Nd Isotopes from North American Epicontinental Seas: Carboniferous Ocean Chemistry and Inter-Basinal Circulation

We present new Carboniferous seawater Nd isotope data from North American epicontinental seaway deposits recorded by the apatite of fossilized conodont elements and fish debris. Nd isotope ratios are useful indicators of water mass composition. The primary source of Nd to the ocean is through rivers and continental runoff. As a result, variations in Nd isotopes in seawater reflect the composition of the continental source being drained into a water mass formation region. Due to the modern oceanic residence time of Nd (approximately 1000 years), which is slightly less than the mixing time of modern global oceans, the ε_Nd (Nd isotope composition normalized to the bulk earth value) of different oceanic water masses reflects the provenance of the water mass formation region. The Nd chemical signatures of coastal and enclosed/semi-enclosed marine systems may be more complicated. These settings have limited circulation, so the Nd isotopic fingerprint likely reflects weathering inputs from the regional geologic setting which can be diluted by the influx of open marine water masses. Therefore, semi-enclosed basins with limited sources of recharge may develop geochemical gradients that can be used as indicators of inter-basinal communication.

Samples collected from the U.S. mid-continent (Iowa, Illinois, Indiana and Texas), North American Cordillera (Arrow Canyon, Nevada), and Mexico (San Salavador – Patlanoaya), provide a general transect stretching from the North American mid-continent west and south to Panthalassa. Kinderhookian to Virgilian age conodonts from the mid-continent yield ε_Nd(t) values (Nd isotope ratios corrected for depositional age) ranging from -3.5 to -6.3, with the most radiogenic values found in Indiana. These values likely reflect a mixture of regional weathering inputs and advected seawater. Mississippian and Pennsylvanian sedimentary rocks in the Ouachita Mountains provide one gauge of regional weathering inputs, and record a wide range of ε_Nd(t) values (-2 to -10) (Gleason et al., 1995).