Petrological Characteristics and Genesis of the Central Indian Ocean Basin Basalts

The Central Indian Ocean Basin (CIOB) basalts are plagioclase-rich, while olivine and pyroxene are very few. The analyses of 41 samples reveal high FeOT (~10–18 wt%) and TiO2 (~1.4–2.7 wt%) indicating a ferrobasaltic composition. The basalts have high incompatible elements (Zr 63–228 ppm; Nb ~1–5 ppm; Ba ~15–78 ppm; La ~3–16 ppm), a similar U/Pb (0.02–0.4) ratio as the normal mid-oceanic basalt (0.16±0.07) but the Ba/Nb (12.5–53) ratio is much larger than that of the normal mid-oceanic ridge basalt (~5.7) and Primitive Mantle (9.56). Interestingly almost all of the basalts have a significant negative Eu anomaly (Eu/Eu*=0.78–1.00) that may have been a result of the removal of feldspar and pyroxene during crystal fractionation. These compositional variations suggest that the basalts were derived through fractional crystallization together with low partial melting of a shallow seated magma.     

Seamounts in the Central Indian Ocean Basin: indicators of the Indian plate movement

The study of seamount parameters in the tectonically most-complicated and least-understood Indian Ocean assumes importance since their properties vary as a function of tectonic setting, physics of lithosphere, conduit geometry and chemical composition of magma. More than 100 such seamounts ranging in summit height (h) from 300 to 2870 m, are indentified in the oceanic crust between Indian continent and Mid-Indian Ridge (MIR) and South-East Indian Ridge (SEIR). Most of the minor seamounts (h > 1000) are found in the southern part of the study area. Major seamounts (h < 1000 m) are roughly distributed in two groups—the northern group on Cretaceous Oceanic Crust and southern group on Pliocene-Miocene Oceanic Crust. On an average northern group seamounts (SM 1 to 6) are taller, wider and flatter than those from the southern group. These seamounts appear to be the result of continuous growth from tapped, moving magma chamber while stress depleted magma and inconsistent Indian Plate movement during Mid-Tertiary are attributed to the origin of southern group of smaller seamounts. Distribution and morphology of seamounts as a whole indicate their formation either from Reunion hotspot or from two separate hotspots in the geological past.     

Physical Properties, Morphology and Petrological Characteristics of Pumices from the Central Indian Ocean Basin

About 400 pumice clasts collected from the Central Indian Ocean Basin(CIOB)were studied for their morphology and were classified based on their shape and size.A majority of the samples range between<1 cm and 36 cm and in the Zinggs shape diagram plot in the equant and oblate fields.The Corey Shape Factor for most of the samples is close to 0.7,which is common for volcaniclastic material. The physical properties such as density,specific gravity,void ratio,porosity,moisture content and degree of saturation...     

Biogenic signature and ultra microfossils in ferromanganese nodules of the Central Indian Ocean Basin

Theories related to the precipitation mechanism of the metallic elements in marine manganese nodules have remained controversial between two schools of thoughts (1) chemical oxidation (abiotic origin) and (2) deposition of the metals through microbial enzymatic processes (biogenic origin). One of the most important evidence in support of the biogenic origin is the occurrence of fossilized microbes. However, well-documented literature in this regard is either lacking or very scanty in case of Indian Ocean nodules. Using high resolution FEG-SEM we have recorded various biogenic signatures and ultra microfossils in the ferromanganese nodule samples from Central Indian Ocean Basin (CIOB) that are presented in this paper. The microfossils are mostly protozoans belonging to varieties of bacteria, diatoms and foraminifera. Some of the features recorded in this study have perhaps never been reported before from any manganese nodules. The chemical compositions of these ultra microfossils indicate a high-level of manganese precipitation in and around them in comparison to the distant surrounding areas. While clumpy microbes are enriched with nickel, the rod shaped bacteria are rich in copper. Up to 4.70 wt.% nickel and 5.31 wt.% Cu have been recorded in the fossilized microbe bodies. The high abundance of biogenic features as well as microfossils in the ferromanganese nodules and their chemical compositions support arguments in favor of a dominant role of the microorganisms in the construction of the nodules of the CIOB.     

