Central Indian Basin Research Articles

The overflows across the Ninetyeast Ridge

Abstract Water deeper than, say, 3800 m in the Central Indian Basin comes from the West Australian Basin across deep saddles on the Ninetyeast Ridge near Lats. 5°S, 10°S, and 28°S. The WOCE Hydrographic Program in the Indian Ocean (1994–1996) provided opportunities for more detailed measurements of water properties in these overflows than any made earlier. With zero-velocity surfaces inferred from contrasts in oxygen and silicic-acid concentrations and from distributions of specific-volume anomaly, geostrophic estimates of these overflow rates are, respectively, 1.0, 1.1, and 0.1×10 6 m 3 s −1 (westward). These constructions also imply strong zonal flows of mid-depth water (between 2000 db and the zero-velocity surfaces) through the passages: 2.2 and 6.7×10 6 m 3 s −1 westward at Lats. 5°S and 10°S, respectively, and 1.8×10 6 m 3 s −1 eastward at Lat. 28°S. As in the southward flow through Denmark Strait, density stratification explains the splitting of westward currents at 5°S and 10°S into mid-depth and deep flows as they pass over their saddles (but not why such vertically extended flows should exist in the first place). The cause of opposed eastward flow at 28°S is less certain, but might be related to the particular geometry of the West Australian Basin.

Bay of Bengal nutrient-rich benthic layer

A nutrient- and carbon-rich, oxygen-poor benthic layer is observed in the lower 100 m of the central and western Bay of Bengal, at depths between 3400 to 4000 m. The observed ratios for the biogeochemical anomalies in the benthic layer water are similar to those observed for phytoplankton blooms in open oceans and hence suggest that the source of the high silica, phosphate, nitrate and carbon is likely to be due to decomposition of marine plankton deposited on the Ganges fan. While similar sediment types are expected to exist across a more extensive area of the Bay of Bengal, accumulation of nutrients only within a confined pool of bottom water is due to a greater degree of ventilation elsewhere. To the north of the nutrient-rich benthic pool, in shallower water, inflow of water from West Australian Basin minimizes anomalous benthic properties. To the south, in deeper water, ventilation by bottom water of the Central Indian Basin lifts the Bay of Bengal nutrient-rich benthic water off the sea floor. Thus the nutrient-rich benthic layer occupies zone between better ventilated regions. A counter-clockwise flow of bottom water is suggested for the Bay of Bengal, with nutrient-rich bottom water flowing westward south of Sri Lanka.

Optimum sampling interval for evaluating ferromanganese nodule resources in the central Indian Ocean

A study to estimate manganese nodule abundance (weight of nodules in kg/m2) was carried out in a small area of the abyssal plains covering a one-degree square block in the central Indian Basin. Abundance was assessed at various intervals by progressively reducing the grid spacing. Sampling the corners of the 1° survey block (approximately110-km spacing), i.e., four stations with 5–7 free-fall operations (sampling locations) in each case, indicated a nodule abundance of 3.50 kg/m2. By reducing the sampling spacing to four grid units (0.5° survey blocks) and sampling the entire block at eight stations (25 locations), the average abundance of the block was 3.36 kg/m2. Further reduction of the grid to 0.25° survey blocks and sampling in 16 grid units (70 sampling locations) increased the abundance to 4.41 kg/m2. For 64 grid units in the one-degree block (sampling in 0.125° survey blocks), a substantially higher value was recorded, i.e., 5.31 kg/m2 or about 1.5 times the abundance obtained at a 1° spacing. Adding 25 more stations in 0.0625° survey blocks (intervals of sampling locations approximately 500 m) resulted in a negligible change in abundance, the average value of the one-degree block being 5.23 kg/m2. These data demonstrate that, for estimating nodule resources in the region, it is important to adopt a close-grid sampling strategy, so that areas with lower abundance can be relinquished and areas with higher abundance can be confidently identified. To ascertain exact nodule abundance for mine-track selection, it may be sufficient to restrict detailed grid surveys to areas with marked variations in topography and nodule abundance, rather than carrying out such detailed (albeit less cost effective) surveys at a very narrow spacing (0.0625°) over the entire pioneer area.

