Abstract

High-precision triple oxygen isotope analysis of water has given rise to a novel second-order parameter, ¹⁷O-excess (often denoted as Δ¹⁷O), which describes the deviation from a reference relationship between δ¹⁸O and δ¹⁷O. This tracer, like deuterium excess (d-excess), is affected by kinetic fractionation (diffusion) during phase changes within the hydrologic cycle. However, unlike d-excess, ¹⁷O-excess is present in paleowater proxy minerals and is not thought to vary significantly with temperature. This makes it a promising tool in paleoclimate research, particularly in relatively arid continental regions where traditional approaches have produced equivocal results. We present new δ¹⁸O, δ¹⁷O, and δ²H data from stream waters along two east–west transects in the Pacific Northwest to explore the sensitivity of ¹⁷O-excess to topography, climate, and moisture source. We find that discrepancies in d-excess and ¹⁷O-excess between the Olympic Mountains and Coast Range are consistent with distinct moisture source meteorology, inferred from air-mass back trajectory analysis. We suggest that vapor d-excess is affected by relative humidity and temperature at its oceanic source, whereas ¹⁷O-excess vapor is controlled by relative humidity at its oceanic source. Like d-excess, ¹⁷O-excess is significantly affected by evaporation in the rain shadow of the Cascade Mountains, supporting its utility as an aridity indicator in paleoclimate studies where δ²H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on d-excess and ¹⁷O-excess along altitudinal transects. We find that subcloud evaporation explains much, but not all of observed increases in d-excess with elevation and a minor amount of ¹⁷O-excess variation in the Olympic Mountains and Coast Range of Oregon.

1. Key Points

2. ¹⁷O-excess correlates spatially with relative humidity across the Pacific Northwest, supporting its use as an aridity indicator in paleoclimate studies.

3. Discrepancies in d-excess and ¹⁷O-excess between the Olympic Mountains and Oregon Coast Range suggest that their moisture source is different.

4. Subcloud evaporation explains most of observed increases in d-excess with elevation, and a minor amount of ¹⁷O-excess variation in the Olympic Mountains and Oregon Coast Range.

摘要

的水高精度三重氧同位素分析已经引起了一种新颖的二阶参数，¹⁷ O型过量（通常表示为Δ ¹⁷ O），其描述了从δ之间的引用关系的偏差¹⁸ O和δ ¹⁷ O.这种示踪剂，例如氘过量（d-过量），在水文循环内的相变过程中会受到动力学分级分离（扩散）的影响。但是，与d-过量不同，古水代用矿物中存在¹⁷ O-过量，并且认为它不会随温度而显着变化。这使其成为古气候研究的有前途的工具，特别是在相对干旱的大陆地区，这些地区的传统方法产生了模棱两可的结果。我们提出了新的δ ¹⁸O，δ ¹⁷ O，和δ ^2个从流水域沿着在西北太平洋2东西断面H数据探索的灵敏度¹⁷ O型过量来地形，气候和水分源。我们发现，奥林匹克山脉和海岸山脉之间的d过量和¹⁷ O过量之间的差异与从空气质量后向轨迹分析推断出的独特的水分源气象学是一致的。我们建议d过量的蒸气受其海洋源的相对湿度和温度的影响，而¹⁷ O过量的蒸气受其海洋源的相对湿度的控制。像d-excess，¹⁷O形过量通过在瀑布山的雨影蒸发显著影响，在支撑其可用作在古气候研究其中δ的干旱指示器² H数据是不可用的。我们使用雨滴蒸发模型和局部气象学来研究亚云蒸发沿垂直样带对d过量和¹⁷ O过量的影响。我们发现，亚云蒸发解释了很多原因，但并不是所有观察到的d超量随海拔升高而增加，在俄勒冈州奥林匹克山脉和俄勒冈州海岸山脉中都有少量的¹⁷ O过量变化。

1. 关键点

2. ¹⁷ O过量与西北太平洋地区的相对湿度在空间上相关，支持其在古气候研究中用作干旱指标。

3. 奥林匹克山脉和俄勒冈州海岸山脉之间的d过量和¹⁷ O过量的差异表明它们的水分来源不同。

4. 亚云蒸发解释了所观察到的大多数d过量随海拔的增加，以及奥林匹克山和俄勒冈州海岸山脉中少量的¹⁷ O过量变化。