碳酸盐团块同位素值受到来自硝酸盐的二氧化氮干扰物的影响

IF 3.6 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Jens Fiebig , Miguel Bernecker , Niels Meijer , Katharina Methner , Philip Tauxe Staudigel , Amelia Jane Davies , Lkhamsuren Bayarjargal , Dominik Spahr , Björn Winkler , Sven Hofmann , Manuel Granzin , Sierra V. Petersen
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引用次数: 0

摘要

基于Δ47的团块同位素测温法能够重建地球表面温度,而不受碳酸盐中氧气来源的影响。据推测,碳酸盐样本中可能含有导致等压干扰的污染物,从而影响Δ47 的测量值和重建的温度。在这里,我们将理论上预测的污染矢量与在 90 °C下普通酸浴中磷酸消化碳酸盐所产生的二氧化碳中的Δ47-Δ48 测量值和 NO2 丰度进行了比较。我们的研究表明,硝酸盐衍生的二氧化氮对萃取的二氧化碳构成了严重的等压干扰。在酸解过程中,硝酸盐分解为 NO 和 NO2。在随后使用低温捕集器(-80 °C)和气相色谱法(-15 °C下的Porapak Q填充柱)纯化碳酸盐衍生的二氧化碳时,这两种化合物都不能被有效去除,从而导致测量的Δ47和Δ48值出现偏差。在双团块同位素空间中,偏差样品的斜率为-0.3,这是离子源中 NO2+ 对 CO2+ 的不同亚ppm贡献的特征。如果考虑到 CO2 比 NO2 优先电离,则在分析级 CO2 中测量到的 NO2 浓度与观测到的Δ47 和Δ48 值偏差一致。硝酸盐污染出现在使用 Ca(NO3)2 沉淀的合成方解石、成足碳酸盐结核、等离子闪烁的回声棘、生物磷灰石(格陵兰鲨鱼牙)以及 ETH-3(最近指定用于碳酸盐Δ47 分析的锚)中。连续漂白测试表明,如果用 3 wt%的次氯酸钠(NaOCl)对碳酸盐样品进行过夜预处理,可以有效去除硝酸盐污染物和 NO2+偏差。在一个完全不同的分析装置中也显示了 ETH-3 衍生的 CO2 中的 NO2+偏差以及通过漂白有效去除 NO2+偏差的情况,该装置使用了单独的反应容器、70 °C 下的酸消化、-60 °C 下的低温捕集器和-30 °C 下的静态 Porapak Q 捕集器。除非有独立的证据表明 NO2+偏差与特定设置无关,否则应漂白 ETH-3,并对未知样品进行进一步的系统性连续漂白测试,以避免任何同位素偏差。我们对 ETH-1 和 ETH-2 (Bernecker 等人,2023 年)以及漂白 ETH-3 的高精度长期 Δ47 (CDES 90)值完全证实了最近为这些标准分配的 Δ47-I-CDES 值,这表明短期内可以用漂白的 ETH-3 代替 ETH-3,以实现准确的 I-CDES 数据归一化。从长远来看,应该用没有污染物的低温碳酸盐锚来替代它。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Carbonate clumped isotope values compromised by nitrate-derived NO2 interferent

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.

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来源期刊
Chemical Geology
Chemical Geology 地学-地球化学与地球物理
CiteScore
7.20
自引率
10.30%
发文量
374
审稿时长
3.6 months
期刊介绍: Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.
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