(Table 4, page 1372) Element abundances in ferromanganese oxide crusts on bioclastic limestones at R.R.S. Shackleton Sta. 1225 (Annan Seamount)

Phosphatized bioclastic limestones of Early Tertiary age have been dredged from the crest of a seamount in the equatorial Atlantic near 9°N, 21°W. The samples were recovered from water depths of 700 to 970 m but clearly were deposited in a high-energy reefal environment, thus indicating significant Tertiary subsidence. The vertical movement is beleived to reflect reactivation of an old structural line, the Guinée Fracture Zone, which may have had important effects on the transfer of bottom waters of polar origin between the Sierra Leone and Gambia basins. Examination of foraminiferal limestone coating the shallow water material suggests that deep-water conditions were established on the seamount by Middle Miocene time. All phosphatized samples are covered with a layer of ferromanganese oxides which, from textural and faunal evidence, is Palaeocene or Eocene in age and of shallow water origin. Pervasive phosphatization of the bioclastic limestones appears to have been accomplished before the end of the Eocene while the crest of the seamount was near sea level, because Upper Eocene limestone of shallow water facies is unaffected by such radical diagenesis. Phosphatization probably was associated with vigorous upwelling near the seamount and has been taken to an unusually advanced stage, resulting in a phosphoritesof high economic grade. P2O5 contents are in the range of 37 to 41%. The purity of the phosphorites may be related to the intensity of upwelling during the Early Tertiary, but other factors such as the paucity of terrigenous detritus, frequent reworking of the sedimentary capping of the seamount, and a high primary porosity of the limestones may have been important in their information. The Early Tertiary period of phosphorite deposition on the seamount and in northwest Africa ended before the close of Eocene time in both areas, while shallow water conditions prevailed. It is argued that the cessation of phosphorite formation reflects major changes in the surface circulation of the Atlantic during the Eocene.在赤道大西洋近9°N、21°W处的一座海山（seamount）峰顶，打捞获取了早第三纪（Early Tertiary）的磷酸盐化生物碎屑灰岩（Phosphatized bioclastic limestones）。样品采自700至970米的水深，但显然沉积于高能礁相环境，这表明该区域经历了显著的第三纪沉降。此次垂直运动被认为反映了古构造线——几内亚断裂带（Guinée Fracture Zone）的复活，该断裂带可能对塞拉利昂盆地（Sierra Leone basin）与冈比亚盆地（Gambia basin）之间极地起源底水的运移产生了重要影响。对覆盖于浅水底物之上的有孔虫灰岩（foraminiferal limestone）的研究表明，该海山在中中新世（Middle Miocene）已处于深水环境。所有磷酸盐化样品均被一层铁锰氧化物（ferromanganese oxides）覆盖，从结构和动物群证据来看，该层形成于古新世（Palaeocene）或始新世（Eocene），且起源于浅水环境。生物碎屑灰岩的广泛磷酸盐化似乎发生在始新世末期之前，当时该海山峰顶接近海平面——因为浅水环境相的上始新统灰岩并未受到这类强烈成岩作用（diagenesis）的影响。磷酸盐化作用可能与海山附近的强烈上升流（upwelling）相关，且演化至异常高级的阶段，最终形成了具有高经济品位的磷块岩（phosphorites）。五氧化二磷（P₂O₅）含量介于37%至41%之间。磷块岩的纯度可能与早第三纪时期的上升流强度有关，但其他因素如陆源碎屑（terrigenous detritus）匮乏、海山沉积盖层频繁遭受改造以及灰岩原生孔隙度（primary porosity）较高，也可能在其形成过程中发挥了重要作用。该海山及北非地区的早第三纪磷块岩沉积作用均在两区域的始新世末期之前结束，当时两地仍处于浅水环境。研究认为，磷块岩形成作用的中止反映了始新世期间大西洋表层环流（surface circulation）的重大变化。