末次冰消期南大洋深部流通性增强的罗斯海沉积记录

SEDIMENTARY RECORDS OF ENHANCED DEEP VENTILATION DURING THE LAST DEGLACIALTION IN THE ROSS SEA,SOUTHERN OCEAN

南大洋因其面积广阔等优势,能够存储更多的热量和二氧化碳(CO_(2)),因此在全球碳循环及气候变化中的地位十分重要。罗斯海作为南大洋第二大边缘海,是研究古海洋演化的理想海域。本研究采用罗斯海陆坡和海盆区的3根插管沉积物岩芯——BC008(水深1063 m,长27 cm,年龄6.0~14.8 ka B.P.)、BC010(水深2055 m,长44 cm,年龄0~15.5 ka B.P.)和BC006(水深2120 m,长54 cm,年龄0~22.3 ka B.P.),通过分析其生物硅含量及浮游有孔虫碳同位素比值(Nps-δ^(13)C)的变化发现,生物硅含量在末次冰消期较高,在约16 ka B.P.达到极大值,这指示了冰消期罗斯海海域深层水上涌增强并在约16 ka B.P.最为剧烈。与此同时,Nps-δ^(13)C的负偏,指示了南大洋上涌的水团将溶解的硅酸盐传递至海洋表层的同时,也将碳同位素等化学信号传递至表层海水。深层水上涌在末次冰消期显著增强的趋势,与大气CO_(2)浓度在冰消期之后的急剧上升十分吻合,这进一步验证了冰消期南大洋深层水上涌的假说及其对大气CO_(2)浓度上升的贡献。此外,本研究进一步讨论了引起末次冰消期南大洋深层水上涌的可能触发机制,主要可能因南北两极热量分布不均,导致南半球西风带位置和强度以及大西洋经向翻转流强度发生变化,进而驱动南大洋深层水上涌。

The Southern Ocean,also known as the Antarctic Ocean,is an enormous body of water.It plays an important role in the global carbon cycle and climate change because the ventilation of deep Southern Ocean is tightly linked to glacial-interglacial variations in atmospheric carbon dioxide(CO_(2)).Being the second largest marginal sea of the Southern Ocean and one source of global ocean deep water,the Ross Sea is an ideal area for deep ocean ventilation and paleoceanography studies.In this study,three sedimentary cores BC008(74.13°S,176.34°W;water depth 1063 m;length 27 cm),BC010(74.10°S,175.77°W;water depth 2055 m;length 44 cm)and BC006(74.04°S,175.67°W;water depth 2120 m;length 54 cm)retrieved from the slope and adjacent basin of the Ross Sea.These cores were mainly composed of sandy silt and silty sand and were selected for biogenic silica content and foraminifera isotope ratio analysis.The sedimentary record is dated using accelerator mass spectrometer radiocarbon(AMS14C)methods on acid insoluble organic matter and foraminiferal calcite.The ages of BC008,BC010 and BC006 cores range from 6.0 ka B.P.to 14.8 ka B.P.,from 0 to 15.5 ka B.P and from 0 to 22.3 ka B.P,respectively.The biogenic silica contents in the sediment are relatively high during the last deglaciation with the maximum value at about 16 ka B.P.indicate that the upwelling of deep water in the Ross Sea increased during this period and reach the most intense at about 16 ka B.P.The similar pattern on planktonic foraminifera Nps-δ^(13)C changes during the last deglaciation indicate that the upwelling water masses in the Southern Ocean transmit not only dissolved silicate,but also some chemical signals such as carbon isotope to the surface water.In addition,the upward trend of deep water upwelling during the last deglaciation is consistent with the sharp increase of atmospheric CO_(2) concentration after the last deglaciation,which further verifies the hypothesis of deep water upwelling in the Southern Ocean and its contribution to the increase of atmospheric CO_(2) concentration.Moreover,the possible triggering mechanisms of deep water upwelling in the Southern Ocean during the last deglaciation are further discussed.The main trigger mechanisms may be the uneven distribution of heat between the north and the south poles,which resulting in changes in the location and intensity of the Southern Hemisphere Westerly Winds belt(SWW)and the strength of Atlantic Meridional Overturning Circulation(AMOC),thus driving the upwelling of deep water in the Southern Ocean.The sediment records of biogenic silica and carbon isotope ratio of planktonic foraminifera in the Ross Sea imply that the second rapid increase in atmospheric CO_(2)(during YD)is driven by processes operating elsewhere in the Southern Ocean,or another region.