中-晚更新世南大洋深部氧化还原条件指示垂向流通状况:以IODP 374航次U1524A孔为例

Deepsea redox conditions in the Southern Ocean indicating vertical ventilation during the Middle-Late Pleistocene: A cause study from IODP Expedition 374 Hole U1524A

  • 摘要: 大洋深部氧化还原条件是诊断大洋深部碳储存、释放的重要参数,在排除/扣除有机质耗氧的基础上,其主要指示大洋深部流通状况。然而,多数研究没有考虑侧向流通和垂向流通的解耦,而笼统地用该指标指示总体或侧向流通,这在垂向流通发育的海区(如南大洋)可能并不正确。针对该科学问题,本文在利用岩芯物理参数划分南极罗斯海IODP 374航次U1524A孔冰期-间冰期旋回基础上,通过蛋白石/Ti、氧化还原敏感元素比值(Mn/Ti和Mo/Ti)和ln (Zr/Rb)分别重建了生产力、深部氧化还原条件和底流流速强度(即侧向流通状况)记录。研究发现,中-晚更新世南极罗斯海生产力、深部氧化还原条件和侧向流通状况表现出明显的冰期旋回特征:冰期低氧化强度(即亚氧化条件)对应低生产力和强侧向流通状况;间冰期高氧化强度(即氧化条件)对应高生产力和弱侧向流通状况。将上述记录与表征环极深层水垂向上涌的南极绕极流强度记录对比,结果表明垂向流通是深部氧化还原条件的主控因素。具体机制为,冰期时西风带北移,同时南极绕极流减弱和海冰扩张,导致环极深层水上涌减弱,从而减弱深部相对氧化强度和深部对大气的碳释放,最终贡献大气pCO2降低。该研究表明,中-晚更新世南大洋深部氧化还原条件指示垂向流通状况,这暗示需谨慎利用大洋深部氧化还原条件笼统地反演深部流通状况。

     

    Abstract: The redox conditions in the deep ocean are important parameters for diagnosing carbon storage/release in abyssal waters. After correcting the oxygen consumption by organic matter, deep-sea redox conditions are regulated mainly by deep-water ventilation. However, in most available studies, the decoupling between lateral and vertical ventilation was often ignored, but instead used this indicator to suggest the overall or lateral ventilation intensity, which may not be correct in sea areas with developed vertical ventilation (such as the Southern Ocean). To address this issue, we reconstructed the productivity (opal/Ti), redox conditions (Mn/Ti and Mo/Ti ratios), and lateral current strength (ln (Zr/Rb)) from International Ocean Discovery Program Hole U1524A in the Ross Sea, Antarctica, dated to the Middle-Late Pleistocene in a glacial-interglacial framework established through physical parameters. Results demonstrate distinct glacial patterns. The glacial periods were characterized by weaker oxidation (suboxic conditions), lower productivity, and stronger lateral currents compared to the interglacial periods that had stronger oxidation (oxic conditions), higher productivity, and weaker lateral current. Comparing these records with the record of the intensity of the Antarctic Circumpolar Current (ACC), which represents the vertical upwelling of deep water of the Circumpolar Deep Water (CDW), the vertical ventilation was proposed to be the dominate process of deep-sea redox condition. The specific mechanism is that during the glacial periods, the westerlies moved northward, while the ACC weakened and sea ice expanded, which collectively suppressed CDW upwelling. The reduced vertical ventilation diminished deep-sea oxidation and carbon release to the atmosphere, and consequently decreased the atmospheric pCO2. These findings demonstrate that the Southern Ocean deep-sea redox conditions in the Middle-Late Pleistocene reflected the vertical ventilation, underscoring the importance of distinguishing between vertical and lateral ventilation to properly interpret deep-sea redox signals.

     

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