RADIOGENIC ISOTOPE COMPOSITION OF CARBONIFEROUS SEAWATER – NORTH AMERICAN TIME SERIES AND GEOGRAPHIC TRANSECT

In an effort to better understand the causes and consequences of climate change during the Carboniferous, we investigate the degree to which the Carboniferous North American epicontinental sea recorded global open-ocean conditions. We have analyzed radiogenic isotope tracers of varying residence times (Sr and Nd) along a transect spanning the Mixteco terrane in present-day southwestern Mexico (most oceanic), Arrow Canyon in NV, the US Midcontinent in KS, the Illinois Basin, and the Appalachian Basin (most interior) to constrain the degree of geochemical coupling between the epicontinental seas and the open ocean (Panthalassa and Rheic Ocean). We have used long time series from Mexico and Nevada to provide a gauge of the evolution of open-ocean compositions through the Carboniferous.

Sr isotope analyses of conodont material from Patlanoaya and Arrow Canyon yield mixed results, likely due to the difference in apatite preservation of the seawater Sr signal versus Nd. Patlanoaya samples yielded values more radiogenic than contemporaneous seawater, suggesting significant alteration. In addition, many of the Arrow Canyon conodont ⁸⁷Sr/⁸⁶Sr values were higher than those recorded by well preserved brachiopods, reflecting the tendency of biogenic apatite to exchange Sr with pore fluids after burial in carbonate-rich sediments.

In contrast, the Nd isotopic composition of biogenic apatite resists diagenesis because the [Nd] in apatite is several orders of magnitude higher than that in porewaters.

Nd isotope values (expressed in epsilon notation and corrected for age) from Patlanoaya, Mexico spanning the upper Mississippian to lower Permian range from –7.1 to –5.1. Upper Mississippian to Pennsylvanian values from Arrow Canyon, NV range from –10.1 to –7.5. These time series provide the context for generating time slices across North America. Gzhelian age conodonts spanning the geographic transect across North America indicate a decrease in ε_Nd(t) values from west to east, progressing from –6.3 to –16.8. The less radiogenic Nd of the continental interior is likely due to local/regional inputs from very old crustal rocks as well as more restricted communication with Panthalassa.