Microfossil abundances and stable isotope composition of foraminifera from DSDP Hole 90-594 off southeastern New Zealand

High-resolution textural, carbonate, microfossil, and oxygen (delta18O) and carbon (delta13C) stable isotopic analyses are presented for the late Quaternary (isotopic stages 1 to 6) interval of a core at Deep Sea Drilling Project site 594, situated just south of the present Subtropical Convergence in northernmost Subantarctic surface waters on the southern flank of Chatham Rise in the southwest Pacific. Downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes, respectively. Interglacial oozes contain a northern Subantarctic assemblage of planktonic foraminifera, with rare cool subtropical species, while glacial oozes are characterized by species typical of southern(most) Subantarctic waters and include radiolaria with affinity for Antarctic waters. The planktonic delta18O record for the site supports a 3°-6°C temperature change in near-surface waters between interglacial and glacial stages and indicates that during stage 5e, the near-surface waters were about 1°C warmer than at present. A pronounced cooling during stage 5d matches that of the Vostok ice core deltaD record, and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation. Benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when delta18O results suggest a temperature drop in glacial stage bottom waters of 2°-4°C, possibly a result of upward displacement of Antarctic Intermediate Water by Circumpolar Deep Water at the site. The foraminiferal delta13C records support a reduced influence of North Atlantic Deep Water in the southwest Pacific during glacial stages, when nutrient enhancement occurred in both bottom and surface waters. Despite the pronounced changes between interglacial and glacial conditions inferred at site 594, contributed to by a substantial northward shift by at least 5° of latitude in the position of the Antarctic Convergence south of New Zealand during glacial episodes, we conclude that the Subtropical Convergence remained locked to Chatham Rise (approximately 44°S) throughout stages 1 to 6. Major compression of the intervening belt of Subantarctic water during glacial episodes, and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest Pacific, account for the harsh, frigid environment reported for on-land southern New Zealand at these times.

本文报道了西南太平洋查塔姆海隆南侧、当前亚热带辐合带（Subtropical Convergence）以南最北端亚南极表层水域内，深海钻探计划（Deep Sea Drilling Project, DSDP）594号站位岩芯晚第四纪（同位素阶段1至6）的高分辨率结构、碳酸盐、微化石以及氧（δ¹⁸O）、碳（δ¹³C）稳定同位素分析数据。该岩芯剖面中远洋软泥与半远洋软泥的交替沉积，分别对应间冰期与冰期事件。间冰期软泥中含有北亚超群浮游有孔虫（planktonic foraminifera）组合，伴生少量亚热带冷水种；而冰期软泥则以最南端亚南极水域的典型物种为特征，还包含具有南极水域亲缘关系的放射虫（radiolaria）。该站位的浮游有孔虫δ¹⁸O记录显示，间冰期与冰期阶段的近表层海水温度相差3℃~6℃，且在5e阶段，近表层海水温度较当前高出约1℃。5d阶段的显著降温与沃斯托克冰芯（Vostok ice core）的δD记录一致；而阶段2和6末期的δ¹⁸O负偏值则表明，北半球冰消作用发生之前，南纬度地区已出现增温现象。当δ¹⁸O结果显示冰期阶段底层海水温度下降2℃~4℃时，半远洋软泥层位中典型冷中层至深层水域的底栖有孔虫（benthic foraminifera）丰度可大幅增加，这可能是该站位环极深层水（Circumpolar Deep Water, CDW）向上置换南极中层水（Antarctic Intermediate Water, AAIW）的结果。有孔虫δ¹³C记录表明，冰期阶段西南太平洋受北大西洋深层水（North Atlantic Deep Water, NADW）的影响减弱，此时底层与表层水域均出现营养盐富集现象。尽管594号站位推断出的间冰期与冰期环境存在显著变化，而该变化由冰期期间新西兰以南的南极辐合带（Antarctic Convergence）至少向北移动5个纬度所导致，但我们仍认为在阶段1至6期间，亚热带辐合带始终稳定位于查塔姆海隆附近（约南纬44°）。冰期期间亚南极水域带的显著压缩，以及西南太平洋该区域由此形成的极强温度梯度、增强的大气与海洋环流模式，共同解释了当时新西兰南部陆地区域所记录的严酷寒冷环境。