Relative abundance and ranges of selected diatoms from Pliocene-Pleistocene sections of ODP Leg 177, Atlantic sector of the Southern Ocean

During Ocean Drilling Program (ODP) Leg 177, seven sites were drilled aligned on a transect across the Antarctic Circumpolar Current in the Atlantic sector of the Southern Ocean. The primary scientific objective of Leg 177 was the study of the Cenozoic paleoceanographic and paleoclimatic history of the southern high latitudes and its relationship with the Antarctic cryosphere development. Of special emphasis was the recovery of Pliocene–Pleistocene sections, allowing paleoceanographic studies at millennial or higher time resolution, and the establishment of refined biostratigraphic zonations tied to the geomagnetic polarity record and stable isotope records. At most sites, multiple holes were drilled to ensure complete recovery of the section. A description of the recovered sections and the construction of a multihole splice for the establishment of a continuous composite is presented in the Leg 177 Initial Reports volume for each of the sites (Gersonde, Hodell, Blum, et al., 1999). Here we present the relative abundance pattern and the stratigraphic ranges of diatom taxa encountered from shore-based light microscope studies completed on the Pliocene–Pleistocene sequences from six of the drilled sites (Sites 1089–1094). No shore-based diatom studies have been conducted on the Pliocene–Pleistocene sediments obtained at Site 1088, located on the northern crest of the Agulhas Ridge, because of the scattered occurrence and poor preservation of diatoms in these sections (Shipboard Scientific Party, 1999b). The data included in our report present the baseline of a diatom biostratigraphic study of Zielinski and Gersonde (2002), which (1) includes a refinement of the southern high-latitude Pliocene–Pleistocene diatom zonation, in particular for the middle and late Pleistocene, and (2) presents a biostratigraphic framework for the establishment of age models of the recovered sediment sections. Zielinski and Gersonde (2002) correlated the diatom ranges with the geomagnetic polarity record established shipboard (Sites 1090 and 1092) (Shipboard Scientific Party, 1999c, 1999d) and on shore (Sites 1089, 1091, 1093, and 1094) by Channell and Stoner (2002). The Pliocene–Pleistocene diatom zonation proposed by Zielinski and Gersonde (2002) relies on a diatom zonation from Gersonde and Bárcena (1998) for the northern belt of the Southern Ocean. Because of latitudinal differentiation of sea-surface temperature, nutrients, and salinity between Antarctic and Subantarctic/subtropical water masses, the Pliocene–Pleistocene stratigraphic marker diatoms are not uniformly distributed in the Southern Ocean (Fenner, 1991; Gersonde and Bárcena, 1998). As a consequence, Zielinski and Gersonde (2002) propose two diatom zonations for application in the Antarctic Zone south of the Polar Front (Southern Zonation, Sites 1094 and 1093) and the area encompassing the Polar Front Zone (PFZ) and the Subantarctic Zone (Northern Zonation, Sites 1089–1092). This accounts especially for the Pleistocene zonation where Hemidiscus karstenii, whose first abundant occurrence datum and last occurrence datum defines the subzonation of the northern Thalassiosira lentiginosa Zone, occurs only sporadically in the cold-water realm south of the PFZ and thus is not applicable in sections from this area. However, newly established marker species assigned to the genus Rouxia (Rouxia leventerae and Rouxia constricta) are more related to cold-water environments and allow a refinement of the Pleistocene stratigraphic zonation for the southern cold areas. A study relying on quantitative counts of both Rouxia species confirms the utility of these stratigraphic markers for the identification of sequences attributed to marine isotope Stages 6 and 8 in the southern Southern Ocean (Zielinski et al., 2002).大洋钻探计划（Ocean Drilling Program, ODP）第177航次期间，科研人员沿横跨南大洋大西洋扇区的南极绕极流（Antarctic Circumpolar Current）断面布设7个钻探站位并完成取芯。本航次的核心科学目标为探究南极高纬度地区新生代古海洋学与古气候演化历史，及其与南极冰圈发育的关联机制。本次研究的重点在于获取上新世-更新世（Pliocene–Pleistocene）沉积序列，以实现千年甚至更高时间分辨率的古海洋学研究，并建立与地磁极性记录（geomagnetic polarity record）及稳定同位素记录（stable isotope records）绑定的高精度生物地层分带（biostratigraphic zonations）体系。多数站位均布设了多个钻孔，以保障沉积序列的完整取芯。各站位的沉积序列描述及用于构建连续复合序列的多钻孔拼接方案，均已刊载于第177航次初始报告卷册（Gersonde、Hodell、Blum等，1999）。 本文基于岸基光学显微镜分析，对6个钻探站位（1089~1094站位）的上新世-更新世沉积序列中的硅藻（diatom）类群进行了研究，现将其相对丰度分布模式与地层延限范围公开发布。位于厄加勒斯海岭（Agulhas Ridge）北峰的1088站位所获取的上新世-更新世沉积物，因其中硅藻分布零散且保存状况较差，尚未开展岸基硅藻相关研究（船上科学工作组，1999b）。 本文所刊载的数据为Zielinski与Gersonde（2002）开展的硅藻生物地层学研究提供了基础数据集，该研究（1）优化了南极高纬度地区上新世-更新世硅藻分带体系，尤其是中更新世与晚更新世的分带方案；（2）为构建已获取沉积序列的年代模型（age models）提供了生物地层框架。Zielinski与Gersonde（2002）将硅藻延限与地磁极性记录进行了关联，其中1090、1092站位的地磁极性记录由船上科学工作组于航次期间测定（Shipboard Scientific Party，1999c、1999d），1089、1091、1093、1094站位的则由Channell与Stoner（2002）通过岸基分析完成。 Zielinski与Gersonde（2002）提出的上新世-更新世硅藻分带体系，以南大洋北部带的Gersonde与Bárcena（1998）硅藻分带为基础。由于南极、亚南极与亚热带水团之间存在海表温度、营养盐与盐度的纬度梯度差异，上新世-更新世地层标志性硅藻在南大洋的分布并不均匀（Fenner，1991；Gersonde与Bárcena，1998）。基于此，Zielinski与Gersonde（2002）提出了两套硅藻分带方案：一套适用于极锋（Polar Front）以南的南极海区（南部分带，对应1093、1094站位），另一套适用于包含极锋带（Polar Front Zone, PFZ）与亚极带（Subantarctic Zone）的海区（北部分带，对应1089~1092站位）。 这一点在更新世分带中尤为明显：作为北部具纹氏篓藻（Thalassiosira lentiginosa）亚带划分依据的首次大量出现基准面与末次出现基准面的卡氏半盘藻（Hemidiscus karstenii），仅在极锋以南的冷水域零星出现，因此该物种不适用于该区域的沉积序列分析。不过，新确立的弯楔藻属（Rouxia）标志性物种（勒文特弯楔藻Rouxia leventerae与缢缩弯楔藻Rouxia constricta）更适配冷水环境，可用于优化南部冷水海区的更新世地层分带体系。针对这两种弯楔藻的定量计数研究证实，该地层标志可用于识别南大洋南部海区的海洋同位素阶段（marine isotope Stage）第6、8阶段沉积序列（Zielinski等，2002）。