Hydrocarbons in the Bay of Bengal and Central Indian Basin bottom sediments: indicators of geochemical processes in the lithosphere

A study on the bulk distributions and molecular structures of n-alkanes and polycyclic aromatic hydrocarbons (PAH) in organic matter of the sediments from the Bay of Bengal and the Eastern and Central Indian Basins was undertaken. The former two regions represent areas characterised by “normal” sedimentation while the third one mainly represents a region of “active tectonism”. Content of the hydrocarbons in the sediments of “normal” sedimentation ranges between 4.6 and 10.5 μg/g and aromatic hydrocarbons ranges between 0 and 0.38 μg/g. n-Alkanes in the sediments of the northern deep part of the Bay of Bengal consist mostly of long-chain structures (total C₂₅–C₃₃ up to 70%) with a high carbon preference index (CPI=3.01–3.43), indicating a large contribution of organic matter from terrigenous sources. The sediments from the Eastern Indian Basin have n-alkane distributions in which the long-chain components did not exceed 52.5% and the CPI was 1.7–1.90, indicating that the hydrocarbons are mostly derived from marine sources. Sharp increases of hydrocarbons are found in the vicinity of the tectonically active region of the Central Indian Basin, particularly in the sediments collected from the fracture zone. The total concentration of hydrocarbons increase to 170 μg/g and the aromatic hydrocarbons fraction to 156.3 μg/g. The proportion of short-chain n-alkanes increases up to 70%, CPI decreases to 0.76–1.12, and high concentrations of n-C₁₆ (16–40%) occur, all of which are absent in the other samples. The molecular content of PAH includes the unsubstituted individual structures: biphenyl, fluorene, pyrene, perylene, benzo(ghi)perylene, and the groups of homologues of naphthalene, benzofluorene, phenanthrene and chrysene. The association of the PAH and composition of paraffin hydrocarbons in the surficial sediments of deformation zone indicate that these are the resultant products of hydrothermal processes. It is, therefore, suggested that the association and composition of the hydrocarbons in sediments can be utilised as a paleoceanographic parameter to decipher the history of tectonism of an area.     

Monitoring the Impact of Simulated Deep-sea Mining in Central Indian Basin

Monitoring of deep-sea disturbances, naturai or man-made, has gained significance due to the associated sediment transport and for the ensuing alterations in environmental conditions. During the Indian Deep-sea Environment Experiment (INDEX), resuspension of deep-sea sediment in the Central Indian Basin (CIB) resulted in an increase and lateral movement of suspended particles, vertical mixing of sediments, changes in sedimentological, biochemical, and geochemical conditions and an overall reduction in benthic biomass. Monitoring the conditions 44 months after the experiment has shown a partial recovery of the benthic ecosystem, with indications of restoration and recolonization.     

Response of Sedimentary Nucleic Acids to Benthic Disturbance in the Central Indian Basin

Information on the response of nucleic acids (i.e., DNA and RNA) to simulated benthic disturbance was obtained from samples collected from eight sediment cores (0–10 cm) located in the Central Indian Basin (CIB). In general the total sedimentary DNA (DNA) increased with depth (from 380.09 to 408.99 μg·g^?1), while total sedimentary RNA (RNA) decreased (from 878.13 to 484.16 μg·g^?1). Following the simulated benthic disturbance brought about by the benthic hydraulic disturber, DNA decreased by 10% along the disturbed track while the RNA increased by around 75%. The RNA/DNA ratio decreased nearly 10%. However, the ratio doubled along the disturbed track suggesting that the activity was stimulated at molecular level following artif icial disturbance. The nucleic acid ratio thus shows that the CIB sediments are metabolically active, and with disturbance the activity tends to increase further.     

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

Abstract

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 (M_zφ) 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 (K_G) 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.     

