SIGNIFICANCE OF d13C, d18O, AND STRONTIUM CONTENT IN THE IDENTIFICATION OF SURFACES OF SUBAERIAL EXPOSURE IN ORDOVICIAN LIMESTONES OF THE NASHVILLE DOME, TENNESSEE, U.S.A
Variation between vertical transects suggests that dissolution of aragonite is not laterally uniform and implies that lateral replicates must be taken in order to characterize the isotopic composition of a horizon. Confidence limits determined for the mean d13C of each stratigraphic horizon indicate that negative excursions in d13C are statistically significant below sequence-stratigraphically defined subaerial exposure surfaces, such as the M4-M5 3rd order sequence boundary and peritidal-capped cycles. Variance in d13C is greatest below exposure surfaces, suggesting that meteoric alteration is not uniform along such surfaces. These data also suggest the presence of a third exposure surface that lacks any macroscopic evidence of subaerial exposure.
Analysis of oxygen isotopes have previously had limited success in identifying exposure surfaces. In this study, confidence limits on the mean oxygen isotope composition of each horizon indicate that d18O increases upward to both the parasequence boundary at -120 cm and the M4-M5 sequence boundary as a result of calcite precipitation from isotopically heavy water. d18O decreases upward toward the erosional surface located at -70 cm, which suggests a short period of precipitation of calcite from isotopically lighter water.
Weight percent Sr displays low mean values and low variance beneath the M4-M5 sequence boundary and the peritidal-capped cycle. Low variance in a diagentically altered section is expected because removal of Sr only requires the fluid be undersaturated with respect to aragonite and that a fluid pathway for migration exists. Low variance in Wt% Sr suggests that removal of Sr during meteoric diagenesis is relatively uniform.