Earth, Ocean and Atomospheric Sciences - Publications

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    2.61 Ga potassic granites and crustal reworking in the western Dharwar craton, southern India: Tectonic, geochronologic and geochemical constraints
    ( 2006-10-20) Jayananda, M. ; Chardon, D. ; Peucat, J. J. ; Capdevila, R.
    We present combined field, structural data and spot image analysis, petrographic data, U-Pb zircon ages, Nd isotopes and whole-rock geochemical data for the late Archaean granite plutons from Arsikere-Banavara (AB suite) and Chitradurga-Jampalnaikankote-Hosdurga (CJH suite), in the western Dharwar craton (WDC). AB plutons are syn-kinematic with respect to the development of dome-and-basin patterns resulting from partial crustal diapiric overturn (D1), whilst CJH suite plutons show deformation patterns resulting from the superimposition of large-scale regional strike-slip shearing (D2) onto D1 syn-emplacement fabrics. SIMS U-Pb zircon ages of 2614 ± 10 Ma for the Chitradurga pluton (CJH) and of 2617 ± 3 Ma for the Arsikere pluton (AB) define the minimum age of the D1 strain pattern. The CJH and AB suites are mainly high-K granites whose origin is to be sought in partial melting of an old (>3.0 Ga) depleted lower crust of intermediate composition. The CJH and AB suites correspond mainly to the CA1- and CA2-types of Archaean calc-alkaline granites and some facies to the transitional TTG. Their heterogeneous chemical compositions indicate involvement of various sources and also significantly different depths of partial melting to produce these granites. These results allow documenting regional partial reworking of the WDC 2615 Ma ago, requiring an HT event to have taken place before 2.61 Ga and thus different from the 2.55 to 2.51 Ga granulite facies metamorphic episode associated to regional-scale strike-slip shearing (D2) that affected the entire craton. Partial reworking of the older lower crust at 2.61 Ga and associated deformation would result from heat advection and/or crustal thickening that may relate to voluminous mafic greenstone volcanism and moderate shortening at that time. © 2006 Elsevier B.V. All rights reserved.
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    2.62Ga high-temperature metamorphism in the central part of the Eastern Dharwar Craton: Implications for late Archaean tectonothermal history
    ( 2012-03-01) Jayananda, M. ; Banerjee, M. ; Pant, N. C. ; Dasgupta, S. ; Kano, T. ; Mahesha, N. ; Mahabaleswar, B.
    Aluminous metasediments occurring in the central part of the Eastern Dharwar Craton (EDC), southern India, develop contrasting mineral assemblages in closely spaced domains. Detailed petrographic, mineral chemical and geothermobarometry showed 'peak' metamorphic conditions of 780-820°C and ~5kbar that resulted in partial melting of biotite-bearing protoliths. Initial bulk compositional heterogeneity and different degrees of melt extraction produced diverse restitic mineral assemblages. P-T pseudosections, constructed from effective bulk composition, and consideration of the sequence of mineral reactions, deduced from textural criteria, define a nearly isobaric heating-cooling P-T trajectory. Monazite in the metasediments has a detrital core age of 3161±78Ma and a metamorphic age of 2625±26Ma (EPMA chemical age), which is the first record of a pre-2500Ma high temperature-low pressure metamorphism event in the EDC. Based on age contemporaneity with the 2.7-2.62Ga hot juvenile magmas of tonalite-trondhjemite-granodiorite (TTG)-greenstone accretion and the deduced P-T history, we conclude that heat advecting from rising TTG magmas and mafic magmas (greenstones) as a consequence of mantle melting, resulted in a hot transient mid-crust during a major late Archaean tectonothermal event in the Eastern Dharwar Craton. All the phenomena are consistent with a recently proposed model involving an arc setting during ridge subduction and consequent opening of a slab window. This study therefore provides new constraints on the thermal evolution of the Eastern Dharwar Craton. © 2011 John Wiley & Sons, Ltd..
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    3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry
    ( 2008-04-05) Jayananda, M. ; Kano, T. ; Peucat, J. J. ; Channabasappa, S.
