Isotopic and sedimentological clues to productivity change in Late Riphean Sea: A case study from two intracratonic basins of India

P P Chakraborty1, A Sarkar2, S K Bhattacharya3 and P Sanyal3
1Department of Applied Geology, Indian School of Mines, Dhanbad 826 004, India.
2Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India.
3Physical Research Laboratory, Ahmedabad 380 009, India.

Enriched ¹³C/¹²C ratios with §¹³ C ~3‰ (w.r.t PDB) of two Late Riphean (~700{610 Ma) intracratonic carbonate successions viz., Bhander Limestone of Vindhyan Basin and Raipur Limestone of Chattisgarh Basin suggest higher organic productivity during this period. This view is supported by sedimentological evidence of higher biohermal growth and consequent increase in depositional relief in the low gradient ramp settings inferred for these basins. Oxygen isotope analysis of these carbonates show distinct segregation between enriched deeper water carbonate mudstone and depleted shallow water stromatolite facies that received fresh water influx. This shows that facies-specific analyses can be useful in understanding the depositional setting of these sediments.

Grenvillian thermal event and remnant charnockite: Isotopic evidence from the Chilka Lake granulite-migmatite suite in the Eastern Ghats belt, India

S Bhattacharya1, M P Deomurari2 and W Teixeira3
1Indian Statistical Institute, 203 B.T.Road, Calcutta 700 035, India.
e-mail: samar@isical.ac.in
2Physical Research Laboratory, Ahmedabad, India.
3Institute of Geosciences, Sao Paulo University, Brazil.

Spectacular exposures of granulite-migmatite occur in the Chilka Lake area of the Eastern Ghats belt. The garnetiferous granite gneiss of peraluminous granitic composition, often contains restitic metapelite inclusions and is demonstrably a product of biotite-dehydration melting in pelitic rocks. On the other hand, older layers and bands of charnockitic rocks frequently occur as dismembered patches within the peraluminous granite, thus imparting a measled appearance of the granite exposures.
The partial melting and emplacement of the peraluminous granite represent the Grenvillian thermal event, as evidenced by Rb-Sr whole rock and Pb-Pb zircon dating. On the other hand, minor patches of charnockite represent migmatized relict, as evidenced by some older zircons, in addition to those of Grenvillian age.

Reflection of P and SV waves at the free surface of a monoclinic elastic half-space

Sarva Jit Singh* and Sandhya Khurana
Department of Mathematics, Maharshi Dayanand University, Rohtak 124 001, India.
*e-mail: s−j−singh@yahoo.com

The propagation of plane waves in an anisotropic elastic medium possessing monoclinic symmetry is discussed. The expressions for the phase velocity of qP and qSV waves propagating in the plane of elastic symmetry are obtained in terms of the direction cosines of the propagation vector. It is shown that, in general, qP waves are not longitudinal and qSV waves are not transverse. Pure longitudinal and pure transverse waves can propagate only in certain specific directions. Closed-form expressions for the reflection coefficients of qP and qSV waves incident at the free surface of a homogeneous monoclinic elastic half-space are obtained. These expressions are used for studying numerically the variation of the reflection coefficients with the angle of incidence. The present analysis corrects some fundamental errors appearing in recent papers on the subject.

Characteristics of low frequency oscillations of the atmosphere-ocean coupling

Randhir Singh, B Simon and P C Joshi
Atmospheric Sciences Division, Meteorology & Oceanography Group, Space Applications Centre (SAC),
ISRO, Ahmedabad 380 015, India

The low frequency oscillation of latent heat flux over the tropical oceans has been studied. The NCEP reanalyzed elds of wind and humidity alongwith Reynolds SST are used to compute the instantaneous as well as monthly mean surface latent heat fluxes (LHF) for the year 1999. The procedure of LHF computation is based on bulk method. Spectral analysis shows that signi-
cant energy is contained in Madden Julian Oscillation band in the winds, SST, moisture and in  the latent heat flux. The global distribution of wind, humidity, SST and LHF oscillation on the time scale of 30-50 days are analyzed. Maximum amplitude of oscillation on this time scale in all the above mentioned parameters were found over the Indian Ocean. The fluctuation of surface wind speed and moisture controls the latent heat flux on this time scale. The fluctuation of SST on this time scale does not seem to be important over most of the oceans.

Detection of marine aerosols with IRS P4-Ocean Colour Monitor

Indrani Das*, M Mohan* and K Krishnamoorthy**
*Marine Science and Coastal Environment Division, Space Applications Centre (ISRO),
Ahmedabad 380 015, India
e-mail: dindrani@rediffmail.com        mannilmohan@yahoo.co
**Space Physics Laboratory, Vikram Sarabhai Space Centre (ISRO), Trivandrum 695 022, India.

