HIGH RESOLUTION CARBON ISOTOPIC VARIATIONS OF BULK ORGANIC MATTER IN PALEOSOLS

Carbon isotopic variations (δ¹³C) of fossil plants are thought to be an adequate proxy for δ¹³C_atmo 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 δ¹³C_{kerogen (}δ¹³C_k) and δ¹³C_{marine carbonates} at different time scales.

Here, we focus on high temporal resolution records (10³-10⁵years) by measuring the δ¹³C_k 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 δ¹³C_kerogen differs fundamentally from these other records of the CIE. The CIE recorded through δ¹³C_k is 2-3‰ smaller in amplitude than δ¹³C_n-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 δ¹³C_k variations. For each paleosol, we characterized kerogen through reflectance of vitrinite. Results demonstrate that kerogen is not homogeneous in paleosols which could explain δ¹³C_k 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.