Isotope stratigraphy, sedimentation rates, deep circulation, and carbonate events in the Labrador Sea during the last ~ 200 ka

Abstract

High-resolution stable-isotope and accelerator mass spectrometry 14C measurements in 25 box and piston cores from the Labrador Sea allow the reconstruction of major paleoceanographical changes during the last ~ 200 ka in this basin and also document the links between the interior of the Laurentide Ice Sheet and the North Atlantic Ocean during this interval. Two deep circulation regimes are indicated by contrasting sedimentation rates on the slopes: (i) the modern situation is characterized by a gyre of the North Atlantic Deep Water (NADW) components into the basin, which is responsible for the high-energy Western Boundary Undercurrent; this regime also prevailed during the former "true" interglacials and possibly during shorter "warm" intervals (e.g., Bølling-Allerød); (ii) a more frequent NADW-free situation during glacial and interstadial intervals is marked by the presence of a vertically homogeneous water mass in the basin. Sedimentological records also indicate that carbonate events (i.e., pulses of detrital Paleozoic carbonates from Hudson Strait) and Heinrich events (sharp increases in ice-rafted lithic fragments), which are observed during the last two glaciations, are due to two distinct mechanisms. We hypothesize that detrital-carbonate events are linked to the triggering of turbidity currents in the western Labrador Sea by surges of the ice margin off Hudson Strait, and that the North Atlantic Mid-Ocean Channel, and possibly the Charlie Gibbs Fracture Zone, played a role in the spreading of these carbonates into the North Atlantic. Rapid calving would have been responsible for the intense ice rafting responsible for the Heinrich events stricto sensu. During such intervals, low productivity in surface waters is inferred from light carbon events and low concentrations in primary productivity indicators.