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

[0074085]斯特恩对德克萨斯州中部和许多其他地区的土壤侵蚀历史知之甚少，但了解这段历史对研究环境变化和土壤保持工作很重要。我们建议量化霍尔洞穴附近土壤厚度的时间演变，该洞穴位于德克萨斯州科尔公司的爱德华兹高原地区。土壤演变将通过应用一种新技术进行量化，该技术利用洞穴填充沉积物中保存的哺乳动物化石的锶同位素组成（87Sr/86Sr）进行量化。霍尔洞穴的沟状沉积物序列包含丰富的古生物记录，并为洞穴中过去16,000年的沉积建立了高分辨率年表。由于大约8000年前爱德华兹高原的土壤被侵蚀，来自石灰岩基岩（87Sr/86Sr值低）的锶对土壤中硅酸盐（87Sr/86Sr值高）的锶交换库的相对贡献应该有所增加。因此，生活在景观上的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，然后将其与温度、湿度和土壤特征变化的独立化学和生物代用物进行比较。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约束；(3)牙釉质和腐殖质的碳同位素比值代表了古生态系统中C4草的比例；(4)地层孢粉学；(5)以丰富的脊椎动物区系为标志的古生态。这些环境指标与土壤侵蚀时间和速率的时间相关性，如Sr同位素变化所示，将为评估严重土壤侵蚀的原因提供必要的证据，这些土壤侵蚀使德克萨斯州中部的景观失去了曾经厚厚的土壤。

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.