Isotopic and trace element compositions of Pennsylvanian brachiopods from northern New Mexico
H. -S. MII, T. E. Yancey, and E. L. Grossman
Stable isotope analyses of well preserved brachiopod shells collected from Missourian and Virgilian cyclic deposits in New Mexico yield information about paleoenvironmental variations in Late Pennsylvanian seas of North America. Samples were collected from shales or shales interbedded within limestones, found in the Manzano and Jemez Mountains of northern New Mexico. The brachiopods Crurithyris, Composita and Neospirifer were analyzed—genera which have been used extensively in studies of Pennsylvanian brachiopods in NorthAmerica. Well preserved shells were recognized by (1) the presence of original microstructure under plane-light microscope, (2) the absence of luminescence under cathodoluminescence microscope, and (3) the absence of detectable Mn and Fe concentrations for non-luminescent areas as measured by electron microprobe. Biological differences in Sr, Na and S concentrations (Crurithyris > Neospirifer > Composita) are preserved in the New Mexico samples and apparently record the original chemical signals. Isotopic analyses of non-luminescent areas of 77 New Mexico shells yield ð18O values from -4.5%o to -2.1%o and VC values from 2.2%o to 4.8%o. The average ð13C value of Composita is about Woo higher than that of co-occurring Crurithyris and Neospirifer, suggesting vital or microhabitat effects. Changes in ð180 and ð13C values generally do not correlate with water depth variations in these depositional cycles. Average ð13C and ð180 values of New Mexico Pennsylvanian brachiopods are lower than values of brachiopods in Texas or Kansas (Grossman et al., 1991, 1993). The lower ð18O values suggest warmer temperatures in New Mexico compared to Texas seas.
- MII, H. -S.; Yancey, T. E.; Grossman, E. L., 1996, Isotopic and trace element compositions of Pennsylvanian brachiopods from northern New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 189-197.