Late Holocene radiocarbon variability in Northwest Atlantic slope waters

Deep-sea gorgonian corals secrete a 2-part skeleton of calcite, derived from dissolved inorganic carbon at depth, and gorgonin, derived from recently fixed and exported particulate organic matter. Radiocarbon contents of the calcite and gorgonin provide direct measures of seawater radiocarbon at depth and in the overlying surface waters, respectively. Using specimens collected from Northwest Atlantic slope waters, we generated radiocarbon records for surface and upper intermediate water layers spanning the pre- and post-bomb-14C eras. In Labrador Slope Water (LSW), convective mixing homogenizes the pre-bomb Δ14C signature (−67±4‰) to at least 1000 m depth. Surface water bomb-14C signals were lagged and damped (peaking at ∼+45‰ in the early 1980s) relative to other regions of the Northwest Atlantic, and intermediate water signals were damped further. Off southwest Nova Scotia, the vertical gradient in Δ14C is much stronger. In surface water, pre-bomb Δ14C averaged −75±5‰. At 250–475 m depth, pre-bomb Δ14C oscillated quasi-decadally between −80 and −100‰, likely reflecting interannual variability in the presence of Labrador Slope Water vs. Warm Slope Water (WSW). Finally, subfossil corals reveal no systematic changes in vertical Δ14C gradients over the last 1200 yr.