Relationship of Sediment-Biochemistry,Bacterial Morphology,and Activity to Geotechnical Properties in the Central Indian Basin

Distribution of biochemical properties, morphological and functional characteristics of bacteria in the extreme sedimentary environment of the Central Indian Basin (CIB) was examined in relation to geotechnical properties and sediment texture. Interrelationships between these parameters in siliceous northern sediments were compared to southern pelagic clay to understand relationships and to appreciate mining implications. Clay impacted lipids negatively and silt positively in the south. Geotechnical properties had the opposite influence on the distribution of coccoidal and rod forms of bacteria. Rods were affected negatively by water content/porosity and positively by wet bulk density in the north suggesting their preference for deeper niches. Shear strength affected coccoidal distribution negatively in the south. Competition for labile substrates by coccoidal forms in the surface layers could also have restricted the rods to the deeper regions. Principal component analysis (PCA) further strengthened our inference that the distribution of rod forms increased with depth. Enzymatic properties of bacterial isolates also showed that the rods were able to degrade substrates like gelatin and DNA. These observations suggest that they are capable of degrading relatively more recalcitrant compounds in the deeper layers. The minor components, like the rod morphotypes and sand content, could wield a large influence on the variability of other parameters.     

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 (M z φ) 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 σI values (2.7 to 2.9). Higher and negative values of skewness (SK I ) indicated a winnowing action of prevailing currents in the area. Average kurtosis (K G ) 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.     

First Phase Monitoring Studies of Simulated Benthic Disturbance Delineating Movement of Fine Particles in the Central Indian Basin

Benthic disturbance due to future deep-sea polymetallic nodule mining would involve expensive sediment plume generation and resedimentation on the sea floor. In order to evaluate the effects of resedimentation on benthic environment, the Indian Deep-sea Environment Experiment (INDEX) was conducted in 1997, and pre-, and post-disturbance studies on grain size were carried out. The initial increase in clay content after the experiment, continued to increase further as measured in the first monitoring phase samples, 44 months later. Increase in clay-sized particles during monitoring-l (M-l) was highest within the simulated (disturbed) zone and to the north of it, which is attributed to the combined effects of disaggregation, abrasion, and powderization of sediments during transportation. Due to this fractionation (breaking up), the particles appear to have remained in suspension over a prolonged period of time after they were discharged in the water column 5 m above the seabed during INDEX. The travel effects of INDEX plume appears to be localized and confined within and around the disturbed zone (DZ) as resettlement of fine particles from the benthic plume was traced up to 2 km south and 12 to 18 km north of the DZ. The evidence does not suggest the existence of strong currents and benthic storms in the CIB     

Restoration of Deep-Sea Macrofauna after Simulated Benthic Disturbance in the Central Indian Basin

Macrofaunal communities of the Central Indian Basin (CIB) were sampled with a spade before (June 1997), and immediately after (August 1997), and 44 months (April 2001) after a simulated benthic disturbance for polymetallic nodule mining. The average density recorded down to a sediment depth of 40 cm ranged from 89 to 799 ind·m^-2 (mean: 373 ± 221 SD; n = 12) and 178-1066 ind·m^-2 (mean: 507 ± 489 SD; n = 3) in the test and reference area, respectively. Most of the macrobenthic animals (64%) were concentrated in the upper 0 to 2 cm sediment layers, whereas, sizeable fauna (6%) inhabited the 20-40 cm sediment section and the deepest 5 cm section from 35-40 cm contributed only about 2% to the total population density. The fauna, comprised of 12 groups, were dominated by the nematodes, which constituted 54% of the total population. The macrofaunal density in the test site showed a significant increase (x:400 ind·m^-2) in the 44 months postdisturbance sampling (x:320 ind·m^-2). The population of nematodes and oligochaetes was nearly restored after 44 months, but the polychaetes and crustaceans did not reach the baseline populations measured in June 1997. The top 0-2 cm sediment layer was severely affected by the disturber, and the study suggests that physically disturbed deep-sea macrofauna may require a longer period for restoration and resettlement than normally believed.     

西南印度洋深海热液区铠甲虾初探

简述了深海热液区铠甲虾的分类研究现状,整理出铠甲虾总科下属科的形态分类检索表,并对2008年10月-2009年2月中国大洋科考第20航次在西南印度洋深海热液区获得的铠甲虾样品进行详细的形态描述和分类,初步鉴定为5种,隶属于2科5属,分别是:劣柱虾科(Chirostylidae)的折尾虾(Uroptychus sp.),...     

