Distribution of benthic and planktic foraminifera in surface sediments of the Indian and Pakistan continental margin, Arabian Sea

During the Indian Ocean Expedition of the German research vessel "Meteor" and the following cruise with the Pakistani fishing vessel "Machhera" in February and March 1965, sediments were sampled from the shelf, continental slope and the Arabian Basin off Pakistan and India. The biostratigraphic studies are based on sedimentary material from 24 sediment cores up to 480 cm long and 100 grab samples.
The faunal residues of the > 160 µ fraction (chiefly foraminifera and pteropods) were determined and counted in order to get an idea of the climatic conditions during the Late Quaternary of this region.
Biostratigraphic correlations of these Late Quaternary deposits are only possible if the thanatocoenosis of the surface sediments are well known. The analysis of the benthonic foraminiferal populations resulted in the definition of several foraminiferal facies. The following sequence of forarniniferal facies, named after their most characteristic members, can be distinguished from the shelf to the deep-sea:
1. Ammonia-Florilus facies ; 2. Ammonia-Cancris facies; 3. Cassidulina-Cibicides facies; 4. Uvigerina-Cassidulina facies ; 5. Buliminacea facies ; 6. deepwater facies, partly with Bulimina aculeata or with Nonionidae. On the upper continental slope there is a zone extremely poor in benthonic foraminifera. In this water depth the oxygen minimum layer (0.05-0.02 ml/l) of the water column reaches the slope.
Almost no connection can be observed between the living and the dead foraminiferal population of the same sample.
The regional distribution of the planktonic foraminifera from plankton tows as well as from the surface sediments shows marked differences in the species composition of faunas from different regions within the area of investigation. That depends on oceanographic conditions such as upwelling, dissolution of carbonate at great depths etc.
Based on the results of faunal analysis of samples from the recent sea-floor, a biostratigraphic subdivision of the sediments in the cores was established. The following biostratigraphically defined sections could be distinguished from the top of the sediment cores downwards : 1. Relatively cool climatic conditions are reflected by the foraminifera
of the uppermost core sections. 2. The next section is characterized by much warmer conditions (Holocene climatic optimum). The C-14 ages of this interval range from 4000 to 10 000 years B.P. according to different authors. C-14 dates on the material investigated do not give reliable clues. 3. Foraminiferal populations adapted to much colder conditions can be observed in the underlying core section. The boundary between the warm climate reflected by the foraminifera of section 2 and the cold climate (section 3) is relatively sharp. It can be correlated from core to core over the whole area investigated. The cold climate sediments of section 3 are underlain by different cool-, warm- and cold-climate sediments which can only be correlated over very short distances.
Since it appears certain that the last really cold conditions ended earlier in the Arabian Sea and its vicinity than in Europe it is recommended not to use the European stratigraphic terms for the Quaternary.
Because of the lack of reliable absolute sediment ages for the cores no exact sedimentation rates can be given. According to rough estimates, however, the rates are 1-2 cm/1000 years in the deep basin and up to 40 cm/1000 years on the upper continental slope. Sedimentation rates are always larger near the mouth of the Indus-River than off South India at stations of about the same water depth.
Planktonic gastropods (mainly pteropods) cannot be used for biostratigraphic purposes in the region under consideration. All of them seem to be displaced from the shelf. Their distribution there is given in.

1965年2月至3月，在德国考察船“流星号”（Meteor）印度洋考察航次及后续巴基斯坦渔船“马彻拉号”（Machhera）的巡航任务期间，科研人员从巴基斯坦与印度近海的大陆架、大陆坡及阿拉伯海盆采集了沉积物样品。本次生物地层学（biostratigraphic）研究依托24根长可达480厘米的沉积岩芯（sediment cores）及100个抓斗取样（grab samples）获得的沉积物质展开。

为明确该区域晚第四纪（Late Quaternary）的气候状况，科研人员对粒径大于160微米的粒级组分中的生物群残体（主要为有孔虫（foraminifera）与翼足类（pteropods））进行了鉴定与计数。

仅当表层沉积物的尸积群（thanatocoenosis）特征被充分掌握时，方可开展上述晚第四纪沉积的生物地层对比研究。通过对底栖有孔虫（benthonic foraminifera）种群的分析，科研人员划定了多种有孔虫相（foraminiferal facies）。从大陆架至深海环境，可区分出以下以特征属种命名的有孔虫相序列：1. 希望虫-圆辐虫（Ammonia-Florilus）相；2. 希望虫-蟹形虫（Cancris）相；3. 袋形虫-卷包虫（Cassidulina-Cibicides）相；4. 优锥虫-袋形虫（Uvigerina-Cassidulina）相；5. Buliminacea相；6. 深水相，部分以刺突袋形虫（Bulimina aculeata）或非子虫科（Nonionidae）为特征。在上部大陆坡区域，存在一个底栖有孔虫极度贫乏的带，该水深区间的水体中，氧最小值层（oxygen minimum layer，0.02~0.05 ml/l）直达大陆坡。

同一采样点的活有孔虫种群与死亡有孔虫种群之间几乎不存在关联。

浮游拖网及表层沉积物中的浮游有孔虫（planktonic foraminifera）区域分布特征显示，研究区域内不同区域的生物群物种组成存在显著差异，这与上升流、深层碳酸盐溶解等海洋学条件密切相关。

基于现代海底沉积物样品的生物群分析结果，科研人员对岩芯中的沉积物开展了生物地层分区。自沉积岩芯顶部向下，可区分出以下生物地层学界定的地层单元：1. 岩芯最上部层段的有孔虫组合反映了相对凉爽的气候条件；2. 下一层段以显著温暖的气候为特征（全新世气候适宜期（Holocene climatic optimum）），不同学者给出的该层段的碳十四（C-14）年龄介于4000至10000年前（B.P.）。但本次研究涉及样品的碳十四测年结果并不可靠；3. 在该层段之下的岩芯层位中，可见适应寒冷气候的有孔虫种群。第2层段有孔虫所反映的温暖气候与第3层段的寒冷气候之间的界线相对清晰，可在整个研究区域的各岩芯之间进行对比。第3层段的寒冷气候沉积物之下，分布着多种受冷、暖、冷气候影响形成的沉积物，这些沉积物仅能在极短距离内进行对比。

现有研究表明，阿拉伯海及其邻近区域的末次极寒气候结束时间早于欧洲，因此建议在该区域第四纪地层研究中避免使用欧洲地层术语。

由于缺乏可靠的沉积物绝对年龄数据，无法给出准确的沉积速率（sedimentation rates）。但粗略估算显示，深海盆地区域的沉积速率为1~2厘米/千年，上部大陆坡区域可达40厘米/千年。在相同水深条件下，印度河河口附近的沉积速率始终高于印度南部近海站位。

浮游腹足类（planktonic gastropods，主要为翼足类）不适用于本研究区域的生物地层学分析，所有样品中的翼足类似乎均源自大陆架，其区域分布情况已在相关文献中记载。