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

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 δ¹³CO_2^. 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.