Abstract

Field study and radiocarbon dating of authigenic carbonate in arid-zone paleosols and calcretes have been carried out in Australia to determine the reliability of this method of soil dating.

Three calcareous paleosols of late Quaternary age are developed within alluvial and eolian deposits on the Lake Torrens plain in arid South Australia. Dates for organic carbon obtained from the sediments show that soil-forming episodes took place shortly after 30,000 yrs B.P., between about 16,000 and 12,000 yrs B.P., and between 6,000 and 1,500 yrs B.P. Respective mean radiocarbon ages determined for pedogenic carbonate are (with approximate scatter) 27,450 (± 7,000) yrs B.P., 13,900 (± 2,000) yrs B.P., and 7,750 yrs (± 1,000) B.P. Although the great majority of the individual carbonate dates and the three mean dates are sound as relative ages, they are less reliable as absolute ages.

A review of the literature reveals that most carbonate dates for arid-zone paleosols and calcretes which have been dated by coexisting organic carbon are from about 500 to 7,000 radiocarbon years too old. This is attributed to an initial low C14/C12 ratio in soil carbonate, reflecting low C14 content of source carbonate. Postformation contamination with C14 commonly appears to be minimal in the arid zone, where dates for micritic carbonate from well-drained sites should approximate to or be up to several thousand years older than true ages of soil formation. Age correction based on δC13 values for soil carbonate is invalid.

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