Solid-state 13C NMR and stable isotopic analysis of petrified wood: Insight into organic matter degradation and Late Triassic climate change

Abstract:

Preserved organic matter (OM) from fossil plants has the potential to be a significant long-term archive of terrestrial paleoclimate information. However, the mechanisms controlling OM preservation and the potential attenuation of geochemical signatures useful for paleoenvironmental reconstructions are poorly understood. To better understand decomposition effects, we applied solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with stable carbon and hydrogen isotopic (δ¹³C_W and δD_W) analyses, to the study of OM of petrified wood from the Late Triassic Chinle Formation in Petrified Forest National Park (PEFO). This study quantifies the influence of long term diagenetic degradation on the environmental signature preserved in OM in petrified wood. Solid state ¹³C NMR results reveal that OM composition of wood samples was dominated by altered lignin structures. During burial diagenesis, the molecular composition of all samples became similar and is dominated by aromatic structures, with lower amounts of methoxyl and alkyl carbon. Stable hydrogen isotopic analysis reveals that, with increasing decomposition, the remaining abundance of OM decreased, H/C ratios decreased, and stable hydrogen isotope ratio is depleted proportionally, suggesting that the decomposition of OM is the controlling factor of δD_W. In contrast, there is no evidence of such impact on δ¹³C_W, and a mean annual precipitation anomaly (MAPA) analysis indicate that the variation of stable carbon isotopes documented by petrified wood agrees well with the timing of the documented climatic trend in both continental and marine records.