Deep ocean circulation modulated glacial–interglacial climates through
feedbacks to the carbon cycle and energy distribution. Past work has
suggested that contraction of well-ventilated North Atlantic Deep Water
during glacial times facilitated carbon storage in the deep ocean and
drawdown of atmospheric CO2 levels. However, the spatial extent and
properties of different water masses remain uncertain, in part due to
conflicting palaeoceanographic proxy reconstructions. Here we combine five
independent proxies to increase confidence and reconstruct Atlantic deep
water distributions during the Last Glacial Maximum (around 21 thousand
years ago) and the following Heinrich Stadial 1—a time when massive ice
rafting in the North Atlantic interfered with deep water formation and caused
global climate shifts. We find that North Atlantic Deep Water remained
widespread in both periods, although its properties shifted from a cold,
well-ventilated mode to a less-ventilated, possibly warmer, mode. This finding
implies a remarkable persistence of deep water formation under these cold
boundary conditions, sustained by compensation between the two formation
modes. Our constraints provide an important benchmark for evaluating Earth
system models, which can enhance confidence in future climate projections.