to be -9.4‰
and -9.8‰, respectively, compared to -9.1‰ and -9.8‰ in the original
study. Following the method in L. Li et al. (2019) where the authors use
different modeling approaches to consider dominant regional moisture
sources, the paleo-water δ18O values correspond to a
mean hypsometric paleoelevation of 2.8±1.1 km and 3.1±1.1 km, which is
very similar to the reconstruction that was published (2.8±1.1 km and
3.2±1.1 km), and ~1.3 km lower than the modern
hypsometric mean elevation (4.2 km) of the Nangqian Basin. Given the
similarities in projected water temperatures between the
material-specific and composite regressions for the sample used to
reconstruct paleoelevation, it is unsurprising that the projected
elevation estimates are identical (Table 4). The cooler temperatures
derived using Anderson calibrations result in more depleted water
δ18O values (-9.5‰ and -10.0‰) and slightly higher
estimates of paleoelevation (2.9±1.1 km and 3.2±1.1 km), but still
within error of the original publication and findings in this study.
The basin floor paleoelevation of the Nangqian Basin was estimated from
the T-Δ47 values of late Eocene lacustrine carbonates in
unit 4. The newly calculated mean T-Δ47 value of these
samples is 29.5°C, 0.5°C higher than what was published. Following the
method in Li et al. (2019), this mean T-Δ47 value is
12.5°C higher than the estimated warm-season lake surface water
temperature (~17oC) for an elevation
at 3.8 km. Benthic foraminiferal records indicate that roughly 6°C of
the temperature decrease can be attributed to the post-Eocene global
cooling (Hansen et al., 2008), while the remaining 6.5°C reflects
paleoelevation increase of the basin floor after the Eocene. Following
the method in L. Li et al. (2019), this indicates 1.1 ± 0.3 km of
post–late Eocene elevation increase, if a lapse rate of –6.1 ± 1.0
°C/km is applied for lake surface-water temperature on the Tibetan
Plateau (Huntington et al., 2015). Therefore, the late Eocene elevation
of the Nangqian Basin floor is estimated as 2.7 km above sea level,
which is 1.1 km lower than the modern elevation of 3.8 km.
4 Conclusions
In order to confidently use proxies to characterize and understand past
environments, it is necessary to have a solid understanding of modern
systems. Thus, in this work, we present an extensive composite dataset
of 135 clumped isotope samples of terrestrial freshwater carbonates from
96 sites, and derive relationships between modern water temperatures and
Δ47. These freshwater Δ47-temperature
calibrations are well constrained, encompass a variety of types of
natural lacustrine, fluvial, and spring carbonates, and span a broad
range of temperatures, elevations, and latitudes. As the carbonates
presented in this study are taken directly from modern freshwater
settings, they are more representative of real-world systems, and may,
in some circumstances, be more appropriate for application to
reconstruct paleotemperatures, than synthetically-derived
Δ47-temperature carbonate calibrations. However, we note
that since our approach utilizes in-situ lake water surface
temperature data, there is an added uncertainty in the timing of
carbonate formation temperature and calcification timeframe for each of
our calibration samples.
Our results show a convergence of slopes but differences in the
intercepts of the Δ47-temperature relationship between
freshwater carbonate groups. Specifically an ANCOVA analysis shows that
material specific calibrations based on grouping freshwater carbonates
(biogenic, biologically mediated, micrite, and travertine) have
statistically indistinguishable slopes between other freshwater
carbonate groups and recently published calibration studies, but in some
cases, where there is strong evidence for biogenicity, detects
differences in intercepts.
Utilizing a single, composite calibration does not always yield the most
accurate results, while in most cases, implementing material-specific
calibrations reduces the magnitude of residuals (offsets between
Δ47-derived temperatures/water δ18O
and measured temperature/water δ18O) and RMSE, and
thus, can provide more robust estimates of temperature. Water
δ18O values derived from utilizing material specific
calibrations can recover independently measured water
δ18O values accurately, with 39% and 74% of lakes
being within 1‰ and 2‰ of measured water δ18O,
respectively, reflecting a 7% and 10% improvement relative to our
composite freshwater calibration.
Additionally, we explore the application of the new calibration
relationships reported in this study by examining three case studies
using Δ47 measurements of freshwater carbonates. First,
we utilize the biologically-mediated and composite calibrations to
explore hydroclimatic changes from the LGM and deglacial at Lake
Surprise in the Western US and derive cooler temperatures using
material-specific, composite, and Anderson calibrations, but with
similar results for evaporation and precipitation rates. Second, we
utilize the micrite calibration to estimate the Eocene paleoelevation of
the Nangqian Basin within the Tibetan Plateau and find that our
material-specific derived elevations are in agreement with the original
publication, but elevations derived from the composite and Anderson
calibration suggest a higher degree of uplift. We also present a new
dataset from a travertine sequence in Austria and show that the origin
of the fluid that allowed for carbonate formation was likely derived
from groundwater sources due to increased percolation of meteoric water
and glacial melt. Overall, this work provides a basis for more accurate
reconstructions of terrestrial paleoclimate, paleohydrology, and
paleoaltimetry using freshwater archives, and opens the door to more
robust understandings of paleoenvironmental processes.