Strontium isotopes, ages and carbonate content of sediments from the Agulhas Current

The Agulhas Leakage represents a significant portion of the warm, surface return flow of the global overturning circulation and thus may be an important feedback in the ocean climate system. Models indicate that reduced leakage could be caused by a stronger Agulhas Current and/or a more upstream (eastward) Agulhas Retroflection, while a weaker Agulhas Current would result in a more westward retroflection and increased leakage. However, data for the Last Glacial Maximum support both a weaker Agulhas Current and less leakage, implying a possible displacement of the retroflection. We present new 87Sr/86Sr results for modern sediments within this region, confirming that the modern pathway of the Agulhas Current, Retroflection, and Leakage can be traced by terrigenous sediment provenance using Sr isotopes. New 87Sr/86Sr data from sediments deposited during the Last Glacial Maximum suggest that the glacial Agulhas Current and Retroflection followed nearly their modern trajectory. The provenance data appear to rule out both a stronger Agulhas Current and a more upstream Agulhas Retroflection. We conclude that the reduced glacial leakage was caused by the weakened Agulhas Current, with no significant change in the retroflection position. This is inconsistent with the model predictions and thus emphasizes the need for further work in this region.

阿古拉斯泄漏（Agulhas Leakage）在全球翻转环流的暖表层回流中占据重要比例，因此可能是海洋气候系统中的关键反馈机制。模型研究显示，更强的阿古拉斯海流（Agulhas Current）和/或位置更靠上游（向东）的阿古拉斯回流（Agulhas Retroflection）可能会削弱阿古拉斯泄漏；而较弱的阿古拉斯海流则会导致回流位置更偏西，进而加剧泄漏。不过，末次盛冰期（Last Glacial Maximum）的观测数据同时表明阿古拉斯海流减弱且泄漏减少，这暗示回流位置可能发生了偏移。本研究报道了该区域现代沉积物的全新87Sr/86Sr同位素测试结果，证实可通过陆源沉积物物源（terrigenous sediment provenance）分析，结合锶同位素示踪阿古拉斯海流、回流及泄漏的现代路径。我们还获得了末次盛冰期沉积沉积物的全新87Sr/86Sr同位素数据，结果显示冰期阿古拉斯海流与回流的运动轨迹与现代几乎一致。物源分析结果似乎排除了两种可能性：一是存在更强的阿古拉斯海流，二是阿古拉斯回流位置更靠上游。我们最终得出结论：冰期阿古拉斯泄漏减弱是由阿古拉斯海流减弱导致的，回流位置未发生显著变化。这一结论与模型预测结果相悖，因此凸显了对该区域开展进一步研究的必要性。