Isotopic constraints of sedimentary inputs and organic carbon burial rates in the Saguenay Fjord, Quebec

Abstract

The Saguenay Fjord (Quebec) has exceptionally high sedimentation rates resulting in up to 100 m-thick Holocene sequences filling some of its basins, partly due the frequent occurrence of rapidly deposited layers (landslides, seismic events, floods). In the present study, special attention is paid on the incidence of such events on organic carbon (OC) fluxes, sources and burial rates, using physical, sedimentological, geochemical and isotopic measurements on 4 sediment cores raised from the inner basin of the fjord and the adjacent Baie des Ha!Ha! In these cores, rapidly deposited layers (RDL) include notably a turbidite linked to a major earthquake in 1663, a landslide layer dating from 1971 (Saint-Jean-Vianney landslide), and a flood layer from 1996. Compared with background sediments, RDL are characterized by low OC contents (<1%), relatively high CaCO 3 contents (>2%), and low δ 13C values in OC (<−27‰). These geochemical properties indicate incorporation of reworked marine sediments from the Laflamme Sea, which occupied the area during postglacial times. δ 13C-OC values (−26.8±0.2‰) and C/N ratios (17.7±1.7) in the pre-industrial, pre-1663 fjord sediments, indicate that most sedimentary organic matter (OM) was then of terrigenous origin. Paper mill activity during the 20th century has resulted in OM fluxes growing almost exponentially until the mid-1960s, when major changes in industrial practices and the implementation of environmental regulations during the 1970s and later on, reduced the amount of industrial OM discharged into the fjord. This industrial OM shows an isotopic signature of −26.34±0.02‰ and C/N ratios >20, not unlike those of the regional terrestrial OM. OC burial rates since the 1663 earthquake varied between ∼20 and 290 gC m −2 yr −1. RDL seem to favour OC preservation in the fjord by reducing bioturbation and both oxic and anoxic OC degradation, and by contributing themselves to higher carbon burial rates. Therefore, transitional environments such as the Saguenay Fjord could represent significant long-term carbon sinks.