Figure 5: Comparison of reconstructed values of temperature and δ18Owater from material-specific and composite calibrations from this study to observations. Use of a composite Δ47-temperature calibration yields less accurate and precise results. A. Comparison of measured temperature (Tobs.) to Δ47-derived temperature (TΔ47) values using the material-specific (top row, blue frame) and composite freshwater calibration (top row, black frame). Bottom panel shows a comparison of temperature residuals (reconstructed-observations) using the composite and material-specific calibrations. Values derived using the composite regression are represented using black squares and material-specific calibrations are represented using blue circles. Horizontal black bars represent the mean and values at the bottom of each dataset show the mean value and standard deviation for residuals along with the RMSE for each dataset using the respective calibrations. Our results show improvement in RMSE of reconstructed temperatures for biogenic carbonates and micrites when applying the material specific calibration compared to the composite calibration. Temperature residuals for biologically mediated carbonates are similar between the composite and material specific calibration, however, there is increased spread when applying the material specific calibrations, reflected in the elevated RMSE relative to the composite calibration. Similar to biologically mediated carbonates, the spread in temperature residuals for travertines, and thus RMSE, increases, when using material specific calibrations. However, we note that the overall mean of the residuals is reduced for travertines when applying a material specific calibration relative to the composite calibration. B. Comparison of measured δ18Owater18Oobs.) to Δ47-derived δ18Owatervalues (δ18Orecon.) using material specific (top row, blue frame) and composite freshwater calibration (top row, black frame). δ18Owater values are calculated using temperatures derived using our composite and material-specific calibrations between Δ47 and temperature, and oxygen isotope mineral-water fractionation factors from Kim and O’Neil (1997) (calcite) or Kim et al. (2007) (aragonite). Bottom panel displays δ18Owater residuals (reconstructed-observations) using the composite and material-specific calibrations. Our results show improvement in RMSE for δ18Owater reconstructions for biogenic carbonates, micrites, and travertines. δ18Owater RMSE for biologically-mediated carbonates is similar for both the composite and material specific calibration, however, we note that the mean of the residuals is reduced, and closer to zero, in the material specific calibration.
3.3 Reconstructing water δ18O
In addition to providing thermodynamic constraints on the temperature of formation of carbonates, Δ47 measurements can be paired with carbonate oxygen isotope ratios to directly calculate source water δ18O. We compare our clumped-isotope derived estimates of source water δ18O derived with our temperature predictions (δ18Ow-reconstructed) to measured modern freshwater δ18O (δ18Ow-measured) values to evaluate if this method can accurately reconstruct the isotopic composition of the water in which the carbonate precipitated. For δ18Ow-measured, we synthesized any available published measurements for the water bodies examined to compare to reconstructed values. δ18Ow-measured data is available for 86 of the 108 sites examined in this study (Supplementary Table 4). Although some sites had long-term measurements of water body oxygen isotope composition, some of the measurements were single point measurements, and thus may not fully represent temporal variability.
We used the equation of Kim & O’Neil (1997) for calcite and Kim et al. (2007) for aragonite in order to constrain the relationship between formation temperature, δ18Ocarbonate, and δ18Owater. We observe a positive relationship between measured and clumped-isotope derived δ18Ow-reconstructed derived from the composite freshwater calibration in this study (p = <0.0001; r2 = 0.7935) and material-specific reconstructed temperatures (p <0.0001; r2 = 0.8267). Figure 4c shows comparison of the residuals for reconstructed δ18Ow using both the material specific and composite calibrations developed within this study. RMSE was reduced using the material specific calibration relative to the composite calibration for biogenic carbonates (material specific RMSE = 1.5‰, composite RMSE = 1.8‰), micrites (material specific RMSE = 1.4‰, composite RMSE = 1.7‰), and travertines (material specific RMSE = 2.1‰, composite RMSE = 2.3‰). However, for biologically mediated carbonates, the composite calibration slightly outperforms the material specific calibration (material specific RMSE = 1.4‰, composite RMSE = 1.5‰), although the mean of the residuals is closer to zero.
