Carbonate clumped isotope values compromised by nitrate-derived NO2 interferent

Abstract

Δ 47 based clumped isotope thermometry has enabled reconstruction of Earth's surface temperatures independent of the source of oxygen within the carbonate. It has been postulated that carbonate samples can contain contaminants that cause isobaric interferences, compromising measured Δ 47 values and reconstructed temperatures. The exact nature of contaminants and isobaric interferents, however, largely remained unidentified. Here, we compare theoretically predicted contamination vectors with measured Δ 47 - Δ 48 values and measured NO 2 abundances in the CO 2 evolved from phosphoric acid digestion of carbonates in a common acid bath at 90 °C. We show that nitrate-derived NO 2 constitutes a serious isobaric interferent for the extracted CO 2 . During acid digestion, nitrate decomposes to NO and NO 2 . Both compounds are not effectively removed during subsequent purification of carbonate-derived CO 2 using cryogenic traps (−80 °C) and gas chromatography (packed Porapak Q column at −15 °C), generating a bias in measured Δ 47 and Δ 48 values. In dual clumped isotope space, biased samples plot along a slope of −0.3 that is characteristic for variable sub-ppm contributions of NO 2 + to CO 2 + in the ion source. Measured NO 2 concentrations in the analyte-grade CO 2 correspond to observed biases in Δ 47 and Δ 48 values if preferential ionization of CO 2 over NO 2 is taken into account. Nitrate contamination occurs in a synthetic calcite precipitated using Ca(NO 3 ) 2 , a pedogenic carbonate nodule, a plasma-ashed echinoid spine, a bioapatite (Greenland shark dentine), and in ETH-3 (a recently assigned anchor for Δ 47 analysis of carbonates). Sequential bleaching tests reveal that nitrate contaminant and NO 2 + bias can be effectively removed if carbonate samples are pre-treated overnight with 3 wt-% sodium hypochlorite (NaOCl). NO 2 + bias in ETH-3-derived CO 2 and its effective removal through bleaching is also indicated in a completely different analytical setup that makes use of individual reaction vessels, acid digestion at 70 °C, cryogenic traps at −60 °C and a static Porapak Q trap at −30 °C. Considering that NO 2 + bias is observed in two fairly conventional analytical setups, we strongly recommend that each laboratory tests to which extent their setup is affected. Unless independent evidence is given that NO 2 + bias is irrelevant for a specific setup, ETH-3 should be bleached and further systematic sequential bleaching tests be carried out on unknown samples in order to avoid any isotopic bias. Our high-precision long-term Δ 47 (CDES 90) values for ETH-1 and ETH-2 ( Bernecker et al., 2023 ) and for bleached ETH-3 exactly confirm recently assigned Δ 47 -I-CDES values for these standards, demonstrating that ETH-3 – in the short term – could be replaced by its bleached counterpart for accurate I-CDES data normalization. In the long term, it should be replaced by a low-temperature carbonate anchor devoid of contaminants. • Trace amounts of nitrate-derived NO 2 can compromise carbonate Δ 47 , Δ 48, Δ 49 values. • Nitrate contamination effective in a pedogenic carbonate, shark dentine and ETH-3. • Compromised results can be avoided bleaching carbonates with 3 wt-NaOCl overnight. • Δ 47 (CDES 90) value of bleached ETH-3 agrees with assigned I-CDES value of 0.6132 ‰.