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R. K. Kotra

Publications and source records attributed to R. K. Kotra.

6 recordsLinked to original sources
USGS1990

Organic matter in hydrothermal metal ores and hydrothermal fluids

Massive polymetallic sulfides are currently being deposited around active submarine hydrothermal vents associated with spreading centers. Chemoautolithotrophic bacteria are responsible for the high production of organic matter also associated with modern submarine hydrothermal activity. Thus, there is a significant potential for organic matter/metal interactions in these systems. We have studied modern and ancient hydrothermal metal ores and modern hydrothermal fluids in order to establish the amounts and origin of the organic matter associated with the metal ores. Twenty-six samples from modern and ancient hydrothermal systems were surveyed for their total organic C contents. Organic C values ranged from 0.01% to nearly 4.0% in these samples. Metal ores from modern and ancient sediment-covered hydrothermal systems had higher organic C values than those from modern and ancient hydrothermal systems lacking appreciable sedimentary cover. One massive pyrite sample from the Galapagos spreading center (3% organic C) had stable isotope values of −27.4% (δ 13 C) and 2.1% (δ 15 N), similar to those in benthic siphonophors from active vents and distinct from seep sea sedimentary organic matter. This result coupled with other analyses (e.g. 13 C NMR, pyrolysis/GC, SEM) of this and other samples suggests that much of the organic matter may originate from chemoautolithotrophic bacteria at the vents. However, the organic matter in hydrothermal metal ores from sediment covered vents probably arises from complex sedimentary organic matter by hydrothermal pyrolysis. The dissolved organic C concentrations of hydrothermal fluids from one site (Juan de Fuca Ridge) were found to be the same as that of background seawater. This result may indicate that dissolved organic C is effectively scavenged from hydrothermal fluids by biological activity or by co-precipitation with metal ores.

Applied Geochemistry↗
USGS1990

Thermal maturity of Jurassic shales from the Newark Basin, U.S.A.: Influence of hydrothermal fluids and implications to basin modeling

Organic geochemical investigations were conducted on a series of cores that systematically sampled the uppermost Jurassic strata from the northern Newark Basin. Each sedimentary unit consists of fluvial red sandstones and siltstones with cyclic deposits of interbedded black lacustrine shales and gray deltaic siltstones. In a suite of organic-rich shales from the Boonton, Towaco and Feltville Formations, organic maturation parameters were used to determine aspects of the thermal history of the Newark Basin. Comparisons of model calculations and measured maturities support 39 Ar/ 40 Ar-geochronometer studies that indicate a hydrothermal event occurred ∼175 Ma ago. An increase in the regional geothermal gradient to ∼7.5°C/100 m for ∼5 Ma best conforms to the organic geochemical observations. Biomarker compounds in Boonton and Towaco strata should have been relatively unaltered by this regional event, but anomalous molecular distributions in the organic-rich rocks may have resulted from localized heating by hydrothermal fluids. The effects of this interaction would be very subtle and may be indistinguishable from variations caused by differences in organic facies. Within this uncertainty, sterane and hopane isomerization and steroid aromatization reactions advanced in the Boonton and Towaco Formation strata primarily because of burial and normal geothermal heating that followed the hydrothermal event. Biomarker kinetic models indicate that ∼2400 m of Boonton and post-Boonton strata were eroded after basinal uplift commenced ∼50 Ma ago.

Applied Geochemistry↗
USGS1988

Pyrolysis g.c.-m.s. of a series of degraded woods and coalified logs that increase in rank from peat to subbituminous coal

Xylem tissue from degraded wood and coalified logs or stems was examined by pyrolysis g.c.-m.s. to improve understanding of the coalification process. The pyrolysis data, when combined with solid-state 13 C n.m.r. data for the same samples, show several stages of evolution during coalification. The first stage, microbial degradation in peat, involves the selective degradation of cellulosic components and preservation of lignin-like components. As coalification increases, the lignin structural units undergo a series of defunctionalization reactions. The first of these involve loss of methoxyl groups, with replacement by phenolic hydroxyls such that catechol-like structures are produced. As the xylem tissue is converted to subbituminous coal, the persistence of phenols and methylated phenols in pyrolysis g.c.-m.s. data of subbituminous coal suggests that the catechol-like structures are being converted to phenol-like structures. The ability to discern detailed changes in the chemical structural composition of a genetically and histologically related series of samples provides an ideal method for developing models of coal structure, especially that of low-rank coal.

Fuel↗