Basinal seamounts and seamount chains of the Central Indian Ocean: Probable near-axis origin from a fast-spreading Ridge

Hydrosweep mapping of crust in the Central Indian Ocean Basin reveals abundant volcanoes ocurring both as isolated seamounts and linear seamount chains parallel to flow lines. Their shapes, sizes and overall style of occurrence are indistinguishable from near-axis seamounts in the Pacific. Evidence from seamount morphology, distributions and petrography of dredged samples suggests that they were generated near the fast-spreading Southeast Indian Ridge at 50–60 Ma. If so, this style of near-axis seamount generation may be a result of fast-spreading rate rather than a peculiarity of the present Pacific spreading ridges. In fact, the results of several recent studies, taken together, suggest that the style of axis/near-axis seamount volcanism varies systematically as a function of spreading rate.     

Segmentation of the Central Indian Ridge between 12°12′ S and the Indian Ocean Triple Junction

The present morphology and tectonic evolution of more than 1500 kilometres of the Central Indian Ridge are described and discussed following the integration of GLORIA side-scan sonographs with conventional geophysical datasets. Segmentation of the ridge occurs by a series of ridge axis discontinuities ranging in periodicity along strike from 275 km to less than 30 km. These segment boundaries we have classified into two types: first order fracture zones of offsets greater than 50 km which bound five major (mega-) segments, and smaller scale structures of a variety of offset styles and amplitudes which cut four of these segments. We refer to these as ridge-axis discontinuities. The frequent opposite sense of offset identified between the first order structures and the subordinate discontinuities between these major structures is interpreted as resulting from the adjustment to new kinematic parameters after magnetic anomaly 20. As far as our data allows us to determine, the central major segment is not subdivided by minor ridge axis discontinuities, which we suggest is a result of its proximity to the Rodriguez hotspot.     

Microbial Biomass and Organic Nutrients in the Deep-Sea Sediments of the Central Indian Ocean Basin

In order to assess the impact of deep-sea mining on the in situ benthic life, we measured the microbial standing stock and concentration of organic nutrients in the deep-sea sediments of the Central Indian Ocean Basin in the Indian pioneer area. Sediments were collected using box core and grab samples during September 1996. The total bacterial numbers ranged from 10 10 -10 11 cells per g -1 dry weight sediment. There was a marginal decrease in the number of bacteria from surface to 30 cm depth, though the subsurface section registered a higher number than did the surface. The highest numbers were encountered at depths of 4-8 cm. The retrievable number of bacteria were two orders less in comparison with the direct total counts of bacteria. An almost homogeneous distribution of bacteria, total organic carbon, living biomass, and lipids throughout the depth of cores indicates active microbial and benthic processes in the deep sea sediments. On the other hand, a uniform distribution of total counts of bacteria, carbohydrates, and total organic carbon in all the cores indicates their stable nature and suggests that they can serve as useful parameters for long-term monitoring of the area after the benthic disturbance. Further studies on temporal variability in this region would not only verify the observed norms of distribution of these variables but would also help to understand restabilization processes after the simulated benthic disturbance.     

Response of Meiofauna to Immediate Benthic Disturbance in the Central Indian Ocean Basin

Abstract

Twenty-four stations (12 each, during pre- and postdisturbance studies) 5300 to 5330 m deep between 10°01' and 10°03'S latitude and between 75°59' and 76°02'E longitude were sampled to study the effect of benthic disturbance on the distribution of meiofauna in the Central Indian Ocean. Bottom-sampling was conducted with a box corer. Total meiofauna density ranged from 35 to 45 organisms per 10 cm² of bottom area during the predisturbance period and 21 to 32 organisms per 10 cm² during the postdisturbance period in the test and reference sites. Differences between pre- and postdisturbance study results were statistically significant (P < 0.05). Nematoda was the most abundant meta-zoan group, on an average representing >55% of the meiofaunal population. The abundance of nematodes and harpacticoid copepods as well as total meiofauna showed marked decreases during postdisturbance sampling. Vertical distribution of meiofauna in the sediment cores revealed that 75% to 90% of the metazoan population was confined to the top 2-cm layer of the sediment. Recolonization experiments suggest that harpacticoid copepods may take more time for recolonization than the nematodes and are more sensitive to the physical disturbance. These findings suggest that nematodes and harpacticoid copepods can be used as indicator organisms in recolonization experiments.     