Distribution pattern and morphochemical relationships of manganese nodules from the Central Indian Basin

In the Central Indian Basin manganese nodule abundance was variable in all sediment types. Mean abundance varied from 1.5 in calcareous ooze to 10.2 kg/m2 in terrigenous-siliceous ooze sediments. Nodule grade and growth rates are positively correlated only up to 10 mm/My (million years), and grade shows no distinct relationship with abundance. Relationships between the morphochemical characteristics of the nodules and host sediment types are subtle. Both hydrogenetic and diagenetic nodules (with smooth and rough surfaces respectively) occur on almost all sediments, but in variable proportions. Thus, the overall distribution pattern shows that small nodules ( 4-cm diameters) of higher grade (average value Ni+Cu+Co=1.80%) with rough surfaces are more prevalent on siliceous ooze, siliceous ooze-red clay, and calcareous ooze-red clay transition-zone sediments. This implies an enhanced supply of trace metals from pore waters to rough-surface nodules during early diagenesis.

Thermohaline circulation in the Central Indian Basin (CIB) during austral summer and winter periods of 1997

Abstract As a part of Indian Deep Sea Environment Experiment (INDEX) aimed at assessing the environmental impact of manganese nodule mining in the Central Indian Basin (CIB), a study on baseline physical conditions of water column viz. potential temperature ( θ ), salinity and potential density together with geostrophic circulation regime in the deeper depths of the basin was conducted. The hydrography data used in the present analysis were collected over a wide area of the western part of CIB (71°–79°E; 9°–14°S) during austral summer (January 1997) from the Indian research vessel ORV Sagar Kanya , while during the austral winter season (June–July 1997), hydrographical stations were occupied by Russian research vessel RV Yuzhmorgeologia in the central part of CIB (75°–77°E; 9°–11°S) where a benthic disturbance on experimental scale was carried out. The spatial variations in the physical parameters decreased below 3500 m, inferring a restricted basin-scale deep circulation. The dynamic topography field at 5000 m relative to 2000 db surface in the central part of CIB, representing the abyssal circulation, was generally characterized by a southwestward weak flow around 10°S flanked by cyclonic and anti-cyclonic eddies on its right and left sides, respectively. This flow regime agreed with the earlier one inferred by Warren (J. Mar. Res. 40 (1982) 823) linking the source of deep water in CIB to a saddle overflow across Ninetyeast Ridge from West Australian Basin around 10°S.

Impact of benthic disturbance on megafauna in Central Indian Basin

Abstract Deep-sea photographs and video data were studied to evaluate the effects of benthic disturbance on megafaunal distribution in the Central Indian Basin. Xenophyophores (41%) and holothurians (30%) are the most abundant taxa, followed by other groups in the area before the disturbance. An overall reduction (32%) in the total megafaunal population after disturbance is direct evidence of the impact on benthic environment. Different groups such as xenophyophores, sea anemones, shrimps, starfish, brittle stars, holothurians and fish show different degrees of reduction (21–48%) in their numbers, depending upon their ability to withstand increased turbidity and sedimentation rates due to disturbance. Faunal groups such as protobranch molluscs, polychaete worms, seafans and squids, observed before the disturbance, were not seen after disturbance, whereas populations of some taxa increased after the disturbance. Increased numbers of mobile taxa could be due to increased levels of organic carbon due to resedimentation, whereas increase in sessile taxa may partly reflect the difficulty in distinguishing live from dead specimens. The impact on faunal assemblages is more severe in the disturbed area than the undisturbed area. Our results indicate that monitoring of megafauna can be used effectively to evaluate the potential impacts of large-scale mining or other disturbance on the seafloor, and may therefore help in developing measures for conservation of the benthic environment.

Bacterial standing stock, meiofauna and sediment–nutrient characteristics: indicators of benthic disturbance in the Central Indian Basin

Abstract As part of the environmental impact assessment studies for polymetallic nodule mining, the effect of simulated “benthic disturbance” caused by a benthic hydraulic disturber was studied in the Central Indian Basin (CIB). We have compared the abundance and distribution of meiofauna, macrofauna, total counts of bacteria (TC) and cultivable bacteria in the sediments, retrieved from depths of 5000–5300 m, using box corer, before and after disturbance. Nine box core samples each in pre- and post-disturbance stages, were analyzed for labile organic matter (LOM constituted by carbohydrates, protein and lipids), total organic carbon (TOC), ATP and sediment enzymes, phosphatase and lipase. Immediately after the benthic disturbance, (within 10 days), we observed a decrease in meiofauna, macrofauna and bacterial numbers, accompanied by a decrease in LOM, ATP and lipase activity, indicating importance of quality food for the deep-sea benthos. On the other hand, there was an increase in TOC, phosphatase activity and cultivable bacteria, suggesting beneficial effect of disturbance. The results show that a benthic disturbance caused by a hydraulic device may have mixed effects, such as bringing up nutrients from the subsurface layers on one hand and blanketing the bottom by the discharged sediment plume resulting in a decrease in the number of benthos on the other hand. The distinct change in nutrient characteristics of the bottom sediments caused by the disturbance which is probably comparable to a magnified bioturbation process, can be used as an indicator of benthic disturbance.

