Stable Isotope Compositions of Caliches in Texas and Their Paleoenvironmental Implications

Analyses of 48 caliche profiles on various bedrocks (limestone, igneous, etc.) across Texas, show that the δ¹⁸O values range from -9.1‰ to 0.2‰, mostly between -6‰ and -2‰, and the δ¹³C values are between -10.6‰ and +0.7‰, mostly distributed evenly between -1‰ and -9‰. From east to west, the δ¹⁸O values of late Quaternary caliches decrease gradually, which mimics the areal variation of δ¹⁸O values of modern meteoric water. This change in meteoric water composition is due to the increase in distance from the Gulf of Mexico (source of meteoric water) as well as a minor increase in elevation. The corresponding δ¹³C values reflect the distribution of vegetation zones. Caliches in the Texas Gulf Coastal Plains, Southern High Plains, and west Texas exhibit greater δ¹³C values than those in the south and central Texas, which indicates a difference in vegetation between C₄-dominated and C₃-dominated plants. The significant increase in δ¹³C values of caliches from the Tertiary strata into the overlying Quaternary strata confirms the invasion of C₄ biomass in the South High Plains since the early Pliocene.

However, in some cases, the stable isotope values of caliches developed on marine limestone and calcareous alluvial deposits in central Texas and south Texas partly inherit significantly different signatures from the associated bedrocks. Similarly, the δ¹³C values of caliche nodules in the Gulf Coastal Plains are also affected by saline groundwater.

Thus, whereas the stable isotope compositions of caliches are, in general, good paleoenvironment indicators, interpretations must be based upon robust datasets and supported by independent sedimentologic evidence.