EOS White Paper

Exploring the complex biogeochemistry of carbon with thermal 14C analysis

Ann McNichol, Woods Hole Oceanographic Institution
Brad E. Rosenheim, University of South Florida
Valier Galy, Woods Hole Oceanographic Institution

Inaugural workshop on Thermal Analysis of Natural Organic Matter, Woods Hole, 15-16 September 2016.

A better understanding of the makeup of natural organic matter (OM) in sediments, soils, and seawater is crucial to interpreting past and present biogeochemical signals. Radiocarbon (14C) dating applied to sedimentary and soil OM has been instrumental in our developing understanding of biogeochemical cycles.  However, there is a growing chasm between our understanding of bulk OM properties and the isotopic composition of individual molecular structures within these systems.

These issues were on center stage at a recent workshop hosted at the Woods Hole Oceanographic Institution by the National Ocean Sciences Accelerator Mass Spectrometer (NOSAMS) facility, sponsored by the U.S. National Science Foundation, and attended by a diverse group of over 30 scientists.  The focus of the meeting was conceptual advances that have benefitted from thermal analysis coupled with 14C dating of OM, a suite of techniques collectively known as Ramped PyrOx 14C. These methods seek to decompose OM into a spectrum of thermal reactivity and attendant radiocarbon contents, unraveling contributions from different pools of carbon.

The meeting aimed at identifying key geoscience questions that can be addressed with Ramped PyrOx 14C as well as attendant technical hurdles and their workarounds.

 Ramped PyrOx 14C was initially used to uncover issues with using bulk OM 14C dating for geochronology. The first applications indeed showed improvement of Antarctic margin sediments dating. However, Ramped PyrOx 14C was never intended to fully isolate autochthonous carbon but rather to unravel the mixture of contrasted organic carbon pools.  Thus, the pertinence and caveats of calibrating Ramped PyrOx 14C data to the calendar years scale was a major topic of discussion.

 In the context of biogeochemistry, the full spectrum of radiocarbon content and thermal stability obtained via Ramped PyrOx 14C analysis is meaningful. This was best illustrated by the wide radiocarbon age spectra observed in sediments, with implications for understanding the source and reactivity of organic matter. In contrast, preliminary application of Ramped PyrOx 14C to soils produced invariant radiocarbon spectra even in cases when individual molecular structures showed different radiocarbon content between them. This represents a possible illustration of the soil continuum model and points to an exciting future of experiments designed to better understand soil OM composition.

 The most important development highlighted at the workshop pertained to the mathematical treatment of complex thermal reactivity data.  The activation energy distribution can be inferred using an inverse model considering a set of parallel first order kinetic reactions. This not only allows deciphering the complex mixtures that make up natural sediment and soil carbon reservoirs but also more meaningful and cost effective sampling of Ramped PyrOx fractions for 14C analysis.

 The first workshop on thermal analysis of natural OM provided concrete direction on an exciting path to a better understanding of biogeochemical cycles in soil, sediment, lakes, and oceans.  For more information on this meeting, please see the Ramped PyrOx 14C White Paper that resulted from this meeting at www.whoi.edu/NOSAMS/RPO_whitepaper.