Composition of macrobenthos from the Central Indian Ocean Basin

The deep sea is well known for its high faunal diversity. But the current interest in its abundant polymetallic nodules, poses a threat to the little known benthic organisms surviving in this unique environment. The present study is the first attempt to document the Indian Ocean abyssal benthic diversity of macroinvertebrates and to investigate its relation to the surface primary production (chl-a), sediment labile organic matter, organic carbon and texture. The present study is based on 87 individuals. Altogether 39 macroinvertebrate genera were obtained from water depths of 4500–5500 m from 23 box cores. Reduction in macrobenthic density was seen towards the southern latitudes. The area was dominated by deposit feeding macrobenthos. Vertically, the fauna was distributed down to 30 cm depth, with the highest faunal density in the top 2–5 cm sediment section. The values for population density were strongly correlated with surface water chl-a and sediment protein, indicating supply of fresh organic matter as a critical factor for maintaining the deep sea benthic diversity and abundance.     

中印度洋海盆表层沉积物稀土元素分布特征及富集规律

对中印度洋海盆14个站位的表层沉积物进行了稀土元素(REE+Y,简称REY)分布特征和富集规律研究.结果表明,样品中REY主要富集于沸石黏土和远洋黏土中(稀土元素总量最高为1239×10?6),且明显富集钇(Y)等重稀土元素(Y富集系数高达14.1,重稀土元素和Y富集系数最高为11.6);富稀土沉积物呈明显Ce亏损,发...     

Relationship between chemical composition and magnetic susceptibility in sediment cores from Central Indian Ocean Basin

Three sediment cores in a north-south transect (3°N to 13°S) from different sediment types of the Central Indian Ocean Basin (CIOB) are studied to understand the possible relationship between magnetic susceptibility (χ) and Al, Fe, Ti and Mn concentrations. The calcareous ooze core exhibit lowest χ (12.32 × 10⁻⁷ m³ kg⁻¹), Al (2.84%), Fe (1.63%) and Ti (0.14%), terrigenous clay core with moderate χ (29.93 × 10⁻⁷ m³ kg⁻¹) but highest Al (6.84%), Fe (5.20%) and Ti (0.44%), and siliceous ooze core with highest χ (38.06 × 10⁻⁷ m³ kg⁻¹) but moderate Al (4.49%), Fe (2.80%) and Ti (0.19%) contents. The distribution of χ and detrital proxy elements (Al, Fe, and Ti) are identical in both calcareous and siliceous ooze. Interestingly, in terrigenous core, the behaviour of χ is identical to only Ti content but not with Al and Fe suggesting possibility of Al and Fe having a non-detrital source. The occurrence of phillipsite in terrigenous clay is evident by the Al-K scatter plot where trend line intersects K axis at more than 50% of total K suggesting excess K in the form of phillipsite. Therefore, the presence of phillipsite might be responsible for negative correlation between χ and Al (r = −0.52). In siliceous ooze the strong positive correlations among χ, Al_exc and Fe_exc suggest the presence of authigenic Fe-rich smectite. High Mn content (0.5%) probably in the form of manganese micronodules is also contributing to χ in both calcareous and siliceous ooze but not in the terrigenous core where mean Mn content (0.1%) is similar to crustal abundance. Thus, χ systematically records the terrigenous variation in both the biogenic sediments but in terrigenous clay it indirectly suggests the presence of authigenic minerals.     

On the impacts of ENSO and Indian Ocean dipole events on sub-regional Indian summer monsoon rainfall

The relative impacts of the ENSO and Indian Ocean dipole (IOD) events on Indian summer (June–September) monsoon rainfall at sub-regional scales have been examined in this study. GISST datasets from 1958 to 1998, along with Willmott and Matsuura gridded rainfall data, all India summer monsoon rainfall data, and homogeneous and sub-regional Indian rainfall datasets were used. The spatial distribution of partial correlations between the IOD and summer rainfall over India indicates a significant impact on rainfall along the monsoon trough regions, parts of the southwest coastal regions of India, and also over Pakistan, Afghanistan, and Iran. ENSO events have a wider impact, although opposite in nature over the monsoon trough region to that of IOD events. The ENSO (IOD) index is negatively (positively) correlated (significant at the 95% confidence level from a two-tailed Student t-test) with summer monsoon rainfall over seven (four) of the eight homogeneous rainfall zones of India. During summer, ENSO events also cause drought over northern Sri Lanka, whereas the IOD events cause surplus rainfall in its south. On monthly scales, the ENSO and IOD events have significant impacts on many parts of India. In general, the magnitude of ENSO-related correlations is greater than those related to the IOD. The monthly-stratified IOD variability during each of the months from July to September has a significant impact on Indian summer monsoon rainfall variability over different parts of India, confirming that strong IOD events indeed affect the Indian summer monsoon.

