Burial rates of organic matter along the eastern Canadian margin and stable isotope constraints on its origin and diagenetic evolution

Abstract

Organic carbon (OC) and nitrogen (N) contents and δ 13 C and δ 15 N values in total organic matter (OM) were measured in sub-surface sediments (0–30 cm sub-bottom) from 21 cores raised from the Laurentian Channel of the Gulf of St. Lawrence and the Labrador Sea, to document OM fluxes and storage along the eastern Canadian margin. Storage rates as high as ∼2.5 g m −2 yr −1 for OC and ∼0.2 g m −2 yr −1 for N are observed in the Laurentian Channel, suggesting that the shelf plays a significant role in terms of OM storage (from 1 to 2% of the primary production). Based on the isotopic composition of the essentially marine OM of the Labrador Sea ( δ 13 C/V-PDB =−21.9±0.4‰; δ 15 N/AIR =7.6±0.6‰; n=12), there is no isotopic evidence for a significant relative input of terrestrial OM along the Laurentian Channel ( δ 13 C/V-PDB =−21.9±0.4‰; δ 15 N/AIR =8.0±0.9‰; n=10), either due to high relative fluxes of marine OM and/or to the trapping of continental OM in the estuary and upstream. High storage rates of OM are also observed on the continental rise of the Labrador Sea (as high as 1.1 g C m −2 yr −1 and ∼0.09 g N m −2 yr −1). They contrast with one order of magnitude lower rates on the slope, due to low sedimentation rates (SR) and sediment winnowing by the Western Boundary Undercurrent (WBUC). Reduced early diagenetic alteration of OM is observed, particularly in the Laurentian Channel. It results in discrete (i) losses of OC and N, (ii) shifts in C/N ratios, suggesting preferential removal of N-bearing OM also highlighted by losses in total hydrolysable amino acids (HAA). In the Labrador Sea slope records, due to low SR, OM concentration changes linked to long term temporal variations may superimpose on these diagenetic trends, and some influence of the WBUC is noticeable.