Indian Deep-sea Environment Experiment (INDEX):

Abstract Indian Deep-sea Environment Experiment (INDEX) is a multi-disciplinary study to establish baseline conditions and evaluate the possible impact of deep-seabed mining in Central Indian Basin. A disturbance was simulated to study the effects of sediment re-suspension and re-settlement in the benthic areas. Monitoring the process of restoration and recolonisation of benthic environment and development of predictive models for environmental impact of deep seabed mining are underway. Significant information on physical, chemical, biological and geological characteristics of water column and benthic baseline conditions has been generated in the programme. Evaluation of impact of simulated disturbance on the seafloor shows vertical mixing of sediment on the seafloor, lateral migration of sediment plume, changes in geochemical and biochemical conditions as well as reduction in biomass in the benthic environment. The results obtained are useful in determining the indicator parameters and standardising the methods for assessment of effects of large-scale mining.

Chemical characteristics of Central Indian Basin waters during the southern summer

Abstract Chemical properties of the water column were examined at the Indian Deep-sea Environment Experiment (INDEX) site in the Central Indian Basin (CIB), as a part of baseline studies prior to the benthic disturbance experiment for the environmental impact assessment of mining of polymetallic nodules. The study shows three equatorward moving water masses. (a) The Subsurface Salinity Maximum in the depth range 125–200 m, characterized by high salinity (34.74–34.77 psu) and oxygen minimum associated with weak maxima in nutrients. (b) The Deep Oxygen Maximum (234–245 μM) in the depth range 250–750 m, associated with minima in nutrients and relatively high pH. (c) The Salinity Minimum Water (34.714–34.718 psu) corresponding to the Antarctic Intermediate Water (AAIW) at depths 800–1200 m in the density (σθ) range 27.2–27.5. Progressive changes in these characteristics are attributed to mixing with waters above and below, and to oxidation of organic detritus en route. Among the three water masses, the oxygen maximum water shows the lowest changes in its properties, which may suggest that this water mass is moving the fastest.

Dissolved organic carbon in the INDEX area of the Central Indian Basin

Abstract Dissolved organic carbon (DOC) concentrations at the Indian Deep-Sea Environment Experiment (INDEX) site of the Central Indian Basin can be divided into three depth intervals. The subsurface layer (25–200 m) shows highly variable distribution with concentrations ranging from 52 to 191 μM. Between 500 and 800 m, DOC shows a minimum (41–57 μM) and is associated with decreased oxygen. The bottom waters (3500–5300 m) show decrease with little variability in the DOC concentrations (41–64 μM) suggesting its refractory nature in the deep ocean. The variable concentration of DOC with depth is related to the biochemical activity as well as to the chemical features of the water column. Both high and low values of DOC with high apparent oxygen utilisation (AOU) indicate no significant correlation between DOC and AOU and suggest simultaneous utilisation of oxygen for the oxidation of other species of organic matter in the water column.

Sediment redistribution during simulated benthic disturbance and its implications on deep seabed mining

Abstract A benthic disturbance experiment was conducted in the Central Indian Basin to evaluate the possible effects of sediment redistribution on marine ecosystem during deep-sea mining. The analyses of disturbance features on the seafloor and particle resettlement indicate that the most intense impact is created within the experiment site, and some of the resuspended sediments that remain in the near-bottom water after the disturbance are transported away from the site, changing the physico-chemical conditions in the area. Resedimentation is observed on either side of the disturbance zone, with an increase in suspended particles by 300% (from 49 to 150 mg/m 2 /day) during the disturbance and a reduction by 33% (to 95 mg/m 2 /day) immediately after the experiment. Higher collection of particles as well as an increase in organic carbon in seafloor sediments S–SW of the disturbance zone indicates directional migration of suspended material and redeposition in the adjacent areas. The implications of increase in turbidity and sedimentation rates in the near bottom water column due to sediment re-suspension, as well as lateral migration of sediment, on the benthic ecosystem have been discussed. These parameters provide important inputs in ascertaining long-term effects of deep-sea mining.