    We present field, petrographic, Sm-Nd whole-rock isochron and whole-rock geochemical data for komatiites from Sargur Group greenstone belts of the western Dharwar craton. Field evidence such as pillow structure indicates their eruption in a marine environment. Petrographic data reveal that the igneous mineralogy has been altered during post-magmatic hydrothermal alteration processes corresponding to greenschist- to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene. A 16-point Sm-Nd whole-rock isochron gives an age of 3352 ± 110 Ma for the timing of eruption of komatiite lavas. About 60% of the studied komatiite samples show Al-depletion whilst the remaining are Al-undepleted. The Al-depleted komatiites are characterised by high CaO/Al2O3 ratios (1.01-1.34) and low Al2O3/TiO2 (5-16) whereas Al-undepleted komatiites show lower CaO/Al2O3 ratios (0.59-0.99) and higher Al2O3/TiO2 (17-26). Trace element distribution patterns of komatiites suggest that most of the primary geochemical and Nd isotopic compositions are preserved with only minor influence of post-magmatic alteration processes and negligible crustal contamination. The chemical characteristics of Al-depleted komatiites, such as high (Gd/Yb)N together with lower HREE, Y, Zr and Hf, imply their derivation from deeper upper mantle with garnet (majorite?) involvement, whereas lower (Gd/Yb)N slightly higher HREE, Y, Zr and Hf suggest derivation from shallower upper mantle without garnet involvement. The observed chemical characteristics (CaO/Al2O3, Al2O3/TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, REE) indicate derivation of the komatiite magmas from different depths in a plume setting, whereas sub-contemporaneous felsic volcanism and TTG accretion can be attributed to an arc setting. In order to explain the spatial association of komatiite volcanism with contemporaneous mafic-felsic volcanism and TTG accretion we propose a combined plume-arc setting. Nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early (>4.53 Ga) global differentiation of the silicate Earth as suggested by Boyet and Carlson [Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (>4.53 Ga) global differentiation of silicate Earth. Science 309, 577-581] or extraction of continental crust during the early Archaean. © 2007 Elsevier B.V. All rights reserved.
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    3D Gravity Analysis in the Spatial Domain: Model Simulation by Multiple Polygonal Cross-Sections Coupled with Exponential Density Contrast
    ( 2019-06-01) Mallesh, K. ; Chakravarthi, V. ; Ramamma, B.
    An automatic 3D modeling technique is developed in the spatial domain to analyze the gravity anomalies produced by a concealed density interface with mass density contrast differing exponentially with depth. The sedimentary column above the interface is described with a stack of multiple vertical polygonal sections of unit thickness each. For such a case, the depth ordinates of the vertices of the cross-sections become the unknown parameters to be estimated from gravity data. Forward solution of the model space is realized in the spatial domain by a technique that combines both analytic and numeric approaches. Initial depths to the interface are calculated based on the Bouguer slab approximation and subsequently improved, iteratively, based on the ratio of the product of the observed gravity anomaly and existing depth parameter to the corresponding model gravity response. The iterative process continues until one of the predefined termination criteria is accomplished. Unlike the existing methods, the advantage of the proposed method is that the observed gravity anomalies need not necessarily be sampled/available at regular spatial grid intervals. The applicability of the proposed model is exemplified with a set of noisy gravity anomalies attributable to a synthetic structure before being applied to a real world gravity data. In the case of the synthetic example, the method has yielded a structure that was compatible with the assumed structure even in the presence of random noise. Application of the proposed method to the gravity data set from the Los Angeles Basin, California, using a prescribed exponential density function has yielded a model that concurs reasonably well with the published models.
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    3D gravity inversion of basement relief - A depth-dependent density approach
    ( 2007-01-01) Chakravarthi, Vishnubhotla ; Sundararajan, Narasimman
    We present a 3D gravity inversion technique, based on the Marquardt algorithm, to analyze gravity anomalies attributable to basement interfaces above which the density contrast varies continuously with depth. The salient feature of this inversion is that the initial depth of the basement is not a required input. The proposed inversion simultaneously estimates the depth of the basement interface and the regional gravity background. Applicability and efficacy of the inversion is demonstrated with a synthetic model of a density interface. We analyze the synthetic gravity anomalies (1) solely because of the structure, (2) in the presence of a regional gravity background, and (3) in the presence of both random noise and regional gravity background. The inverted structure remains more or less the same, regardless of whether the regional background is simulated with a second-degree polynomial or a bilinear equation. The depth of the structure and estimated regional background deviate only modestly from the assumed ones in the presence of random noise and regional background. The analyses of two sets of real field data, one over the Chintalpudi subbasin, India, and another over the Pannonian basin, eastern Austria, yield geologically plausible models with the estimated depths that compare well with drilling data. © 2007 Society of Exploration Geophysicists.
