Salinity Indicators in Sediment Through the Fluvial-to-Marine Transition (Fraser River, Canada)

The transition from non-marine to marine conditions along rivers (fluvio–tidal transition) is characterized by the interaction of salt- and freshwater and fluvial and tidal currents. Determining the salinity of water under which sediment is deposited through the fluvio-tidal transition has been attempted using a wide range of sediment- and geochemical proxies with varying degrees of success. Herein, six proxies (Sr/Ba-HAc, Sr/Ba-NH4Ac, δ13Corg, C/N, and the relative abundances and concentrations of dinoflagellate cysts) are compared from the Fraser River Delta, Canada and surrounding coastal areas. Proxies are compared by depositional position along the fluvio–tidal transition, by salinity, and by sedimentological characteristics. Most proxies exhibit distinct trends between parts of the river that experience sustained marine water (saltwater) influence over seasonal and tidal timeframes, and parts that experience only freshwater or periodic saltwater influence. No attributes are reliable indicators of depositional position where saltwater incursion is short lived or where water is fresh. Where marine influence is sustained, Sr/Ba-HAc and Sr/Ba-NH4Ac are the most reliable positional indicators along the fluvio–tidal transition. When compared strictly to salinity, Sr/Ba-HAc, Sr/Ba-NH4Ac, and δ13Corg all correlate predictably except in delta front and prodelta settings. Our data show that all six sediment attributes are heavily impacted by river-derived sedimentation, and it is not appropriate to compare values from strongly river-influenced settings (e.g., deltas) with those from weakly river-influenced settings (e.g., bays and estuaries).