Implications of the deep circulation and ventilation of the Indian Ocean on the renewal mechanism of North Atlantic Deep Water

In the Indian Ocean sector of the “great ocean conveyor” scheme the North Atlantic Deep Water (NADW) replacement is realized by the transformation of the sinking Circumpolar Deep Water (CDW) from the Southern Ocean to a prevailingly upward flow in the northern Indian Ocean. This water‐mass transformation scenario has been studied by using hydrographie data including potential temperature, salinity, dissolved oxygen, and nutrients in a water‐mass mixing model and by calculation of dianeutral velocity. The model comprises a mixing system of three major deep water masses, NADW, CDW, and North Indian Deep Water. This third water mass is introduced as a virtual water mass to represent the aged CDW. The assumption is feasible as the artificial water mass can be eliminated on the deepest σN = 28.12 neutral surface, characterized by isopycnal mixing. The experiment is fulfilled when a conservative variable, initial phosphate PO40, is introduced and all conservative parameters are used in the mixing scheme. The σN = 28.12 surface provides a major isopycnal transition path of CDW to the northern Indian Ocean, and thus CDW is transformed to an upward flow. Dominant dianeutral up welling is detected on all four neutral surfaces north of 30°S, showing increasingly strong dianeutral velocity toward shallower surfaces. With the water‐mass mixing pattern and spreading path, dianeutral circulation, and dynamical information of the acceleration potential (10 m2 s−2) mapped on neutral surfaces a schematic of deep water circulation and ventilation of the Indian Ocean emerges from this study. The contribution of NADW is largely limited to the southwest Indian Ocean. The northward CDW is found mainly in two paths: one in the western Indian Ocean (apparent between 2000 and 3000 m) through the western Crozet and Madagascar Basins, and farther north into the Somali Basin and Arabian Basin and another in the Central Indian Basin (apparent below 3000 m where ridge‐blocking becomes effective in the west and NADW salinity reaches a maximum). While the major path switches from west to the central basin in the deeper layer (3000–3500 m), the flow in the western Indian Ocean reverses southward. North of the Southeast Indian Ridge, the deep path bifurcates, from which point the eastern branch feeds the deep western boundary flow in the West Australian Basin. Except for part of the transformed CDW, which is contributed to the upward flow in the north, the return flow of CDW is found on the eastern side of these basins.

Handbook of Marine Mineral Deposits

Handbook of Marine Mineral Deposits delves into marine mineral exploration and mining, covering both the fields economics and the state of the art of its science. The 1982 Law of the Sea Convention had a chilling effect on the deep‐sea mining industry Later in the 1980s, most nations adopted the 370‐km exclusive economic zone, which rekindled interest in the industry. Amendments made to the law in 1994 have also improved the outlook since then. India has agreed to abide by the Law of the Sea Convention by setting aside reserves of manganese nodules in the Central Indian Basin for international use that equal those it has claimed for itself.

Distribution of Grain Size and Clay Minerals in Sediments from the INDEX Area, Central Indian Basin

The granulometry of 21 box core sediments from five selected sites in the Central Indian Basin (CIB) have been studied to understand the sediment size characteristics in the Pioneer area before the benthic disturbance experiment. The sediments are predominantly clayey silt. Statistical parameters were determined to understand the control and variations in the grain size. The mean grain size (Mzφ) ranged from 7.0 to 8.6 for the surface and from 6.6 to 8.6 for the subsurface sediments, indicating deposition of very fine fractions during the Neogene period. The sediments were poorly sorted as revealed from the higher ρ1 values (2.7 to 2.9). Higher and negative values of skewness (SK1) indicated a winnowing action of prevailing currents in the area. Average kurtosis (KG) values for the surface (0.4 to 0.8) and subsurface (0.5 to 1.2) showed that sediments were very platykurtic and leptokurtic. Clay mineralogical studies showed that smectite had a wider range (from 6% to 55%) and decreased southward because of its terrigenous origin. The terrigenous influence of the sediments could be documented as far as 12°S. Smectite and kaolinite increased eastward and chlorite decreased southward. This observation positively correlated with the mean current direction in the area. However, the influence of Antarctic Bottom Water (AABW) in the area could be predicted from the southward increase in chlorite content.