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    3D spatial domain gravity inversion with growing multiple polygonal cross-sections and exponential mass density contrast
    ( 2021-06-01) Ramamma, B. ; Mallesh, K. ; Chakravarthi, V.
    Abstract: An automatic 3D spatial domain inversion technique is developed to estimate basement depths of sedimentary basins from observed gravity anomalies using a prescribed exponential mass density contrast. A collage of vertical polygonal cross-sections, each one with unit thickness, in which the density contrast differs exponentially with depth describes the model space. The proposed technique estimates the optimum depth ordinates of the vertices of polygonal cross-sections from a given set of gravity anomalies following predefined convergence criteria. Initial depths to basement interface at plurality of observations are calculated presuming that the density contrast within the Bouguer slab at each observation is also varying exponentially with depth. A previously reported algorithm that make use of both analytic and numeric approaches to compute the gravity response of such 3D model space with exponential mass density contrast is adopted for forward modelling. The proposed inversion is efficient even when the gravity anomalies are available at non-uniform spatial grid intervals. Recovery of basement depths with modest error from a set of gravity anomalies attributable to a synthetic model in the presence of pseudorandom noise and also the fact that the estimated depth structure of the Almazán Basin in NE Spain correlates reasonably well with the information derived from seismic data demonstrates the applicability of the proposed inversion method. The snags associated with other existing density models in the analysis of gravity anomalies are demonstrated on both synthetic and real field anomalies. Research Highlights: 1.It is a 3D inversion technique to analyse the gravity anomalies of sedimentary basins.2.Density contrast variation is automatically ascribed by an exponential function in the algorithm.3.The interpretation technique does not require initial model specification to start with.4.The algorithm is fully automatic.
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    A biological switch at the ocean surface as a cause of laminations in a Precambrian iron formation
    ( 2016-07-15) Hashizume, K. ; Pinti, D. L. ; Orberger, B. ; Cloquet, C. ; Jayananda, M. ; Soyama, H.
    Banded iron formations (BIFs) exhibit alternating silica- and iron-rich laminae, potentially reflecting the dynamics of the paleo-environments in which they were formed, although the exact mechanism remains unclear. Here the formation of a 2.7-2.9 Ga BIF from Dharwar Craton, India, is deciphered by analyzing the inter-band variations of the redox-sensitive isotope biomarkers, 15N/14N and 56Fe/54Fe. Organic matter with δ15N values as high as +12.0±0.8‰ appears to be trapped in silica. Iron oxides exhibit systematically positive δ56Fe values, ranging between +0.80±0.05‰ and +1.67±0.02‰. Compared to the iron-rich bands, silica-rich bands, which show higher δ56Fe values, exhibit an order of magnitude higher concentrations of 15N-rich organic nitrogen, normalized by the abundances of its host silica. The presence of 15N-rich organic matter may imply the emergence of a modern-like biological nitrogen cycle that requires the formation of oxidized nitrogen compounds. The higher concentration of 15N-rich organic nitrogen for the silica-rich bands possibly suggests that the photosynthetic activity was higher during the formation periods of these bands. The heavier iron isotope compositions of the silica-rich bands cannot be explained alone by iron oxidation through probable pathways. The relative 56Fe-enrichment in silica-rich bands is explained here by the progressive dissolution of iron oxides to the ocean, through iron reduction by 15N-rich organic matter actively produced at the ocean surface. The formation of iron-rich bands possibly corresponds to periods of reduced biological productivity, when precipitated iron was not effectively dissolved to the ocean. The observed shift in the organic concentration between Fe- and Si-rich bands could be the switch that triggered the BIF laminations. This shift could conceivably represent periodic fluctuations in the oxygen generation, which possibly occurred over periods of millennia, at the dawn of the Great Oxidation Event.
