EARTH SURFACE TEMPERATURES DETERMINED FROM THE OXYGEN ISOTOPIC COMPOSITION OF PEDOGENIC CARBONATES: APPLICATION TO WEST TEXAS LATE CRETACEOUS PALEOSOLS

Using the oxygen isotopic composition of pedogenic carbonates to calculate Earth-surface paleotemperatures is fraught with problems. These include: 1) the isotopic modification of soil water compared to meteoric water, 2) isotopic changes due to pedogenic carbonate recrystallization, and 3) the necessity of assuming a soil water oxygen isotopic composition when calculating temperatures. Modification to soil water is caused by evaporation and is extremely difficult to document once pedogenic carbonates have formed. Furthermore, there is no consensus on criteria that are useful for determining recrystallization of pedogenic carbonates although texture, cathodoluminesence, Sr isotopic composition, and trace element concentrations may all prove to be useful. We suggest that the necessity of assuming a soil water oxygen isotopic composition can be circumvented through the simultaneous solution of two equations that both describe the relationship between ambient temperatures and the oxygen isotopic composition of local precipitation. These published equations are: 1) the fractionation of oxygen isotopes from water into precipitating calcite and 2) the relationship between the oxygen isotopic composition of meteoric water and Earth surface temperature. The resulting equation is: –0.498 (K) ³ + (d¹⁸O _{calcite (SMOW)} + 153.04) (K) ² – 2.78 x 10⁶=0. This equation calculates surface temperatures from the measured oxygen isotopic compositions of pedogenic calcite. Reasonable Earth-surface temperatures can be calculated when this equation is applied to pedogenic carbonates that have oxygen isotopic values heavier than –10 per mil (PDB). We have applied this equation to pedogenic carbonates from a succession of late Cretaceous paleosols in west Texas. Our results show that mean annual temperatures during this time period hovered between 17 and 18 °C but were punctuated by two greenhouse events where temperatures rose by 4 °C. Paleobotanical studies document temperature fluctuations of the same magnitude during the late Cretaceous.