Biostratigraphic Analysis of the Top Layer of Sediment Cores from the Reference and Test Sites of the INDEX Area

Radiolarian fossil study in the sediment cores collected during the pre- and postdisturbance cruises of the Environmental Impact Assessment (EIA) Indian Ocean Experiment (INDEX) program of deep sea mining in the Central Indian Ocean Basin suggests a pronounced directional deposition of fossil radiolarians exhumed during the deep sea benthic disturbance experiment. The relative occurrences of the Stylatractus universus species that became extinct ∼0.425 million years before present were mostly confined to the older and deeper strata of the sediment of the disturbance tract in the southwestern direction. This pattern is remarkable and suggests that the disturbance plume has been preferentially redeposited in the southwestern direction. This observation is in concurrence with the prevailing southwestern abyssal current during the disturbance experiment in the Central Indian Basin.

Distribution of Grain Size and Clay Minerals in Sediments from the INDEX Area, Central Indian Basin

Distribution of Grain Size and Clay Minerals in Sediments from the INDEX Area, Central Indian Basin

Increased Particle Fluxes at the INDEX Site Attributable to Simulated Benthic Disturbance

Particle fluxes were measured 7 m above the sea bottom during the predisturbance, disturbance, and postdisturbance periods by using time series sediment traps attached to seven deep-sea moorings deployed in the INDEX experiment site in the Central Indian Basin. The predisturbance particle fluxes varied between 22.3 to 55.1 mg m−2 day−1. Increased and variable particle fluxes were recorded by the sediment traps during the disturbance period. The increase observed was 0.5 to 4 times more than the background predisturbance fluxes. The increases in particle fluxes (∼4 times) recorded by the sediment trap located in the southwestern direction (DMS-1) were the greatest, which could be the result of preferential movement of resuspended particles generated during the deep-sea benthic disturbance along the general current direction prevailing in this area during the experimental period. Also, the traps located closer to the disturbance area recorded greater fluxes than did the traps far away, across the Deep Sea Sediment Resuspension System path. This variability in recorded particle fluxes by the traps around the disturbance area clearly indicates that physical characteristics such as grain size and density of the resuspended particles produced during the disturbance had an important effect on particle movement. The postdisturbance measurements during ∼5 days showed a reduction in particle fluxes of ∼50%, indicating rapid particle settlement.

Distribution of Deep-Sea Benthos in the Proposed Mining Area of Central Indian Basin

During the first stage of the project work on the Indian Deep-Sea Environment Experiment (INDEX), the abundance and distribution of deep-sea benthos were surveyed in the Central Indian Basin for the collection of baseline data. The deep-sea community of the sediment was characterized by a moderately high standing crop and diverse fauna. The macrofaunal component was dominated by polychaetes (100% prevalence) and peracarid crustaceans, whereas the meiofauna was dominated by nematodes and harpacticoid cope-pods. The results of this study conform to the general distribution reported elsewhere. The macrofaunal abundance showed an inverse relation to the abundance of polymetallic nodules. However, the relation between meiofaunal vertical distribution and the vertical profile of the total organic matter and total labile matter was positive.

Benthic Disturbance and Impact Experiments in the Central Indian Ocean Basin

As a part of the Environmental Impact Assessment studies for nodule mining, a long-term program has been initiated in the Central Indian Basin. Multidisciplinary studies on geological, biological, physical, and chemical parameters were carried out in an area selected on the basis of baseline data collected in the first phase of the program. A benthic disturbance was simulated with a hydraulic device also used in the previous experiments in the Pacific Ocean. A site of 3,000 × 200 m was repeatedly disturbed by a combination of fluidizing pump and suction pump to dislodge and discharge sediment from the seafloor into the water column 5 m above the seafloor. During 9 days of operation, 26 tows were carried out for 47 h of disturbance, resuspending about 6,000 m3 of sediment along an 88-km line. Data for postdisturbance impact assessment were collected with sediment traps, deep-towed cameras, seafloor samples, and conductivity-temperature depth sensor (CTD)-rosette observations. Seafloor data, sediment samples, and water column studies were aimed at evaluating the impact of benthic disturbance, on the basis of pre- and post-disturbance data collected during the experiment. Observations show that vertical mixing of sediment as well as its lateral movement and resedimentation because of plume migration alters various parameters and leads to changes in the environment around the area.

Geotechnical Properties of Surface Sediments in the INDEX Area

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0–10 cm) have a shear strength of 0–1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312–577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm3. Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.