T199 High-Resolution Terrestrial and Marine Geochemical Proxies of Global Change: Progress, Problems, and Utility II

Tuesday, 20 October 2009
1:30 PM - 5:30 PM
Portland Ballrooms 251/258

A 7 MA RECORD OF ATMOSPHERIC δ13CO2 DERIVED FROM ORGANIC MATTER IN LATE CRETACEOUS AND EARLY TERTIARY PAELOSOLS

Paper number: T199-10
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

Measuring the carbon isotopic composition of dispersed organic matter in a succession of paleosols from west Texas provides the opportunity to document secular variations in atmospheric δ13CO2 during the late Cretaceous and early Tertiary. If the paleosol organic matter reflects atmospheric δ13CO2, it would serve as a convenient way of obtaining an important variable that is needed for paleo-pCO2 calculations.

We first investigate the reliability of dispersed organic matter in these paleosols to serve as a proxy for atmospheric δ13CO2. This was carried out by comparing the carbon isotopic composition of dispersed organic matter in a west Texas paleosol that formed prior to the K/T boundary to the carbon isotopic composition of organic matter from five different terrestrial locations of similar age. If ancient organic matter that formed in different locations at the same time preserves the same isotopic composition, it supports the idea that these fossil plants reflect the atmospheric δ13CO2 from which they formed. Of the six locations, five (including the west Texas paleosol) have organic matter with very similar carbon isotopic compositions (average δ13C = -26.5 +/- 0.5 ‰) while plants at the sixth location apparently formed under environmental stress. This suggests that dispersed organic matter preserved in these paleosols record atmospheric δ13CO2 as faithfully as bulk organic matter from coals or carbonaceous mudrocks.

Therefore, we have measured the abundance and character of dispersed organic matter in B-horizons from a succession of 39 paleosols from across the K/T boundary in west Texas. Organic carbon concentrations vary between 0.07 and 0.28 wt % and average 0.14 wt%. The δ13C of this organic matter varies between -28.1 and -25.1‰. This corresponds to atmospheric δ13CO2 values between -8.6 and – 5.8 ‰. In general, the atmospheric δ13CO2 exhibits a gradual change from about -8‰ at 71 m.y.a. to about -7‰ at 63.5 m.y.a.

Lastly, we use these atmospheric δ13CO2 values in conjunction with the carbon isotopic composition of coeval pedogenic carbonates to calculate atmospheric pCO2. Across the study interval, atmospheric pCO2 ranges from about 300 ppmV to 1200 ppmV with the highest values occurring during two previously identified green house events.