What do benthic δ 13 C and δ 18 O data tell us about Atlantic circulation during Heinrich Stadial 1?

Approximately synchronous with the onset of Heinrich Stadial 1 (HS1), δ 13 C decreased throughout most of the upper (~1000–2500 m) Atlantic, and at some deeper North Atlantic sites. This early deglacial δ 13 C decrease has been alternatively attributed to a reduced fraction of high‐δ 13 C North Atlantic Deep Water (NADW) or to a decrease in the NADW δ 13 C source value. Here we present new benthic δ 18 O and δ 13 C records from three relatively shallow (~1450–1650 m) subpolar Northeast Atlantic cores. With published data from other cores, these data form a depth transect (~1200–3900 m) 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 δ 13 C and δ 18 O occurred in upper waters containing the highest NADW fraction during the LGM. We show that the δ 13 C decrease can be explained entirely by a lower NADW δ 13 C 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 δ 13 C values in the upper North Atlantic. We suggest that the benthic δ 18 O gradients above ~2300 m at both core transects indicate the depth to which heat and North Atlantic deglacial freshwater had mixed into the subsurface ocean by early HS1.