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

Abstract

Coastal upwelling zones are among the most productive regions in the world and play a major role in the global carbon cycle. Radiocarbon (as Δ14C) 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 Δ14C signature of surface DIC, or that of POM or DOM remains almost completely unknown. Here we present a time series of stable carbon (δ13C) and Δ14C isotopic data of major water column carbon pools, including dissolved inorganic carbon (DIC), large (0.7–500μm) and small (0.1–100μm) POM, and high molecular weight (HMW; ∼1nm–0.1μm) DOM from an upwelling center along the Big Sur coast. We show that DIC Δ14C values (ranging between +29‰ and −14‰) are strongly correlated to coastal upwelling processes, and that this 14C-signal readily propagates into both the POM and HMW DOM pool. However, the presence of negative POM and HMW DOM Δ14C values (ranging between +46‰ and −56‰, +6‰ and −123‰ and −1‰ and −150‰, respectively) suggests contributions of “pre-aged” OM, complicating the direct use of “bulk” Δ14C for tracing upwelling-derived carbon production/export. Using a triple-isotope mixing model (δ13C, δ15N, Δ14C) 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 Δ14C within this upwelling system, possibly representing a new tool for modeling ocean C and N biogeochemical cycles.