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.

研究人员从南极威德尔海东部海域以及南加州外海东太平洋海域的3个站位的箱式岩心中分样获取了5根短岩心，这些岩心的采集水深范围为500~2000 m，对其开展分析以探究岩心垂向上活体（经玫瑰红（Rose Bengal）染色）与死亡底栖有孔虫（benthic foraminifera）的分布特征。在加州大陆边缘海域，Planulina ariminensis、Rosalina columbiensis以及Trochammina spp.的个体附着于沉积物-水界面上方的胶结型多毛类栖管生存。Bolivina spissa仅栖息于表层沉积物内部或表面。经玫瑰红染色的Chilostomella ovoidea标本可在沉积物内下至6 cm深度处被发现，而Globobulimina pacifica标本的最大发现深度可达8 cm。活体G. pacifica的δ13C（Delta13C）值随沉积物表面向下至7 cm岩心深度的增加而降低，表明该内栖底栖有孔虫物种利用孔隙水中的贫13C碳源。在以钙质壳体为主的死亡底栖有孔虫组合中，上部沉积物柱内小型纤细壳体的选择性溶蚀导致物种占比呈现连续变化。在威德尔海东部海域，钙质物种Bulimina aculeata仅栖息于碳酸盐溶蚀环境的表层沉积物内部或表面。砂质胶结型的Cribrostomoides subglobosum、Recurvoides contortus以及部分Reophax属物种常分布于沉积物表层4 cm范围内，而Haplophragmoides bradyi、Glomospira charoides以及Cribrostomoides wiesneri的染色标本在表层1.5 cm以下的沉积物中丰度达到最高。以砂质胶结壳体为主的死亡底栖有孔虫组合的物种占比呈现三个显著变化阶段：第一阶段由沉积物-水界面处的方解石溶蚀作用导致；第二阶段与强烈生物扰动带的下界重合；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梯度变化所引起。