Freshwater sources to the coastal ocean off northeastern North America: Evidence from H218O/H216O

Oxygen isotope (H 2 18 O/H 2 16 O)/salinity data identify freshwater sources along the northeastern North American continental margin. The oxygen isotope (δ 18 O)‐salinity ( S ) properties of various water types are distinguished. Sea ice formation on the Labrador Shelf is shown to influence δ 18 O‐ S values. It is estimated that 2–3 m of freshwater is extracted from the water column to form sea ice. It is hypothesized that waters on the Scotian Shelf, Gulf of Maine, and the Middle Atlantic Bight are composed of slope water diluted by an upstream low‐salinity source. This upstream source is a mixture of brine‐enriched Labrador Shelf Water and St. Lawrence River water. The δ 18 O value of the apparent freshwater component (δ 18 O S =0) of waters on the Scotian Shelf and farther south is ≈−20‰, which has been used to suggest a sole high‐latitude freshwater source. We show instead that the St. Lawrence River contributes ≈35% of the freshwater on the Scotian Shelf, in agreement with the physical oceanographic evidence. The remaining freshwater is supplied by high‐latitude rivers dominated by Arctic runoff. Finally, the isotope evidence identifies Baffin Bay as an important pathway by which freshwater from the Arctic Ocean can reach the Labrador Sea.