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    A look at the relationship between the ENSO and the Indian Ocean Dipole
    ( 2003-02-01) Ashok, Karumuri ; Guan, Zhaoyong ; Yamagata, Toshio
    Using observed sea surface temperature data from 1871-1998, and observed wind data from 1958-1998, it is confirmed that the recently discovered Indian Ocean Dipole (IOD) is a physical entity. Many IOD events are shown to occur independently of the El Niño. By estimating the contribution from an appropriate El Niño index based on sea surface temperature anomaly in the eastern Pacific, it is shown that the major fraction of the IOD Mode Index is due to the regional processes within the Indian Ocean. Our circulation analysis shows that the Walker circulation during the pure IOD events over the Indian/ Pacific Ocean is distinctly different from that during the El Niño events. Our power spectrum analysis, and wavelet power spectrum analysis show that the periodicities of El Niño and IOD events are different. The results from the wavelet coherence analysis show that, during the periods when strong and frequent IOD events occurred, the Indian Ocean Dipole Mode Index is significantly coherent with the equatorial zonal winds in the central Indian Ocean, suggesting that these events are well coupled. During the periods when there seems to be some relationship between the equatorial zonal winds in the central Indian Ocean and ENSO index, no significant coherence is seen between the Indian Ocean Dipole Mode Index and the equatorial zonal winds in the central Indian Ocean, except after 1995, suggesting that most of the IOD events are not related to ENSO.
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    A method and a GUI based JAVA code for interactive gravity modeling of strike limited listric fault sources with arbitrary density-depth variations
    ( 2014-01-01) Chakravarthi, V. ; Rajeswara Sastry, S. ; Pramod Kumar, M.
    A method coupled with a GUI based computer program, FRGMLSTRK, coded in JAVA, has been developed to interactively model the gravity anomalies of strike-limited listric fault sources where the detached hanging wall of the structure consists of several geologic formations that have different densities and thicknesses. The program is simple and user friendly in the sense that it allows interactive model construction and modification, the display of fault geometry, depth and the densities of various sub-surface formations, and real-time computation of the gravity anomalies arising from the model. The non-planar fault planes are analytically constructed by fitting a polynomial function of arbitrary but specific degree to a set of points selected on the fault plane by means of a few mouse clicks in the structure panel of the graphical layout. Further, input parameters pertaining to depths and densities of formations can be specified by means of mouse clicks in respective panels. Subsequent changes in these parameters can be realized by simple drag and drop mouse operations. The modeled gravity anomalies are automatically updated and displayed whenever changes are made to: (i) the geometry of the fault plane; (ii) depths of density interfaces; and (iii) densities of formations. These changes can be made either independently or in combination. In addition to displaying the results in graphical form, the code also generates the output in ASCII format. The applicability of the code is demonstrated with both a simple synthetic model and real-world gravity anomalies from the margin of the Chintalpudi sub-basin in India. © 2014 Geological Society of India.
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    A numerical simulation study of the Indian summer monsoon of 1994 using NCAR MM5
    ( 2004-12-01) Bhaskar Rao, Dodla Venkata ; Ashok, Karumuri ; Yamagata, Toshio
    In this study, the National Center for Atmospheric Research Mesoscale Model, NCAR MM5 adopted for the Indian region has been integrated for a four month period from 1 May 1994, to study the Indian summer monsoon during the months of June, July and August. This version of the MM5 mesoscale model has a horizontal resolution of 30 km, and 23 vertical levels. The initial and boundary conditions are taken from NCEP/NCAR reanalysis data available at 2.5° grid interval, and interpolated to the model domain. The results indicate that the NCAR MM5 model simulates many observed features of the Indian summer monsoon on a regional scale, which otherwise cannot be simulated using a global general circulation model. The simulated features of sea level pressure, 925 hPa temperature, low level wind flow are compared with NCEP and precipitation fields with GPCP. Significant features of the monsoon circulation, such as the monsoon trough, heat low over northwest India, and mesoscale precipitation patterns are well simulated. The model precipitation is also well simulated with the strengths and locations of the maxima and minima agreeing with the observations. The advancement of the monsoon current during the onset phase, as simulated by the model, is evaluated by comparing the pentad rainfall during June, 1994, with the advancement of monsoon reported by the India Meteorological Department. Model simulated area averaged time series of precipitation, and 850 hPa geopotential for five different zones covering the Indian subcontinent are evaluated through correlation, root mean square error, bias and threat score. © 2004, Meteorological Society of Japan.
