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McKenzie N.R.,University of California at Riverside | Hughes N.C.,University of California at Riverside | Myrow P.M.,Colorado College | Xiao S.,Virginia Polytechnic Institute and State University | Sharma M.,Birbal Sahni Institute of Palaeobotany
Earth and Planetary Science Letters | Year: 2011

A common view in Himalayan geology is that differences in detrital zircon age distributions and whole-rock neodymium isotopic compositions (εNd) distinguish lithotectonic zones within the system. Such differences are used to map these zones and to locate their modern boundaries, as well as to infer ancient terrane boundaries. We test the utility of this approach using integrated geochemical, geochronological, and sedimentological data from the Himalayan successions of northern India and relatively undeformed, age-equivalent successions of the Indian craton. U-Pb geochronology of detrital zircons from cratonic successions of the Vindhyan, Ganga, and Marwar supergroups and the "inner" and "outer" Lesser Himalaya lithotectonic zones show that rocks of similar depositional age bear strikingly similar detrital zircon age distributions throughout the entire region. A sharp change in εNd occurs within the "inner" Lesser Himalaya and correlates with a regional unconformity recognized on the craton, here constrained to span a period of ~. 500. million years. Results demonstrate that isotopic differences among the lithotectonic zones relate primarily to differences in the depositional ages of their constituent rocks, and that all parts of the Himalaya were in sediment-source continuity with the Indian craton from the late Paleoproterozoic to the early Cambrian. Isotopic "signatures" may vary as much within individual Himalayan lithotectonic zones as between such zones and no lithotectonic zone can be characterized by such data alone. © 2011.

Clementz M.,University of Wyoming | Bajpai S.,Indian Institute of Technology Roorkee | Ravikant V.,Indian Institute of Technology Roorkee | Ravikant V.,Indian Institute of Science | And 3 more authors.
Geology | Year: 2011

The timing of initiation of continent-continent collision between Asia and India is controversial, but this major tectonic event is generally thought to have occurred in the Early Eocene, ca. 50 Ma. New and independent data from strontium isotopes, stable carbon isotopes, microfossil biostratigraphy, and mammal fossils from an Early Eocene marginal marine sequence (Cambay Shale) at the Vastan Lignite Mine of western India indicate that terrestrial faunal exchanges, and therefore continental collision, between Asia and the Indian subcontinent took place before 53.7 Ma. This age coincides with the second Eocene Thermal Maximum (ETM2), a short-lived warming episode that followed the Paleocene-Eocene Thermal Maximum (PETM) ca. 55.5 Ma. Our data also document, for the first time, a clear record of the ETM2 in terrestrial organic material from a lowlatitude site, which is represented by a 3‰-4‰ carbon isotope excursion (CIE) in lignite and dispersed organic carbon δ13C values. The magnitude of the CIE at this location closely matches that observed in marine cores from the Arctic Ocean, which supports an interpretation that this hyperthermal event, though of lower magnitude, was similar in character to that of the PETM, being a global phenomenon that affected both terrestrial and marine ecosystems. © 2011 Geological Society of America.

Crosby C.H.,University of Minnesota | Bailey J.V.,University of Minnesota | Sharma M.,Birbal Sahni Institute of Palaeobotany
Geology | Year: 2014

The oxygenation of Earth's atmosphere allowed for the diversification of metabolisms to include those that rely on oxygen and its derivatives. For example, chemolithotrophic oxidation of sulfide and iron both require oxygen or nitrate as terminal electron acceptors. A growing number of oxygen-utilizing chemolithotrophs are known to accumulate intracellular polyphosphate as an energy reserve that allows them to adapt to the fluctuating redox conditions in their distinctive-gradient habitats. Polyphosphate is also thought to play an important role in the formation of phosphatic mineral deposits. Here we present fossil evidence of iron-oxidizing bacteria preserved as filamentous iron oxides within phosphatic Paleoproterozoic stromatolites. The filaments include twisted stalks similar to those produced by modern iron-oxidizing bacteria that are known to metabolize polyphosphate and inhabit steep redox gradients. Fossil iron-oxidizing bacteria preserved within some of the oldest known phosphorites serve as indicators of O2-Fe(II) gradients that may have supported microbially mediated phosphogenesis via polyphosphate metabolism and/or an active iron redox pump. © 2014 Geological Society of America.