Microscale Response of Sediment Variables to Benthic Disturbance in the Central Indian Ocean Basin

Abstract

The response of sediment bacteria and biochemical variables to benthic disturbance was investigated in the Central Indian Ocean Basin from cores at nine stations. Generally, bacterial density and biochemical variables declined vertically with depth in the upper layers of the cores. Mechanical disturbance caused by a hydraulic disturber brought about a substantial decrease in total bacterial numbers. However, the numbers of retrievable bacteria increased by two orders of magnitude. The biochemical parameters decreased in quantity with a shift in the linear relationship. The data suggest that abiotic artifacts will directly affect the biomass and biochemistry of the sediment.     

Microscale Response of Sediment Variables to Benthic Disturbance in the Central Indian Ocean Basin

The response of sediment bacteria and biochemical variables to benthic disturbance was investigated in the Central Indian Ocean Basin from cores at nine stations. Generally, bacterial density and biochemical variables declined vertically with depth in the upper layers of the cores. Mechanical disturbance caused by a hydraulic disturber brought about a substantial decrease in total bacterial numbers. However, the numbers of retrievable bacteria increased by two orders of magnitude. The biochemical parameters decreased in quantity with a shift in the linear relationship. The data suggest that abiotic artifacts will directly affect the biomass and biochemistry of the sediment.     

Thorium isotopes as tracers of particles dynamics and deep water circulation in the Indian sector of the Southern Ocean (ANTARES IV)

Dissolved and particulate samples were collected to study the distribution of thorium isotopes (²³⁴Th, ²³²Th and ²³⁰Th) in the water column of the Indian sector of the Southern Ocean (from 42°S to 47°S and from 60°E to 66°E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess ²³⁰Th (²³⁰Th_xs) increases linearly with depth in surface water (0–100 m) and a model was applied to estimate a residence time relative to the thorium scavenging (τ_scav). Low τ_scav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the ²³⁰Th_xs scavenging efficiency and explain this difference. An innovative coupling between ²³⁴Th and ²³⁰Th_xs was then used to simultaneously constrain the settling velocities of small (0.6–60 μm) and large (above 60 μm) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60 ± 2 m d^− 1, station 8) than south of the Agulhas Return Current (119 ± 23 and 130 ± 5 m d^− 1 at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the ²³⁰Th_xs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the ²³⁰Th_xs concentrations. A specific ²³⁰Th_xs transport model is applied in the deep water column and allows us to assess a “travel time” of NADW ranging from 2 to 15 years.     

Facies character and depositional architecture of hydrothermal travertine slope aprons (Pleistocene,Acquasanta Terme,Central Italy)

Hydrothermal travertines develop various depositional geometries, from tabular to high-relief mounds or aprons with steep slopes, under the control of local topography, location and geometry of the vents, fault activity, hydrology, water physico-chemical properties, rates of thermal water flow and carbonate precipitation rates. This study focuses on two Pleistocene, tens of metres thick, travertine slope aprons accumulated on fluvial terraces in the Tronto River Valley (Acquasanta Terme, Central Italy) to investigate their facies character, geochemical signature, porosity and evolution of the depositional geometry through time.The two travertine aprons consist of four aggradational-progradational units, vertically and laterally stacked with onlap and downlap stratal terminations, separated by erosional unconformities produced by events of non-deposition and erosion, due to temporary interruptions of vent activity, shifts of the vent location and/or deviation of the flow directions. The travertine units include various depositional environments: 1) smooth slope, 2) terraced slope with metre-scale sub-horizontal pools separated by rounded rims and vertical walls, and 3) sub-horizontal, tens of metres wide ponds. Smooth slope clinoforms are made of centimetre to decimetre thick layers of crystalline dendrite cementstone, laminated boundstone and radial coated grain grainstone, precipitated by fast-flowing water on inclined substrates. Rims and walls of terraced slopes are built by crystalline dendrites and laminated boundstone. Sub-horizontal layers of terrace pools and ponds consist of facies precipitated by slow-flowing to standing water (clotted peloidal micrite dendrite, coated bubble boundstone, raft rudstone) associated with radial coated grains and laminated boundstone. Carbonate coated reeds occur in distal ponds adjacent to toe of slopes or overlie packstone/rudstone with travertine intraclasts and substrate extraclasts, marking events of subaerial exposure and erosion. Travertine facies porosity and permeability range from 4 to 21% and 0.03 to 669 mD, respectively, showing no direct correlation. Stable isotope values (δ¹³C: 5.7–9.3‰; δ¹⁸O −9.6‰ to −12.2‰) are similar to other travertines precipitated by thermal water in Central Italy.This study identifies the centimetre-scale travertine facies variability linking it to the environment of deposition and to the depositional geometry of travertine units affected by the substrate topography and lateral shifts of the active springs. Despite the different scale and facies composition, the geometry of aggradational-progradational units of travertine aprons might resemble marine flat-topped high-relief carbonate platforms. Travertine units in the subsurface, if present with sizes that can be seismically resolved, might be wrongly interpreted as carbonate platforms with steep slopes without a detailed facies analysis.     

