Calibrating the Carbonate Clumped Isotope Thermometer from 7 to 70 ˚C by Automated Laser Spectroscopy

Abstract:

Measurements of ¹³C-¹⁸O clumping in CO₂ derived from carbonates have expanded robust paleothermometry research, but measurement is slow, requires large sample sizes, and is expensive. In order to increase sample analysis rates by an order of magnitude and reduce sample size, we have developed an isotope ratio laser spectrometry (IRLS) method for clumped CO₂ measurements by tunable infrared laser differential absorption spectroscopy (TILDAS). Our previously published results using gas samples demonstrate that the IRLS is competitive with the best isotope ratio mass spectrometry (IRMS) systems and surpasses typical IRMS measurements in several key respects, such as small sample size (15 μmol of CO₂, 1.5 mg equivalent calcite), rapid measurement (precision of 0.01‰ within 20 minutes, 1 S.E.), and no isobaric interference problems. The reported TILDAS ∆_16O13C18O values, approximately equivalent to ∆₄₇, show a linear relationship with theoretical values, with a weak dependence on bulk isotope compositions. During the past 9 months, a manually operated gas extraction line was constructed for phosphoric acid digestion of carbonates at 90 ˚C, and at the same time the TILDAS instrument was upgraded with addition of an automated bellows sample introduction system. More than 240 gas samples and 470 carbonates have been tested, including laboratory standards (Carrara marble), interlaboratory standards (ETH1, ETH2, ETH3, ETH4), and 2 natural samples. Synthetic carbonates, precipitated at temperatures from 7 to 70 ˚C, have also been measured. In total 96 measurement of 14 synthetic samples were used to produce a linear temperature calibration. Although this relationship is specific to our laser system and manual extraction line at present, the slope of the ∆_16O13C18O–temperature calibration is in agreement with other published relationships.