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    A PMIP3 narrative of modulation of ENSO teleconnections to the Indian summer monsoon by background changes in the Last Millennium
    ( 2019-09-13) Tejavath, Charan Teja ; Ashok, Karumuri ; Chakraborty, Supriyo ; Ramesh, Rengaswamy
    Using nine model simulations from the PMIP3, we study simulated mean Indian summer (June–September) climate and its variability during the Last Millennium (LM; CE0850-1849) with emphasis on the Medieval Warm Period (MWP; CE1000-1199) and Little Ice Age (LIA; CE1550-1749), after validation of the simulated ‘current day (CE1850-2005)’ climate and trends. We find that the simulated above (below) mean-LM summer temperatures during the MWP (LIA) are associated with relatively higher (lower) moisture, and relatively higher (lower) number of concurrent El Niños (La Niñas). Importantly, the models simulate higher (lower) Indian summer monsoon rainfall (ISMR) during the MWP (LIA) compared to the LM-mean, notwithstanding a strong simulated negative correlation between NINO3.4 index and the area-averaged ISMR. Interestingly, the percentage of the simulated strong El Niños (La Niñas) associated with negative (positive) ISMR anomalies is higher (lower) in the LIA (MWP). This nonlinearity is explained by the simulated background climate changes, as follows. Distribution of simulated anomalous 850 hPa boreal summer velocity potential during MWP in models indicates, relative to the mean LM conditions, a zone of anomalous convergence in the central tropical Pacific flanked by two zones of divergence, i.e. a westward shift in the Walker circulation. The anomalous divergence centre in the west during the MWP also extends into the equatorial eastern Indian Ocean, triggering in an anomalous convergence zone over India and relatively higher moisture transport therein and therefore excess rainfall during the MWP as compared to the LM-mean, and hence an apparent weakening in the El Niño impact.
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    Abandoned Paleocene spreading center in the northeastern Indian Ocean: Evidence from magnetic and seismic reflection data
    ( 2000-01-15) Krishna, K. S. ; Gopala Rao, D.
    Magnetic data from the northeastern Indian Ocean reveal east-west trending magnetic lineations 28 through 34, an abandoned spreading center (ASC) and part of the Cretaceous Magnetic Quiet Zone (CMQZ), which are used to reconstruct past tectonic processes involved in the evolution of the ocean. Offsets in the magnetic lineations outline the existence of four N-S trending fracture zones: 80°E, Indira, 84.5°E, and 86°E. The 84.5°E Fracture Zone (F-Z) acts as a boundary separating oceanic crust with different magnetic isochron patterns. The pairs of magnetic lineations 30 through 32n.2 between the 86°E FZ and the Ninetyeast Ridge reveal an ASC of about 65 Ma age, parallel to ≃ 0.5°S latitude. The structure of the ASC is well-identified in seismic reflection data as an undulating basement topographic rise covered by up to 2 km of Bengal Fan sediments. The spreading center might have initiated its activity along with other spreading centers of the Wharton Ridge and India-Antarctica Ridge, after the first major plate reorganization of the Indian Ocean (about 95 ± 5 Ma), but ceased shortly after formation of anomaly 30 (about 65 Ma). Then it jumped southward between anomalies 32n.2 and 33. The jump captured the oceanic crust of anomalies 30 through 32n.2 that initially formed on the Antarctica plate, but were later transferred to the Indian plate. The spreading centers in the vicinity of the Ninetyeast Ridge jumped several times southward, to maintain a close proximity to the Kerguelen hotspot. (C) 2000 Elsevier Science B.V.
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    Acetylene C2H2 retrievals from MIPAS data and regions of enhanced upper tropospheric concentrations in August 2003
    ( 2011-10-19) Parker, R. J. ; Remedios, J. J. ; Moore, D. P. ; Kanawade, V. P.
