NOAA/WDS Paleoclimatology - Natural Bridge Caverns, Texas 6700 Year Speleothem Stable Isotope Data

Delineating the climate processes governing precipitation variability in drought-prone Texas is critical for predicting and mitigating climate change effects, and requires the reconstruction of past climate beyond the instrumental record. We synthesize existing paleoclimate proxy data and climate simulations to provide an overview of climate variability in Texas during the Holocene. Conditions became progressively warmer and drier transitioning from the early to mid Holocene, culminating between 7 and 3 ka (thousand years ago), and were more variable during the late Holocene. The timing and relative magnitude of Holocene climate variability, however, is poorly constrained owing to considerable variability among the different records. To help address this, we present a new speleothem (NBJ) reconstruction from a central Texas cave that comprises the highest resolution proxy record to date, spanning the mid to late Holocene. NBJ trace-element concentrations indicate variable moisture conditions with no clear temporal trend. There is a decoupling between NBJ growth rate, trace-element concentrations, and d18O values, which indicate that (i) the often direct relation between speleothem growth rate and moisture availability is likely complicated by changes in the overlying ecosystem that affect subsurface CO2 production, and (ii) speleothem d18O variations likely reflect changes in moisture source (i.e., proportion of Pacific-vs. Gulf of Mexico-derived moisture) that appear not to be linked to moisture amount.

厘清干旱频发的得克萨斯州境内调控降水变率的气候过程，对于预测并减缓气候变化影响至关重要，同时需要重建仪器观测记录之外的古气候序列。本研究整合现有古气候代用资料（paleoclimate proxy data）与气候模拟结果，对全新世（Holocene）期间得克萨斯州的气候变率特征进行了系统梳理。全新世早中期，区域气候逐步向暖干方向演变，并在7至3 ka（千年前，thousand years ago）达到气候极值；晚全新世时期气候变率显著增强。然而，由于不同代用记录间存在显著差异，全新世气候变率的发生时序与相对强度仍缺乏精确约束。为解决这一科学问题，本研究报道了来自得克萨斯州中部一处洞穴的新型石笋（speleothem，NBJ）重建序列，该序列是目前分辨率最高的全新世中晚期代用记录。NBJ的微量元素浓度显示区域水汽条件存在显著波动，无明确的长期变化趋势。研究发现NBJ的生长速率、微量元素浓度与δ¹⁸O值（d18O values）之间存在解耦现象，这表明：（i）通常认为的石笋生长速率与水汽可获得性之间的直接关联，可能因影响地下二氧化碳生成的上覆生态系统变化而变得复杂；（ii）石笋δ¹⁸O值的变化大概率反映了水汽来源的改变（即太平洋与墨西哥湾来源水汽的占比变化），而这类变化似乎与水汽总量无关。