南印度洋海区营养盐的分布特征

探讨了南印度洋海区总有机磷(TOP)、总有机氮(TON)以及溶解无机营养盐的分布规律。分析结果表明：研究海区内溶解无机营养盐受水体中生物活动和物理过程的综合影响,表层水体由于生物活动的消耗,其磷酸盐等无机营养元素的含量一般是采样水深范围内最低的;中深层水体由于生物活动的降低以及有机质矿化作用的影响,无机营养元素的变化范围较小。表层水体中TOP和TON含量占TP和TN的主要部分,说明表层水体中的氮和磷主要以有机态形式存在,且沿着37.8°S从西向东,TOP和TON的含量以及TOP/TP和TON/TN的比值呈降低的趋势。研究海区叶绿素a的分析结果表明,初级生产力的变化可能是控制研究海区TON和TOP空间分布的主要因素。不同形态氮、磷营养元素的相关分析表明,有机营养盐和无机营养盐之间互为补充,且表层水体中有机氮和磷是水体初级生产所需营养盐的重要来源,总氮、总磷的关系表明研究区初级生产力并不受氮、磷的限制。     

Historic records of teletsunami in the Indian Ocean and insights from numerical modelling

Following the catastrophic “Great Sumatra–Andaman” earthquake- tsunami in the Indian Ocean on the 26th December 2004, questions have been asked about the frequency and magnitude of tsunami within the region. We present a summary of the previously published lists of Indian Ocean Tsunami (IOT) and the results of a preliminary search of archival materials held at the India Records Office, at the British Library in London. We demonstrate that in some cases, normal tidal movements and floods associated with tropical cyclones have been erroneously listed as tsunami. We summarise archival material for tsunami that occurred in 1945, 1941, 1881, 1819, 1762 and a little known tsunami in 1843. We present the results of modelling of the 2004, 1861 and 1833 tsunami generated by earthquakes off Sumatra and the 1945 Makran earthquake and tsunami, and examine how these results help to explain some of the historical observations. The highly directional component to tsunami propagation illustrated by the numerical models may explain why we are unable to locate archival records of the 1861 and 1833 tsunami at important locations like Rangoon, Kolkata (formally Calcutta) and Chennai (formally Madras), despite reports that these events created large tsunami that inundated western Sumatra. The numerical models identify other areas (particularly the central and southern Indian Ocean islands) where the 1833 tsunami may have had a large enough effect to produce a historic record. We recommend further archival research, coastal geological investigations of tsunami impacts and detailed modelling of tsunami propagation to better understand the record and effects of tsunami in the Indian Ocean and to estimate their likelihood of occurring in the future.     

Biological productivity, terrigenous influence and noncrustal elements supply to the Central Indian Ocean Basin: Paleoceanography during the past ∼ 1 Ma

A 2m-long sediment core from the siliceous ooze domain in the Central Indian Ocean Basin (CIOB; 13‡03′S: 74‡44′E; water depth 5099m) is studied for calcium carbonate, total organic carbon, total nitrogen, biogenic opal, major and few trace elements (Al, Ti, Fe, K, Mg, Zr, Sc,V, Mn, Cu, Ni, Zn, Co, and Ba) to understand the productivity and intensity of terrigenous supply. The age model of the sediment core is based on U-Th dating, occurrence of Youngest Toba Tuff of ∼ 74 ka and Australasian microtektites of ∼ 770 ka. Low carbonate content (< 1%) of sediment core indicates deposition below the carbonate compensation depth. Organic carbon content is also very low, almost uniform (mean 0.2 wt%) and is of marine origin. This suggests a well-oxygenated bottom water environment during the past ∼ 1100ka. Our data suggest that during ∼ 1100 ka and ∼ 400 ka siliceous productivity was lower, complimented by higher supply of terrigenous material mostly derived from the metasedimentary rocks of High Himalayan crystalline. However, during the last ∼ 400 ka, siliceous productivity increased with substantial reduction in the terrigenous sediment supply. The results suggest that intensity of Himalayan weathering, erosion associated with monsoons was comparatively higher prior to 400 ka. Manganese, Ba, Cu, Ni, Zn, and Co have around 90% of their supply from noncrustal (excess) source and their burial to seafloor remained unaffected throughout the past ∼ 1100 ka.     

The warm pool in the Indian Ocean

The structure of the warm pool (region with temperature greater than 28°C) in the equatorial Indian Ocean is examined and compared with its counterpart in the Pacific Ocean using the climatology of Levitus. Though the Pacific warm pool is larger and warmer, a peculiarity of the pool in the Indian Ocean is its seasonal variation. The surface area of the pool changes from 24 × 10⁶ km² in April to 8 × 10⁶ km² in September due to interaction with the southwest monsoon. The annual cycles of sea surface temperature at locations covered by the pool during at least a part of the year show the following modes: (i) a cycle with no significant variation (observed in the western equatorial Pacific and central and eastern equatorial Indian Ocean), (ii) a single maximum/minimum (northern and southern part of the Pacific warm pool and the south Indian Ocean), (iii) two maxima/minima (Arabian Sea, western equatorial Indian Ocean and southern Bay of Bengal), and (iv) a rapid rise, a steady phase and a rapid fall (northern Bay of Bengal).     

