Down-core distribution of live and dead benthic foraminifera in deep sea sediments from the Weddell Sea and the Californian continental borderland

Five short cores sub-sampled from box cores from three sites in the eastern Weddell Sea off Antarctica and in the eastern Pacific off southern California, covering a range in water depth from 500 to 2000 m, were analysed for the down-core distribution of live (stained with Rose Bengal) and dead benthic foraminifera. In the California continental borderland, Planulina ariminensis, Rosalina columbiensis and Trochammina spp. live attached to agglutinated polychaetes tubes that rise above the sedimentwater interface. Bolivina spissa lives exclusively in or on the uppermost sediment. Stained specimens of Chilostomella ovoidea are found down to 6 cm within the sediment and specimens of Globobulimina pacifica down to a maximum of 8 cm. Delta13C values of live G. pacifica decrease with increasing depth from the sediment surface down to 7 cm core depth, indicating that this infaunal species utilizes13C-depleted carbon from pore waters. In the dead, predominantly calcareous benthic forminiferal assemblage, selective dissolution of small delicate tests in the upper sediment column causes a continuous variation in species proportions. In the eastern Weddell Sea, the calcareous Bulimina aculeata lives in a carbonate corrosive environment exclusively in or on the uppermost sediment. The arenaceous Cribrostomoides subglobosum, Recurvoides contortus and some Reophax species are frequently found within the top 4 cm of the sediment, whereas stained specimens of Haplophragmoides bradyi, Glomospira charoides and Cribrostomoides wiesneri occur in maximum abundance below the uppermost 1.5 cm. Species proportions in the dead, predominantly arenaceous, benthic foraminiferal assemblage change in three distinct steps. The first change is caused by calcite dissolution at the sediment-water interface, the second coincides with the lower boundary of intense bioturbation, and the third results from the geochemical shift from oxidizing to reducing conditions below a compacted ash layer.

本研究对采自南极威德尔海东部以及南加州外海东太平洋三个站位的箱式岩芯中分取的5根短岩芯（采样水深范围为500~2000 m）开展分析，旨在探究活体（经玫瑰红（Rose Bengal）染色）与死亡底栖有孔虫（benthic foraminifera）的岩芯垂向分布特征。在加利福尼亚大陆边缘海域，阿里米嫩抱环虫（Planulina ariminensis）、哥伦比亚卷瓣虫（Rosalina columbiensis）以及编织虫属（Trochammina spp.）的活体个体附着于伸出沉积物-水界面的胶结型多毛类栖管之上。厚壁卷管虫（Bolivina spissa）仅存活于沉积物表层之内或表层之上。经玫瑰红染色的卵形类圆盘虫（Chilostomella ovoidea）标本在沉积物中的分布深度可达6 cm，而太平洋球泡虫（Globobulimina pacifica）标本的最大采样深度可达8 cm。活体太平洋球泡虫的δ¹³C值随沉积物表层向下至岩芯7 cm深度处的深度增加而逐渐降低，表明该内生底栖物种利用孔隙水中贫¹³C的碳源。在以钙质壳为主的死亡底栖有孔虫组合中，沉积物上层柱体内细小脆弱的壳体发生选择性溶解，导致物种相对丰度呈现连续变化。在威德尔海东部海域，钙质壳的刺状泡轮虫（Bulimina aculeata）仅存活于沉积物表层之内或表层之上，其栖息环境为碳酸盐溶蚀环境。砂质壳的近球形筛管虫（Cribrostomoides subglobosum）、扭曲反旋虫（Recurvoides contortus）以及部分串珠虫属（Reophax spp.）物种常被发现于沉积物表层0~4 cm范围内；而经玫瑰红染色的布雷迪单房虫（Haplophragmoides bradyi）、球形球旋虫（Glomospira charoides）以及威氏筛管虫（Cribrostomoides wiesneri）的标本，则在沉积物表层1.5 cm以下区域丰度达到峰值。在以砂质壳为主的死亡底栖有孔虫组合中，物种相对丰度的变化可划分为三个显著阶段：第一阶段由沉积物-水界面处的方解石溶解作用引发；第二阶段与强烈生物扰动的下限边界相契合；第三阶段则由压实火山灰层下方从氧化环境到还原环境的地球化学转变所导致。