Radiocarbon signatures and size-age-composition relationships of major organic matter pools within a unique California upwelling system

Coastal upwelling zones are among the most productive regions in the world and play a major role in the global carbon cycle. Radiocarbon (as Delta C-14) is a powerful tool for tracing the source and fate of suspended particulate and dissolved organic matter (POM, DOM), and has the potential to reconcile key carbon budgets within upwelling systems. However, the extent to which upwelling processes influence the Delta C-14 signature of surface DIC, or that of POM or DOM remains almost completely unknown. Here we present a time series of stable carbon (delta C-13) and Delta C-14 isotopic data of major water column carbon pools, including dissolved inorganic carbon (DIC), large (0.7-500 mu m) and small (0.1-100 mu m) POM, and high molecular weight (HMW; similar to 1 nm-0.1 mu m) DOM from an upwelling center along the Big Sur coast. We show that DIC Delta C-14 values (ranging between +29 parts per thousand and -14 parts per thousand) are strongly correlated to coastal upwelling processes, and that this 14 C-signal readily propagates into both the POM and HMW DOM pool. However, the presence of negative POM and HMW DOM Delta C-14 values (ranging between + 46 parts per thousand and -56 parts per thousand, + 6 parts per thousand and -123 parts per thousand and -1 parts per thousand and -150 parts per thousand, respectively) suggests contributions of pre-aged OM, complicating the direct use of bulk Delta C-14 for tracing upwelling-derived carbon production/export. Using a triple-isotope mixing model (delta C-13, delta N-15, Delta C-14) we estimate that 50-90% and 45-51% of large and small POM is newly-produced OM, while between 6-22% and 12-44% of large and small POM are derived from pre-aged re-suspended sediments. Finally, we observe quantitative relationships between OM size, composition (C:N ratio) and Delta C-14 within this upwelling system, possibly representing a new tool for modeling ocean C and N biogeochemical cycles. (C) 2013 Elsevier Ltd. All rights reserved.