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At least 415 records · Page 23Linked to original sources
PubMed1997

A blast of gas in the latest Paleocene: simulating first-order effects of massive dissociation of oceanic methane hydrate.

Carbonate and organic matter deposited during the latest Paleocene thermal maximum is characterized by a remarkable -2.5% excursion in delta 13C that occurred over approximately 10(4) yr and returned to near initial values in an exponential pattern over approximately 2 x 10(5) yr. It has been hypothesized that this excursion signifies transfer of 1.4 to 2.8 x 10(18) g of CH4 from oceanic hydrates to the combined ocean-atmosphere inorganic carbon reservoir. A scenario with 1.12 x 10(18) g of CH4 is numerically simulated here within the framework of the present-day global carbon cycle to test the plausibility of the hypothesis. We find that (1) the delta 13C of the deep ocean, shallow ocean, and atmosphere decreases by -2.3% over 10(4) yr and returns to initial values in an exponential pattern over approximately 2 x 10(5) yr; (2) the depth of the lysocline shoals by up to 400 m over 10(4) yr, and this rise is most pronounced in one ocean region; and (3) global surface temperature increases by approximately 2 degrees C over 10(4) yr and returns to initial values over approximately 2 x 10(6) yr. The first effect is quantitatively consistent with the geologic record; the latter two effects are qualitatively consistent with observations. Thus, significant CH4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum. This conclusion is of broad interest because the flux of CH4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present-day anthropogenic CH4 sources.

Atmosphere↗
PubMed2000

Life associated with a 2.76 Ga ephemeral pond?: evidence from Mount Roe #2 paleosol.

Dark sericitic material at and near the top of the 2.765 +/- 0.01 Ga Mount Roe #2 paleosol in Western Australia contains 0.05-0.10 wt% organic carbon with delta 13C values between -33% and -51% PDB (Peedee belemnite). Such negative isotopic values strongly indicate that methanotrophs once inhabited this material. The textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds, that these ponds became desiccated, and that heavy rains transported the material to its present sites. The discovery of methanotrophs associated with the Mount Roe #2 paleosol may extend their geologic record on land by at least 1.5 b.y. Methanotrophy in this setting is consistent with the notion that atmospheric methane levels were > or = 20 (mu)atm during the Late Archean. The radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the Late Archean.

Atmosphere↗
PubMed2000

Chuar Group of the Grand Canyon: record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma.

The Chuar Group (approximately 1600 m thick) preserves a record of extensional tectonism, ocean-chemistry fluctuations, and biological diversification during the late Neoproterozoic Era. An ash layer from the top of the section has a U-Pb zircon age of 742 +/- 6 Ma. The Chuar Group was deposited at low latitudes during extension on the north-trending Butte fault system and is inferred to record rifting during the breakup of Rodinia. Shallow-marine deposition is documented by tide- and wave-generated sedimentary structures, facies associations, and fossils. C isotopes in organic carbon show large stratigraphic variations, apparently recording incipient stages of the marked C isotopic fluctuations that characterize later Neoproterozoic time. Upper Chuar rocks preserve a rich biota that includes not only cyanobacteria and algae, but also heterotrophic protists that document increased food web complexity in Neoproterozoic ecosystems. The Chuar Group thus provides a well-dated, high-resolution record of early events in the sequence of linked tectonic, biogeochemical, environmental, and biological changes that collectively ushered in the Phanerozoic Eon.

Animals↗
PubMed1997

The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front.

