Reports: AC2
47836-AC2 Use of Geochemical Proxies to Evaluate Paleosalinity and Carbon Flux in the Bell Canyon Formation, Guadalupe Mountains West Texas
We collected and analyzed samples from the middle Permian Lamar Limestone and Reef Trail Member of the Bell Canyon Formation for stable carbon and oxygen isotopes, elemental geochemistry, and organic biomarker analysis. These analyses are serving as geochemical proxies to reconstruct basin controls on variations on the radiolarian microfossil assemblages, a group of siliceous microplankton that was very prevalent in the basin. We have hypothesized that the observed faunal variations are driven by changes in paleoenvironment. A MS student spent 2 months this Spring at Kyudai University Fukuoka, Japan learning how to process samples for organic carbon isotopes and biomarkers. She was able to process samples from the Reef Trail member and is currently calculating and analyzing her data. This student also conducted fieldwork in the Guadalupe Mountains National Park for isotope analysis and acquired additional samples from the Apache Mountains for isotope work. A PhD student was partially funded on this grant for her work on the Lamar Limestone Member analyzing geochemical data and radiolarian faunas to reconstruct changes in carbon flux that may be paleoclimatically driven.
Our major results are as follows:
1. Isotope and organic biomarker analysis of the Lamar Member shows positive correlation to faunal shifts and are interpreted to represent changes in climate state. One state has increased siliclastic input, higher TOC, a terrestrial organic carbon isotope signature, and high radiolarian richness (principally spumellarians), where the second state has a more marine signature that is dominated by albaillellid radiolarians. The positive correlation between the radiolarian richness and higher carbon flux suggests that riverine input may be generating a slight productivity spike by fertilizing the basin with additional organic matter.
2. Wavelet analyses of the geochemical proxies reveal periodicities (e.g. ~1 m/cycle) that mimic orbital variations. Similar rhythms also exist in the relative abundance record of Lamar albaillellid radiolarians, probably indicating an ultimate orbital control on the Lamar radiolarian variations.
3. Isotope data run on the Reef Trail Member show a large negative excursion that we interpret to be related to the End Guadalupian extinction, one of the larges extinctions in EarthÕs history. Good biostratigraphic control of this section ties it in to the Global Stratotype section in China and interestingly, a negative excursion in that section occurs above the Guadalupian-Lopingian boundary, after the extinction. In our sections it occurs before the boundary indicating that the excursion may not be globally synchronous, or that there may be multiple perturbations leading up to the extinction.
We will continue to investigate the biomarker data from the Reef Trail Member and develop our model for climate fluctuations and their effect on the basin. We will also expand our isotope analysis to a section from the Apache Mountains to correlate the large negative excursion associated with the End Guadalupian extinction.
