SEASONAL INFLUENCE OF CHANGING HYDROLOGY ON PEDOGENIC CARBONATE FORMATION IN VERTISOLS, DANCE BAYOU, BRAZORIA COUNTY, TX: IMPLICATIONS FOR THE QUANTIFICATION OF PAST ATMOSPHERIC pCO2

Quantification of past atmospheric CO₂ compositions from stable carbon isotopic values of pedogenic carbonate in paleosols requires an understanding of the processes influencing calcite precipitation in modern soil analog systems. Application of the pCO₂ paleobarometer assumes that the δ¹³C value of the pedogenic calcite represents equilibration with both soil-respired CO₂ and with diffusion of atmospheric CO₂. Screening for isotopic validity is typically transmitted light petrographic inspection of pedogenic carbonates for original pedogenic textures. This study focuses on the influence of seasonal environmental change on the isotopic geochemistry of pedogenic carbonates formed in epi-aquic Vertisols in southeastern Texas. The Churnabog soil series forms in clay-filled channels and oxbow lakes on the San Bernard River floodplain and varies between ponded (winter) and well-drained (summer) soil conditions. Pedogenic carbonate nodules from the Churnabog soils in transmitted light display complex, yet original pedogenic textures appropriate for isotopic assays. Evaluation of the same carbonate nodules with cathodoluminescence (CL) microscopy shows that there are both luminescent (lum.) and nonluminescent (nonlum.) micrites (in the same nodule) as well as lum. spar cements. Lum. phases have an average δ¹³C value of ~ -11‰ PDB, whereas the nonlum. phases have an average δ¹³C value of ~ -4‰ PDB; both phases have a constant δ¹⁸O value of ~ -3.3‰ PDB. Lum. carbonate phases likely formed during saturated soil conditions when mobile manganese, in a low Eh environment, can be incorporated into the crystal lattice. Atmospheric CO₂ pressures have little interaction or influence over carbonate precipitation during these ponded soil conditions leaving only soil-respired CO₂ in the system creating a relatively more negative isotopic value. Nonluminescent phases likely formed during well-drained conditions in which atmospheric gases are free to mix with soil derived CO₂, driving the isotopic value of the carbonate to less negative. Pedogenic carbonates in the rock record derived from paleo-Vertisols with similar hydrologic histories are common and should be investigated with both transmitted and CL microscopy to eliminate seasonal influences and accurately determine past atmospheric CO₂ pressures.