USE OF 14C AND 13C NATURAL ABUNDANCES FOR EVALUATING RIVERINE, ESTUARINE, AND COASTAL DOC AND POC SOURCES AND CYCLING: A REVIEW AND SYNTHESIS

Abstract

Though not typically regarded as ''biomarkers'' in the traditional sense of the word, the radioactive and stable isotopes of carbon (14C and 13C, respectively) can serve as powerful tools for identifying sources and estimating turnover times of organic matter in aquatic systems. Paired 14C and 13C measurements of carbon pools can provide an additional degree of specificity for studies of organic matter cycling as a result of: (1) the lower susceptibility of natural isotopes to diagenetic effects that can alter organic biomolecules; (2) the ''dual'' isotopic nature of the approach; (3) the unique input functions for each isotope; and (4) the greater dynamic range in Δ14C (-1000 to ~+200%%) compared to δ13C (~-32 to -12%%). Relatively few geochemical studies in rivers, estuaries and the coastal ocean waters have employed 14C and 13C analyses of organic matter. In this paper we summarize the available data on 14C and 13C measurements in dissolved and particulate organic carbon (DOC and POC, respectively) in these systems. A brief review is presented of current methods for the separation and oxidation of DOC and POC from water samples, for subsequent Δ14C and δ13C analyses. We also compile the existing datasets on paired 14C and 13C measurements across the riverine to coastal marine continuum in order to elucidate sources, ages, and transformations of organic matter within each system, and during transport from rivers to the coastal ocean. The natural range in the Δ14C values of both DOC and POC across similar system types was 500 and 1000%%, respectively. In general, riverine DOC was enriched in 14C relative to POC in rivers and estuaries, but the opposite generally held for coastal marine waters. This is indicative of the different sources and transport mechanisms for DOC and POC within and across these three general types of systems. During river and estuarine transport, DOC generally becomes enriched in 13C and depleted in 14C due to simultaneous additions from autochthonous production and removals from heterotrophic bacteria and abiotic processes. Bacterial utilization experiments indicate that bacteria preferentially utilize a 14C enriched (i.e. young) DOC fraction and, therefore, DOC utilization is a partial explanation for the 14C-depeleted riverine and estuarine DOC. It is concluded that through the use of paired 14C and 13C measurements in DOC and POC, a more robust interpretation of sources, sinks, and residence times of organic matter may be attained than by using either isotope separately.