POC fluxes measured by sediment traps and 234Th:238U disequilibrium in Saanich Inlet, British Columbia

A 2-year time series sampling with moored sediment traps deployed in Saanich Inlet, British Columbia, provides a monthly record of particulate organic carbon (POC) flux at three depths (50 m, 115 m, 180 m), which is compared to concurrent POC flux estimates obtained from 234 Th: 238 U disequilibrium. The results obtained by both methods are in general agreement, supporting the validity of both approaches for measuring POC flux in these coastal waters. Closer comparison indicates that the 234 Th-derived fluxes at 50 m are lower than measured by sediment traps. This difference is attributed to particle focusing from the narrowing of the fjord with depth, which leads to overestimating fluxes with sediment traps while underestimating these same fluxes with 234 Th. Higher fluxes measured at greater depths and better agreement between the two methods are consistent with lateral input of sediment winnowed from the sill by tidal currents. The very high scavenging intensity at our study site results in large 234 Th deficits and low dissolved 234 Th activities. This limits lateral 234 Th transport and the system largely maintains steady state. Our results indicate that the sinking flux of POC could be reasonably estimated by simply measuring 234 Th and organic carbon in suspended particles at our study site. Such a relatively simple method may be applicable to other coastal settings with similar high scavenging intensity and limited exchange with the open ocean and would facilitate the implementation of sampling programs to monitor carbon cycling or contaminants dispersion in this type of environment.