深海钻探揭示构成环礁的浅水碳酸盐建隆覆盖沉积在太平洋西北部平顶海山之上.相对于现今珊瑚-藻类生物构成的环礁而言,白垩纪和古新世孤立的环礁的四周边缘生物含量甚少,造礁能力下降.这些环礁浅水碳酸盐沉积在火山基座的顶部,在Albian初期、Albian末期、Maastrichtian晚期和始新世中期发生沉没.早期研究认为这些碳酸盐体系经历了地面暴露阶段以及随后的海侵淹没阶段.然而,一个事实是,环礁沉没仅发生在当太平洋板块背负着平顶海山向北漂移过程中,并且是经过南纬7°以后.几个不同的假说用来解释该时期环礁沉没现象,包括环礁暴露、喀斯特化、地面侵蚀、增强赤道上升洋流、缺氧和/或者富营养的浑浊水的出现等.文中在太平洋平顶海山深海钻孔的沉积、生物地层和古纬度数据综合研究基础上,提出一种新模式用于解释晚白垩世-古新世浅水碳酸盐建造的沉没.与以前的模式不同的是,文中提出,这些晚白垩世-古新世"环礁"之所以发生沉没,原因是受到原始环南赤道洋流(pSEC)的影响,导致一种对分泌碳酸盐的生物不利环境的产生.而且,当这些平顶海山漂移到南纬5°时,即赤道古纬度带,许多浅水碳酸盐建造之上,铁锰结核开始沉积.同时,平顶海山下沉到溶解氧极小层(海平面之下400～1 100 m)和原始环南赤道中层洋流(pEIC)区域,碳酸盐生产停滞.当平顶海山漂移经过赤道后(北纬0～3°),并且下沉到南赤道中层洋流(pEIC)深部海平面之下～1 100 m,远洋沉积才开始.因此,沉积学数据提供确切证据表明,太平洋中部赤道地区在过去100 Ma大部分时期内,和现在非常类似地受到赤道地转洋流系的影响.%Combination of sedimentary, biostratigraphic and paleolatitudinal data from deep boreholes drilled through Pacific guyots-flat-topped seamounts-explains the drowning of late Cretaceous-Paleogene atolls. These formed through an accumulation of biogenic and inorganic carbonate sediments in mid-oceanic regions. Deep-sea drilling has documented that shallow-water carbonate buildups resembling atolls are capping many of the northwest Pacific guyots. In contrast to the coral-algal bulwarks of modern atolls, the perimeter ridges of these isolated Cretaceous and Paleogene atolls have negligible biohermal content and thus lower up-building capacity. These atolls drowned during the earliest Albian, latest Albian, late Maastrichtian and middle Eocene. The carbonate systems recovered from earlier brief episodes of subaerial exposure and subsequent transgressive flooding. However, when the northward drift of the Pacific plate carried piggy-back these guyots to within ～7° S paleolatitude, the atolls drowned. Several different hypotheses such as atolls emergence, karsting and subaerial erosion, enhanced equatorial upwelling, anoxia and/or presence of nutrient-rich, turbid waters were suggested as the causes of atoll drowning. Study of sedimentary sequences capping the guyots, comprised of a shallow water carbonate buildup, Fe-Mn crusts coating the top of the carbonate buildups and or volcanic pedestal surfaces, and of the overlying pelagic carbonate cap, provide evidence of an overwhelming effect of an equatorial current system on sediment deposition. These late Cretaceous-Paleogene in age atolls drowned when they came into the influence of the proto-South Equatorial Current (pSEC), which created an inhospitable environment for carbonate secreting organisms. A combination of continuing guyot subsidence and northwestward drift of the Pacific plate brought submerging surfaces of guyots into the influence of strong proto-Equatorial Undercurrent (50～400 m bsl), indicated by nondeposition and erosion on guyot surfaces. Precipitation of ferromanganese crusts was initiated between paleolatitude of ～5°S and the equator, when the tops of the guyots subsided into an oxygen minimum zone (400～1 100 m bsl) and the domain of proto-Equatorial Intermediate Current (pEIC). Pelagic carbonate cap deposition began only after the guyot crossed the equator (0～3°N) and subsided below ～1 100 m bsl and into deeper part of the pEIC , suggested by occasional winnowing of deposited foraminiferal oozes at the base of pelagic caps. Sedimentary data therefore provide convincing evidence that the central equatorial Pacific has been under the influence of an ocean currents system similar to the present, for most of the past 100 Ma.
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