Triple oxygen isotope compositions of globally distributed soil carbonates record widespread evaporation of soil waters
Triple oxygen isotope compositions of globally distributed soil carbonates record widespread evaporation of soil waters
复制标题
DOI：
10.1016/j.gca.2023.06.034
复制
发表时间：
2023-08
影响因子：
5
通讯作者：
J. Kelson;T. Huth;B. Passey;N. Levin;S. Petersen;P. Ballato;E. Beverly;D. Breecker;G. Hoke;A. Hudson;Haoyuan Ji;A. Licht;Erik J. Oerter;J. Quade
中科院分区：
文献类型：
--
作者：
J. Kelson;T. Huth;B. Passey;N. Levin;S. Petersen;P. Ballato;E. Beverly;D. Breecker;G. Hoke;A. Hudson;Haoyuan Ji;A. Licht;Erik J. Oerter;J. Quade
成壤碳酸盐的稳定同位素组成是过去气候和地形研究的核心，为我们了解地球的陆地历史提供了基础。成土碳酸盐氧同位素值（δ 18 O）的许多应用的核心假设是它们反映了降水（雨/雪）的δ 18 O值。如果土壤碳酸盐在蒸发的土壤沃茨中形成，则违反了这一假设。在这项工作中，我们开发了一种方法来识别蒸发在古土壤中使用的三重氧同位素组成（16 O-17 O-18 O）的成壤碳酸盐。蒸发过程中同位素动力学的理论预测和其他地质材料中三重氧同位素的研究都表明，δ 17 O和δ 18 O之间的关系偏离参考线，用Δ′ 17 O参数评估，对蒸发是敏感的。作为在古成土碳酸盐中开发Δ′ 17 O应用的第一步，我们报告了来自全球分布的不同干旱环境（超干旱到潮湿）的47个近现代成土碳酸盐样品的Δ′ 17 O值。成壤碳酸盐的Δ′ 17 O值范围为−154至−60/meg（以CaCO 3计，通过O2，VSMOW-S测量），对应于计算的土壤水值为−66至+27/meg（VSMOW-S测量）（使用碳酸盐-水三重氧同位素分馏指数0.5250和凝聚同位素衍生的碳酸盐生长温度）。Δ′ 17 O值表明，土壤水蒸发改性形成成土碳酸盐是普遍的，特别是在干旱环境中。干旱环境中的土壤沃茨蒸发程度从高到低不等，形成成土碳酸盐，而潮湿环境中的土壤水蒸发程度很低，形成成土碳酸盐。在相同干燥度的环境中，Δ′ 17 O的变化可能与成土碳酸盐仅记录碳酸盐矿化时期的土壤条件有关，这可能偏离年度条件。因此，Δ′ 17 O可能有助于理解成土碳酸盐形成的具体情况，但可能无法提供环境干旱程度的无可争议的证据。用Δ′ 17 O可以检测成土碳酸盐δ 18 O值的蒸发改变，从而改进未蒸发沃茨δ 18 O的估算。我们的数据表明，蒸发必须（重新）考虑所有的古气候推断的基础上成壤碳酸盐的δ 18 O。Δ′ 17 O的加入将为使用（或避免使用）成土碳酸盐δ 18 O的古气候研究注入新的活力。
The stable isotopic composition of pedogenic carbonates is central to many studies of past climate and topography, providing a basis for our understanding of Earth's terrestrial history. A core assumption of many applications of oxygen isotope values (δ18O) of pedogenic carbonate is that they reflect the δ18O value of precipitation (rain/snow). This assumption is violated if soil carbonates form in evaporated soil waters. In this work, we develop a means to identify evaporation in ancient soils using the triple oxygen isotope composition (16O-17O-18O) of pedogenic carbonates. Both theoretical predictions of isotope kinetics during evaporation and studies of triple oxygen isotopes in other geological materials show that the deviation in the relationship between δ17O and δ18O from a reference line, evaluated using the parameter Δ′17O, is sensitive to evaporation. As a first step in developing the use of Δ′17O in ancient pedogenic carbonates, we report Δ′17O values from 47 near-modern pedogenic carbonate samples from globally distributed environments that vary in aridity (hyper-arid to humid). The Δ′17O values of pedogenic carbonate range from −154 to −60 per meg (as CaCO3, measured via O2, VSMOW-SLAP), corresponding to calculated soil water values of −66 to +27 per meg (VSMOW-SLAP) (using a carbonate-water triple oxygen isotope fractionation exponent of 0.5250 and clumped isotope-derived carbonate growth temperatures). The Δ′17O values indicate that evaporative modification of soil water from which pedogenic carbonate forms is common, especially in arid environments. Arid environments host pedogenic carbonates formed from soil waters ranging from highly to minimally evaporated, while humid environments host pedogenic carbonates formed from waters that are only minimally evaporated. The variability in Δ′17O within environments classified by the same aridity may relate to the fact that pedogenic carbonates record soil conditions only during times of carbonate mineralization, which may deviate from annual conditions. Thus, Δ′17O may be useful in understanding the specific circumstances of pedogenic carbonate formation but may not provide incontrovertible evidence of the magnitude of environmental aridity. Evaporative modification of δ18O values of pedogenic carbonates can be detected with Δ′17O, thereby improving estimates of δ18O of unevaporated waters. Our data show that evaporation must be (re)considered for all paleoclimate inferences based on the δ18O of pedogenic carbonate. The addition of Δ′17O will re-energize paleoclimate studies that use (or have avoided using) δ18O of pedogenic carbonate.