A framework for triple oxygen isotopes in speleothem paleoclimatology
A framework for triple oxygen isotopes in speleothem paleoclimatology
复制标题
洞穴古气候学中的三氧同位素框架
DOI：
10.1016/j.gca.2021.11.002
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发表时间：
2021
影响因子：
5
通讯作者：
N. Levin
中科院分区：
文献类型：
--
作者：
T. Huth;B. Passey;Julie E. Cole;M. Lachniet;D. McGee;R. Denniston;S. Truebe;N. Levin
洞穴沉积物氧同位素（δ 18 O）记录为研究晚第四纪陆地古气候变化的速率和时间提供了重要依据。然而，很难将δ 18 O信号反卷积为单个分量，其中包括与水分来源，水分输送，温度，降水量，渗透和洞穴环境相关的过程。我们开发了一个框架，使用三重氧同位素分布在洞穴沉积物中，以完善解释的δ 18 O洞穴沉积物记录。该框架通过Δ′ 17 O与δ′ 18 O空间中的特征（尽管不一定是唯一的）趋势来确定主导过程对δ 18 O值随时间的影响，其中Δ′ 17 O = δ′ 17 O- 0.528δ′ 18 O和δ′xO = ln（δxO + 1）。继郭和周（2019 a）之后，我们预计"洞穴动力学“过程（例如，滴注点快速脱气，方解石沉淀之前）将驱动δ′ 18 O和Δ′ 17 O之间的正趋势。相比之下，我们可以识别水文过程，从近水平的趋势，反映瑞利型大气降水过程和负趋势的变化，在水分源区或在洞穴网站，矿化温度和季节性的降水/渗透量的蒸发过程的驱动。我们应用这个框架，美国西部的钟洞（亚利桑那州），利维坦洞（内华达州），雷曼洞穴（内华达州）的洞穴。钟洞和利维坦的数据有接近水平的负趋势，表明δ 18 O的变化主要是由大气水分和水分来源条件的瑞利蒸馏的变化驱动的，支持先前的解释。我们分析了两个雷曼洞穴的记录，因为它们可能受到非平衡过程的影响，数据显示出弱到中等的负面趋势。对于样本LMC-12 b，由于其极端的7.5‰ δ 18 O范围而被选择，该趋势在统计上不同于近水平瑞利过程趋势，并且与作为主要驱动因素的局部蒸发强度和渗透季节性的变化最为一致。这些记录都没有显示δ′ 18 O和Δ′ 17 O之间的正协变斜率，表明洞穴动力学过程随时间的变化有限，或者郭和周（2019 a）的动力学模型存在未知的局限性。此外，所有站点的重建地层沃茨都落在Δ′ 17 O与δ′ 18 O局部大气水线附近，我们建议将这种相关性作为洞穴动力学过程引起的同位素偏移绝对大小的新测试。更广泛地说，我们的框架为碳酸盐洞穴沉积物三重氧同位素组成的唯一其他研究增加了背景（Sha等人，2020年）。我们发现，在洞穴沉积物数据中可能存在正到负的Δ′ 17 O与δ′ 18 O的趋势，这可能是合理的预期，从区域气候过程，并结合其他代用数据，三重氧同位素数据将是有用的限制解释δ 18 O洞穴沉积物记录。
Speleothem oxygen isotope (δ18O) records provide key insight into the rate and timing of terrestrial paleoclimate changes during the late Quaternary. However, it can be difficult to deconvolve the δ18O signal into individual components, which include processes related to moisture source, moisture transport, temperature, precipitation amount, infiltration, and the cave environment. We developed a framework that uses triple oxygen isotope distributions in speleothems to refine interpretations of δ18O speleothem records. This framework identifies the influence of dominant processes on δ18O values through time by their characteristic (although not necessarily unique) trends in Δ′17O vs. δ′18O space, where Δ′17O = δ′17O – 0.528δ′18O and δ′xO = ln(δxO + 1). Following Guo and Zhou (2019a), we expect that ‘cave kinetic’ processes (e.g., fast degassing at the drip site, prior calcite precipitation) will drive positive trends between δ′18O and Δ′17O. In contrast, we can identify hydrologic processes from near-horizontal trends that reflect Rayleigh-type meteoric water processes and negative trends driven by changes in evaporation processes at the moisture source region or at the cave site, mineralization temperature, and seasonality in precipitation/infiltration amount. We applied this framework to four western USA speleothems from Cave of the Bells (Arizona), Leviathan Cave (Nevada), and Lehman Caves (Nevada). The Cave of the Bells and Leviathan data have near-horizontal to negative trends indicating δ18O variability was driven largely by changes in Rayleigh distillation of atmospheric moisture and moisture source conditions, supporting prior interpretations. We analyzed two Lehman Caves records because they were likely influenced by non-equilibrium processes and the data show weak to moderate negative trends. For sample LMC-12b, chosen for its extreme 7.5‰ δ18O range, the trend is statistically distinct from the near-horizontal Rayleigh-process trend and most consistent with changes in local evaporation intensity and infiltration seasonality as primary drivers. None of these records displays a positive covariation slope between δ′18O and Δ′17O, suggesting limited variability in cave kinetic processes through time or unknown limitations to the kinetic model of Guo and Zhou (2019a). Additionally, reconstructed formation waters for all sites fall near the Δ′17O vs. δ′18O Local Meteoric Water Line, a correlation we suggest as a novel test of the absolute magnitude of isotopic offset due to cave kinetic processes. More broadly, our framework adds context to the only other study of carbonate speleothem triple oxygen isotope composition (Sha et al., 2020). We find that positive to negative Δ′17O vs. δ′18O trends likely exist in speleothem data that may reasonably be expected from regional climate processes and that, combined with other proxy data, triple oxygen isotope data will be useful in constraining interpretations of δ18Ospeleothemrecords.