Age model of ODP Site 113-697 (Table 2)

We examined diatom preservation patterns in Pliocene age sediments of Jane Basin (ODP Site 697) and compared them with diatom distribution in more northerly sites at various sectors of the Southern Ocean. Our data from Site 697, as well as other sites from around the Southern Ocean, support the view that there was significant ice growth on Antarctica during the late Pliocene. DSDP Site 514 in the Atlantic sector shows increased relative abundance of Eucampia antarctica, an ice-related form, in the upper part of the Gauss Chron with a larger increase just above it. With one exception, all sites included in the present study show increased relative abundance of E. antarctica in the upper part of the Gauss. Our view that there was ice growth on Antarctica during the late Gauss Chron is supported by the results from ODP Site 697. While diatoms are present and percent opal is high in the early and middle Gauss Chron (suggesting more open-ocean conditions), late Gauss sediments contain low percentages of opal and few or no diatoms. This is also true for the early Matuyama Chron. If we accept spring and summer sea-ice cover as the major suppressant of diatom productivity in the Southern Ocean, then we conclude that sea-ice covered the region around Site 697 through much of the year during this interval. Further, the absence of diatoms and the low percentages of opal in middle and late Matuyama chron sediments suggests increased sea-ice cover over the Jane Basin during this time. Although warmer openocean intervals are inferred for intervals near the Olduvai and Jaramillo Subchrons, most of the Matuyama Chron was marked by extensive sea-ice cover with low seasonal contrast. Our results for the early part of the Brunhes Chron are similar, at least for the Jane Basin. During this time, sea-ice cover over the basin apparently extended well into the growing season. In contrast, the later Brunhes Chron is marked by alternating open water (during the growing season) and extensive, almost year-round, sea-ice.

本研究针对简盆地（Jane Basin，大洋钻探计划（Ocean Drilling Program, ODP）697站位）的上新世（Pliocene）沉积物中的硅藻保存模式展开分析，并将其与南大洋（Southern Ocean）不同区域更靠北（纬度更低）的站位的硅藻分布特征进行对比。我们从ODP 697站位及南大洋其他站位获取的数据，均支持上新世晚期南极地区存在大规模冰体扩张的观点。大西洋海域的深海钻探计划（Deep Sea Drilling Project, DSDP）514站位显示，高斯极性时（Gauss Chron）上部地层中，与冰相关的硅藻物种南极弯角藻（Eucampia antarctica）的相对丰度有所升高，且在该层位之上丰度增幅更为显著。本次研究涉及的所有站位，仅一处例外，均在高斯极性时上部地层中呈现出南极弯角藻相对丰度升高的特征。ODP 697站位的研究结果进一步验证了我们的观点：高斯极性时晚期南极地区存在冰体扩张现象。尽管高斯极性时早、中期地层中硅藻大量产出，且蛋白石（opal）百分含量较高（指示更为开阔的大洋环境），但高斯极性时晚期的沉积物蛋白石百分含量较低，硅藻数量稀少甚至完全缺失。松山极性时（Matuyama Chron）早期地层亦呈现出相同特征。若将春夏季节海冰覆盖视作南大洋硅藻生产力的主要抑制因素，则可推断在此时间段内，ODP 697站位周边海域全年多数时间均被海冰覆盖。此外，松山极性时中、晚期沉积物中硅藻缺失且蛋白石百分含量偏低，表明此时间段简盆地周边海域的海冰覆盖范围进一步扩大。尽管奥杜威亚极性时（Olduvai Subchron）与贾拉米洛亚极性时（Jaramillo Subchron）附近的地层被推断为温暖开阔的大洋环境，但松山极性时的大部分时期均以大范围海冰覆盖、季节温差较小为特征。至少针对简盆地而言，布容极性时（Brunhes Chron）早期的研究结果与上述特征相似。该时期，简盆地的海冰覆盖范围明显延伸至硅藻生长季的中后期。与之相反，布容极性时晚期则呈现出交替变化的环境特征：生长季为开阔水域，其余时段则被大范围、近乎全年的海冰覆盖。