南海北部近6 Ma以来中层水体演化的底栖有孔虫证据

Evolution of Intermediate Water in the Northern South China Sea During the Last 6 Million Years:Evidence from Benthic Foraminifera

本文对南海北部陆坡SH7B孔底栖有孔虫群落结构组成及壳体氧碳同位素值变化特征进行研究,旨在了解该区晚中新世以来的中层水体演化历史。利用因子和聚类分析对该钻孔128个样品中的35个底栖有孔虫优势属种的相对百分含量数据进行统计分析,识别出4个组合:Globocassidulina subglobosa-Stilostomella spp.(Gs-St),Chilostomella mediterranensis-Globobulimina spp.(Cm-Gl),Hoglundina elegans-Pyrgo spp.(He-Py),Melonis affinis-Pyrgo spp.(Ma-Py)。晚中新世晚期至上新世早期(5.8~2.8 Ma)相对高海平面温暖期,SH7B孔底栖有孔虫为相对稳定的Gs-St组合,反映了低到中等营养物质供给的低氧水体环境。其中5.8~3.79 Ma底栖有孔虫壳体碳同位素偏负和强烈的碳酸盐溶解现象在全球其他海区均有发现,指示当时南海北部陆坡底层水体低氧环境可能受到因全球大洋环流格局变化影响的太平洋偏腐蚀性的缺氧中层水体影响。3.79~2.8 Ma底栖有孔虫丰度和喜氧类表生种含量逐渐增加,指示水体流通性趋于增强,水体氧含量稍有增加。晚上新世2.4 Ma以来,底栖有孔虫组合与氧碳同位素值波动变化频繁,反映了中等到高表层输出生产力的富氧到缺氧中层水体环境,南海北部陆坡底栖有孔虫的分布主要受陆源有机质输入量的多寡和南海中深层水体的循环状况共同控制。

In order to investigate the evolution of intermediate water in Shenhu area on the northern slope of the South China Sea(SCS) from Late Miocene,benthic foraminiferal assemblage composition and stable oxygen and carbon isotope values from core SH7B were studied.Factor and cluster statistical analysis were used to perform based on the quantitative census data of benthic foraminiferal fauna with 35 dominance genus or species in 128 samples.Four assemblages were recognized including assemblage of Globocassidulina subglobosa-Stilostomella spp.(Gs-St),Chilostomella mediterranensis-Globobulimina spp.(Cm-Gl),Hoglundina elegans-Pyrgo spp.(He-Py) and Melonis affinis-Pyrgo spp.(Ma-Py).During the warm period and higher eustatic sea level stage from Late Miocene to early Pliocene(5.8~2.8 Ma),benthic foraminiferal assemblage(Gs-St) is stable and relative medium-low nutrient environment and low oxygenated situation in this region.Caused by the current status of global ocean,the relative negative excursion ofδ13C values and marked dissolution of carbonate in Core SH7B were also recognized in other marine regions from 5.8 ~3.79 Ma,which indicate that the low oxygenated situation of intermediate water in the northern slope of the SCS may be controlled by the presence of oxygen-depleted and corrosive pacific intermediate water.From the Early Pliocene3.79 to 2.8 Ma,the increasing of benthic foraminiferal abundance and aerobic epifaunal species suggests the intensification of the intermediate water circulation and the increasing of oxygen content.Since Late Pliocene 2.4Ma,fluctuating changes of benthic foraminiferal assemblages and stable isotope values indicate medium-high seasonality of surface water productivity and well-oxygenation to oxygen-depleted bottom-water situation.The benthic foraminiferal distribution is mainly controlled by the organic matter flux to the seafloor and the circulation of intermediate water.