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The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation

机译：冰川中深度放射性碳凸起及其对倾覆环流的影响

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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.
机译：已发布的南大洋大西洋地区放射性碳重建资料表明，在上一次冰河最高期（LGM）期间，放射性碳年龄处于中等深度。这与现代海洋相反，在现代海洋中，水团之间的强烈混合导致相对均匀的放射性碳剖面。法拉利等。 （2014）认为，南极洲海啸期间南极海冰覆盖范围的扩大，需要使子午线翻转环流的上支和下支之间的边界更浅。浅滩边界位于主要的地形特征之上，而这些特征与强烈的斜向混合有关，从而在倾覆环流的下部分支中隔离了稠密的南方水源。这种隔离会使放射性碳衰变，从而为中等深度的放射性碳年龄膨胀提供了可能的解释。我们使用全球翻转环流的理想化二维残差平均动力学模型检验了这一假设。将模拟的倾覆平移的衰变示踪剂的浓度分布与已发布的放射性碳数据进行比较。我们发现，海冰范围增加了600 km（约等于纬度5度），使45°S倾覆环流的上支和下支之间的边界减少了600 m，并减少了北大西洋深水对流的深度50度N到2500m。即使在LGM期间深水源区地表放射性碳年龄没有增加，仅这种环流结构的变化也使中大西洋南部45度S处的放射性碳含量降低了40％。

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