T258 Advances in Terrestrial Paleoclimatology and Paleoecology: Geochemical Techniques and Examples Using Inorganic and Organic Molecules in Fossil Soils, Plants, Invertebrates, and Vertebrates (Posters)

Wednesday, 12 October 2011
9:00 AM - 6:00 PM
Hall C

HIGH RESOLUTION CARBON ISOTOPIC VARIATIONS OF BULK ORGANIC MATTER IN PALEOSOLS

Paper number: T258-14
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Carbon isotopic variations (δ13C) of fossil plants are thought to be an adequate proxy for δ13Catmo because plant take up their carbon from atmospheric CO2. Although organic-rich material such as fossil wood provide abundant material for paleoecology, they are not ubiquitous which limits the resolution of continental records. Using organic poor but widely available plant-derived organic matter (kerogen) dispersed in ancient buried soils (paleosols) increases the potential to obtain high-resolution paleoenvironmental datasets in continental settings. Several chemostratigraphic studies have demonstrated the existence of correlations between δ13Ckerogen (δ13Ck) and δ13Cmarine carbonates at different time scales.

Here, we focus on high temporal resolution records (103-105years) by measuring the δ13Ck from 44 paleosols deposited in the Wilwood Bighorn Basin during the Paleocene-Eocene Thermal Maximum (PETM). Although the content of organic matter is generally very low in these paleosols (between 0.05 and 0.25%), we identified the carbon isotope excursion (CIE) characteristic of the PETM. We compared these results to existing CIEs measured in n-alkanes from the same section and from marine carbonates and found that δ13Ckerogen differs fundamentally from these other records of the CIE. The CIE recorded through δ13Ck is 2-3‰ smaller in amplitude than δ13Cn-alkanes and displays high resolution fluctuations which are not present in the other records. These differences could reflect: 1) variability in ecosystem response to climate, 2) variations in organic matter sources and preservation among sites, 3) chemical taphonomy and/or diagenesis related to burial. We present combined organic petrography, mineralogy and organic chemistry analysis on 12 paleosols aiming to untangle these potential factors of δ13Ck variations. For each paleosol, we characterized kerogen through reflectance of vitrinite. Results demonstrate that kerogen is not homogeneous in paleosols which could explain δ13Ck fluctuations. For the same paleosols, we separated different kerogen fractions through density fractionation at 1.2g.cm-3 and 1.8g.cm-3 and centrifugation and analyzed their respective δ13C. The results underline the importance of identifying kerogen characteristics before interpreting kerogen-derived δ13C.