31 results match your criteria American Journal Of Science[Journal]

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Geology and geochemistry of paleosols developed on the Hekpoort Basalt, Pretoria Group, South Africa.

Authors:
R Rye H D Holland

Am J Sci 2000 Feb;300(2):85-141

Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.

The Hekpoort paleosols comprise a regional paleoweathering horizon developed on 2.224 +/- 0.021 Ga basaltic andesite lavas at the top of the Hekpoort Formation of the Pretoria Group, Transvaal Supergroup, South Africa. Read More

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February 2000
6 Reads

Paleosols and the evolution of atmospheric oxygen: a critical review.

Authors:
R Rye H D Holland

Am J Sci 1998 Oct;298(8):621-72

Harvard University, Department of Earth and Planetary Sciences, Cambridge, Massachusetts 02138, USA.

A number of investigators have used chemical profiles of paleosols to reconstruct the evolution of atmospheric oxygen levels during the course of Earth history (Holland, 1984, 1994; Kirkham and Roscoe, 1993; Ohmoto, 1996). Over the past decade Holland and his co-workers have examined reported paleosols from six localities that formed between 2.75 and 0. Read More

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October 1998
1 Read

The carbon-isotopic composition of Proterozoic carbonates: Riphean successions from northwestern Siberia (Anabar Massif, Turukhansk Uplift).

Am J Sci 1995 Sep;295(7):823-50

Botanical Museum, Harvard University, Cambridge, Massachusetts 02138, USA.

Thick carbonate-dominated successions in northwestern Siberia document secular variations in the C-isotopic composition of seawater through Mesoproterozoic and early Neoproterozoic (Early to early Late Riphean) time. Mesoproterozoic dolomites of the Billyakh Group, Anabar Massif, have delta 13C values that fall between 0 and -1.9 permil versus PDB, with values in the upper part of the succession (Yusmastakh Formation) consistently higher than those of the lower (Ust'-Il'ya and Kotuikan formations). Read More

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September 1995

Organic acids in hydrothermal solutions: standard molal thermodynamic properties of carboxylic acids and estimates of dissociation constants at high temperatures and pressures.

Authors:
E L Shock

Am J Sci 1995 May;295(5):496-580

Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA.

Experimental standard partial molal volumes, heat capacities, and entropies as well as apparent standard partial molal enthalpies and Gibbs free energies of mono- and dicarboxylic acids and their anions at low temperatures and pressures are used to generate correlations for predicting the same properties at high temperatures and pressures for 59 carboxylic and 18 hydroxyacid species with the revised Helgeson-Kirkham-Flowers (HKF) equation of state. Predicted equilibrium dissociation constants are compared with experimental values from the literature and tabulated as functions of pressure and temperature for 25 carboxylic acids and nine hydroxyacids. Close agreement between independent predictions and experimental data supports the generality of the computational techniques and the accuracy of predicted data. Read More

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May 1995
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Modelling the Phanerozoic carbon cycle and climate: constraints from the 87Sr/86Sr isotopic ratio of seawater.

Am J Sci 1992 Feb;292(2):81-135

Department of Atmospheric, Oceanic, and Space Sciences, The University of Michigan, Ann Arbor 48109.

A numerical model describing the coupled evolution of the biogeochemical cycles of carbon, sulfur, calcium, magnesium, phosphorus, and strontium has been developed to describe the long-term changes of atmospheric carbon dioxide and climate during the Phanerozoic. The emphasis is on the effects of coupling the cycles of carbon and strontium. Various interpretations of the observed Phanerozoic history of the seawater 87Sr/86Sr ratio are investigated with the model. Read More

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February 1992
1 Read

Sulfate reduction in deep-sea sediments.

Authors:
D E Canfield

Am J Sci 1991 Feb;291(2):177-88

NASA-Ames Research Center, Moffett Field, California 94035-1000, USA.

Sulfate reduction rates calculated from about 200 DSDP pore water sulfate profiles have been contoured and plotted on a map covering most areas of the world ocean. Rates show a remarkable spatial consistency, with high rates observed near the continental margins, becoming progressively lower toward the central ocean basins. Relatively elevated rates are also found in the eastern equatorial Pacific, a site of upwelling and correspondingly high rates of primary organic production. Read More

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February 1991
1 Read

Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic.

Authors:
S W Grant

Am J Sci 1990 ;290-A:261-94

Botanical Museum, Harvard University, Cambridge, Massachusetts 02138, USA.

Cloudina-bearing biosparites and biomicrites in the lower part of the Nama Group, Namibia, contain a wide morphological diversity of shell fragments that can all be attributed to the two named species C. hartmannae and C. riemkeae. Read More

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July 1996
1 Read

Carbonate deposition during the late Proterozoic Era: an example from Spitsbergen.

Authors:
A H Knoll K Swett

Am J Sci 1990 ;290-A:104-32

Botanical Museum, Harvard University, Cambridge, Massachusetts 02138, USA.

Carbonate sediments reflect the physico-chemical and biological circumstances of their formation; thus, features of limestones and dolomites may provide insights into both environmental and evolutionary change through geological time. The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale. Although Proterozoic carbonates are often seen as predominantly dolomitic, the Akademikerbreen Group is about 45 percent limestone. Read More

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July 1996
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A paleoweathering profile from Griqualand West, South Africa: evidence for a dramatic rise in atmospheric oxygen between 2.2 and 1.9 bybp.

Am J Sci 1990 ;290-A:1-34

Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.

