Historical and geographical variations of sources and transport of terrigenous organic matter within a large-scale coastal environment

Elemental and molecular analyses indicate that the sources and inputs of terrigenous organic matter (OM) to the upper St. Lawrence system have been influenced by increased discharges of industrial solid organic wastes from the pulp and paper industry following its expansion in the 1920–1940ʹs. Moderately altered lignin-rich particles from a combination of natural and anthropogenic sources predominate within recent sediments of this system, with anthropogenic fractions ranging 10–70% and 2–30% for the Fjord and the Lower Estuary, respectively. Compositional and isotopic signatures of sedimentary OM show that the sediments within the Lower Estuary are dominated by inputs of allochthonous OM (60–80%), whereas terrigenous OM inputs are minor (15–30%) sources of OM to the Gulf/Shelf sediments. In this latter environment, the terrigenous OM pool is composed exclusively of highly altered lignin-poor soil OM with no substantial influence from anthropogenic lignin. A global mass balance calculation suggests that about half of the global annual riverine flux is degraded, leaving only the remaining half to accumulate predominantly (98%) within shelf and slope sediments. This estimate suggests that lignin does not behave conservatively within the marine environment but supports some sort of organic matter degradation.