A deep‐sea coral record of North Atlantic radiocarbon through the Younger Dryas: Evidence for intermediate water/deepwater reorganization

Our record of Younger Dryas intermediate‐depth seawater Δ 14 C from North Atlantic deep‐sea corals supports a link between abrupt climate change and intermediate ocean variability. Our data show that northern source intermediate water (∼1700 m) was partially replaced by 14 C‐depleted southern source water at the onset of the event, consistent with a reduction in the rate of North Atlantic Deep Water formation. This transition requires the existence of large, mobile gradients of Δ 14 C in the ocean during the Younger Dryas. The Δ 14 C water column profile from Keigwin (2004) provides direct evidence for the presence of one such gradient at the beginning of the Younger Dryas (∼12.9 ka), with a 100‰ offset between shallow (<∼2400 m) and deep water. Our early Younger Dryas data are consistent with this profile and also show a Δ 14 C inversion, with 35‰ more enriched water at ∼2400 m than at ∼1700 m. This feature is probably the result of mixing between relatively well 14 C ventilated northern source water and more poorly 14 C ventilated southern source intermediate water, which is slightly shallower. Over the rest of the Younger Dryas our intermediate water/deepwater coral Δ 14 C data gradually increase, while the atmosphere Δ 14 C drops. For a very brief interval at ∼12.0 ka and at the end of the Younger Dryas (11.5 ka), intermediate water Δ 14 C (∼1200 m) approached atmospheric Δ 14 C. These enriched Δ 14 C results suggest an enhanced initial Δ 14 C content of the water and demonstrate the presence of large lateral Δ 14 C gradients in the intermediate/deep ocean in addition to the sharp vertical shift at ∼2500 m. The transient Δ 14 C enrichment at ∼12.0 ka occurred in the middle of the Younger Dryas and demonstrates that there is at least one time when the intermediate/deep ocean underwent dramatic change but with much smaller effects in other paleoclimatic records.