Should Stable Isotope Values of Pedogenic Carbonates Be Used in Paleoclimatic Reconstructions? The Effects of Diagenesis on the Geochemistry of Pedogenic Carbonates

Pauline Deutz, Isabel P. Montanez, Greg H. Mack, H. Curtis Monger

Paleoclimatological and paleoenvironmental reconstructions commonly are based on "whole-rock" isotopic analysis of individual pedogenic carbonate components. These studies, however, overlook the potential effects of diagenesis on the microfabrics and geochemical compositions of pedogenic and paleosol carbonates. Recrystallization and/or overgrowth of low Mg calcite by diagenetic fluids may result in changes in the stable isotope values and elemental contents of these carbonates, thus their geochemical compositions record their fluid-rock interaction history rather than the physio-chemical conditions that governed pedogenesis. This study of soil and paleosol carbonates from terrigenous siliciclastics in southern New Mexico investigates the textural and geochemical heteroge eity of pedogenic carbonates and the effects of diagenetic modification on their textural and geochemical composition.

Standard petrography, epi-fluorescence, cathodoluminescence, and scanning electron microscopy all indicate that the textures associated with pedogenic carbonates in soils are not perfectly preserved in paleosols. Soil carbonates are brightly fluorescent, nonluminescent and display such textures as needle-fiber calcite, peloids, and fine grained micrite. Paleosol carbonates are less brightly fluorescent, variably luminescent, contain ferroan calcite cements, and are predominantly coarser micrite and microspar. These textural differences suggest a strong possibility of geochemical overprinting, which has been further evaluated by carrying out coupled, high-resolution microtextural, stable isotopic and geochemical studies. Variations in the stable isotope values (1 to 3^pmil) of multiple microsamples collected from single pedogenic carbonate components indicate that analyses of entire pedogenic components (e.g. nodules) may artificially homogenize their natural geochemical heterogeneity. Furthermore, homogeneous geochemical compositions of paleosol carbonates may record diagenetic overprinting rather than the physio-chemical conditions which governed during pedogenesis. The preliminary results of this study suggest the need for rigorous evaluation of the post pedogenic, fluid rock interaction history of paleosol carbonates prior to applying their geochemical compositions to paleoclimatological and paleoenvironmental reconstruction.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995