晚更新世冰期旋回成因的南大洋机制及其对北极冰盖形成的启示

The Southern Ocean mechanism of the Late Pleistocene glacial cycling and its implications for the formation of the northern hemisphere ice sheet

目前,学术界普遍认为南大洋对调节晚更新世冰期/间冰期大气二氧化碳分压(pCO_(2))变化发挥了重要作用。在晚更新世,冰期大气pCO_(2)比间冰期大气pCO_(2)下降约90×10^(−6)。而在约2.7 Ma,随着北极冰盖快速扩张(intensification of Northern Hemisphere Glaciation,iNHG),冰期旋回振幅增大,大气pCO_(2)也骤降。探究晚更新世冰期及iNHG时期大气pCO_(2)下降的原因,对构建完整的冰期旋回理论意义重大。本文综合晚更新世冰期南大洋北部的亚南极区(Subantarctic Antarctic Zone,SAZ)和南部的南极区(Antarctic Zone,AZ)的洋流、海冰、生产力等记录,分析了两个区域在晚更新世冰期可能的储碳机制,并结合iNHG时期各项地质记录,讨论南大洋洋流、碳储库在iNHG期间所发生的变化。结果认为,SAZ和AZ使冰期大气pCO_(2)下降的机制不完全相同,铁肥输入增强导致SAZ生物泵效率增强,增加了海洋固碳,而在AZ,深水通风减弱、海冰扩张、深海分层增强是加强深海碳封存的关键机制。同时,iNHG时期洋流、碳储库等记录表明,南源水向北大西洋、北太平洋深部显著扩张,南大洋海冰扩张,铁肥输入增强,太平洋碳储库增大,暗示iNHG时期南大洋机制可能是大气pCO_(2)下降的原因,直接造成了晚上新世北极冰盖的最终形成。

It has been generally believed that the Southern Ocean has played an important role in modulating glacial/interglacial changes of the atmospheric partial pressure of carbon dioxide(pCO_(2))during the Late Pleistocene.In the late Pleistocene,the atmospheric pCO_(2)during the glacial periods was about 90×10^(−6) lower than that during the interglacial periods.Furthermore,in around 2.7 Ma,with the intensification of the Northern Hemisphere Glaciation(iNHG),the amplitude of glacial cycles increased,while the atmospheric pCO_(2)greatly decreased.Exploring the reasons for the decline in atmospheric pCO_(2)during the Late Pleistocene glaciation and the iNHG period is of great significance for constructing a complete theory of the Ice Ages.We combined the records of ocean currents,sea ice,and productivity in the Subantarctic Antarctic Zone(SAZ)in the northern part of the Southern Ocean and the Antarctic Zone(AZ)in the southern part of the Southern Ocean during the Late Pleistocene glaciation,investigated the possible carbon storage mechanisms in these two regions during this period,and discussed the changes in the Southern Ocean currents and carbon reservoirs during the iNHG period by integrating geological records.We proposed that SAZ and AZ had different carbon storage mechanisms during ice ages.The enhancement of iron fertilization increased the biological pump efficiency in the SAZ,thus increasing ocean carbon sequestration.Meanwhile,in the AZ,weakened deep-water ventilation,sea ice expansion,and enhanced deep-sea stratification were the key mechanisms for enhancing deep-sea carbon sequestration.Additionally,records of ocean currents and carbon reservoirs during the iNHG period indicate that southern ocean sourced waters expanded significantly towards the deep North Atlantic and North Pacific,with an expansion of sea ice in the Southern Ocean,enhancement in iron fertilization,and the increase in the Pacific carbon reservoir.We infer that the Southern Ocean mechanisms of the Late Pleistocene ice glacial cycling had probably contributed greatly to the decrease in the atmospheric pCO_(2)during the period of the iNHG,which triggered the final formation of the northern hemisphere glaciation.