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.

厘清易受干旱影响的德克萨斯州内控制降水变率的气候过程，对于预测与减缓气候变化影响至关重要，而这需要重建仪器观测记录之外的古气候信息。本研究整合现有古气候代用资料与气候模拟结果，对全新世（Holocene）时期德克萨斯州的气候变率进行了系统性综述。气候条件在早全新世向中全新世过渡期间逐渐趋于暖干化，并在7至3千年前（ka）达到鼎盛阶段，晚全新世的气候变幅则更大。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严格约束。为解决这一问题，本研究报道了来自德克萨斯州中部一处洞穴的全新洞穴碳酸盐沉积物（speleothem，编号NBJ）重建序列，该序列为目前分辨率最高的古气候代用记录，时间跨度覆盖中全新世至晚全新世。NBJ的微量元素浓度数据显示，研究区水汽条件存在显著波动，且无明确的长期变化趋势。NBJ的生长速率、微量元素浓度与δ¹⁸O值之间存在解耦现象，这表明：（1）通常认为的洞穴碳酸盐沉积物生长速率与水汽可获得性之间的直接关联，可能因上覆生态系统变化影响地下CO₂产生而变得复杂；（2）洞穴碳酸盐沉积物的δ¹⁸O变化或反映水汽来源的改变（即太平洋来源水汽与墨西哥湾来源水汽的占比差异），且该变化似乎与水汽总量无关。