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