Here we argue that life emerged on Earth from a redox and pH front at c. 4.2 Ga. This front occurred where hot (c. 150 degrees C), extremely reduced, alkaline, bisulphide-bearing, submarine seepage waters interfaced with the acid, warm (c. 90 degrees C), iron-hearing Hadean ocean. The low pH of the ocean was imparted by the ten bars of CO2 considered to dominate the Hadean atmosphere/hydrosphere. Disequilibrium between the two solutions was maintained by the spontaneous precipitation of a colloidal FeS membrane. Iron monosulphide bubbles comprising this membrane were inflated by the hydrothermal solution upon sulphide mounds at the seepage sites. Our hypothesis is that the FeS membrane, laced with nickel, acted as a semipermeable catalytic boundary between the two fluids, encouraging synthesis of organic anions by hydrogenation and carboxylation of hydrothermal organic primers. The ocean provided carbonate, phosphate, iron, nickel and protons; the hydrothermal solution was the source of ammonia, acetate, HS-, H2 and tungsten, as well as minor concentrations of organic sulphides and perhaps cyanide and acetaldehyde. The mean redox potential (delta Eh) across the membrane, with the energy to drive synthesis, would have approximated to 300 millivolts. The generation of organic anions would have led to an increase in osmotic pressure within the FeS bubbles. Thus osmotic pressure could take over from hydraulic pressure as the driving force for distension, budding and reproduction of the bubbles. Condensation of the organic molecules to polymers, particularly organic sulphides, was driven by pyrophosphate hydrolysis. Regeneration of pyrophosphate from the monophosphate in the membrane was facilitated by protons contributed from the Hadean ocean. This was the first use by a metabolizing system of protonmotive force (driven by natural delta pH) which also would have amounted to c. 300 millivolts. Protonmotive force is the universal energy transduction mechanism of life. Taken together with the redox potential across the membrane, the total electrochemical and chemical energy available for protometabolism amounted to a continuous supply at more than half a volt. The role of the iron sulphide membrane in keeping the two solutions separated was appropriated by the newly synthesized organic sulphide polymers. This organic take-over of the membrane material led to the miniaturization of the metabolizing system. Information systems to govern replication could have developed penecontemporaneously in this same milieu. But iron, sulphur and phosphate, inorganic components of earliest life, continued to be involved in metabolism.

Bacteria↗
PubMed1999

Stromatolites in Precambrian carbonates: evolutionary mileposts or environmental dipsticks?

Stromatolites are attached, lithified sedimentary growth structures, accretionary away from a point or limited surface of initiation. Though the accretion process is commonly regarded to result from the sediment trapping or precipitation-inducing activities of microbial mats, little evidence of this process is preserved in most Precambrian stromatolites. The successful study and interpretation of stromatolites requires a process-based approach, oriented toward deconvolving the replacement textures of ancient stromatolites. The effects of diagenetic recrystallization first must be accounted for, followed by analysis of lamination textures and deduction of possible accretion mechanisms. Accretion hypotheses can be tested using numerical simulations based on modem stromatolite growth processes. Application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during Archean and older Proterozoic times, but that younger Proterozoic stromatolites grew largely through the accretion of carbonate sediments, most likely through the physical process of microbial trapping and binding. This trend most likely reflects long-term evolution of the earth's environment rather than microbial communities.

Biological Evolution↗
PubMed2000

Understanding oblique impacts from experiments, observations, and modeling.

Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters' rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.

Computer Simulation↗
PubMed1997

Dissecting global diversity patterns: examples from the Ordovician Radiation.

Although the history of life has been characterized by intermittent episodes of radiation that can be recognized in global compilations of biodiversity, it does not necessarily follow that these episodes are caused by processes that occurred uniformly around the world. Major diversity increases could be generated by the cumulative effects of different mechanisms operating simultaneously at several geographic or environmental scales. The purpose of this review is to describe ongoing research on the manifestations, at several scales, of the Ordovician Radiation, which was among the most extensive intervals of diversification in the history of life. Through much of the period, diversity was concentrated most heavily near regions of active mountain building and volcanism; differences in diversity patterns from continent to continent, and among regions within continents, reflect this overprint. While this suggests a linkage of the Radiation and tectonic activity, this is by no means the only mediating agent. Outcrop-based research in North America has demonstrated that tectonic activity was detrimental to some biotic elements, in contrast to its effects on other organisms. Moreover, in the Great Basin of North America where the local stratigraphic record is of particularly high quality, biotic transitions characteristic of the period occurred far more rapidly than observed in global compilations of diversity, suggesting that the global rate of transition may represent the aggregate sum of transitions that occurred abruptly, but at different times, around the world. Finally, it has been demonstrated that, in concert with an increase in average age, the environmental and geographic ranges of Ordovician genera both increased significantly through the period, indicating a role for intrinsic factors in producing Ordovician biotic patterns.

Animals↗
PubMed1998

A composite reference section for terminal proterozoic strata of southern Namibia.

Integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal Proterozoic to Cambrian Witvlei and Nama Groups of Namibia. Coupled with precise U-Pb zircon age constraints, these correlations make it possible to construct a composite reference section for use in calibrating terminal Proterozoic chronostratigraphy. The Namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the Proterozoic-Cambrian boundary. The two glacial horizons may represent each of two distinct Varanger-age glaciations better known from the North Atlantic region. From the higher of the two glacial horizons up, the composite stratigraphy preserves one of the thickest and most complete available records of carbon-isotope variability in post-Varanger terminal Proterozoic seawater. Four carbon-isotope chemostratigraphic intervals are recognized: (1) a postglacial negative delta 13C excursion (Npg interval); (2) a rising interval (Pr interval) of increasing positive delta 13C values; (3) a falling interval (Pf interval) characterized by decreasing positive delta 13C and culminating in near zero or negative values; and (4) an interval of moderately positive, relatively invariant delta 13C values (I interval) that extends up to the unconformity that contains the Proterozoic-Cambrian boundary. Each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world. By comparing sediment accumulation rate in the radiometrically calibrated Namibian stratigraphy with sediment accumulation rates in correlative sections in Arctic Canada and Oman, a maximum age of 564 Ma is estimated for the end of the younger Varanger glaciation, 25 m.y. younger than previous estimates.

Carbon Isotopes↗
PubMed1997

[Detection of organic compounds on Mars].

McKay et al. detected polycyclic aromatic hydrocarbons (PAHs) in Martian meteorite ALH 84001 by two-step laser mass spectrometry. From the presence of PAHs, together with other results, they concluded that there were past life of Mars. On the other hands, no organisms nor organic compounds were detected in Martian regolith in Viking experiments in 1976. In order to obtain solid evidence for organisms or bioorganic compounds compounds on Mars, further analyses of Martian samples are required. There may be four classes of organic compounds on Mars, which are (i) organic compounds abiotically formed from primitive Mars atmosphere, (ii) Organic compounds delivered out of Mars, (iii) Organic compounds biotically formed by Mars organisms, and (iv) Organic compounds abiotically formed from the present Mars atmosphere. Possible organic compounds on Mars and analytical methods for them are discussed.

Evolution, Chemical↗
PubMed1997

[Martian environment and life].

Five conditions for life to arise are discussed with referring to early Martian environment. The key to determine whether any life form appeared on Mars is found to be the early Martian carbon dioxide atmospheric pressure and then the temperature. The importance to determine the heat flow is indicated. The items to be measured for future Martian exploration are listed. The surface materials, which has been poorly understood, are emphasized for further exploration. Two strategies for search for life on Mars, "step by step" strategy and quick strategy, are suggested.

Atmosphere↗
PubMed1998

Search for life on Mars.

A multi-user integrated suite of instruments designed to optimize the search for evidence of life on Mars is described. The package includes: -Surface inspection and surface environment analysis to identify the potential Mars landing sites, to inspect the surface geology and mineralogy, to search for visible surficial microbial macrofossils, to study the surface radiation budget and surface oxidation processes, to search for niches for extant life. -Subsurface sample acquisition by core drilling -Analysis of surface and subsurface minerals and organics to characterize the surface mineralogy, to analyse the surface and subsurface oxidants, to analyse the mineralogy of subsurface aliquots, to analyse the organics present in the subsurface aliquots (elemental and molecular composition, isotopes, chirality). -Macroscopic and microscopic inspection of subsurface aliquots to search for life's indicators (paleontological, biological, mineralogical) and to characterize the mineralogy of the subsurface aliquots. The study is led by ESA Manned Spaceflight and Microgravity Directorate.

Europe↗
PubMed2000

[Biomineralization at hot springs and mineral springs, and their significance in relation to the Earth's history].

