When organisms die, they leave behind a wealth of information about carbon cycling and climate. The organic matter in soils, sediments, and water may come from decomposed land plants, dead plankton (tiny marine animals and plants), or burned wood or fossil fuels, and it offers clues about Earth’s past and present environments. These clues are critical to understanding Earth’s current and future responses to a changing climate.
However, the myriad forms of organic matter make it difficult to access the information stored within it. Analyzing the entire organic carbon pool blends information from a wide variety of sources into one often misleading signal. Zeroing in on specific organic compounds provides detailed information on those compounds but misses potentially important context. Promisingly, a new technique bridges these approaches and offers researchers both the broad and narrow analyses they formerly had to choose between.
The new technique, called ramped pyrolysis/oxidation (ramped PyrOx or RPO), takes advantage of the way different kinds of organic matter react to heat. For example, when younger organic matter from plankton is heated using RPO, it generally reacts at lower temperatures than much older organic matter from eroded bedrock. By combining RPO results with analyses of radiocarbon (a radioactive isotope of carbon with a half-life of 5,730 years) and stable carbon isotopes, scientists can identify the sources and ages of organic matter in the environment.
In September 2016, the National Ocean Sciences Accelerator Mass Spectrometry Facility hosted a 2-day workshop on the thermal analysis of natural organic matter. A diverse group of more than 30 scientists from the United States, the United Kingdom, Switzerland, and China gathered to discuss the scientific potential of combining RPO and radiocarbon analyses. The meeting included panel discussions on the history and applications of RPO and breakout groups to discuss future technical and scientific priorities for the method.
In one session, meeting participants considered the best ways to date sediments using RPO. In environments like Antarctica that seldom preserve foraminifera—tiny organisms whose shells are the gold standard for dating ocean sediments—researchers have relied on dating bulk organic matter. This approach is not ideal, however, because Antarctic sediments contain both marine plankton and land-derived organic matter of vastly different ages. Combining RPO and radiocarbon dating, scientists can link the most-heat-reactive organic matter to the youngest carbon dates, thereby obtaining more accurate sediment ages, which correspond to the ages of surface water plankton in the sediments. This information yields radiocarbon ages that can be equivalent to foraminifera ages, representing the time since these organisms were alive at the surface of the ocean and thus the age of sedimentation.
Other sessions focused on applications that do not use radiocarbon composition for dating. For instance, RPO alone can be used to track organic carbon cycles in rivers and their watersheds, in soils, and in ocean water. Linking RPO results from riverine and coastal marine sediments will enhance our understanding of what material survives transport from the terrestrial environment to the ocean.
Breakout groups discussed the need for better interpretations of RPO results when combined with radiocarbon dating; the potential to use RPO to study compounds other than carbon, such as nitrogen and sulfur; and several other topics.
This was the first such workshop on the analysis of natural organic matter using thermal techniques coupled with radiocarbon dating and stable isotope measurements, and it was sponsored by the National Science Foundation. The workshop provided concrete direction on a new technique that illuminated an exciting path toward a better understanding of biogeochemical cycles in soils, lakes, rivers, and oceans. For more information about this meeting, see the RPO radiocarbon white paper on the Woods Hole Oceanographic Institution website.
—Ann McNichol (email: [email protected]; @ann_mcnichol), Woods Hole Oceanographic Institution, Mass.; Brad Rosenheim (@Rosenheim_group), University of South Florida, Tampa; and Valier Galy (@valiergaly), Woods Hole Oceanographic Institution, Mass.