EARLY PALEOCENE ENVIRONMENTAL RECONSTRUCTION USING THE ISOTOPIC COMPOSITION OF LEAF COMPRESSIONS FROM THE SAN JUAN BASIN

This study investigates the geochemistry of organic matter from early Paleogene rocks of the San Juan Basin in order to evaluate ecologic, climatic, and diagenetic conditions related to these plants. Carbon isotopes can be used to reconstruct canopy cover, water stress, light intensity, and the carbon isotopic composition of the atmosphere. Recent work has demonstrated that carbon isotopic analyses can be applied to compression fossils to reconstruct environmental conditions in which the plants lived as well as to asses changes related to decomposition.

Leaf compressions from two species of early Paleocene trees from two stratigraphic horizons, separated by ~300 ky, in the San Juan Basin were analyzed for their isotopic composition and C/N ratio to reconstruct canopy structure and paleoclimate. Bulk organic matter immediately adjacent to the leaf compressions was also sampled to assess organic matter decomposition.

Leaf fossils from different stratigraphic horizons exhibit narrow ranges of carbon isotope ratios (<2 ‰) suggesting open canopy conditions in early Paleocene forests of the San Juan Basin. Leaf compressions of species Averhoites affinis within the Ojo Alamo Sandstone Formation have an average δ¹³C value of -26‰, while the same species in the overlying Nacimiento Formation have a mean value of -28‰. Leaf compressions of Platanites raynoldsii have the same carbon isotopic composition of -28‰ from both formations. This difference in carbon isotopic composition between the two species, as well as the different stratigraphic horizons is most likely due to a differing physiologic response to water stress.

Both the carbon and nitrogen isotopic compositions of leaf compressions are different than adjacent bulk organic matter probably due to processes associated with decomposition and contribution from other organic matter sources. In general, dispersed organic matter in adjacent bulk sediment has lower C/N ratios than leaf compressions suggesting a higher degree of decomposition. In addition to smaller C/N ratios, bulk organic matter consistently has higher δ¹³C values and lower δ¹⁵N values than adjacent leaf fossils. These diagenetic changes in bulk organic matter indicate that the isotopic composition of a leaf compression is a better indicator of environmental conditions than bulk organic matter.