MAGNETIC AND GEOCHEMICAL VARIATIONS AS INDICATORS OF PALEOCLIMATE ANDARCHAEOLOGICAL SITE EVOLUTION - EXAMPLES FROM 41TR68, FORT-WORTH, TEXAS

The River Bend site, 41TR68, is located on the West Fork of the Trinit y 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 sig nificant magnetic anomalies. Test excavations were then conducted. Sev eral physical properties measurements, including magnetic susceptibili ty, 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 cul tural and palaeoclimatic controls during sediment accumulation. Distin ctive peaks in all parameters were exhibited by samples associated wit h hearths and shell concentrations. Broad magnetic susceptibility high s, indicative of high magnetic minerals concentrations, are closely as sociated with hearths and result from the production of new magnetic m inerals by chemical redox reactions during heating. Variations in red spectral reflectance percentages, indicative of hematite mineral conce ntration, appear to be the result of pedogenesis at the site and there fore are controlled primarily by climate. Susceptibility highs are cor related with reduced hematite concentrations, apparently resulting fro m the chemical reduction of hematite during heating. Calcium carbonate percentage peaks are indicative of shell debris concentrations from m ollusks which were an abundant food source at those times. Superimpose d 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 palaesols. While overall variation s appear to be the result of palaeoclimate, we interpret the local var iations associated with hearths to result from several factors. First, the presence of abundant carbonate and sulphur at the site has result ed in the formation of authigenic iron carbonate and iron sulphide min erals from dissimulatory iron reduction by bacteria during pedogenesis . Second, at low temperatures these iron carbonates and sulphides oxid ize to highly magnetic mineral phases, including magnetite and maghemi te, thus increasing the observed magnetic susceptibility in samples ac quired from the site. And third, at moderate temperatures, produced in sediments surrounding and beneath hearths, the abundant hematite at t he site is reduced to maghemite, thus further increasing the magnetic susceptibility in the samples collected.