A B C D E F G H I J K L M N O P Q R S T U V W X Y Z All
Padmakumari, V. M.
- Evolution of Strontium Isotopes in Seawater During the Middle Miocene: New Results from ODP Site 758A
Authors
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 55, No 3 (2000), Pagination: 307-316Abstract
Thc 87Sr/86Sr ratio of seawater has increased significantly over the last 40 Ma. This increase was marked by very high rate of increase in 87Sr/86Sr and almost zero growth rate. The middle Miocene is one such period when considerable changes in the slope of 87Sr/86Sr curve have been reported. This study tests and improves on previously published strontium isotopic records for this period.We report 34 high precision strontium isotopic analyses of well preserved planktonic foraminifera from Ocean Drilling Program (ODP) site 758A in the eastern Indian Ocean. This site was selected because of the high quality magnetostratigraphy, suitable sedimentation rate (0.3 to 1.5 cm/ka) and excellent preservation of calcareous microfossils in sediments. Samples studied represent the period from 15.2 Ma to 10.3 Ma with approximately one sample per 100-200 ka.
The results show that the seawater 87Sr/86Sr ratio during this period increased from 0.70881 to 0.70892. The rare of increase in 87Sr/86Sr during 15.2 Ma to 14.3 was -31 x 10-6/m.y., whereas significantly from 14.3 to 12.8 Ma it was about a factor of two higher (-59x10-6/ m.y.). High rate of growth in seawater 87Sr/86Sr during this time interval indicates exceptionally high rates of continental weathering. However, during 12.8 to 10.3 Ma the growth in 87Sr/86Sr of seawater remained almost constant with an average value of 0.708914. This constant value may he due to decrease in weathering rates of continents as a result ofextensive glaciation. It is suggested that the tectonicuplift and denudation of Himalayas coupled with climatic changes have resulted in the variations of seawater strontium isotopes during this period. The inflection points in the curve may be used as event marker for stratigraphic corretation of marine sediments.
Keywords
Marine Geology, Stratigraphy, Strontium Isotopes, Miocene, Planktonic Foraminifera.
- Potassium - Argon Ages of Dolerite Dykes in Late Archean/Early Proterozoic Closepet Granite
Authors
1 National Geophysical Research Institute, Hyderabad-500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 46, No 1 (1995), Pagination: 47-51Abstract
Emplacement of the Closepel granite around 2.6 to 2.5 Gaage was the last major event in the crustal formation process in the Dharwar Craton. This granitic body has been intruded by a number of dykes mainly trending E-W or NE-SW between Madhugiri and Tumkur. Potassium-argon ages on these dykes range from 2.5 Ga to 1.2 Ga and can be classified into three different phases. The first phase (2.5-2.4 Ga) dyke activity might be correlated with emplacement of the Closepet Granite. The second phase (1.8-1.6 Ga) of dyke activity could be associated with hypabyssal magmatism connected with the Proterozoic Cuddapah Basin formation and volcanism. The last phase (1.4-1.2 Ga) represents the middle Proterozoic dyke activity during which emplacement of Rapakivi granites and alkaline magmatism were reported.Keywords
Geochronology, Dykes, Archean, Proterozoic, Karnataka.- Changes in Seawater 87Sr/86Sr Curve during Early Miocene to Early Late Miocene: Implications of ODP Site 758A Record
Authors
1 National Geophysical Research institute, Uppal Road, Hyderabad - 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 65, No 2 (2005), Pagination: 147-157Abstract
A seawater 87Sr/86Sr record is generated for early Miocene to early late Miocene based on measurements of 114 samples of cleaned planktonic foraminifera from Ocean Drilling Program site 758 (Hole 758A) in the eastern Indian Ocean. Age model for this Site is based on nannofossil events and interpolated numerical ages are compared with Sr Isotopically-Derived ages, There is a fairly good agreement between the two stratigraphic tools (<±1.5 Ma). 87Sr/86Sr ratio during ∼23.7 Ma to ∼8.1 Ma increased from 0.70828 to 0.70895, with variations in the rate of 87Sr/86Sr increase. Seventh-Order polynomial fit curve shows the 87Sr/86Sr increase rates of 44 ×10-6m .y.-1 for -23.7 to ∼19.2 Ma, 41 × 10-6 m.y.-1 for -18.2 to - 10.8 Ma, and 25 × 10-6 m.y.-1 for ∼10.8 to ∼8.1 Ma. These rates are broadly consistent with the growth rates of 87Sr/86Sr reported from other DSDP and ODP sites with some minor differences.A composite Sr isotope curve constructed for the 24-8 Ma is broken into three linear segments with a different linear regression equation. The overall rate of 87Sr/86Sr increase for ∼24-8 Ma at site 758A (40 x 10-6 m.y.-1) is in agreement with the rate of 87Sr/86increase of the composite curve (42 x 10-6 m.y.-1).
Keywords
Sr Isotopes, Miocene, Stratigraphy, Planktonic Foraminifera, Eastern Indian Ocean.- Seawater Neodymium Isotopic Composition in the Northeastern Indian Ocean during the LGM to Holocene: Response to Glacial and Monsoonal Weathering in Himalaya-Tibet
Authors
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007