Entrainment of circumpolar water in the Indian Ocean region of the Antarctic

The net influx of the circumpolar water on the western (approximately along 10°E) and eastern (approximately 115°E) boundaries of the Indian Ocean, adopting the method of Montgomery and Stroup is computed on bivariate distribution of potential thermosteric anomaly and salinity to identify the characteristics of the flux. The zonal flux at both the boundaries indicates an alternate strong easterly and westerly flow between 36°S and 45°S, south of which the flow is mainly easterly but weak up to 56°S. At the western boundary the easterly flow is 146 Sv and westerly is 98.07 Sv, while at the eastern boundary (115°E) the corresponding fluxes are 123.46 Sv and 27.20 Sv respectively, indicating a net outflux of 48.33 Sv. This water should have been accounted by the melting of ice and influx of the Equatorial Pacific Ocean Water.     

Sea surface heights obtained from Modular Ocean Model simulation and comparison with Topex/Poseidon data for the Indian Ocean

The Modular Ocean Model (MOM) is perhaps the most versatile ocean model available today for the simulation of the large scale circulation of the ocean. The Topex/Poseidon altimeter which has been operating since September 1992 has been providing sea surface heights (SSH) of the accuracy of 5–10 cms with a repeat cycle of 10 days. We examine in this paper, the SSH in the Indian Ocean obtained from a global simulation of MOM with a resolution of 1° in the longitude, 1/3° in the latitude between 30°S and 30°N and 20 levels in the vertical with climatological windforcing and restoring conditions on temperature and salinity. They are compared with the SSH from the Topex/Poseidon altimeter after suitable filtering in the time domain to remove smaller time and length scales. In addition, unfiltered data from both sources are analysed by estimating the cross-spectral density to find the coherence and crossphase at different frequencies. The agreement between the two, over most of the Northern Indian Ocean, especially the Arabian Sea and the Bay of Bengal is quite good.     

Geochemistry and mineralogy of REY-rich mud in the eastern Indian Ocean

Deep-sea sediments in parts of the Pacific Ocean were recently found to contain remarkably high concentrations of rare-earth elements and yttrium (REY) of possible economic significance. Here we report similar REY-rich mud in a core section from Deep Sea Drilling Project Site 213 in the eastern Indian Ocean. The sediments consist mainly of siliceous ooze, with subordinate zeolitic clay that contains relatively high REY concentrations. The maximum and average total REY (ΣREY) contents of this material are 1113 and 629 ppm, respectively, which are comparable to those reported from the Pacific Ocean. The REY-rich mud at Site 213 shows enrichment in heavy rare-earth elements, negative Ce anomalies, and relatively low Fe₂O₃/ΣREY ratios, similar to those in the Pacific Ocean. In addition, the major-element composition of the Indian Ocean REY-rich mud indicates slight enrichment in lithogenic components, which probably reflects a contribution from southern African eolian dust. A volcaniclastic component from neighboring mid-ocean ridges or intraplate volcanoes is also apparent. Elemental compositions and X-ray diffraction patterns for bulk sediment, and microscopic observation and elemental mapping of a polished thin section, demonstrate the presence of phillipsite and biogenic apatite, such as fish debris, in the REY-rich mud. The strong correlation between total REY content and apatite abundance implies that apatite plays an important role as a host phase of REY in the present deep-sea sediment column. However, positive correlations between ΣREY and elements not present in apatite (e.g., Fe₂O₃, MnO, and TiO₂) imply that the REY-rich mud is not formed by a simple mixture of REY-enriched apatite and other components.     

The biogeochemical distribution of trace elements in the Indian Ocean

The present review deals with the distributions of dissolved trace metals in the Indian Ocean in relation with biological, chemical and hydrographic processes. The literature data-base is extremely limited and almost no information is available on particle processes and input and output processes of trace metals in the Indian Ocean basin and therefore much research is needed to expand our understanding of the marine chemistries of most trace metals. An area of special interest for future research is the Arabian Sea. The local conditions (upwelling induced productivity, restricted bottom water circulation and suboxic intermediate waters) create a natural laboratory for studying trace metal chemistry.     

