13C NMR CHARACTERIZATION AND STABLE ISOTOPE INVESTIGATION OF ORGANIC MATTER IN SILICIFIED WOOD, PETRIFIED FOREST NATIONAL PARK (ARIZONA, USA): EVIDENCE FOR LATE TRIASSIC CLIMATE CHANGE

Rapid silica permineralization of wood in the Chinle Formation at Petrified Forest National Park (PEFO) in Arizona, resulted in organic matter that has well-preserved cell structure that is surrounded by and impregnated with quartz. Stable isotopic and solid state ¹³C nuclear magnetic resonance spectroscopy (NMR) analysis of this organic matter provides insights into Late Triassic environmental conditions and information regarding the linkage between climate, burial diagenesis and character of fossil wood. This study investigates the isotope composition and chemical structure of organic matter in petrified wood with the goal of using these potential proxies to monitor the evolution of environmental conditions during Chinle deposition.

Collected silicified organic matter includes silicified logs, roots, and organic mats from multiple stratigraphic horizons spanning the time interval of 225 to 207 Ma in the Chinle Formation. ¹³C NMR results shows the two most significant peaks belong to alkyl C and aromatic C components indicating that the biogeochemical and diagenetic alterations involve degradation of hemicellulose and cellulose components of wood with the selective preservation of lignin components. The residual lignin in the fossil wood undergoes a certain degree of geochemical decomposition such as demethylation and dehydroxylation although there is still a significant lignin methoxyl group that can be recognized.

Organic matter occurring in silicified wood with variable cell preservation has hydrogen isotopes that vary stratigraphically between -144‰ to -64‰ (SMOW) with the significantly enriched δD values in the lower portion of the Chinle Formation. These values of bulk organic matter could have resulted from trees receiving meteoric precipitation with hydrogen isotope ratios between -90‰ to -30‰ (SMOW). This suggests an environment for the Chinle that was less humid and cooler than typical modern equatorial areas most likely resulting from high altitudes. The stratigraphic trend of the stable hydrogen isotope values indicates that PEFO was wetter and warmer during early Chinle deposition and become gradually dryer in the Late Triassic. This result is consistent with our climatic interpretations based on petrographic and geochemical analysis of Chinle paleosols and sandstones.