Carbonate clumped isotope values compromised by nitrate-derived NO2 interferent

Abstract

Δ₄₇ 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 Δ₄₇ 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 Δ₄₇-Δ₄₈ values and measured NO₂ abundances in the CO₂ evolved from phosphoric acid digestion of carbonates in a common acid bath at 90 °C. We show that nitrate-derived NO₂ constitutes a serious isobaric interferent for the extracted CO₂. During acid digestion, nitrate decomposes to NO and NO₂. Both compounds are not effectively removed during subsequent purification of carbonate-derived CO₂ using cryogenic traps (−80 °C) and gas chromatography (packed Porapak Q column at −15 °C), generating a bias in measured Δ₄₇ and Δ₄₈ values. In dual clumped isotope space, biased samples plot along a slope of −0.3 that is characteristic for variable sub-ppm contributions of NO₂⁺ to CO₂⁺ in the ion source. Measured NO₂ concentrations in the analyte-grade CO₂ correspond to observed biases in Δ₄₇ and Δ₄₈ values if preferential ionization of CO₂ over NO₂ is taken into account.

Nitrate contamination occurs in a synthetic calcite precipitated using Ca(NO₃)₂, a pedogenic carbonate nodule, a plasma-ashed echinoid spine, a bioapatite (Greenland shark dentine), and in ETH-3 (a recently assigned anchor for Δ₄₇ analysis of carbonates). Sequential bleaching tests reveal that nitrate contaminant and NO₂⁺ bias can be effectively removed if carbonate samples are pre-treated overnight with 3 wt-% sodium hypochlorite (NaOCl). NO₂⁺ bias in ETH-3-derived CO₂ 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 NO₂⁺ 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 NO₂⁺ 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 Δ₄₇ (CDES 90) values for ETH-1 and ETH-2 (Bernecker et al., 2023) and for bleached ETH-3 exactly confirm recently assigned Δ₄₇-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.