Stable Isotope Ratios of Carbonate and Organic Carbon from Silicified Tree Trunks, Petrified Forest, New Cairo, Egypt – Possible Interpretations of Palaeoenvironment

Abstract

The silicified tree trunks of the New Cairo Petrified Forest – evidence of fossilized remains of Paleogene angiosperms also abundant in other sites in Egypt – are found in horizontal positions on the surface and buried completely or partly by Oligocene sand and gravel underlain by late Eocene sediments. Differences in petrographic characteristics allowed for grouping these silicified woods into two types: (1) Type A composed of microgranular quartz ± some goethite ± minor moganite and (2) Type B composed of chalcedony or a mixture of chalcedony and cryptograined quartz ± some goethite ± minor gypsum. The two type groups contain traces of carbonate and organic matter: 0.004 to 0.35 wt % CaCO3 and 0.004 to 0.16 wt % organic carbon. The carbonate of each type has characteristic δ13C and δ18O values: –3.2 ± 5.7‰ VPDB and –0.88 ± 5‰ VPDB for silicified wood Type A and –10.9 ± 6.8‰ VPDB and –7.64 ± 3.3‰ VPDB for silicified wood Type B, respectively. Covariation between carbonate δ13C and δ18O values of both silicified wood Type A and B samples indicate a hydrologically closed system with different inorganic carbon and meteoric water signatures. These interpretations are supported by mineralogical and chemical data from the samples. Various factors likely influenced the carbonate δ13C and δ18O values of the silicified wood samples during the time they formed, including biogenic CO2, water residence effect, salinity, and evaporation. The δ13C values of bulk organic matter of silicified wood Type A and B samples ranging from –24.04 to –26.89‰ VPDB are within a characteristic range of C3-type biomass. The shift to a less negative bulk organic matter δ13C value in these samples (particularly those of the silicified wood Type B) has been attributed to post-depositional changes.