Probably the most striking feature of the record during this CHPG time is the ∼250‰∼250‰ drop in intermediate water Δ14C between 27.6–26.2 ka. The trajectory of this drop closely matches that of 14C decay, implicating in situ ageing of a water mass. It also closely matches the initial portion of a Δ14C drop from the Brazil Margin observed by Mangini et al. starting at 27.9 ka (Supplemental Fig. 3) (Mangini et al., 2010). Mangini et al. suggest that this drop in intermediate water Δ14C is related to a slowdown in the meridional overturning circulation during Heinrich 2. Yet a circulation slowdown that would give decay-like Δ14C trajectories implies complete stagnation of the water, or an unlikely mixing scenario between multiple water masses. In addition, deep circulation changes in the North Atlantic and southern sourced intermediate waters are not expected to be in phase if the ‘seesaw’ mechanism (Broecker, 1998) is still at work during this time. Another possible mechanism is mixing with 14C-dead water. While zone of physiological stress would be difficult to simulate a decay-like Δ14C trajectory by mixing water masses that contain radiocarbon, a 50:50 mixture with 14C-dead water over 5730 yr naturally mimics decay. If mixing with 14C-dead water was the cause of the decay-like drop in Δ14C at ∼27.6 ka, however, the source of this water remains elusive.
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