    Acetylene (C2H2) volume mixing ratios (VMRs) have been successfully retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Level 1B radiances during August 2003, providing the first global map of such data and ratios to CO in the literature. The data presented here contain most information between 300 hPa and 100 hPa with systematic errors less than 10% at the upper levels. Random errors per point are less than 15% at lower levels and are closer to 30% at 100 hPa.

    Global distributions of the C2H2 and C2H2/CO ratio confirm significant features associated with both the Asian monsoon anticyclone and biomass burning for this important hydrocarbon in a characteristic summer month (August 2003), showing tight correlations regionally, particularly at lower to medium values, but globally emphasising the differences between sources and lifetimes of CO and C2H2. The correlations are seen to be particularly disturbed in the regions of highest C2H2 concentrations, indicating variability in the surface emissions or fast processing. A strong isolation of C2H2 within the Asian monsoon anticyclone is observed, evidencing convective transport into the upper troposphere, horizontal advection within the anticyclone at 200 hPa, distinct gradients at the westward edge of the vortex and formation of a secondary dynamical feature from the eastward extension of the anticyclone outflow over the Asian Pacific. Ratios of C2H2/CO are consistent with the evidence from the cross-sections that the C2H2 is uplifted rapidly in convection. Observations are presented of enhanced C 2H2 associated with the injection from biomass burning into the upper troposphere and the outflow from Africa at 200 hPa into both the Atlantic and Indian Oceans. In the biomass burning regions, C2H 2 and CO are well correlated, but the uplift is less marked and peaks at lower altitudes compared to the strong effects observed in the Asian monsoon anticyclone. Ratios of C2H2/CO clearly decay along transport pathways for the outflow, indicating photochemical ageing of the plumes. Overall, the data show the distinctive nature of C2H 2 distributions, confirm in greater detail than previously possible features of hydrocarbon enhancements in the upper troposphere and highlight the future use of MIPAS hydrocarbon data for testing model transport and OH decay regimes in the middle to upper troposphere. © 2011 Author(s).
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    Active faulting on the Ninetyeast Ridge and its relation to deformation of the Indo-Australian plate
    ( 2013-08-01) Sager, W. W. ; Bull, J. M. ; Krishna, K. S.
    The ∼4500 km long Ninetyeast Ridge (NER) in the northeastern Indian Ocean crosses a broad zone of deformation where the Indo-Australian plate is fracturing into three smaller plates (India, Capricorn, Australia) separated by diffuse boundaries whose extents are poorly defined. New multichannel seismic reflection profiles image active faults along the entire length of the NER and show spatial changes in the style of deformation along the ridge. The northern NER (0°N-5°N) displays transpressional motion along WNW-ESE faults. Observed fault patterns confirm strike-slip motion at the western extent of the April 2012 Wharton Basin earthquake swarm. In the central NER (5°S-8°S), deformation on WNW-ESE-trending thrust faults implies nearly N-S compression. An abrupt change in fault style occurs between 8° and 11°S, with modest, extension characterizing the southern NER (11°S-27°S). Although extension is dominant, narrow zones of faults with strike-slip or compressional character also occur in the southern NER, suggesting a complex combination of fault motions. At all sites, active faulting is controlled by the reactivation of original, spreading-center formed, normal faults, implying that deformation is opportunistic and focused along existing zones of weakness, even when original fault trend is oblique to the direction of relative plate motion. Observed faulting can be interpreted as India-Australia deformation in the northern NER and Capricorn-Australia deformation in the southern NER. The India-Capricorn boundary is directly adjacent to the northern NER and this juxtaposition combined with a different style of faulting to the east of the NER imply that the ridge is a tectonic boundary. © 2013. American Geophysical Union. All Rights Reserved.
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    Advances in marine geophysical studies of the Indian Ocean: Contributions from India (2010-2015)
    ( 2016-07-01) Krishna, K. S. ; Kamesh Raju, K. A. ; Ramprasad, T. ; Chaubey, A. K. ; Dewangan, P. ; Yatheesh, V.
