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，含自生粘土的岩钙土层覆盖着地貌表面，它们也以不整合面形式出现在岩石记录中。自生海泡石和坡缕石粘土形成于II期碳酸盐岩垂坠和III - VI期油钙层。在地貌学和沉积学中，准确测定岩钙土壤的年代仍然是最大的科学障碍之一。我们提出了一种未经测试的新方法，使用40Ar/39Ar地质年代学来确定自生海泡石和坡缕石的年代。我们的理论考虑表明，我们在这里提出的技术是可行的，适用于全球许多土壤，并可能有助于量化土壤形成，景观演变和气候变化的速度。如果成功，我们的方法将不仅适用于现存的石钙土壤景观，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。我们建议分析两个年代久远的石钙土壤地点的样本：内华达州的摩门教台地和新墨西哥州的拉台地。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)胶结土样、散装土样和2)微钻孔亚样中提取海泡石和坡纹石。成功的微钻方法可以加快样品制备，用于40Ar/39Ar测年，因为新的微观形态学证据（Brock & Buck, 2006）表明自生粘土通常形成于集中的、宏观可见的层状中。在每个萃取步骤后，我们将使用SEM， XRD和ICP-OES分析样品等分，以确定或排除对粘土晶格完整性或K含量的不利影响。透射电镜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）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摩门教台地和拉台地表面的已建立的地层和形态地层年龄进行比较。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性粘土矿物的晶体完整性或晶内K和Ar含量？(2)从岩钙样品中微钻纤维粘土是否影响晶体完整性或晶内K和Ar含量？(3)要使40Ar/39Ar的不确定度降到最低，需要多大质量的海泡石或坡缕石？(4)成土海泡石/坡缕石的40Ar/39Ar年代学是否是一种可行的岩钙土壤地貌面数值定年方法？这项研究的更广泛的影响包括博士研究生的论文研究和研究生培训，科林·罗宾斯，和一个（可能是女性）本科生的培训。NIGL人员，包括研究生研究助理，将改进细粒度材料定年的方法和知识。这将加强其他前沿研究应用，如断层泥定年，以确定浅层地壳断层的运动时间。与国立大学工作人员的合作将加强区域范围内的旱地研究。

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.