Sheltered coastal environments as archives of paleo-tsunami deposits: Observations from the 2004 Indian Ocean tsunami

The 2004 earthquake left several traces of coseismic land deformation and tsunami deposits, both on the islands along the plate boundary and distant shores of the Indian Ocean rim countries. Researchers are now exploring these sites to develop a chronology of past events. Where the coastal regions are also inundated by storm surges, there is an additional challenge to discriminate between the deposits formed by these two processes. Paleo-tsunami research relies largely on finding deposits where preservation potential is high and storm surge origin can be excluded. During the past decade of our work along the Andaman and Nicobar Islands and the east coast of India, we have observed that the 2004 tsunami deposits are best preserved in lagoons, inland streams and also on elevated terraces. Chronological evidence for older events obtained from such sites is better correlated with those from Thailand, Sri Lanka and Indonesia, reiterating their usefulness in tsunami geology studies.     

A coupled physical-biological-chemical model for the Indian Ocean

A coupled physical-biological-chemical model has been developed at C-MMACS. for studying the time-variation of primary productivity and air-sea carbon-dioxide exchange in the Indian Ocean. The physical model is based on the Modular Ocean Model, Version 2 (MOM2) and the biological model describes the nonlinear dynamics of a 7-component marine ecosystem. The chemical model includes dynamical equation for the evolution of dissolved inorganic carbon and total alkalinity. The interaction between the biological and chemical model is through the Redfield ratio. The partial pressure of carbon dioxide (pCO₂) of the surface layer is obtained from the chemical equilibrium equations of Penget al 1987. Transfer coefficients for air-sea exchange of CO₂ are computed dynamically based on the wind speeds. The coupled model reproduces the high productivity observed in the Arabian Sea off the Somali and Omani coasts during the Southwest (SW) monsoon. The entire Arabian Sea is an outgassing region for CO₂ in spite of high productivity with transfer rates as high as 80 m-mol C/m² /day during SW monsoon near the Somali Coast on account of strong winds.     

西南印度洋洋盆演化和岩浆地球化学印迹

西南印度洋中脊（SWIR）平均扩张速率约为14 mm/yr,是全球洋中脊系统的重要组成端元,因其具有慢速-超慢速扩张特征,引起全球科学家的广泛关注。基于前人对SWIR的综合研究成果,从构造和岩浆作用两个角度出发,系统地回顾了SWIR的形成和演化历史,探讨了岩浆的分布特征和地幔不均一性成因。SWIR的形成始于冈瓦纳大陆的裂解,中段洋脊区域（26~42°E）是印度洋最早开启的窗口,历经多次洋脊跃迁和扩展作用形成了斜向扩张展布,多分段的构造格局。地幔热点在冈瓦纳大陆裂解过程中扮演了关键角色,并对SWIR的洋底地貌和岩浆作用具有显著影响,其中Bouvet和 Marion热点在SWIR的西段和中段岩浆均留下了地球化学印迹。SWIR西段岩浆除却Bouvet热点影响之外表现出与大西洋-太平洋型玄武岩相似的同位素地球化学特征。在SWIR中段,39~41°E附近的岩浆具有显著的DUPAL异常特征,与冈瓦纳大陆的初始形成、裂解紧密相关。受俯冲改造的中—新元古代的造山带岩石圈地幔拆沉是造成SWIR中段地幔不均一性的重要根源。在SWIR东段,46~52°E区域内的局部岩浆组成异常,推测具有大陆地壳物质混染的成因。而在Melville转换断层以东,洋脊形成时间最晚,玄武岩的地幔源区受到了富集组分的交代作用,其同位素组成与相邻的中印度洋中脊（CIR）和东南印度洋中脊（SEIR）地幔源区具有亲缘性。     

印度洋多金属结核地质特征与资源潜力

通过在中印度洋海盆结核区外的印度洋其他海域内收集到的298处多金属结核站位的分布、成分和赋存环境等地质特征,圈定了5处资源潜力区.文章对这些区域内海洋长周期沉积速率、底层水含氧量、底质类型、夏季海面平均生物生产力、底栖宏生物量密度、海底地形地貌特征和海底表层沉积物有机碳含量等数据信息进行加权评估,揭示各区域结核分布密度的高低状况,辅以结核主要有用组分含量的分类,确定了印度洋内各结核区资源潜力的划分标准.笔者认为加斯科因平原结核区为印度洋多金属结核高资源潜力区,马达加斯加海盆结核区和南澳大利亚海盆西部结核区为中等资源潜力区,克洛泽海盆结核区和南澳大利亚海盆东部结核区为低资源潜力区.未来在这些区域内,尤其是加斯科因平原结核区中有希望通过进一步调查研究,精确锁定具有更高资源潜力的次级面积结核勘探区,检验和完善资源潜力评估方法,精细量化揭示这些区域的资源潜力.