Late Miocene to earliest Pliocene stable isotope record and sedimentology of ODP Site 114-704

Carbonate stratigraphy and stable isotopic ratios of benthic and planktonic foraminifers were used to study paleoceanographic changes that occurred during the late Miocene to earliest Pliocene in the subantarctic South Atlantic, between 9.8 and 4.5 Ma in ODP Hole 704B on the Meteor Rise (47°S, 7°E; 2532 m water depth). During the late Miocene, between 9.8 and 6.4 Ma, carbonate content was high with little variability (generally 84.5% ± 10%), with sustained productivity dominated by foraminifers and calcareous nannoplankton in surface waters north of the Subantarctic Front. Decreased carbonate (40%), along with first significant occurrence of biogenic opal, occurred between 8.45 and 8.2 Ma. The first signals of increased cooling occurred between 8.8 and 8.0 Ma. The interval from 6.3 to 4.5 Ma represents low carbonate values with high variability (61.7% ± 17%), suggesting markedly fluctuating conditions in the production and/or dissolution of carbonate. The onset of this interval in Hole 704B is marked by a decrease in carbonate values and a well-defined 0.85 per mil decrease in d13C values of both planktonic and benthic foraminifers between 6.4 and 6.0 Ma, correlated to the Chron C3AR (upper reversed of Chron 6) "carbon shift." The interval of the carbon shift (6.4 to 6.0 Ma) is characterized by decreasing d18O values, anomalously low d18O minima in planktonic foraminifers, and intervals dominated by temperate and low-latitude diatom and silicoflagellate assemblages, suggesting warm interglacial conditions with brief events of extreme warming or low salinity in the subantarctic South Atlantic. An anomalously low planktonic d18O minimum at 6.15 Ma in Hole 704B is correlated with the deposition of the laminated organic-rich Neobrunia ooze found in Hole 701 during this leg and the Ethmodiscus ooze drilled at DSDP Site 520 on Leg 73. Suboxic bottom water may have formed in response to a meltwater lid that temporarily halted the production of Antarctic Bottom Water during the rapid deglaciation events that were occurring in West Antarctica. The benthic d18O record displays a strong glacial interval between 5.8 and 5.4 Ma, which is coincident with the time of a major increase in an upwelling diatom assemblage between 6.0 and 5.4 Ma. The earliest Gilbert Chron (5.35 to 4.77 Ma) was marked by an intense carbonate dissolution event and low surface productivity. This dissolution event, recognized globally, ended abruptly near the base of the Thvera Subchron at 4.8 Ma when carbonate values increased to 75.3% ± 5.2%. The low carbonate content and highly variable surface water conditions between 6.3 and 4.8 Ma are time equivalent to the Messinian Stage in the Mediterranean, suggesting a possible link between the Messinian salinity crisis and the Southern Ocean during the latest Miocene.本研究以底栖与浮游有孔虫的碳酸盐地层学与稳定同位素比值为分析手段，对南大西洋亚南极区9.8~4.5 Ma期间（晚中新世至早上新世）的古海洋环境变化展开研究，采样位点为流星海岭上的大洋钻探计划(Ocean Drilling Program, ODP)704B井，地理位置为南纬47°、东经7°，水深2532米。 晚中新世9.8~6.4 Ma期间，碳酸盐含量较高且波动极小（平均约84.5%±10%），亚南极锋(Subantarctic Front)北侧表层水体中以有孔虫与钙质超微浮游植物为主的初级生产力持续旺盛。8.45~8.2 Ma期间，碳酸盐含量下降至40%，同时首次出现大量生物成因蛋白石；8.8~8.0 Ma期间则出现了首次显著的降温信号。 6.3~4.5 Ma期间，碳酸盐含量整体偏低且波动幅度较大（平均61.7%±17%），指示碳酸盐的生成与/或溶解过程处于显著波动状态。704B井该层位的起始标志为6.4~6.0 Ma期间碳酸盐含量下降，以及浮游与底栖有孔虫的δ13C值均出现明确的0.85千分比下降，该事件与C3AR地质年代带（第6地质年代带的上部反向带）的‘碳同位素漂移’相对应。6.4~6.0 Ma的碳同位素漂移层位以δ18O值持续降低、浮游有孔虫出现异常偏低的δ18O极小值，以及以温带、低纬度硅藻与硅鞭藻组合为主的沉积段为特征，指示南大西洋亚南极区存在温暖的间冰期环境，伴随短期极端增温或低盐度事件。704B井6.15 Ma处出现的异常偏低浮游有孔虫δ18O极小值，与本航次701井中发现的纹层状富有机质Neobrunia软泥，以及第73航次中在深海钻探计划(Deep Sea Drilling Program, DSDP)520站位钻取的Ethmodiscus软泥的沉积时期相吻合。西南极洲快速冰消事件期间，曾出现融水盖层暂时抑制南极底层水(Antarctic Bottom Water)生成的情况，由此可能形成了缺氧底层水。 底栖有孔虫的δ18O记录显示，5.8~5.4 Ma期间存在显著的冰期层位，该时段与6.0~5.4 Ma期间上升流硅藻组合大幅繁盛的时期相吻合。吉尔伯特最早地质年代带（5.35~4.77 Ma）以强烈的碳酸盐溶解事件与表层生产力低下为特征。这一全球均有记录的溶解事件在4.8 Ma（Thvera亚年代带底部附近）突然结束，此时碳酸盐含量回升至75.3%±5.2%。6.3~4.8 Ma期间的低碳酸盐含量与表层水体剧烈波动的环境，与地中海的墨西拿阶(Messinian Stage)时限相当，暗示晚中新世末期墨西拿盐度危机与南大洋之间可能存在关联。