Carbonate clumped isotope values compromised by nitrate-derived NO2 interferent

∆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 ∆47 values and reconstructed temperatures. The exact nature of contaminants and isobaric interferents, however, largely remained unidentified. Here, we demonstrate that compromised measurements can be identified through high-precision analysis of ∆48 alongside ∆47. We provide evidence that nitrate constitutes a serious contaminant even if present in the carbonate in sub-wt-% quantities only. During phosphoric acid digestion of carbonates at 90°C, nitrate decomposes to NO2, which, in turn, is not effectively removed during subsequent purification of carbonate-derived CO2 using cryogenic traps and gas chromatography (packed Porapak Q column at -15°C). In dual clumped isotope space, samples affected by variable sub-ppm contributions of NO2 to CO2 plot along a characteristic slope of -0.3, in agreement with theoretical predictions. Nitrate contamination occurs in a synthetic calcite precipitated using Ca(NO3)2, a pedogenic carbonate nodule, a plasma-ashed echinoid spine, ETH-3 (a recently assigned anchor for ∆47 analysis of carbonates) and, presumably, in a bioapatite (Greenland shark dentine). The extent of NO2 generation may depend on analytical setup and additional parameters such as nitrate concentration inside the carbonate, acid digestion temperature, reaction time, filament current and filament age. Sequential bleaching tests reveal that nitrate contaminant can be effectively removed from carbonates if samples are pre-treated overnight with 3 wt-% sodium hypochlorite (NaOCl). Our high-precision long-term ∆47 (CDES 90) values for ETH-1 and ETH-2 (Bernecker et al., 2023) and for bleached ETH-3 exactly confirm recently assigned ∆47-I-CDES values for these standards. Unless independent evidence is given that NO2 contamination is irrelevant for a specific analytical setup we strongly recommend that ETH-3 is bleached with 3 wt-% NaOCl overnight in order to enable accurate projection of raw data to the I-CDES. Moreover, systematic bleaching tests should be carried out on unknown samples in order to avoid any isotopic bias.