Articles | Volume 14, issue 22
Biogeosciences, 14, 5099–5114, 2017
Biogeosciences, 14, 5099–5114, 2017

Research article 15 Nov 2017

Research article | 15 Nov 2017

Technical note: An inverse method to relate organic carbon reactivity to isotope composition from serial oxidation

Jordon D. Hemingway1,2,a, Daniel H. Rothman3, Sarah Z. Rosengard1,2,b, and Valier V. Galy1 Jordon D. Hemingway et al.
  • 1Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA
  • 2Massachusetts Institute of Technology – Woods Hole Oceanographic Institution Joint Program in Oceanography and Applied Ocean Science and Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
  • 3Lorenz Center, Department of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
  • apresent address: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
  • bpresent address: Departments of Geography and Earth, Ocean, and Atmospheric Sciences, University of British Columbia, 2207 Main Mall, Vancouver, BC V6T 1Z4, Canada

Abstract. Serial oxidation coupled with stable carbon and radiocarbon analysis of sequentially evolved CO2 is a promising method to characterize the relationship between organic carbon (OC) chemical composition, source, and residence time in the environment. However, observed decay profiles depend on experimental conditions and oxidation pathway. It is therefore necessary to properly assess serial oxidation kinetics before utilizing decay profiles as a measure of OC reactivity. We present a regularized inverse method to estimate the distribution of OC activation energy (E), a proxy for bond strength, using serial oxidation. Here, we apply this method to ramped temperature pyrolysis or oxidation (RPO) analysis but note that this approach is broadly applicable to any serial oxidation technique. RPO analysis directly compares thermal reactivity to isotope composition by determining the E range for OC decaying within each temperature interval over which CO2 is collected. By analyzing a decarbonated test sample at multiple masses and oven ramp rates, we show that OC decay during RPO analysis follows a superposition of parallel first-order kinetics and that resulting E distributions are independent of experimental conditions. We therefore propose the E distribution as a novel proxy to describe OC thermal reactivity and suggest that E vs. isotope relationships can provide new insight into the compositional controls on OC source and residence time.

Short summary
The balance between organic matter (OM) fixation and decay is a major control on atmospheric CO2 concentrations. Understanding the environmental, chemical, and physical mechanisms that control the distribution of OM decay rates is therefore critical for constraining the global carbon cycle. In this manuscript, we derive a method to relate OM reactivity to its isotope composition using a kinetic model and provide a novel framework to discern the controls on OM decay rates.
Final-revised paper