Temperature Estimates of Soil Processes Based on δd and δ18O Values of Phyllosilicate: Implications for CO2 Forcing of Atmospheric Surface Temperatures

We present here δD and δ¹⁸O values of pedogenically-formed phyllosilicates from soil B-horizons along climate-transects from western U.S.A. Temperature estimates (±3°C) of phyllosilicate crystallization are calculated from δD and δ¹⁸O values, assuming (1) equilibrium between mineral and soil water, and (2) δD and δ¹⁸O values of soil water lie upon the meteoric water line of Craig (1963). Estimates of phyllosilicate crystallization temperatures are indistinguishable from mean annual surface air temperatures (MAAT) for areas with mean annual temperatures above ~10°C. Cooler sites have δD and δ¹⁸O values which correspond to significantly higher temperatures of crystallization than MAAT, and may be related to the presence of amorphous solids in addition to phyllosilicates.

Application of this phyllosilicate stable isotope paleothermometer to 115 samples of Upper Cretaceous (~70 Mybp) through Miocene (~5 Mybp) paleopedogenic kaolinites that formed between ~65°N and ~65°S provides the following results: (1) Data from low-latitude paleotropical sites is sparse (n=2), and appears to be similar to (but perhaps slightly warmer than) modern surface temperatures. (2) Data from mid-latitude sites indicate paleotemperatures that were consistently higher than modern, iso-latitudinal, sites. Specifically, kaolinite ?D and ?¹⁸O values provide paleotemperature estimates that are +10°C in Cretaceous, +7°C in Paleocene, +9°C in Eocene, +6°C in Oligocene, and +7°C in Miocene samples compared to modern MAAT. Carbon-mass flux estimates of paleoatmospheric PCO₂ are poorly correlated (r² = ~0.5) with these stable isotope estimates of paleotemperature, whereas geochemical estimates of paleoatmospheric PCO₂ from pedogenic minerals are much better correlated with these paleotemperature estimates (r² = 0.7), and suggest that temperatures in the mid-latitudes increase by ~2.4°C with each doubling of atmospheric PCO₂ concentrations compared to modern, pre-industrial, values.