Rapid and precise measurement of carbonate clumped isotopes using laser spectroscopy

Abstract

Carbonate clumped isotope abundance is an important paleothermometer, but measurement is difficult, slow, and subject to cardinal mass (m/z) interferences using isotope ratio mass spectrometry (IRMS). Here, we describe an optical spectroscopic measurement of carbonate clumped isotopes. We have adapted a tunable infrared laser differential absorption spectrometer (TILDAS) system to measure the abundances of four CO₂ isotopologues used for clumped isotope thermometry. TILDAS achieves the same precision (0.01‰ SE) as IRMS measurements rapidly (∼50 min per carbonate analysis) and using small samples (<2 mg of calcite), without making assumptions about ¹⁷O abundance in the sample. A temperature calibration based on 406 analyses of CO₂ produced by digestion of 51 synthetic carbonates equilibrated at 6° to 1100°C is consistent with results for natural carbonates and previous calibrations. Our system results were indistinguishable from IRMS systems after replicating the InterCarb interlaboratory calibration. Measurement by TILDAS could change the landscape for clumped isotope analysis.

Fig. 1.. System schematic.

A diagram of the fully automated carbonate clumped isotope analyzer. The system is composed of a CO₂ extraction and cleanup section, a sample collection, mixing and delivery system, and an inlet and spectroscopic analysis component. P sensor, pressure sensor; LN₂, liquid nitrogen Dewar; N₂, nitrogen gas; WR, working reference gas; concentric circles, critical orifice. See Materials and Methods for a detailed description of the analysis protocol.

Fig. 2.. Bulk isotope dependence.

The empirical relationship between ∆_638raw and δ_636raw (A) and between ∆_638raw and δ_628raw (B) in the equilibrated and heated gases. Symbols: individual measurement (four sample-WR comparisons); vertical and horizontal error bars: 1 SD uncertainty in ∆_638raw, δ_636raw, and δ_628raw measurements; r: Pearson correlation coefficient. In (A), only samples with equal δ_628raw composition and different δ_636raw composition are shown. In (B), only samples with equal δ_636raw composition and different δ_628raw composition are shown.

Fig. 3.. InterCarb standards comparison.

Δ₄₇ and Δ₆₃₈ results from IRMS and TILDAS measurements of seven Interlaboratory standards: ETH-1, ETH-2, ETH-3, ETH-4, IAEA-C1, IAEA-C2, and MERCK. IRMS results are from the InterCarb study (5). In (A) to (D), raw data are projected into the CDES reference frame using equilibrated and heated gases as anchors. IRMS results are ordered from left to right in the order of laboratories A to J as reported in table 1 of (5) with regular intervals. Each circle represents the mean value for a standard reported by a single laboratory. The blue line represents the weighted mean value for all IRMS laboratories. Red diamonds represent TILDAS mean values for each standard for powders received in March 2019, analyzed 40 times for ETH-1 and 41 times for ETH-2,3,4. Purple diamonds are TILDAS mean values for the same standards but for the exact powders distributed to UW IsoLab for the InterCarb study, analyzed six times each. Error bars are ±1 SE. In (E) to (G), results are projected into the I-CDES reference frame using carbonate standards ETH-1,2,3 for IRMS results and ETH-1,2,3,4 for TILDAS results. The results from IRMS instruments 1 to 26 as reported in table 3 of (5) are ordered from left to right. Note that not all mass spectrometers 1 to 26 reported data for all three standards. In all panels, IRMS values are corrected to 70°C acid digestion by adding a factor of 0.022‰.

Fig. 4.. Carbonate standards stability.

(A) Repeated analyses of intralaboratory (car-2, MK) and interlaboratory (ETH) carbonate standards between 22 July 2021 and 2 November 2021. Clumped isotope values are projected into CDES using equilibrated and heated gases. Carbonates are digested in 70°C acid, and Δ_638CDES values are not corrected to 25°C. Error bars are ±1 SD of individual analyses. Number of analyses: car-2, 82; MK, 130; ETH-1, 40; ETH-2, 41; ETH-2, 41; ETH-4, 41. (B) Deviation from the mean value of each standard for the same analyses as (A). Error bars removed for clarity.

Fig. 5.. Conventional calcite-water ¹⁸O fractionation-temperature calibration.

Black circles are 1000lnα_{calcite-water} values based on mean δ¹⁸O values of 41 synthetic calcite samples prepared for this study and the δ¹⁸O values of the water they precipitated from. Calcite powders were analyzed 5 to 11 times each. Horizontal and vertical error bars are smaller than markers. The red line is a linear regression based on TILDAS δ¹⁸O values of the carbonates. The brown line is the regression based on IRMS δ¹⁸O values of the same samples. Blue diamonds are data points from (22). The blue line is the linear regression of (22).

Fig. 6.. Empirical temperature calibration.

(A) The black circles are mean Δ₆₃₈ CDES values of synthetic calcite prepared for this study, plotted against their precipitation/reequilibration temperatures. Uncertainties in Δ₆₃₈ and in temperatures are smaller than circles. The red line is a weighted linear regression based on the synthetic samples precipitated between 6° and 70°C, as shown in main figure. The dashed brown line is a regression based on both the low-temperature samples and samples reequilibrated at 450° to 1100°C, shown in the bottom left of the inset. The colored symbols are mean Δ₆₃₈ values of natural carbonates plotted against their known formation temperatures. Vertical error bars are smaller than symbols, and horizontal error bars represent uncertainties in formation temperatures. The blue line is the IRMS calibration of (6) with the intercept adjusted to 70°C acid digestion temperature. (B) Same as (A) but samples are projected into the I-CDES reference frame using ETH anchors. The blue line is the calibration of (7).