Das S.,University of Calcutta | Ghosh R.,Birbal Sahni Institute of Palaeobotany | Bera S.,University of Calcutta
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2013

Phytoliths have been proved as reliable indicators of an ecosystem's structure in both modern days as well as in the past. Quantitative phytolith studies from modern plants in India are negligible and most of the data comes from grasses excepting few pteridophytes. Data on non-grass angiospermous phytoliths in India is however completely lacking. In order to interpret the fossil phytolith assemblages and their depositional sub-environments from the deltaic Sunderbans, the largest mangrove vegetation of the world, modern analogue is a prerequisite. The objectives of this study are to establish a relationship between modern deltaic non-grass plant communities of the Indian part of the Sunderbans along salinity gradient with their predominant phytolith contents and this study is the first step in identifying the environment associated with mangrove development from phytoliths in ancient sediments and further studies would ascertain which of these phytoliths would survive erosion and burial. Modern phytolith assemblages were studied from different parts of 33 non-grass species representing 27 genera under 23 families collected from different deltaic sub-environments of the study area. Retrieved leaf phytolith assemblages were subjected to Principal Components Analysis and Cluster Analysis. Elemental analysis of some of these morphotypes done using an Energy Dispersive X-ray Spectrometer (EDX) to authenticate chemical composition of some newly described forms of phytoliths. Twenty five phytolith morphotypes identified under light and Scanning Electron microscopes represent a modern reference non-grass phytolith database of deltaic plant communities of the Sunderbans. Extracted morphotypes are described and their taxonomic and environmental implications are interpreted considering quantitative data and statistical analyses. The taxonomic implications of these morphotypes are mostly poor as observed through both Principal Components Analysis and Cluster Analysis except for monocotyledonous and few dicotyledonous families which produce distinctive morphotypes. However, their role in environmental interpretations could be established using this modern phytolith data. The data would provide a basis for identification of deltaic vegetation of an area using non-grass phytoliths.© 2013 Elsevier B.V.

Gertsch B.,Massachusetts Institute of Technology | Keller G.,Princeton University | Adatte T.,University of Lausanne | Garg R.,Birbal Sahni Institute of Palaeobotany | And 3 more authors.
Earth and Planetary Science Letters | Year: 2011

The Um Sohryngkew section of Meghalaya, NE India, located 800-1000km from the Deccan volcanic province, is one of the most complete Cretaceous-Tertiary boundary (KTB) transitions worldwide with all defining and supporting criteria present: mass extinction of planktic foraminifera, first appearance of Danian species, δ13C shift, Ir anomaly (12ppb) and KTB red layer. The geochemical signature of the KTB layer indicates not only an extraterrestrial signal (Ni and all Platinum Group Elements (PGEs)) of a second impact that postdates Chicxulub, but also a significant component resulting from condensed sedimentation (P), redox fluctuations (As, Co, Fe, Pb, Zn, and to a lesser extent Ni and Cu) and volcanism. From the late Maastrichtian C29r into the early Danian, a humid climate prevailed (kaolinite: 40-60%, detrital minerals: 50-80%). During the latest Maastrichtian, periodic acid rains (carbonate dissolution; CIA index: 70-80) associated with pulsed Deccan eruptions and strong continental weathering resulted in mesotrophic waters. The resulting super-stressed environmental conditions led to the demise of nearly all planktic foraminiferal species and blooms (>95%) of the disaster opportunist Guembelitria cretacea. These data reveal that detrimental marine conditions prevailed surrounding the Deccan volcanic province during the main phase of eruptions in C29r below the KTB. Ultimately these environmental conditions led to regionally early extinctions followed by global extinctions at the KTB. © 2011 Elsevier B.V.

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