CARBON ISOTOPE RATIOS IN BULK ORGANIC MATTER FROM PALEOSOLS OF THE LATE TRIASSIC CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK, ARIZONA, USA

A succession of 90 stacked paleosols that developed on Chinle Formation overbank mud have been evaluated for the carbon isotopic composition of bulk organic matter, charcoal, and plant compressions. Bulk organic matter reveals temporal changes in carbon isotope ratios during Chinle deposition that can be interpreted both in terms of changes in the isotopic composition of atmospheric CO₂ as well as environmental stresses.

Bulk organic matter in Chinle Formation paleosols has carbon isotopic compositions that vary between -26 and -21 per mil VPDB. The youngest member of the Chinle, the Mesa Redondo, exhibits relatively positive carbon isotope ratios of about -22 per mil. An abrupt -2 per mil excursion occurs at the contact with the overlying Blue Mesa Member whose carbon ratios subsequently increase to an average composition of -23 per mil. A second, and more pronounced carbon isotopic excursion of -3 per mil occurs between the Blue Mesa and overlying Sonsela Member. Bulk organic matter of the Sonsela exhibit the most negative carbon isotope ratios within paleosols of the Chinle Formation and average about -26 per mil. Both of these isotope excursions are interpreted in terms of changing atmospheric CO₂ isotopic composition because there are no geochemical or morphologic aspects of the paleosols indicating a change in temperature or precipitation.

In the overlying Petrified Forest Member, bulk organic matter in successive paleosols exhibits a gradual increase in carbon isotopic composition until this trend is punctuated by another negative excursion near the Adamanian-Revueltian land vertebrate faunachron boundary. This in turn is followed by a rapid increase until a decline to values between -24 to -25 per mil is achieved for the remainder of Petrified Forest deposition. The significance of the isotope variations associated with the faunal turnover will be evaluated with respect to paleosol geochemistry and pedogenic features.

Lastly, carbon isotope ratios in bulk organic matter within Owl Rock Member paleosols exhibit a trend toward positive ratios that we interpret as being driven by increasing temperature and declining precipitation. The occurrence of this environmental change is also supported by molecular oxide ratios and the presence of abundant pedogenic calcite.