What do benthic δ13C and δ18O data tell us about Atlantic circulation during Heinrich Stadial 1?

Approximately synchronous with the onset of Heinrich Stadial 1 (HS1), δC decreased throughout most of the upper (~1000–2500m) Atlantic, and at some deeper North Atlantic sites. This early deglacial δC decrease has been alternatively attributed to a reduced fraction of high-δC North Atlantic Deep Water (NADW) or to a decrease in the NADW δC source value. Here we present new benthic δO and δC records from three relatively shallow (~1450–1650m) subpolar Northeast Atlantic cores. With published data from other cores, these data form a depth transect (~1200–3900m) in the subpolar Northeast Atlantic. We compare Last Glacial Maximum (LGM) and HS1 data from this transect with data from a depth transect of cores from the Brazil Margin. The largest LGM-to-HS1 decreases in both benthic δC and δO occurred in upper waters containing the highest NADW fraction during the LGM. We show that the δC decrease can be explained entirely by a lower NADW δC source value, entirely by a decrease in the proportion of NADW relative to Southern Ocean Water, or by a combination of these mechanisms. However, building on insights from model simulations, we hypothesize that reduced ventilation due to a weakened but still active Atlantic Meridional Overturning Circulation also contributed to the low δC values in the upper North Atlantic. We suggest that the benthic δO gradients above ~2300m at both core transects indicate the depth to which heat and North Atlantic deglacial freshwater had mixed into the subsurface ocean by early HS1.