Recently, there is strong interest on microbe-mineral interactions. This is related also to recent expanded knowledges on extremely severe environments in which microbes live. Interaction between microbes and minerals contains biomineralization processes. Varieties of biomineralization products are found not only in various geologic materials and processes in the earth's history but also in present surface environments. Some hot springs represent such environments similar to those of unique and extremely severe environments for life. In this short review, the author briefly shows some examples of biomineralizations at some hot springs and mineral springs, Japan. In such environments, iron ore was formed and some varieties of growing stromatolites were found. The varieties of stromatolite are siliceous, calcic and manganese types. Cyanobacteria and the other bacteria are related to form the stromatolite structure. In the Gunma iron ore, sedimentary iron ores were mineralogically described in order to evaluate the role of microorganisms and plants in ore formation. The iron ore is composed of nanocrystalline goethite. Algal fossils are clearly preserved in some ores. Various products of biomineralization are found in the present pH 2-3, Fe2(+)- and SO4(2-)-rich streams. Bacterial precipitation had variations from amorphous Fe-P-(S) precipitates near the outlet of mineral spring, to Fe-P-S precipitates and to Fe-S-(P) precipitates. Mosses and green algae are also collecting Fe precipitates in and around the living and dead cells. The Gunma Iron Ore can be said as Biologically Induced Iron Ore. At Onikobe and Akakura hot springs, growing stromatolites of siliceous and calcareous types, were found, respectively. At Onikobe, The stromatolites grow especially near the geyser. Cyanobacterial filaments in stromatolite were well preserved in the siliceous and calcic stromatolites. The filaments oriented in two directions which form the layered structures were found. At Yunokoya hot spring, black and brittle stromatolitic structures which were composed of amorphous Mn minerals are growing. The form of these structures are hemispherical. Many bacteria that were coated with amorphous Mn minerals were found on these structures. Furthermore, Precambrian (Proterozoic : Wittenoom-Chichester region, western Australia) manganese stromatolite was briefly shown in comparison. The black stromatolite has been clarified to be composed of todorokite. Small spotty and donuts-like shaped todorokite aggregates which are very similar to biologically induced Mn-precipitates were found in massive dolomite layers.

Australia↗
PubMed1984

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

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 0.5. The contradiction is probably caused by oversimplifications in the BLAG model, such as the neglect of the cycles of organic carbon and sulfur.

Atmosphere↗
PubMed1986

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

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. The dissolved carbon content and alkalinity of the ocean as a whole are controlled by weathering and metamorphism of sedimentary rocks. Equilibrium solutions are derived for the preindustrial atmosphere/ocean system and for a system that may be similar to that existing during the Late Cretaceous Period. The model is then used to determine how the modern and ancient marine biospheres might be affected by an oceanic impact of a large asteroid or comet. Such an event could perturb the carbon cycle in several different ways. Global darkening caused by stratospheric dust veil could destroy most of the existing phytoplankton in a period of several weeks to several months. At the same time, dissolution of atmospheric NOx compounds synthesized during the impact would lower the pH of ocean surface waters and release CO2 into the atmosphere. Both effects might be enhanced by an influx of CO2 released from upwelling of deep ocean water near the hot impact site, from oxidation of dead organic matter, and from the comet itself. The net result could be to raise surface temperatures by several degrees and to make the surface ocean uninhabitable by calcareous organisms for as much as 20 yrs (the time scale for mixing with deep ocean). It appears unlikely, however, that an impact could create a "Strangelove ocean," in which surface waters remained corrosive to calcium carbonate for thousands or tens of thousands of years. Thus, disruption of the carbon cycle by an impact event cannot by itself explain the scarcity of calcium carbonate in sediments found within the first few centimeters above the K/T boundary.

Atmosphere↗
PubMed1989

A new model for atmospheric oxygen over Phanerozoic time.

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. This agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial, with consequently faster burial of C and S and greater O2 production, must be accompanied by a corresponding increase in erosion and increased exposure of C and S on the continents to O2 consumption via weathering. It is the redistribution of sediment between the three different rock types, and not total sedimentation rate, that is important in O2 control. To add stability to the system, negative feedback against excessive O2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks, resulting in rapid recycling of C and S. We did not use direct O2 negative feedback on either weathering of C and S or burial of C because weathering rates are assumed to be limited by uplift and erosion, and the burial rate of C limited by the rate of sediment deposition. The latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers. Results of the modeling indicate that atmospheric O2 probably has varied appreciably over Phanerozoic time. During the Late Carboniferous and Permian periods O2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps. A large decrease in O2 during the Late Permian was due probably to the drying-up of the coal swamps and deposition of a large proportion of total sediment in C and S-free continental red beds. Sensitivity study shows that major parameters affecting results are the mean C concentration in coal basins and the relative sizes of the reservoirs of young (rapidly recycled) versus old rocks. Less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct O2 negative feedback on marine carbon burial. Good agreement for rates of C burial calculated via our model and via independent models, which are based on the use of stable carbon isotopes, indicates that the dominant factor that has brought about changes in atmospheric O2 level (and the isotopic composition of dissolved inorganic carbon in seawater) over Phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt-seawater reaction.

Atmosphere↗