北大西洋墨西拿期古海洋学:有孔虫和稳定同位素证据  

MESSINIAN PALEOCEANOGRAPHY IN THE NORTH ATLANTIC OCEAN:FORAMINIFERAL AND STABLE ISOTOPIC EVIDENCE

作　　者：章纪君 ZHANG Jijun(The Micropaleontology Project,Queens College,6530 Kissena Boulevard,Flushing,New York,NY 11367,USA)

机构地区：[1]美国微体古生物出版社,美国纽约11367

出　　处：《微体古生物学报》2023年第3期262-291,共30页Acta Micropalaeontologica Sinica

摘　　要：墨西拿期(7.2-5.3 Ma)被认为是中新世最重要的时期之一。在上世纪七十年代初,大洋钻探计划(DSDP)对地中海的首次大洋钻探发现整个地中海盆地存在着大量晚墨西拿期的盐类沉积,其上为上新世早期的正常大洋沉积所覆盖,科学家们称之为灾难性的“墨西拿盐度危机”。在接下来的几十年里,科学家们积极尝试寻找地中海盐度危机不容置疑的影响与全球海洋水体历史之间的关系,但收效甚微。在这项研究中,我们对北大西洋的三个DSDP钻孔(552A、608和547A)中169个晚中新世–早上新世岩芯样品中的有孔虫进行了详细研究,包括时常被忽略的小型底栖有孔虫,并对其中82个底栖有孔虫样品和70个浮游有孔虫样品中的特别物种进行了^(18)O和^(13)C分析,以重建北大西洋中新世末期深层环流模式和评估深海环流和墨西拿盐度危机的演变历史。根据δ^(18)O曲线,我们识别出了墨西拿期有九个气候低谷期(M-9到M-1,按地层顺序),其中M-1和M-5期表明冰川加剧使海平面降低了至少60-73米。在这些事件中,随着冰锋从格陵兰岛向冰岛南部扩展,北大西洋深层水强度明显减弱,甚至断流,导致北大西洋深水环流系统发生重大变化。然而,在大部分墨西拿期时间里,我们发现有四个明显的底栖有孔虫Epistominellaexigua丰度主峰(EEP-Ⅳ至EEP-Ⅰ),表明此时的大西洋中部深水区主要被NADW充灌,而此时的南极底层水(AABW)则仅至~43oN附近。此外,这些EEP主峰的碳同位素正偏当归因于高海平面和较弱的大陆侵蚀。通过有孔虫及稳定同位素的综合分析,可以识别出大西洋墨西拿时期具有三个主要古海洋学阶段:1)第一阶段,相当于墨西拿盐度危机,相对于NADW, AABW在北大西洋(522A钻孔)进一步减弱。在EEP-Ⅱ中NADW两次主要低谷期之间, E. exigua丰度陡增表明此时有一短暂的“间冰期”回返, NADW也从大西洋流入地中海盆地,产The Messinian Stage(7.2−5.3 Ma)is considered to be one of the most important periods in the Miocene.In the 1970s,the results of the first Mediterranean deep-sea drillings were interpreted to mean that the entire Mediterranean Sea had extensively evaporated during a catastrophic“Messinian Salinity Crisis”,evidenced by a giant body of late Messinian salts covering the floor of the basin,overlain by normal deep sea deposits of earliest Pliocene age.In the following decades,scientists have vigorously tried to find a relationship between the unquestioned impact of the Mediterranean Salinity Crisis and the history of the global ocean water mass,with very limited results.In this study,we processed 169 core samples of Late Miocene–Early Pliocene age from North Atlantic DSDP sites 552A,608 and 547A in order to extract foraminiferal materials including the sometimes ignored smaller sized species.Selected species from a total of 82 samples of benthic foraminifera and 70 samples of planktonic foraminifera were analyzed for ^(18)O/^(16)O and 13C/12C ratios to reconstruct end-Miocene deep-water circulation in the North Atlantic Ocean and to evaluate the relationship between this abyssal circulation and the evolution of the Messinian Salinity Crisis.Based on theδ^(18)O curves,nine climate lows(M-9 to M-1,in stratigraphic order)of Messinian age were recognized,with episodes M-1 and M-5 indicating glacial increases that lowered sea-levels by at least 60~73 m.With glacial fronts expanding from Greenland to the south of Iceland during these episodes,NADW(North Atlantic Deep Water)bodies were eliminated or reduced,leading to major changes in North Atlantic deep water circulation.For most of the Messinian,however,four extended peaks in the abundance of benthic Epistominella exigua(EEP-IV to EEP-I)indicate that the depths of the central Atlantic Ocean were predominantly filled with NADW,while Antarctic Bottom Water(AABW)only reached as far as~43oN.The positive shifts in the carbon isotope ratios during the EEP periods are att

关 键 词：地中海盐度危机 ^(18)O和^(13)C 偏移 干枯 古海洋学 生物地层学 有孔虫 超微化石 冰川海平面 北大西洋深层水 南极底层水 地中海外流水 速溶层 碳酸盐补偿深度 墨西拿期 中新世 

分 类 号：Q911[天文地球—古生物学与地层学] P597[生物学—古生物学]