The atmospheric correction bands 7 and 8 (765nm and 865nm respectively) of the Indian Remote Sensing Satellite IRS P4-OCM (Ocean Colour Monitor) can be used for deriving aerosol optical depth (AOD) over the oceans. A retrieval algorithm has been developed which computes the AOD using band 7 data by treating the ocean surface as a dark background after removing the Rayleigh path radiance in the sensor-detected radiances. This algorithm has been used to detect marine aerosol distributions at different coastal and offshore locations around India. A comparison between OCM derived AOD and the NOAA operational AOD shows a correlation ~0:92 while that between OCM derived AOD and the ground-based sun photometer measurements near the coast of Trivandrum shows a correlation of ~0:90.

Study on the sensitivity of the vertical cooling (heat sink) in the displacements of the mid-tropospheric ridge using a linear model

A Chandrasekar
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India

A linear model of the response of a stratified atmosphere to isolated heat sources in spherical coordinates is used to study the maintenance of the mean position of the mid tropospheric ridge and its displacement. It is well known that the performance of the southwest Indian monsoon is related to the latitudinal position of the April 500 hPa ridge along 75 ⁰ E. It was demonstrated that an anomalous cooling associated with the increased snow cover in Eurasia can result in moderate southward displacement of the mid-tropospheric ridge. The results of this study indicate that the vertically integrated cooling rate (strength of heat sink) has more effect on the southward displacement of the ridge when the sink is closer to the ridge.

Characteristics of monsoon low level jet (MLLJ) as an index of monsoon activity

N V Sam and K P R Vittal Murty*
Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi, Hauz Khas,
New Delhi 110 016, India
e-mail: nelson@cas.iitd.ernet.in
*Department of Meteorology and Oceanography, Andhra University, Visakhapatnam 530 003, India.


Temperature and wind data are used to describe variation in the strength of the Monsoon Low Level Jet (MLLJ) from an active phase of the monsoon to a break phase. Also estimated are the characteristics of turbulence above and below MLLJ.

Multidimensional scaling technique for analysis of magnetic storms at Indian observatories

M Sridharan1 and A M S Ramasamy2
1IIG Magnetic Observatory, Pondicherry 605 014, India.
2Professor of Mathematics, Pondicherry University, Pondicherry 605 014, India.
 
Multidimensional scaling is a powerful technique for analysis of data. The latitudinal dependenceof geomagnetic field variation in horizontal component (H) during magnetic storms is analysed in this paper by employing this technique.

Magnetic rock properties of the gabbros from the ODP Drill Hole 1105A of the Atlantis Bank, Southwest Indian Ridge

D Gopala Rao* and K S Krishna
National Institute of Oceanography, Dona Paula, Goa 403 004, India.
*e-mail: gopalrao@csnio.ren.nic.in

Laboratory studies of 30 samples from 158m long drill core of the Hole 1105A (ODP Leg 179) of the Atlantis Bank, Southwest Indian Ridge have revealed magnetic properties of the gabbros, olivine gabbros, oxide gabbros and olivine oxide gabbros down the core. Comparison of modal proportions of the oxides, grain sizes and magnetization parameters of the rocks has confirmed that most coarse-grained oxide mineral bearing rocks record low Koenigsberger ratio (2 to 5) and median
destructive fields (5 to 7 mT). Average natural remanent magnetization (Jnrm) and stable remanent magnetization (Jst) of the core samples are 5.8A/m and 1.9 A/m, respectively. Their mean stable magnetic inclination is 66 ⁰  ±4⁰, about 14⁰ steeper than the expected dipole inclination of the area similar to the one reported at Hole 735 B. The excess inclination perhaps marks a tectonic block rotation of the reversely magnetized rocks of the bank. We interpret that gabbros and serpentinites devoid of basaltic carapace significantly contribute to seafloor spreading anomalies of the bank.


Comments on `Generation of Deccan Trap magmas' by Gautam Sen
(Proc. Indian Acad. Sci. (Earth Planet. Sci.), 110, December 2001, 409{431)

H C Sheth
Department of Geology & Geophysics, School of Ocean & Earth Science & Technology (SOEST), University of  awaii at Manoa, Honolulu HI 96822 U.S.A.

Giant Plagioclase Basalts, eruption rate versus time Response to Sheth's comments and some additional thoughts

Gautam Sen
Florida Center for Electron Microscopy, Florida International University, University Park,
Miami, FL 33199.

Comments on `No K/T boundary at Anjar, Gujarat, India: Evidence from magnetic susceptibility and carbon isotopes' by H J Hansen, D M Mohabey and P Toft
(Proc. Indian Acad. Sci. (Earth Planet Sci.), 110, June 2001, 133{142)

A D Shukla* and P N Shukla
Physical Research Laboratory, Ahmedabad 380 009, India
*e-mail: anilds@prl.ernet.in


Reply to the comments by A D Shukla and P N Shukla

H J Hansen*, D M Mohabey** and P Toft*
*Geological Institute, Oster Voldgade 10, DK-1350, Denmark.
**Geological Survey of India, Seminary Hills, Nagpur 440 006, India.