RADIOGENIC ISOTOPE GEOCHEMISTRY OF CARBONIFEROUS DETRITAL SEDIMENTS, ARROW CANYON, NEVADA

Global climate underwent a fundamental transition from greenhouse to icehouse state over the course of the Carboniferous Period, and this transition may have impacted terrestrial weathering processes. To investigate the nature of such weathering changes in the tropics, we generated a record of detrital sediment geochemistry (Sr and Nd isotopes) spanning the upper Mississippian (Chesterian) to upper Pennsylvanian (Desmoinesian) at Arrow Canyon, Nevada. The resulting Sr isotope values range from 0.709429 to 0.730182, with the exception of one data point that was considerably more radiogenic (0.739551). The ⁸⁷Sr/⁸⁶Sr values show no apparent trend upsection and all values are significantly more radiogenic than contemporaneous seawater, suggesting that the regional detrital input to the basin differed from the dissolved Sr inventory of well-mixed seawater. Nd isotope values, expressed in epsilon notation and corrected for age, vary from –5.8 to –9.8 (except one sample that had an ε_Nd(t) value of -0.3). There is a subtle trend toward more unradiogenic Nd isotope values with stratigraphic age, with upper Pennsylvanian values generally more unradiogenic than older samples.

A crossplot of ⁸⁷Sr/⁸⁶Sr vs ¹⁴³Nd/¹⁴⁴Nd suggests mixing of detrital sediments from two potential end members. In our data set, the end members range from a relatively radiogenic Sr/unradiogenic Nd composition of 0.730182/0.511757 to a relatively unradiogenic Sr/radiogenic Nd value of 0.709429/0.511935. This spread in values implies mixing between an older continental source and a younger volcanic source(s). Weathering inputs from the Antler Orogen to the west of the Arrow Canyon basin likely contributed the more radiogenic Sr/unradiogenic Nd signature, while arc volcanic inputs from the south or west may have contributed the relatively unradiogenic Sr/radiogenic Nd component(s). We will test these end member sources and develop a quantitative mixing curve with additional isotope data as well as Sr, Rb, Nd and Sm concentration data. However, results to date do not suggest any change in provenance that correlates to overall climate state.