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

在世界上许多干旱和半干旱地区，含有自生粘土的岩钙土层覆盖着地貌表面，它们也作为不整合出现在岩石记录中。自生海泡石和凹凸棒石粘土形成于II期碳酸盐悬垂和III-VI期岩石钙化层中。准确测年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方法，利用40Ar/39Ar年代学从岩石钙化层中测定自生海泡石和坡缕石的年龄。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，并可能有助于量化土壤发生、景观演变和气候变化的速率。如果成功，我们的方法不仅适用于现存的岩石钙质土壤景观，也适用于古土壤记录。缺乏初步的成功仍将加强进一步的土壤地质年代学研究。我们建议分析来自两个年代良好的岩钙土壤地区的样品：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、参考的海泡石和坡缕石样品。我们将从1)胶结的、大块的土壤样品和2)微钻孔的次样品中提取海泡石和坡缕石。成功的微钻探方法可能会加速40Ar/39Ar测年的样品制备，因为新的微形态证据表明，自生粘土经常形成于浓缩的、宏观可见的纹层中。在每个提取步骤之后，我们将使用扫描电子显微镜、X射线衍射仪和电感耦合等离子体发射光谱仪对样品等分进行分析，以确定或消除对粘土晶格完整性或K含量的不利影响。透射电子显微镜分析将限制提取粘土的成分可变性。成功提取的粘土的40Ar/39Ar测量将在内华达州同位素地质年代学实验室(NIGL)使用真空封装进行。我们最终的40Ar/39Ar数据和土壤年龄解释将与摩门台地和拉梅萨表面已建立的年代和地形地层学年龄进行比较。在我们的项目结束时，我们将回答以下问题：(1)化学去除成壤碳酸盐、无定形二氧化硅、(2)岩钙样品中纤维状粘土的显微钻孔是否影响晶体完整性或晶体内K和Ar含量？(3)需要多大质量的海泡石或凹凸棒石才能将40Ar/39Ar的不确定性降至最低？(4)成土海泡石/凹凸棒石的40Ar/39Ar年代学是一种可行的岩石钙质土壤地貌表面数字测年方法吗？这项研究的更广泛影响包括博士生Colin Robins的毕业论文研究和研究生培训以及本科生(可能是女性)的培训。NIGL的工作人员，包括研究生研究助理，将改进测定细粒材料年代的方法和知识。这将加强其他尖端研究应用，例如确定断层泥的年代，以确定地壳浅断层的运动时间。与NMSU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.