SGER: Novel Approach to Arid Soil Dating: Extraction and 40Ar/39Ar Geochronology of Pedogenic Sepiolite and Palygorskite

SGER：干旱土壤测年新方法：成土海泡石和坡缕石的提取和 40Ar/39Ar 地质年代学

• 批准号: 0715030
• 负责人: Brenda Buck
• 金额: $ 2.99万
• 依托单位: University of Nevada Las Vegas
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-08-01 至 2008-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0715030&HistoricalAwards=false
• 关键词: SGER; Novel; Approach; Arid; Soil

Petrocalcic soil horizons containing authigenic clays cap geomorphic surfaces in many arid and semi-arid regions worldwide, and they also occur in the rock record as unconformities. Authigenic sepiolite and palygorskite clays form in stage II carbonate pendants and stage III - VI petrocalcic horizons. Accurately dating petrocalcic soils remains one of the largest scientific hurdles in both geomorphology and sedimentology.We propose an untested, novel approach to date authigenic sepiolite and palygorskite from petrocalcic horizons using 40Ar/39Ar geochronology. Our theoretical considerations suggest that the techniques we present herein are feasible, are applicable to many soils worldwide, and may help quantify rates of pedogenesis, landscape evolution, and climate change. If successful, our methodology would apply not only to extant petrocalcic soil landscapes, but also to the paleosol record. Lack of initial success will still enhance further soil geochronology research.We propose to analyze samples from two well-dated petrocalcic soil localities: Mormon Mesa, NV, and La Mesa, NM. We will also study purchased, reference sepiolite and palygorskite samples. We will extract sepiolite and palygorskite from 1) cemented, bulk soil samples and 2) microdrilled subsamples. Successful microdrill methods may expedite sample preparation for 40Ar/39Ar dating, given new micromorphological evidence (Brock & Buck, 2006) that authigenic clays often form within concentrated, macroscopically visible laminae. After each extraction step, we will analyze sample aliquots using SEM, XRD, and ICP-OES to identify or discount adverse effects on clay crystal lattice integrity or K content. TEM analyses will constrain compositional variability of extracted clays. 40Ar/39Ar measurements on successfully extracted clays will be made at the Nevada Isotope Geochronology Laboratory (NIGL) using vacuum encapsulation. Our final 40Ar/39Ar data and soil age interpretations will be compared to established tephrachonologic and morphostratigraphic ages for the Mormon Mesa and La Mesa surfaces. At the conclusion of our project, we will have answered the following questions: (1) Does chemical removal of pedogenic carbonate, amorphous silica, and iron-oxide cements affect crystal integrity or intracrystalline K and Ar content of fibrous clay minerals? (2) Does microdrilling of fibrous clays from petrocalcic samples affect crystal integrity or intracrystalline K and Ar content? (3) What mass of sepiolite or palygorskite is needed to minimize 40Ar/39Ar uncertainty? (4) Is 40Ar/39Ar geochronology of pedogenic sepiolite/palygorskite a viable means of numerically dating petrocalcic soilgeomorphologic surfaces?Broader impacts of this research include the dissertation research and graduate training of a Ph.D. student, Colin Robins, and the training of a (probable female) undergraduate student. NIGL personnel, including graduate research assistants, will improve methodologies and knowledge of dating fine-grained materials. This will enhance other cutting-edge research applications such as dating fault gouge to determine timing of motion on shallow crustal faults. Collaboration with NMSU personnel will strengthen arid-lands research on a regional scale.

含有自生粘土的岩钙土层覆盖了全世界许多干旱和半干旱地区的地貌表面，它们也作为不整合面出现在岩石记录中。自生海泡石和坡缕石粘土形成于 II 期碳酸盐悬垂物和 III - VI 期钙质层中。准确测定含钙土壤的年代仍然是地貌学和沉积学中最大的科学障碍之一。我们提出了一种未经测试的新颖方法，利用 40Ar/39Ar 地质年代学来测定含钙层中自生海泡石和坡缕石的年代。我们的理论考虑表明，我们在此提出的技术是可行的，适用于世界各地的许多土壤，并且可能有助于量化成土、景观演化和气候变化的速率。如果成功，我们的方法不仅适用于现存的岩钙土壤景观，还适用于古土壤记录。缺乏初步成功仍将加强进一步的土壤地质年代学研究。我们建议分析两个年代明确的含钙土壤地点的样本：内华达州摩门梅萨和新墨西哥州拉梅萨。我们还将研究购买的参考海泡石和坡缕石样品。我们将从 1) 胶结的大块土壤样品和 2) 微钻孔子样品中提取海泡石和坡缕石。成功的微钻方法可以加快 40Ar/39Ar 测年的样品制备，因为新的微形态证据（Brock & Buck，2006）表明自生粘土通常在集中的、宏观可见的纹层中形成。每个提取步骤后，我们将使用 SEM、XRD 和 ICP-OES 分析样品等份，以确定或消除对粘土晶格完整性或 K 含量的不利影响。 TEM 分析将限制提取粘土的成分变化。内华达同位素地质年代学实验室 (NIGL) 将使用真空封装对成功提取的粘土进行 40Ar/39Ar 测量。我们最终的 40Ar/39Ar 数据和土壤年龄解释将与已确定的摩门台地和拉台地表面的地层学和地貌年龄进行比较。在我们的项目结束时，我们将回答以下问题：（1）化学去除成土碳酸盐、无定形二氧化硅和氧化铁胶结物是否会影响纤维状粘土矿物的晶体完整性或晶内 K 和 Ar 含量？ (2) 对岩钙样品中的纤维粘土进行微钻孔是否会影响晶体完整性或晶内 K 和 Ar 含量？ (3) 需要多少质量的海泡石或坡缕石才能最大限度地减少 40Ar/39Ar 的不确定性？ (4) 成土海泡石/坡缕石的 40Ar/39Ar 地质年代学是否是对含钙土壤地貌表面进行数值测年的可行方法？这项研究的更广泛影响包括博士论文研究和研究生培训。学生科林罗宾斯，以及一名（可能是女性）本科生的培训。 NIGL 人员，包括研究生研究助理，将改进细粒度材料测年的方法和知识。这将增强其他前沿研究应用，例如测定断层泥的年代，以确定浅层地壳断层的运动时间。与新墨西哥州立大学人员的合作将加强区域范围内的干旱地区研究。