    Indian scientists carrying marine geophysical research at different Earth Science institutions have made significant advances during the last six years (2012-2015) for understanding evolution of continental margins of India and deep sea regions. The investigations were mostly focused on continental margins of India encompassing deep-water regions, mid-ocean ridges, aseismic ridges and Andaman Sea including the back-arc basin. The studies were aimed at unraveling the tectonic and physical processes associated with evolution of the margins, ocean basins, volcanic structures, etc. In addition, industryrelated research for mapping of energy resources such as gas hydrates and hydrocarbons was also carried out in rifted basins on Eastern Continental Margin of India. The investigations were successfully carried out with integration of several geophysical datasets available at National Institute of Oceanography, India, National Geophysical Data Center, USA and petroleum industries, and by acquiring new geophysical data and geological sampling. In this report we describe major outcomes of the investigations focused on specific geological aspects of the Indian Ocean. Investigations carried out by individual works are also included in the bibliography. Geophysical studies carried out over the Ninetyeast Ridge led to understand how the Kerguelen plume interacted with the spreading centers as well as with the lithosphere during emplacement of volcanic edifices, isostatic compensation mechanisms of the ridge and magma production rates in response to the Indian plate motion. The analyses of seismic data over the 85°E Ridge revealed the internal structure consisting of volcanic plug and several stratigraphic units such as lava-fed deltas and prograding clinoforms. The lava deltas along with intervening erosional surfaces and mass wasting on ridge flanks suggested that the ridge was built by both subaqueous and multiphase submarine volcanism during the late Cretaceous. Study of seismic reflection data of the Eastern Continental Margin of India and adjacent deep-water region revealed two different phases of sediment deposition piled in the Bay of Bengal. The sediment distribution pattern, thickness and the bay architecture unveiled an occurrence of major change in sediment source from the peninsular India to the Bengal Basin at ∼23 Ma as a result of uplift of Himalayas and onset of Asian monsoon system and this eventually led to the commencement and growth of the Bengal Fan. The gas hydrates studies in Krishna-Godavari (KG) offshore basin revealed that the hydrate distribution in the basin is controlled mainly by fault systems. The velocity analysis of sediment strata within the basin suggested occurrence of high concentration of gas hydrate close to the large-scale fault system. Thus, the fault system in KG Basin provides a conducive environment for the migration of methane, which upon entering the hydrate stability zone forms the fracture-filled gas hydrate deposits. Marine geophysical data of both the continental margins of India and western Andaman region were studied for extending the country's limit of the continental shelf beyond the Exclusive Economic Zone. The scientific and technical information extracted from the studies would serve its primary purpose of delineating the outer limits of India's continental shelf beyond 200 nautical miles. The teleseismic studies of the back-arc basin, Andaman Sea revealed that the back-arc spreading fabric is undergoing readjustment to be in equilibrium with the current plate motions. A passive Ocean bottom seismometer (OBS) experiment in the back-arc basin recorded several events including teleseismic, microseismic and local events. Geological and geophysical studies over the Carlsberg and Central Indian Ridges have brought out finer scale segmentation of the ridge system and delineated zones of magmatic and sparsely magmatic sections of the ridges. The water column and deep-tow investigations carried out over the Carlsberg Ridge provided evidences for a prominent plume signatures emanating from a seafloor hydrothermal vent system. A model for the early opening of the Arabian Sea for the period 88 to 56.4 Ma is proposed with the consideration of the Gop and Laxmi Basin spreading centers as two arms of a triple junction and Narmada Rift on the Indian peninsula in the east as third arm of the triple junction. The reconstruction model did provide a new view on dispersals of Madagascar, Seychelles and India, during their early drift period, wherein the Laxmi Ridge and the Laccadive Plateau have been accommodated as intervening continental slivers.
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    Aerosol characteristics in the upper troposphere and lower stratosphere region during successive and contrasting Indian summer monsoon season
    ( 2018-01-01) Srivastava, A. K. ; Kumar, D. ; Misra, A. ; Kanawade, V. P. ; Pathak, V. ; Tiwari, S. ; Devara, P. C.S.
    Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)-derived aerosol vertical profiles were studied in the upper troposphere and lower stratosphere (UTLS) over the Indian summer monsoon (ISM) region during two successive and contrasting monsoon years (2008–2009). An enhanced aerosol layer was observed in the UTLS between 15 and 19 km altitude, in the vicinity of tropopause during both years. However, the optical characteristics of aerosol layers were found to be dissimilar during the two contrasting consecutive summer monsoon seasons. While the depolarization ratio of enhanced aerosol layer (exceeding 0.2) during both years suggested anisotropic nature of particles, the aerosol backscatter coefficient was observed to be more intensified with a sharp peak during the active monsoon year (2008) whereas it was relatively broader with lower magnitude during a drought year (2009). The enhanced backscatter coefficient in the UTLS was found to be closely associated with the variability in tropopause height and convection during both years, which is more pronounced during the active monsoon year as compared to a drought year. Deep convection over the ISM region may inject boundary layer aerosols into the upper troposphere as evidenced from the analysis of the outgoing long-wave radiation (OLR). Our results also showed an enhanced integrated backscatter coefficient (IBC) of about 30%, which is associated with a decrease in OLR of about 7% during the active monsoon year as compared to drought year. These findings were further corroborated using NCEP-NCAR vertical velocity and HYSPLIT air-mass backward trajectory analyses.
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    Aerosol characteristics in the UTLS region: A satellite-based study over north India
    ( 2016-01-01) Srivastava, A. K. ; Misra, A. ; Kanawade, Vijay P. ; Devara, P. C.S.
    Vertical profiles of aerosol backscatter coefficient and depolarization ratio, obtained from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, were studied in the upper troposphere and lower stratosphere (UTLS) region over North India (21-30° N and 72-90° E), covering the highly polluted Indo-Gangetic Plain (IGP) for one-year period from December 2011 to November 2012. An enhanced aerosol layer was observed between 15 and 18 km altitude, in the vicinity of tropopause, with a broad layer depth of about 2 km. The aerosol layer showed strong seasonal, monthly as well as day and night time variability, with a peak value of backscatter coefficient during monsoon season (~5.54 × 10-3 sr-1 in September). The corresponding depolarization ratio indicates anisotropic (non-spherical) nature of particles. The aerosol layer was found to be highly linked with the variability in tropopause height, showing a positive correlation between tropopause height and the height of maximum backscatter coefficient (correlation coefficient of 0.8). However, it was found to be negatively correlated with the integrated backscatter coefficient (IBC), with a correlation coefficient of 0.3. We further analyzed outgoing long-wave radiation (OLR) data during the study period to investigate the link between the observed enhanced aerosol layer in the UTLS region and prevailing deep convective activities over the study region. Low values of OLR during monsoon (about 214 W m-2) indicate the occurrence of deep convection over this region, which may cause a large-scale circulation-driven vertical transport of boundary-layer pollution into the atmosphere of UTLS region. Results may have potential implications for better understanding and assessing the chemical and radiative impacts of these aerosols in the tropical UTLS region.
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    Aerosol-induced intensification of cooling effect of clouds during Indian summer monsoon
    ( 2018-12-01) Sarangi, Chandan ; Kanawade, Vijay P. ; Tripathi, Sachchida N. ; Thomas, Abin ; Ganguly, Dilip
    Measurements and models show that enhanced aerosol concentrations can modify macro- and micro-physical properties of clouds. Here, we examine the effect of aerosols on continental mesoscale convective cloud systems during the Indian summer monsoon and find that these aerosol–cloud interactions have a net cooling effect at the surface and the top-of-atmosphere. Long-term (2002–2016) satellite data provide evidence of aerosol-induced cloud invigoration effect (AIvE) during the Indian summer monsoon. The AIvE leads to enhanced formation of thicker stratiform anvil clouds at higher altitudes. These AIvE-induced stratiform anvil clouds are also relatively brighter because of the presence of smaller sized ice particles. As a result, AIvE-induced increase in shortwave cloud radiative forcing is much larger than longwave cloud radiative forcing leading to the intensified net cooling effect of clouds over the Indian summer monsoon region. Such aerosol-induced cooling could subsequently decrease the surface diurnal temperature range and have significant feedbacks on lower tropospheric turbulence in a warmer and polluted future scenario.