A deep-sea coral record of the North Atlantic

Our record of Younger Dryas intermediate-depth seawater D14C 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 14C-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 D14C in the ocean during the Younger Dryas. The D14C 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 per mil offset between shallow (<~2400 m) and deep water. Our early Younger Dryas data are consistent with this profile and also show a D14C inversion, with 35 per mil more enriched water at ~2400 m than at ~1700 m. This feature is probably the result of mixing between relatively well 14C ventilated northern source water and more poorly 14C ventilated southern source intermediate water, which is slightly shallower. Over the rest of the Younger Dryas our intermediate water/deepwater coral D14C data gradually increase, while the atmosphere D14C drops. For a very brief interval at ~12.0 ka and at the end of the Younger Dryas (11.5 ka), intermediate water D14C (~1200 m) approached atmospheric D14C. These enriched D14C results suggest an enhanced initial D14C content of the water and demonstrate the presence of large lateral D14C gradients in the intermediate/deep ocean in addition to the sharp vertical shift at ~2500 m. The transient D14C 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.

本研究依托北大西洋深海珊瑚获取的新仙女木事件（Younger Dryas）中层海水Δ¹⁴C记录，证实了突发性气候突变与中层海洋变率之间存在关联。数据显示，在该事件起始阶段，约1700米深度的北半球源中层海水被贫¹⁴C的南半球源海水部分取代，这与北大西洋深层水形成速率降低的结论一致。这一转变过程表明，新仙女木期海洋中存在显著且可移动的Δ¹⁴C梯度。Keigwin（2004）发表的Δ¹⁴C水柱剖面数据，为新仙女木期起始阶段（约12.9 ka）存在此类梯度提供了直接证据，该剖面显示浅于约2400米的水体与深水区之间存在100‰的差值。本研究的新仙女木早期数据与该剖面结果吻合，同时还观测到Δ¹⁴C倒置现象：约2400米深度处的水体Δ¹⁴C较1700米深度处富集35‰。这一特征大概率是由¹⁴C交换充分的北半球源海水与¹⁴C交换程度较低的南半球源中层海水混合所致，后者的分布深度略浅。在新仙女木期剩余时段，本研究获取的中层/深层珊瑚Δ¹⁴C数据呈逐渐升高趋势，而大气Δ¹⁴C含量则持续下降。在约12.0 ka的极短时段以及新仙女木期结束阶段（11.5 ka），约1200米深度的中层海水Δ¹⁴C接近大气Δ¹⁴C水平。这些Δ¹⁴C富集的观测结果表明，该水体初始Δ¹⁴C含量有所升高，同时证实了在中层/深层海洋中，除约2500米深度处的显著垂直偏移外，还存在显著的侧向Δ¹⁴C梯度。约12.0 ka处出现的瞬时Δ¹⁴C富集事件发生于新仙女木期中期，这表明至少存在一个时段，中层/深层海洋发生了剧烈变化，但该变化在其他古气候记录中体现的效应却相对微弱。