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

在世界范围内的许多干旱和半干旱地区，含有自生粘土的岩钙土覆盖着地貌表面，它们也以不整合面的形式出现在岩石记录中。自生的海泡石和坡缕石粘土形成于第II阶段碳酸盐悬垂层和第III-VI阶段岩钙层。准确测年岩钙土仍然是地貌学和沉积学中最大的科学障碍之一，我们提出了一个未经测试的，新的方法，从岩钙层自生海泡石和孢粉使用40 Ar/39 Ar地质年代学。我们的理论考虑表明，我们在这里提出的技术是可行的，适用于世界各地的许多土壤，并可能有助于量化成土作用，景观演变和气候变化的速率。如果成功的话，我们的方法将不仅适用于现存的岩钙土壤景观，而且对古土壤记录。缺乏初步的成功仍将进一步提高土壤地质年代学研究。我们建议分析样本从两个良好的日期岩钙土壤的地方：摩门教梅萨，内华达州和拉梅萨，NM。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）微钻子样品中提取海泡石和坡缕石。成功的微钻方法可以加快40 Ar/39 Ar测年的样品制备，给出了新的微形态学证据（Brock Buck，2006），即自生粘土通常形成于集中的、宏观可见的纹层内。在每个浸提步骤后，我们将使用SEM、XRD和ICP-OES分析样品等分试样，以确定或排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异性。将在内华达州同位素地质年代学实验室（NIGL）采用真空封装法对成功提取的粘土进行40 Ar/39 Ar测量。我们最终的40 Ar/39 Ar数据和土壤年龄的解释将进行比较，以建立tephrachonologic和morphostructural年龄的摩门教梅萨和拉梅萨表面。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影响纤维状粘土矿物的晶体完整性或晶内K和Ar含量？(2)从岩钙样品中对纤维状粘土进行微钻是否会影响晶体完整性或晶内K和Ar含量？(3)需要多大质量的海泡石或坡缕石才能使40 Ar/39 Ar的不确定性最小化？(4)成土海泡石/坡缕石的40 Ar/39 Ar年代学是一种可行的岩石钙质土壤地貌面数字定年方法吗？本研究的更广泛影响包括博士学位论文研究和研究生培养。学生科林·罗宾斯和一名（可能是女）本科生的训练。NIGL人员，包括研究生研究助理，将改进方法和知识的约会细粒材料。这将增强其他前沿研究应用，如确定断层泥年代以确定地壳浅层断层运动的时间。与国立密西根州立大学人员的合作将加强区域范围内的干旱土地研究。

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.