ENVIRONMENTAL AND GLOBAL CARBON CYCLE SIGNALS RECORDED IN 6 MILLION YEAR CARBON ISOTOPE RECORD FROM THE PALEOCENE BLACK PEAK FORMATION, BIG BEND NATIONAL PARK

The Black Peaks Formation (BPF) from the Tornillo Group in Big Bend National Park (BBNP) is comprised of a series of stacked paleosols and sandstone channels deposited by a fluvial system in a sub-tropical intramountain basin during the Laramide orogeny. Paleosols constituting the BPF display alternating drainage and development conditions. The BPF is bounded by the Late Cretaceous Javelina Formation and the Early Eocene Canoe Formation and is therefore thought to be of Paleocene age (65.5-55.8 million years ago). The BPF is an interesting target for paleoenvironmental reconstruction because few data have been generated from the Paleocene for sub-tropical regions, limiting the validation of global climate model predictions in these regions. However, previous attempts to resolve the age of the formation using biostratigraphy, magnetostratigraphy and chemostratigraphy gave ambiguous results limiting the potential of the BPF for paleoenvironmental reconstruction. We analyzed the carbon isotope ratio (δ¹³C ) of carbonate nodules collected from pedologically distinct paleosols throughout the BPF. δ¹³C of carbonates nodules found in reduced black paleosols are systematically 3-4‰ lower than δ¹³C from non-black paleosols from neighboring stratigraphic intervals. We hypothesize that this isotopic difference is related to the lower contribution of atmospheric CO₂ to soil CO₂ in water-logged and/or poorly drained black soils. Large-scale stratigraphic patterns of carbon isotope variations in carbonate nodules from non-black paleosols throughout the BPF strongly resemble well-documented secular changes in δ¹³C values of marine carbonates for the Paleocene. Several recognizable features are present in both curves, including the Paleocene Carbon Isotope Maximum (PCMI) and possibly the Late Danian carbon isotope excursion (LDE). These features provide a new basis for correlation of the BPF to the global geological timescale, and suggest that the Formation preserves a 6 million year record of deposition and paleoenvironmental conditions spanning the majority of Paleocene time (63 to 57 million years ago).