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National Science Foundation (USA) 2007

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

Arid Soil Dating

NATIONAL SCIENCE FOUNDATION

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

Organismes NSF : Division of Earth Sciences (EAR)

Durée : August 1, 2007 - July 31, 2008

Description
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 ?

Partenaires : Brenda Buck buckb unlv.nevada.edu (Principal Investigator) Terry Spell (Co-Principal Investigator)

Financement : $29,936.00

Présentation (National Science Foundation )

Page publiée le 9 juillet 2017, mise à jour le 5 novembre 2017