Evaluation of Variability in Soil Organic Carbon Isotope Record and Implications for Organic Matter Preservation

Open Access
Author:
Chung, Angela Hsien
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 20, 2014
Committee Members:
  • Katherine Haines Freeman, Thesis Advisor
Keywords:
  • Organic carbon
  • isotopes
  • residence time
  • PETM
Abstract:
A large carbon isotope excursion (CIE) is one of the main signatures of the PETM (Paleocene-Eocene Thermal Maximum). Throughout the warming of the PETM, negative shifts in the carbon isotope record on land and in the sea can be observed in various magnitudes. In particular, while lipid and carbonate carbon archives on land display larger CIE magnitudes compared to marine records, the total organic carbon record exhibits a smaller and more variable CIE. This study examines the terrestrial bulk organic carbon isotope record to evaluate factors influencing δ13C changes and their variability. The differences in CIE magnitudes provide an opportunity to investigate the carbon cycle in soils during the PETM. Ancient floodplain sediments from the Wasatch Formation in western Colorado were examined for lithology, degree of pedogenesis and analyzed for stable isotope ratios of organic carbon (reported as δ13Corg) and total organic carbon content (TOC%). δ13Corg values are independent of lithology and degree of pedogenesis of the sample, denoting a lack of a relationship between δ13C of organic matter and depositional environment. No correlation exists between δ13Corg and TOC(%) or δ13Corg and one over total organic carbon (1/TOC). The absence of any relationship makes the evidence for effects of decomposition and mixing of refractory organic carbon and indigenous carbon not readily apparent. The attenuated CIE in terrestrial organic matter archives suggest decreased soil organic matter residence times in the study area during the PETM. Studies utilizing carbon isotopes of bulk organic matter would benefit from separating out different fractions of organic matter and compound specific isotope analysis. Given the extremely low TOC(%) preserved in these samples and attenuated CIE, we suggest that in the hyper-greenhouse conditions of the PETM, the residence time of carbon in soils was reduced.