Sources and distribution of CuO-derived benzene carboxylic acids in soils and sediments

Cupric oxide (CuO) oxidation is a powerful tool for tracing different forms of organic carbon (OC), particularly vascular plant-derived OC, through the environment. The method produces a suite of benzene carboxylic acids (BCAs) whose sources have been unclear, but some of which have been used as tracers of soil OC in the ocean. In particular, some evidence suggested they might derive from charcoal, making them a potentially powerful tracer of combustion-derived OC within the environment. In order to constrain their sources and distributions in the environment, we analyzed the concentrations of an extended suite of 18 BCAs from over 75 samples. The entire suite of compounds was produced from most pure organic samples, as well as from charcoal, indicating that a variety of processes lead to their production. Both terrestrial and marine geochemical samples yielded the suite, and OC content, not charcoal, appears to control the total BCA yield. The compounds cannot, therefore, be used as specific tracers of charcoal in soils and sediments. Types of OC produced by marine biota (i.e. glucose, protein and degraded phytoplankton) produce many of the BCAs, but not 3,5-dihydroxybenzoic acid, suggesting that previous applications of this compound for tracing soil OC in the ocean may be robust. Additionally, we present a new ratio based on the entire BCA suite, which may provide a further constraint on the amount of soil OC present in ocean sediments.