Optical and geochemical characterization of Upper Sinemurian (Lower Jurassic) fossil wood from the Lusitanian Basin (Portugal)

Abstract

It is hypothesized that carbon and nitrogen isotopic compositions (δ¹³C and δ¹⁵N) of fossil wood fragments will provide useful information regarding conditions prior, during, and after deposition. Eleven fossil wood samples were collected from the Sinemurian-lowermost Pliensbachian marly hemipelagic deposits of the Água de Madeiros Formation at S. Pedro de Moel (Lusitanian Basin, Portugal), and analyzed using optical and geochemical methods to confidently determine their use in palaeoenvironmental studies. Organic petrography observations show that the fossil wood samples contain a wide variety of particles mostly related to the phytoclast group, but also include resin impregnations, palynomorphs, and/or marine amorphous organic matter. A significant positive correlation between total carbon content and δ¹³C is observed (defined by 8 out of 11 samples), and most samples have high δ¹⁵N. These data suggest that isotopic compositions of the studied samples were severely affected by sedimentary and diagenetic processes (biological or related to early diagenesis and coalification). It is evident that various processes of alteration either acted on different samples of the studied stratigraphic interval, or occurred with different magnitudes. It is considered that future stable isotopic studies on fossil wood should involve detailed screening as presented in this study, to ensure a proper understanding of the biological (mostly biodegradation) or diagenetic processes affecting samples and their impact on the determined chemical proprieties.