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81.
In the Central Indian Basin manganese nodule abundance was variable in all sediment types. Mean abundance varied from 1.5 in calcareous ooze to 10.2 kg/m2 in terrigenous-siliceous ooze sediments. Nodule grade and growth rates are positively correlated only up to 10 mm/My (million years), and grade shows no distinct relationship with abundance. Relationships between the morphochemical characteristics of the nodules and host sediment types are subtle. Both hydrogenetic and diagenetic nodules (with smooth and rough surfaces respectively) occur on almost all sediments, but in variable proportions. Thus, the overall distribution pattern shows that small nodules (<4-cm diameters) of lower grade (average value Ni+Cu+Co=1.21%) with smooth surfaces are more common on red clay, terrigenous, and terrigenous-siliceous ooze transition-zone sediments. By contrast, large nodules (>4-cm diameters) of higher grade (average value Ni+Cu+Co=1.80%) with rough surfaces are more prevalent on siliceous ooze, siliceous ooze-red clay, and calcareous ooze-red clay transition-zone sediments. This implies an enhanced supply of trace metals from pore waters to rough-surface nodules during early diagenesis. 相似文献
82.
Buried nodules from siliceous sediments in the central Indian Basin are morphologically variable and mineralogically consist
ofd-MnO2 and incipient todorokite. Compositionally they are weakly diagenetic. The sediment coarse fractions (>63 μm) at different
depths show variable abundances of micronodules, volcanic glass shards and biodebris. Dissolution of biodebris increases and
abundance of micronodules decreases with increasing depth. Enrichment in Mn, Fe, Cu, Ni, Co, together with a decrease in organic
carbon in the sediment column, may result from diagenetic metal remobilization. Diagenetically remobilized trace metals might
have been utilized for the growth of micronodules over the buried nodules.
The details of the chemical analyses of buried nodules and associated sediments can be obtained from the first author. 相似文献
83.
Intercalated volcanic ash layers in two deep-sea sediment cores from the Central Indian Basin (CIB) are examined for the possibility of an in situ source of suboceanic volcanism. An in situ source has been predicated based on the bottom morphological structures and activation of seamounts during the geological past. The tentative correlation between depositional ages of the ash layer horizons and the period of global climatic cooling events reinforces the hypothesis that the suboceanic volcanic episode might have been initiated during the onset of glaciation. 相似文献
84.
Relative paleointensities are obtained from a 6-m sediment core from Lake St. Croix, Minnesota, spanning the time range from 445 to 1740 years B.P. To normalize the natural remanent magnetization (NRM) for variations in the magnetic content, a laboratory-induced remanence is chosen, whose alternating field (AF) demagnetization curves most closely resemble the NRM demagnetization curves. By plotting the ratio of the NRM to the normalizing remanence versus AF demagnetizing field, HAF, for samples of the same sediment horizon, as well as for samples from different horizons, estimates are obtained for expected uncertainties in the relative paleointensities. For the Lake St. Croix sediments the anhysteretic remanence (ARM) demagnetization curves are very similar to those of the NRM's, and ARM is therefore used as the normalization parameter. Because the sediment exhibits homogeneous remanence properties throughout, and HAF = 100Oe is the optimum “cleaning” field for the entire core, NRM100/ARM100 is evaluated to represent the fluctuations of the relative paleointensity. Our relative paleointensity data exhibit the same general features as obtained from archeomagnetic studies. The intensity increases as one goes back in time with a peak near 800 years B.P., representing an increase in the intensity of up to 60%. Apparent periodicities in the intensity of 300–400 years are observed. 相似文献
85.
New values of critical grain-size thresholds, ds and d0, for magnetite and Fe have been obtained using a rigorous theoretical approach. The temperature dependence of these thresholds is interesting in that they point up the existence of certain grains which, when cooled from above their blocking temperature, pass sequentially through the following magnetization states: superparamagnetic→single-domain→non-uniform magnetization. Such grains will yield correct paleointensities by the Wilson or Shaw method but not by the well-known method due to Thellier and Thellier. 相似文献
86.
87.
The early Mesoproterozoic Rohtas Limestone in the Son valley area of central India represents an overall shallowing-upward
carbonate succession. Detailed facies analysis of the limestone reveals outer- to inner-shelf deposition in an open marine
setting. Wave-ripples, hummocky cross stratifications and edgewise conglomerates argue against a deep marine depositional
model for the Rohtas Limestone proposed earlier. Stable isotope analysis of the limestone shows that δ13C and δ18O values are compatible with the early Mesoproterozoic open seawater composition. The ribbon limestone facies in the Rohtas
Limestone is characterized by micritic beds, each decoupled in a lower band enriched and an upper band depleted in dissolution
seams. Band-wise isotopic analysis reveals systematic short-term variations. Comparative enrichment of the heavier isotopes
in the upper bands is attributed to early cementation from sea water and water derived from the lower band undergoing dissolution
because of lowering of pH at depth. The short-term positive shifts in isotopic compositions in almost every upward gradational
transition from a seamed band to a non-seamed band support the contention that dissolution seams here are of early diagenetic
origin, although their formation was accentuated under overburden pressure. 相似文献
88.
Gravitational field of a stationary circular cosmic string loop has been studied in the context of full nonlinear Einstein's
theory of gravity. It has been assumed that the radial and tangential stresses of the loop are equal to the energy density
of the string loop. An exact solution for the system has been presented which has a singularity at a finite distance from
the axis, but is regular for any other distances from the axis of the loop.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
89.
The LC50 of Malathion for Channu punctatus is determined for 24… 96 h of exposure as 10.95… 3.22 mg/l. At exposure to 2.5 mg./l Malathion, growth–measured by increase in weight–is significantly reduced from the 19th day and reduced by 10 % till the 31st day. During the same period of time, the RNA and protein contents of the muscle is lowered by 10 and 15 %, respectively, as compared with the control. In addition to an acute toxic effect, in sublethal concentrations Malathion has also pronounced effects on growth and metabolism. 相似文献
90.
Santanu Banerjee S.K. Bhattacharya Subir Sarkar 《Journal of Asian Earth Sciences》2007,29(5-6):823-831
Variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a Paleoproterozoic peritidal stromatolite succession. The upper division of the Kajrahat Limestone, Vindhyan Supergroup of central India consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size. Normally, larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle. Desiccation cracks are found in all the facies indicating subaerial exposure. We investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence. Carbon and oxygen isotopic values of the limestones, in general, are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level. The δ13C and δ18O values of the limestones vary within an individual stromatolite cycle; depleted values characterize the topmost part of the cycles. The isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure. These variations also probably caused the observed scatter in δ13C and δ18O values of supratidal microbial laminites. 相似文献