Magnetic and Geochemical Variations as Indicators of Palaeoclimate and Archaeological Site Evolution: Examples from 41TR68, Fort Worth, Texas

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The River Bend site, 41TR68, is located on the West Fork of the Trinity River near Fort Worth, Texas, in the Upper Trinity River floodplain. A magnetometer survey was performed at this site to identify hearths, and utilizing these data backhoe trenches were placed adjacent to significant magnetic anomalies. Test excavations were then conducted. Several physical properties measurements, including magnetic susceptibility, an indicator of magnetic mineral concentration, calcium carbonate percentages, and percent spectral reflectance in the red colour range were performed on samples from stratigraphic sections adjacent to two excavation units at the site. The data are interpreted in terms of cultural and palaeoclimatic controls during sediment accumulation. Distinctive peaks in all parameters were exhibited by samples associated with hearths and shell concentrations. Broad magnetic susceptibility highs, indicative of high magnetic mineral concentrations, are closely associated with hearths and result from the production of new magnetic minerals by chemical redox reactions during heating. Variations in red spectral reflectance percentages, indicative of hematite mineral concentration, appear to be the result of pedogenesis at the site and therefore are controlled primarily by climate. Susceptibility highs are correlated with reduced hematite concentrations, apparently resulting from the chemical reduction of hematite during heating. Calcium carbonate percentage peaks are indicative of shell debris concentrations from mollusks which were an abundant food source at those times. Superimposed on these peaks are broad variations in calcium carbonate percentages which reflect palaeosol development at the Riverbend Site and suggest the presence of several indistinct palaeosols. While overall variations appear to be the result of palaeoclimate, we interpret the local variations associated with hearths to result from several factors. First, the presence of abundant carbonate and sulphur at the site has resulted in the formation of authigenic iron carbonate and iron sulphide minerals from dissimulatory iron reduction by bacteria during pedogenesis. Second, at low temperatures these iron carbonates and sulphides oxidize to highly magnetic mineral phases, including magnetite and maghemite, thus increasing the observed magnetic susceptibility in samples acquired from the site. And third, at moderate temperatures, produced in sediments surrounding and beneath hearths, the abundant hematite at the site is reduced to maghemite, thus further increasing the magnetic susceptibility in the samples collected. © 1995 Academic Press Limited.

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Journal of Archaeological Science

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