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

Organic carbon (OC) and nitrogen (N) contents and delta(13)C and delta(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 t he Gulf of St. Lawrence and the Labrador Sea, to document OM fluxes and sto rage along the eastern Canadian margin. Storage rates as high as similar to 2.5 g m(-2) yr(-1) for OC and similar to 0.2 g m(-2) yr(-1) for N are obse rved in the Laurentian Channel, suggesting that the shelf plays a significa nt role in terms of OM storage (from 1 to 2% of the primary production). Ba sed on the isotopic composition of the essentially marine OM of the Labrado r Sea (delta(13)C/V-PDB = -21.9 +/- 0.4%; delta(15)N/AIR = 7.6 +/- 0.6%; n = 12), there is no isotopic evidence for a significant relative input of te rrestrial OM along the Laurentian Channel (delta(13)C/V-PDB = -21.9 +/- 0.4 %; delta(15)N/AIR = 8.0 +/- 0.9%; n = 10), either due to high relative flux es 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 ri se of the Labrador Sea (as high as 1.1 g C m(-2) yr(-1) and similar to 0.09 g N m(-2) yr(-1)). They contrast with one order of magnitude lower rates o n the slope, due to low sedimentation rates (SR) and sediment winnowing by the Western Boundary Undercurrent (WBUC). Reduced early diagenetic alterati on 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 pre ferential removal of N-bearing OM also highlighted by losses in total hydro lysable amino acids (HAA). In the Labrador Sea slope records, due to low SR , OM concentration changes Linked to long term temporal variations may supe rimpose on these diagenetic trends, and some influence of the WBUC is notic eable. (C) 1999 Elsevier Science B.V. All rights reserved.