Calcareous nannofossil abundance in ODP Site 185-1149 sediments@en

Basal carbonate sediments recovered at Ocean Drilling Program (ODP) Site 1149 lie directly on magnetic Anomaly M12. They contain abundant and moderately well preserved calcareous nannofossils. Two nannofossil zones are recognized: the lower Calcicalathina oblongata Zone and the upper Lithraphidites bollii Zone, indicating a late Valanginian-late Hauterivian age. The close occurrence of two significant bioevents, the first occurrence (FO) of L. bollii and the FO of Rucinolithus terebrodentarius in Core 185-1149B-20R, together with dip data recorded during in situ geophysical logging, suggest the presence of an unconformity that corresponds to the lower Hauterivian sedimentary section. The continuous occurrence of L. bollii is reported for the first time in sediments from the Pacific Ocean. Other marker species regarded as cosmopolitan (e.g., C. oblongata) have a sporadic occurrence. Nannoconids, very useful zonal markers for Tethyan areas, are virtually absent. The presence of an unusually high abundance of Diazomatolithus lehmanii is also recorded and correlates with the Valanginian 13C positive excursion.大洋钻探计划（Ocean Drilling Program, ODP）1149站位获取的基底碳酸盐沉积物直接赋存于磁异常M12之上。该沉积物中含有丰富且保存中等完好的钙质超微化石（calcareous nannofossils）。已识别出两个超微化石带：下部为椭圆钙冠藻带（Calcicalathina oblongata Zone），上部为博利氏石藻带（Lithraphidites bollii Zone），指示地层年代为凡兰吟阶（Valanginian）晚期至豪特里维阶（Hauterivian）晚期。在185-1149B-20R岩心中，博利氏石藻（Lithraphidites bollii）的首现事件（First Occurrence, FO）与锥齿石藻（Rucinolithus terebrodentarius）的首现事件这两处重要生物事件紧密伴生，结合原位地球物理测井获取的倾角数据，表明存在一处对应下豪特里维阶沉积段的不整合面。本研究首次在太平洋沉积物中报道了博利氏石藻的连续产出记录。其余被视作广布型的标志种（如椭圆钙冠藻（Calcicalathina oblongata））仅呈零星产出。纳米颗石类（nannoconids）作为特提斯域极具实用价值的分带标志，在本沉积物中几乎完全缺失。本次研究还记录到莱氏重盘石藻（Diazomatolithus lehmanii）出现异常高丰度，该现象与凡兰吟阶的碳同位素正偏移事件存在对应关系。