Overall, material-specific calibrations perform better at reconstructing δ18Ow-measured (Figure 5). Out of the 87 samples from sites with measured δ18Owater values, 33 samples fall within ±1‰ of hydrographic data when using the material-specific calibrations, a 7% improvement from using the composite calibration (Fig. 5). δ18Ow-reconstructed values for biologic samples generally recover δ18Ow-measured within 2‰. However, this method yields, for micrites, lower δ18Owater values than observations for more enriched δ18Ow-measured values, which could be due to either kinetic effects and/or changes in surface water chemistry during carbonate precipitation events (Fig. 5). If the latter, it is unlikely to be due to evaporative enrichment of δ18Owater which would produce the opposite trend, but it may arise from changes in carbonate chemistry. Biologically-mediated carbonates and travertines show a positive offset from the 1:1 line, overestimating δ18Owater relative to the measured value, that may also arise from pH related effects on isotopic fractionation or kinetic isotope effects (e.g., Beck et al., 2005; Tripati et al., 2015).
3.4 Comparison of multiple materials at individual sites
Five sites in this study contain a combination of two different types of materials analyzed. Three sites in China (Daija Co, Cuona Lake, and Wulungu Lake) have micrite (Li et al., 2021) and freshwater mollusks (this study), and both the North Arm and the South Arm of the Great Salt Lake in Utah contain modern microbialite and ooid samples (Pace et al., 2016). Comparing results from different carbonate types provides another method of assessing overall calibration performance, therefore we compare clumped isotope-derived water temperature estimates using both material-specific and our composite freshwater calibrations at these sites where different materials are present.
Figure 6 shows increased agreement between different sample types for the same location for temperature and water δ18O in a majority of cases when a material-specific calibration is used. The lakes in China containing dual-materials are terminal lakes where authigenic carbonate precipitation is expected to occur in the later part of summer, when temperatures are most elevated and carbonate supersaturation occurs in the surface waters (Hren & Sheldon, 2012). Both Daija Co and Cuona Lake are high elevation lakes (>4.5 km) where the monthly average air temperature doesn’t exceed 0°C until May, thus, temperature requirements for mollusk and micrite growth are met during similar conditions, yielding similar estimated temperatures between both archives when using the material-specific calibrations. However, applying our composite calibration to samples in these lakes results in higher calcification temperatures projected for mollusks relative to the micrite. We find more of a disparity between Δ47-derived and directly measured temperature values when taking into account material type at Wulungu Lake, an inland, low elevation and high latitude (47°N) lake with a large range of intra-annual temperatures (~36°C), however, our estimates of water temperature are in agreement with typical seasonality of precipitation from the two archives given the lake’s setting. Our results using a material specific calibration suggest that micrite precipitated during a more narrow interval, with higher temperatures and δ18O values than the mollusk samples, consistent with temperature-induced carbonate precipitation and evaporative enrichment. The temperatures recorded by mollusks at the same lake using the material specific calibrations suggest that the shells may be reflecting dominant calcification during comparatively cooler temperatures in spring or early summer.
Modern microbialite and ooid samples from the Great Salt Lake yield similar results, with a 1.5°C difference between archives in both the North and South Arms using the material specific calibration. Both archives in the Northern and Southern arms of the lake are consistent with summer water temperatures when taking into account material type. Furthermore, the separation of the North and South arms of the Great Salt Lake by a causeway results in a more restricted Northern arm with less freshwater input from rivers and more evaporation (Gwynn, 2007). Our Δ47-derived temperature data also shows that our carbonate samples from the Great Salt Lake record more elevated heating in the North Arm relative to the South Arm when using the material specific (0.6°C) compared to the composite (0.1°C) calibrations. Further study is needed to fully understand the differences in calcification in between archives in the same settings, but these findings may support the use of a material-specific calibration when developing reconstructions from multiple types of carbonates for ancient freshwater systems.