Data from: The application of an oxygen isotope aridity index to terrestrial paleoenvironmental reconstructions in Pleistocene North America

Geochemical tools, including the analysis of stable isotopes from fossil mammals, are often used to infer regional climatic and environmental differences. We have further developed an oxygen isotope aridity index and used oxygen (δ18O) isotope values and carbon (δ13C) isotope values to assess regional climatic differences between the southeastern and southwestern United States during the Pleistocene. Using data collected from previously published studies, we assigned taxa to evaporation-sensitivity categories by quantifying the frequency and magnitude of aridity index values (i.e., an average taxon δ18O value minus a site specific proboscidean δ18O value). Antilocapridae, Camelidae, Equidae, and Cervidae were identified as evaporation-sensitive families, meaning that a majority of their water comes from the food they eat, thus indicating that they are more likely to capture changing climatic conditions. Bovidae, Tayassuidae, and Tapiridae were identified as less sensitive families, possibly because of increased or more variable drinking behavior. While it is difficult to tease out individual influences on δ18O values in tooth enamel, the use of an aridity index will provide a more in-depth look at relative aridity in the fossil record. Greater aridity index values in the Southwest suggest a drier climate than in the Southeast during the Pleistocene, and δ13C values suggest that diet does not determine evaporation sensitivity. The combination of more-positive δ13C values and the lack of forest indicator taxa in the Southwest suggest that landscapes were more open than in the Southeast. Inferred higher aridity in the Southwest may indicate that aridity or seasonal aridity/precipitation, not temperature or pCO2, was a greater driver of C4 abundance during the Pleistocene. Collectively, these data suggest that regional climatic and environmental interpretations can be improved by using an aridity index and a more detailed understanding of mammalian paleobiology.

包括化石哺乳动物稳定同位素(stable isotopes)分析在内的地球化学手段，常被用于推断区域气候与环境差异。本研究进一步构建了氧同位素干旱指数(oxygen isotope aridity index)，并利用氧同位素(δ¹⁸O)与碳同位素(δ¹³C)数值，评估了更新世(Pleistocene)时期美国东南部与西南部的区域气候差异。
本研究基于已发表文献收集的数据，通过量化干旱指数数值的出现频率与幅度（即某一分类群(taxa)的平均δ¹⁸O值减去对应遗址长鼻类(proboscidean)的δ¹⁸O值），将不同分类群划分为蒸发敏感性类别(evaporation-sensitivity categories)。研究认定叉角羚科(Antilocapridae)、骆驼科(Camelidae)、马科(Equidae)与鹿科(Cervidae)属于蒸发敏感类群，这类动物的大部分水分来源于摄入的食物，因此更能反映气候条件的变化。牛科(Bovidae)、西猯科(Tayassuidae)与貘科(Tapiridae)则被划分为低敏感类群，这可能与其饮水行为更多或饮水行为波动更大有关。
尽管难以厘清牙釉质(tooth enamel)中δ¹⁸O值的各项独立影响因素，但干旱指数的应用能够更深入地揭示化石记录中的相对干旱程度。更新世时期美国西南部的干旱指数数值更高，表明其气候较东南部更为干旱；而δ¹³C数值则显示，饮食并非决定蒸发敏感性的因素。西南部δ¹³C值偏正程度更高，且缺乏森林指示类群(forest indicator taxa)，这表明其生境较东南部更为开阔。研究推断西南部干旱程度更高，这表明更新世时期，决定C4植物(C4)丰度的主要因素是干旱或季节性干旱/降水，而非温度或大气二氧化碳分压(pCO₂)。综合来看，这些数据表明，通过应用干旱指数并深化对哺乳动物古生物学(paleobiology)的认识，能够优化区域气候与环境的解读结果。