A core drilled near Wolhaarkop in Griqualand West, South Africa, intersected highly oxidized Kuruman Iron Formation below red beds of the Gamagara Formation. The lateral equivalents of the Kuruman Iron Formation in this drill hole consist largely of siderite, ankerite, magnetite, greenalite, and quartz. The oxidation of the Kuruman Iron Formation in WOL 2 occurred almost certainly during weathering prior to the deposition of the Gamagara Formation. Read More

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The post-Paleozoic chronology and mechanism of 13C depletion in primary marine organic matter.

Am J Sci 1989 Apr;289(4):436-54

Department of Chemistry, Indiana University, Bloomington 47405-5101, USA.

Carbon-isotopic compositions of geoporphyrins have been measured from marine sediments of Mesozoic and Cenozoic age in order to elucidate the timing and extent of depletion of 13C in marine primary producers. These results indicate that the difference in isotopic composition of coeval marine carbonates and marine primary photosynthate was approximately 5 to 7 permil greater during the Mesozoic and early Cenozoic than at present. In contrast to the isotopic record of marine primary producers, isotopic compositions of terrestrial organic materials have remained approximately constant for this same interval of time. Read More

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The Flin Flon paleosol and the composition of the atmosphere 1.8 BYBP.

Am J Sci 1989 Apr;289(4):362-89

Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.

Within the 1800 to 1900 my old Flin Flon-Snow Lake greenstone belt, Amisk Group volcanics are overlain by Missi Group fluvial sediments. Several localities along the Missi-Amisk contact, the volcanics show evidence of subaerial weathering. Field relationships, mineralogical evidence, and chemical analyses confirm that this alteration zone is a paleosol. Read More

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A new model for atmospheric oxygen over Phanerozoic time.

Am J Sci 1989 Apr;289(4):333-61

Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA.

A mathematical model has been constructed that enables calculation of the level of atmospheric O2 over the past 570 my from rates of burial and weathering of organic carbon (C) and pyrite sulfur (S). Burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance, and C and S contents, of the three rock types: marine sandstones and shales, coal basin sediments, and other non-marine clastics (red beds, arkoses). By our model, values of O2 versus time, using a constant total sedimentation rate, agree with those for variable sedimentation derived from present-day rock abundances and estimates of erosional losses since deposition. Read More

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http://earth.geology.yale.edu/~ajs/1989/04.1989.01Berner.pdf
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April 1989
46 Reads

A hybrid model of the CO2 geochemical cycle and its application to large impact events.

Am J Sci 1986 May;286(5):361-89

NASA Ames Research Center, Moffett Field, California 94035, USA.

A hybrid model of the carbonate-silicate geochemical cycle is presented which is capable of calculating the partitioning of carbon dioxide between the atmosphere, ocean, and sedimentary rocks. The ocean is subdivided into a shallow, mixed layer, which remains in equilibrium with the atmosphere, and a massive, deep layer which does not. Gradients in dissolved carbon content are established between the mixed layer and the deep ocean as a consequence of downward fluxes of fecal matter and of dead planktonic organisms. Read More

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May 1986
4 Reads

Comments on the BLAG model: the carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years.

Authors:
J F Kasting

Am J Sci 1984 Dec;284(10):1175-82

Space Science Division, NASA Ames Research Center, Moffett Field, California 94035, USA.

A self-consistent method of determining initial conditions for the model presented by Berner, Lasaga, and Garrels (1983) (henceforth, the BLAG model) is derived, based on the assumption that the CO2 geochemical cycle was in steady state at t = -100 my (million years). This initialization procedure leads to a dissolved magnesium concentration higher than that calculated by Berner, Lasaga, and Garrels and to a low ratio of dissolved calcium to bicarbonate prior to 60 my ago. The latter prediction conflicts with the geologic record of evaporite deposits, which requires that this ratio remain greater than 0. Read More

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December 1984

Geology in 10. century Arabic literature.

Authors:
R SAID

Am J Sci 1950 Jan;248(1):63-6

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January 1950

The radium content of varved clay and a possible age of the Hartford, Connecticut, deposits.

Authors:
W D URRY

Am J Sci 1948 Nov;246(11):689-700

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November 1948

Radiographic methods in paleontology; a progress report.

Authors:
R A M SCHMIDT

Am J Sci 1948 Oct;246(10):615-27

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October 1948

Evidence for a steepening of geothermal gradients in some deep mines and drill holes.

Authors:
J A NOBLE

Am J Sci 1948 Jul;246(7):426-40

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The rift valleys of Africa.

Authors:
G F S HILLS

Am J Sci 1948 Mar;246(3):171-81

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Menatotherium, eocene mammal from France.

Authors:
G G SIMPSON

Am J Sci 1948 Mar;246(3):165-70

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Temperature and heat flow in a well near Colorado Springs.

Authors:
F BIRCH

Am J Sci 1947 Dec;245(12):733-53

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December 1947

Distribution of those starred in American men of science.

Authors:
S S VISHER

Am J Sci 1947 Nov;245(11):714-24

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November 1947

Radium content of ultramafic igneous rocks; laboratory investigation.

Authors:
G L DAVIS

Am J Sci 1947 Nov;245(11):677-93

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November 1947

Origin of the hot springs at Hot Springs, North Carolina.

Authors:
G W STOSE A J STOSE

Am J Sci 1947 Oct;245(10):624-44

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October 1947

Spectrochemical analysis of rocks and minerals.

Authors:
E W CLAFFY

Am J Sci 1947 Jan;245(1):35-48

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January 1947

Studies in the mica group; the biotite-phlogopite series.

Authors:
E W HEINRICH

Am J Sci 1946 Dec;244(12):836-48

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December 1946

Neotypes.

Authors:
G G SIMPSON

Am J Sci 1945 Dec;243:680-94

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December 1945

The chalicotheres as a biological type.

Authors:
A BORISSIAK

Am J Sci 1945 Dec;243:667-79

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December 1945
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