Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ₄₇) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ⁴⁷ and Δ₄₇ values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ₄₇ (I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale.

In order to reconstruct the activity of the Indian Summer Monsoon (ISM) during the last ∼4 kyr, two stalagmites from Baratang cave in Andaman Islands have been investigated for their temporal variations in δ¹⁸O. The chronology is provided by radiocarbon ages on the stalagmites. During 1800–2100 cal yr BP, we observed a significant increase in stalagmite δ¹⁸O that we infer is caused by the amount effect. This increase implies a strong reduction in the strength of the ISM that is correlated with the Roman Warm Period. Other proxy records in the region confirm a reduction in ISM activity during this time. Reduction in the ISM is also observed around 1500 and 400–800 cal yr BP; the latter period is the transition from the Medieval Warm Period to the Little Ice Age. The strongest monsoon in the last ∼4 kyr is observed during 800–1200 cal yr BP, the Medieval Warm Period.

Rationale: Determination of δ ¹⁷ O values directly from CO ₂ with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of ¹³ C ¹⁶ O ¹⁶ O on ¹² C ¹⁷ O ¹⁶ O. The methods developed so far use either chemical conversion or isotope equilibration to determine the δ ¹⁷ O value of CO ₂ . In addition, δ ¹³ C measurements require correction for the interference from ¹² C ¹⁷ O ¹⁶ O on ¹³ C ¹⁶ O ¹⁶ O since it is not possible to resolve the two isotopologues. Methods: We present a technique to determine the δ ¹⁷ O, δ ¹⁸ O and δ ¹³ C values of CO ₂ from the fragment ions that are formed upon electron ionization in the ion source of the Thermo Scientific 253 Ultra high‐resolution isotope ratio mass spectrometer (hereafter 253 Ultra). The new technique is compared with the CO ₂ ‐O ₂ exchange method and the ¹⁷ O‐correction algorithm for δ ¹⁷ O and δ ¹³ C values, respectively. Results: The scale contractions for δ ¹³ C and δ ¹⁸ O values are slightly larger for fragment ion measurements than for molecular ion measurements. The δ ¹⁷ O and Δ ¹⁷ O values of CO ₂ can be measured on the ¹⁷ O ⁺ fragment with an internal error that is a factor 1–2 above the counting statistics limit. The ultimate precision depends on the signal intensity and on the total time that the ¹⁷ O ⁺ beam is monitored; a precision of 14 ppm (parts per million) (standard error of the mean) was achieved in 20 hours at the University of Göttingen. The Δ ¹⁷ O measurements with the O‐fragment method agree with the CO ₂ ‐O ₂ exchange method over a range of Δ ¹⁷ O values of −0.3 to +0.7‰. Conclusions: Isotope measurements on atom fragment ions of CO ₂ can be used as an alternative method to determine the carbon and oxygen isotopic composition of CO ₂ without chemical processing or corrections for mass interferences.