URANIUM CONCENTRATION AND ISOTOPE RATIO PROFILES WITHIN MERCENARIA SHELLS - GEOCHRONOLOGICAL IMPLICATIONS

Shells of nineteen specimens of Mercenaria from unconsolidated sedimen ts of the southeastern USA coast, ranging in age from alive to Pliocen e, were each separated into three to six sections parallel to the shel l surface. The uranium concentration and isotope ratio of each subsamp le was determined. The results confirm those of previous studies that whole fossil shells have about 8 times as much uranium (300 vs. 40 ppb ) and considerably higher U-234/U-238 ratios than do whole live-collec ted shells. However, it was also found that uranium concentrations fal l very steeply (by factors of 300 and 15 in live and fossil shells, re spectively) from the exterior of the shell to about two-thirds of the shell thickness. Between this depth and the interior surface, the uran ium concentrations of live shells increase by about a factor of 5 whil e fossils have gradients which vary from moderately positive to modera tely negative. The most surprising finding of the study is that the in terior portions of the shells become closed chemical systems by no mor e than about 15 ka after their death. This is known from the observati on that these portions of Late Pleistocene shells still have oceanic u ranium isotope ratios while the exterior portions of the same shells h ave ratios which reflect an uptake of uranium from continental groundw aters (the water the shells have been in contact with for the vast maj ority of their existence). Thus, the U-series ages of the interior por tions of Mercenaria shells may be valid after a small correction. Thou gh shells older than about 200 ka seem less promising, firm conclusion s cannot be drawn regarding them because of the great scatter in the d ata.