Constructing a 3D structural block diagram of the Central Basin in Marmara Sea by means of bathymetric and seismic data

In this study we made a comparative interpretation of multibeam bathymetric and seismic reflection data with different resolutions and penetration properties collected in the Central Basin of the Marmara Sea. Our main objectives were (i) to investigate and compare the active tectonic deformation observed on the sea bottom and within the uppermost sedimentary layers to that of the deep-seated deformation within the limits of resolution and penetration of the available geophysical data and (ii) to build a three-dimensional (3D) block diagram of the active tectonic and buried features by means of a sliced mapping technique. In this approach, we produced slice maps of the active and buried structural features at selected depths and then combined them to form a 3D structural block diagram. Motivation for our work was to produce a 3D structural diagram to derive a more detailed image of the structural features in the Central Basin where there is no available 3D seismic data. The observations from the bathymetry and seismic data and developed 3D diagram support the presence of a through-going strike-slip fault that forms a rotational depression zone against a right-stepping strike-slip faulting causing a pull-apart basin in the Central Depression zone.     

Indian summer monsoon variability during the holocene as recorded in sediments of the Arabian Sea: Timing and implications

Indian monsoon precipitation fluctuated significantly during the Holocene and a reliable reconstruction of the timing of the events and their implications is of great benefit to our understanding of the effect and response of low latitude climate systems to the forcing factors. We have carried out high-resolution terrigenous proxy studies on a laminated sediment core from the Oxygen Minimum Zone of the eastern Arabian Sea margin to reconstruct the summer monsoon-controlled precipitation changes during the Holocene. The temporal variation in the terrigenous proxy indicators of this core, in combination with other high-quality cores from the Arabian Sea, suggests several abrupt events in monsoon precipitation throughout the Holocene. The early Holocene monsoon intensification occurred in two abrupt steps at 9500 and 9100 years BP and weakened gradually thereafter, starting at 8500 years BP. A weakening in precipitation recorded at ∼7000 years BP, synchronous with similar conditions in India. One of the most significant weak monsoon periods recorded in our studies lies between 6000 and 5500 years BP. Spectral analysis of the precipitation records reveals statistically significant periodicities at 2200, 1350, 950, 750, 470, 320, 220, 156, 126, 113, 104 and 92 years. Most of these millennial-to-centennial cycles exist in various monsoon records as well as the tree ring Δ¹⁴C data and/or other solar proxy records. We suggest that throughout the Holocene, externally, small changes in solar activity controlled the Indian monsoon to a large extent, whereas internally, non-solar causes could have influenced the amplitude of decadal-to-centennial oscillations.     

Distribution and Origin of Seamounts in the Central Indian Ocean Basin

Approximately 200 seamounts of different dimensions have been identified, from multibeam bathymetry maps of the Central Indian Ocean Basin (CIOB) (9°S to 16°S and 72°E to 80°E), of which 61% form eight chains that trend N-S. The seamounts are clustered above and below 12°S latitude. Area II (9°–12°S) shows a concentration of smaller seamounts (≤400 m height), and area I (12°–15°S) has a mixed population (including both less and more than 400 m height). Inspite of the differences in their height, the seamounts of these eight chains are morphologically (slope angle, flatness, basal width) corelatable. Furthermore, we suggest that height-width ratio could be useful in identifying the style of seamount eruption. The seamount chains in the CIOB probably originated from propagative fractures and were produced between 61 and 52 Ma (chrons A26 to A23) as a result of the interaction between the conjugate crusts of the Central Indian and Southeast Indian Ridges during the Indo-Eurasian collision event.