Impact of flood events on the transport of terrestrial organic matter to the ocean: A study of the Têt River (SW France) using the BIT index

We analyzed suspended particulate matter (SPM) from waters of the Têt River (SW France) to reconstruct variations in the BIT (branched and isoprenoid tetraether) index, a tracer for soil organic matter (OM) input. The river system is characterized by the occurrence of flood events, which supply substantial amounts of terrestrial OM to the NW Mediterranean. The mean stable carbon isotopic composition of particulate organic carbon (δ13CPOC) and C/N ratio of SPM were −26.1‰ and 6.5, respectively, consistent with an origin predominantly from soil OM. The average BIT value of river water SPM (0.85) was substantially higher than that of the offshore seawater SPM (< 0.01). Such high values for riverine SPM, in combination with δ13CPOC and C/N values, support the conclusion that the increased BIT index values reported for the Têt prodelta surface sediment time series [Kim et al., Geochemistry Geophysics Geosystems, 7, Q11017, doi:10.1029/2006GC001306, 2006.] were indeed due to high input of soil-derived OM from land transported via rivers to the sea. Our results further suggest that the BIT index is an appropriate proxy for soil OM input into coastal oceans such as the Gulf of Lions in the NW Mediterranean, where the mountains are adjacent to the shore and flood events play an important role in delivering terrestrial OM to the sea.