Radiolarian biostratigraphy in the central Indian Ocean

Identifiable radiolarians of stratigraphic importance were recovered at eight of the sites drilled on Leg 115. The assemblages range in age from Holocene to middle Eocene (Dictyoprora mongolfieri Zone, about 48 Ma). Faunal preservation is particularly good in two stratigraphic intervals: the Holocene through upper Miocene (0-9 Ma), and the lowermost Oligocene to middle Eocene (35-48 Ma). Fluctuating rates of silica accumulation at these drill sites during the Cenozoic reflect changing tectonic and paleoceanographic conditions. In particular, the gradual closure of the Indonesian and Tethyan seaways and the northward migration of the Indian subcontinent severely restricted zonal circulation and silica accumulation in tropical latitudes during the late Oligocene through middle Miocene. By the late Miocene the Indian subcontinent had moved sufficiently north of the equator to allow trans-Indian zonal circulation patterns to become reestablished, and biosiliceous sedimentation resumed.
The composition of the radiolarian assemblages in the tropical Indian Ocean is closely comparable with that of the 'stratotype' sequences in the equatorial Pacific. However, there are some notable exceptions in Indian Ocean assemblages: (1) the scarcity of the genera Pterocanium and Spongaster in the Neogene; (2) the absence of the stratigraphically important Podocyrtis lineage, P. diamesa -> P. phyxis -> P. ampla, in the middle Eocene; and (3) the scarcity of taxa of the genus Dorcadospyris, with the exception of D. ateuchus.
The succession of radiolarian events was tabulated for those stratigraphic intervals where the assemblages were well preserved. We identified 55 events in the middle Eocene to earliest Oligocene, and 31 events in the late Miocene to Holocene. The succession of events is closely comparable with that of the tropical Pacific. However, there are exceptions that appear to be real, rather than artifacts of sample preservation, mixing, and core disturbance.

在第115航次（Leg 115）钻探的8个站位中，采获了具有地层学意义的可鉴定放射虫。其组合的地质年代范围涵盖全新世至始新世中期（网孔虫（Dictyoprora）mongolfieri带，约48 Ma）。两处地层间隔内的放射虫化石群保存状况尤为优良：一是全新世至晚中新世（0~9 Ma），二是渐新世最早期至始新世中期（35~48 Ma）。新生代期间，这些钻探站位的硅质沉积积累速率存在波动，反映出构造与古海洋环境条件的变化。尤其在渐新世晚期至中新世中期，印尼与特提斯海道的逐步闭合，以及印度次大陆的北向漂移，严重限制了热带纬度的纬向环流与硅质沉积积累。至中新世晚期，印度次大陆已北移至赤道以北足够远的位置，使得跨印度洋纬向环流模式得以重建，生物硅质沉积也随之恢复。
热带印度洋的放射虫组合组成，与赤道太平洋的“层型”剖面序列具有高度可比性，但印度洋放射虫组合存在若干显著差异：（1）新近纪时期翼星虫属（Pterocanium）与海绵星虫属（Spongaster）较为稀少；（2）始新世中期缺失具有地层学重要性的波多西里虫属（Podocyrtis）演化支：P. diamesa → P. phyxis → P. ampla；（3）除D. ateuchus外，多角星虫属（Dorcadospyris）的各类群均较为稀少。
针对放射虫组合保存良好的地层间隔，我们将放射虫演化事件序列进行了制表汇总。在始新世中期至渐新世早期，共识别出55个演化事件；在中新世晚期至全新世，则识别出31个演化事件。这些事件序列与热带太平洋的序列具有高度可比性，但也存在一些确属真实的差异，而非样品保存、混合以及岩心扰动所造成的假象。