The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation
The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation
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DOI：
10.1002/2015pa002778
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发表时间：
2015-07-01
期刊：
影响因子：
--
通讯作者：
Thompson, Andrew F.
中科院分区：
文献类型：
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作者：
Burke, Andrea;Stewart, Andrew L.;Thompson, Andrew F.
已发表的南大洋大西洋部分放射性碳重建表明，在末次盛冰期（LGM）期间，存在放射性碳年龄的中层最大值。这与现代海洋形成对比，现代海洋中水团之间的强烈混合导致相对均匀的放射性碳分布。Ferrari等人（2014年）认为，末次盛冰期期间南极海冰覆盖范围的扩大使得赤道翻转环流的上下分支之间的边界必须较浅。这浅滩边界奠定以上的主要地形特征与强大的diapycnal混合，隔离在较低的翻转环流的分支密集的南方水源水。这种隔离可能会使放射性碳衰变，从而为中等深度的放射性碳年龄凸起提供了一个可能的解释。我们使用一个理想化的，2-D的，剩余平均动力学模式的全球翻转环流测试这一假设。浓度分布的衰减示踪剂是平流模拟翻转发表的放射性碳数据进行比较。我们发现，600公里（类似于5度的纬度）的海冰范围增加浅滩之间的边界翻转环流的上，下分支在45度S 600米和浅滩的深度北大西洋深水对流在50度N 2500米。这种变化在流通配置单独减少了40%的南大西洋中深45度的放射性碳含量，即使没有增加在LGM期间的深沃茨的源区的表面放射性碳年龄。
Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate that there is amid-depth maximum in radiocarbon age during the Last Glacial Maximum (LGM). This is in contrast to the modern ocean where intense mixing between water masses results in a relatively homogenous radiocarbon profile. Ferrari et al. (2014) suggested that the extended Antarctic sea ice cover during the LGM necessitated a shallower boundary between the upper and lower branches of the meridional overturning circulation. This shoaled boundary lay above major topographic features associated with strong diapycnal mixing, isolating dense southern sourced water in the lower branch of the overturning circulation. This isolation would have allowed radiocarbon to decay and thus provides a possible explanation for the mid-depth radiocarbon age bulge. We test this hypothesis using an idealized, 2-D, residual-mean dynamicalmodel of the global overturning circulation. Concentration distributions of a decaying tracer that is advected by the simulated overturning are compared to published radiocarbon data. We find that a 600 km (similar to 5 degrees of latitude) increase in sea ice extent shoals the boundary between the upper and lower branches of the overturning circulation at 45 degrees S by 600 m and shoals the depth of North Atlantic Deep Water convection at 50 degrees N by 2500m. This change in circulation configuration alone decreases the radiocarbon content in the mid-depth South Atlantic at 45 degrees S by 40%, even without an increase in surface radiocarbon age in the source region of deep waters during the LGM.