A New Approach to Understanding the Timing, Rates, and Causes of Quaternary Soil Denudation

得克萨斯州中部和许多其他地区的土壤侵蚀历史鲜为人知，但了解这段历史对于研究环境变化和土壤保护工作很重要。我们建议量化霍尔洞附近土壤厚度的时间演变，霍尔洞位于德克萨斯州科尔公司的爱德华兹高原。土壤演化将通过应用一种新的技术来量化，该技术使用锶同位素组成(87Sr/86Sr)，其中包括保存在洞穴充填沉积物中的哺乳动物化石。霍尔洞的一条沟状沉积物序列包含了丰富的古生物记录，并为过去16,000年的洞穴沉积建立了高分辨率的年代学。由于土壤从8000年前开始受到爱德华兹高原的侵蚀，来自石灰岩基岩(低87Sr/86Sr值)的可交换锶矿库的相对贡献应该相对于土壤中的硅酸盐(高87Sr/86Sr值)有所增加。因此，景观上生物群的锶同位素比值会随着时间的变化而变化，反映土壤中可交换锶库的87Sr/86Sr值，从而反映土壤厚度。我们将构建土壤厚度的历史，然后将其与独立的化学和生物指标进行比较，以改变温度、水分和土壤特性。具体指标包括：(1)洞穴沉积物的矿物学和元素化学对土壤侵蚀历史的制约；(2)化石(浆果、蜗牛和洞穴)的氧同位素比值作为过去降水模式的记录；(3)牙釉质和腐殖质的碳同位素比值作为C4草在古生态系统中的比例的替代；(4)地层的孢粉学；以及(5)丰富的脊椎动物群所指示的古生态。这些环境指标与土壤侵蚀时间和速率的时间相关性，如锶同位素变化所表明的，将为评估剥蚀德克萨斯州中部曾经厚实土壤的严重土壤侵蚀的原因提供必要的证据。

0074085Stern The history of soil erosion in central Texas and many other regions is poorly known, yet understanding this history is important for studies of environmental change and efforts in soil conservation. We propose to quantify the temporal evolution of soil thickness in the vicinity of Hall's Cave, which is located in the regionally-extensive Edwards Plateau in Kerr Co., TX. Soil evolution will be quantified through application of a novel technique the uses the strontium isotope composition (87Sr/86Sr of mammalian fossils preserved in the cave-filling sediment. A trenched sediment sequence at Hall's Cave contains a rich paleontologic record, and a high resolution chronology was constructed for the past 16,000 years of deposition in the cave. As soils were eroded from the Edwards Plateau beginning around 8,000 years ago, the relative contribution to the exchangeable strontium pool of Sr derived from the limestone bedrock (low 87Sr/86Sr values) should have increased with respect to the silicates (high 87Sr/86Sr values) in the soil. Therefore, the strontium isotope ratio of biota living on the landscape will change through time and reflect the 87Sr/86Sr value of the exchangeable Sr pool in the soil, and thus the soil thickness. WE will construct a history of soil thickness, which we will then compare to independent chemical and biotic proxies for changing temperature, moisture, and soil character. Specific proxies included: (1) the mineralogy and elemental chemistry of the cave sediments as a constraint on the soil erosion history; (2) the oxygen isotope ratio of fossils (backberries, snails and speleothems as a record of precipitation patterns in the past; (3) the carbon isotope ratio of tooth enamel and humus as a proxy for the proportion of C4 grass in the paleoecosystem; (4) the palynology of the strata; and (5) the paleoecology indicated by the rich vertebrate fauna. Temporal correlation of these environmental proxies with the soil erosion timing and rate, as indicated by the Sr isotope variations, will provide essential evidence to evaluate the causes of the profound soil erosion that denuded the central Texas landscape of its once thick soil.