Despite its importance for climate and the carbon cycle, the deep ocean circulation during the Last Glacial Maximum (LGM) remains poorly understood. Whereas most studies suggest a shallower Atlantic Meridional Overturning Circulation (AMOC) than at present day, there is disagreement about its transport rate, with estimates ranging from stronger to weaker than today. Older deep ocean radiocarbon ages have been suggested to imply a more sluggish circulation. Here we use a global isotope-enabled ocean-climate model to systematically explore the different effects of AMOC depth and strength on carbon isotope (¹³C and radiocarbon) distributions and constraints provided by sediment data. We find that existing data constrain the AMOC depth well, favoring simulations with a shallower-than-present LGM AMOC reaching $2000-2500$ m of depth. However, they provide weaker constraints on AMOC strength. Comparisons with two high vertical resolution LGM ${\delta }^{13}$C profiles suggest LGM AMOC strength between 11 and 18 Sv, but more data are needed to refine this estimate. Contrary to past conjectures, we find radiocarbon age to be only weakly related with deep water transport rates, but strongly dependent on Southern Ocean surface reservoir ages, which are highly correlated with AMOC depth. In addition, upon changes of deep transport rates and/or water mass geometry, variations in modeled ${\delta }^{13}$C and radiocarbon age are highly correlated, suggesting that they do not act as independent traces for physical ocean processes.

尽管它对气候和碳循环很重要，但对最后一次冰河最高峰（LGM）期间的深海循环仍然知之甚少。尽管大多数研究表明，大西洋经络翻转环流（AMOC）比今天要浅，但对其运输速度却存在分歧，估计范围比今天要强。有人认为较深的深海放射性碳年龄暗示着更慢的循环。在这里，我们使用全球同位素使能的海洋气候模型来系统地探索AMOC深度和强度对沉积物数据提供的碳同位素（¹³ C和放射性碳）分布和约束的不同影响。我们发现现有数据很好地限制了AMOC的深度，有利于模拟LGM AMOC小于当前值$2000-2500$米的深度。但是，它们对AMOC强度的约束较弱。与两个高垂直分辨率LGM的比较${\delta }^{13}$C曲线表明LGM AMOC强度在11至18 Sv之间，但是还需要更多数据来完善此估算值。与过去的推测相反，我们发现放射性碳年龄仅与深水运输速率弱相关，而强烈依赖于与AMOC深度高度相关的南大洋表面水库年龄。此外，随着深层运输速度和/或水团几何形状的变化，模型的变化${\delta }^{13}$碳和放射性碳年龄高度相关，表明它们并不充当海洋物理过程的独立痕迹。