Publication Date:
2016-09-01
Description:
The Paleocene-Eocene thermal maximum was a period of abrupt, transient global warming, fueled by a large release of 13 C-depleted carbon and marked globally by a negative carbon isotope excursion. While the carbon isotope excursion is often identified in the carbon isotope ratios of bulk soil organic matter ( 13 C org ), these records can be biased by factors associated with production, degradation, and sources of sedimentary carbon input. To better understand these factors, we compared 13 C org values from Paleocene-Eocene thermal maximum rocks in the southeastern Bighorn Basin, Wyoming, with those derived from leaf wax n -alkanes ( 13 C n -alk ). While both 13 C n -alk and 13 C org records indicate an abrupt, negative shift in 13 C values, the carbon isotope excursions observed in bulk organic matter are smaller in magnitude and shorter in duration than those in n -alkanes. To explore these discrepancies, we modeled predicted total plant tissue carbon isotope ( 13 C TT ) curves from the 13 C n -alk record using enrichment factors determined in modern C 3 plants. Measured 13 C org values are enriched in 13 C relative to predicted 13 C TT , with greater enrichment during the Paleocene-Eocene thermal maximum than before or after. The greater 13 C enrichment could reflect increased degradation of autochthonous organic matter, increased input of allochthonous fossil carbon enriched in 13 C, or both. By comparing samples from organic-rich and organic-poor depositional environments, we infer that microbial degradation rates doubled during the Paleocene-Eocene thermal maximum, and we calculate that fossil carbon input increased ~28%–63%. This approach to untangling the controls on the isotopic composition of bulk soil carbon is an important development that will inform not only future studies of global carbon cycle dynamics during the Paleocene-Eocene thermal maximum hyperthermal event, but also any study that seeks to correlate or estimate duration and magnitude of past events using soil organic carbon.
Print ISSN:
0016-7606
Electronic ISSN:
1943-2674
Topics:
Geosciences
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