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PubMed1991

The mid-Cretaceous super plume, carbon dioxide, and global warming.

Carbon-dioxide releases associated with a mid-Cretaceous super plume and the emplacement of the Ontong-Java Plateau have been suggested as a principal cause of the mid-Cretaceous global warming. We developed a carbonate-silicate cycle model to quantify the possible climatic effects of these CO2 releases, utilizing four different formulations for the rate of silicate-rock weathering as a function of atmospheric CO2. We find that CO2 emissions resulting from super-plume tectonics could have produced atmospheric CO2 levels from 3.7 to 14.7 times the modern pre-industrial value of 285 ppm. Based on the temperature sensitivity to CO2 increases used in the weathering-rate formulations, this would cause a global warming of from 2.8 to 7.7 degrees C over today's global mean temperature. Altered continental positions and higher sea level may have been contributed about 4.8 degrees C to mid-Cretaceous warming. Thus, the combined effects of paleogeographic changes and super-plume related CO2 emissions could be in the range of 7.6 to 12.5 degrees C, within the 6 to 14 degrees C range previously estimated for mid-Cretaceous warming. CO2 releases from oceanic plateaus alone are unlikely to have been directly responsible for more than 20% of the mid-Cretaceous increase in atmospheric CO2.

Atmosphere↗
PubMed1995

Influence of variable rates of neritic carbonate deposition on atmospheric carbon dioxide and pelagic sediments.

Short-term imbalances in the global cycle of shallow water calcium carbonate deposition and dissolution may be responsible for much of the observed Pleistocene change in atmospheric carbon dioxide content. However, any proposed changes in the alkalinity balance of the ocean must be reconciled with the sedimentary record of deep-sea carbonates. The possible magnitude of the effect of shallow water carbonate deposition on the dissolution of pelagic carbonate can be tested using numerical simulations of the global carbon cycle. Boundary conditions can be defined by using extant shallow water carbonate accumulation data and pelagic carbonate deposition/dissolution data. On timescales of thousands of years carbonate deposition versus dissolution is rarely out of equilibrium by more than 1.5 x 10(13) mole yr-1. Results indicate that the carbonate chemistry of the ocean is rarely at equilibrium on timescales less than 10 ka. This disequilibrium is probably due to sea level-induced changes in shallow water calcium carbonate deposition/dissolution, an interpretation that does not conflict with pelagic sedimentary data from the central Pacific.

Atmosphere↗
PubMed1997

Hydrothermal hydration of Martian crust: illustration via geochemical model calculations.

If hydrothermal Systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water-to-rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150 degrees C to 250 degrees C, water-to-rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water-to-rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low-grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential.

Evolution, Planetary↗
PubMed1997

Titan under a red giant sun: a new kind of "habitable" moon.

We explore the response of Titan's surface and massive atmosphere to the change in solar spectrum and intensity as the sun evolves into a red giant. Titan's surface temperature is insensitive to insolation increases as the haze-laden atmosphere "puffs up" and blocks more sunlight. However, we find a window of several hundred Myr exists, roughly 6 Gyr from now, when liquid water-ammonia can form oceans on the surface and react with the abundant organic compounds there. The window opens due to a drop in haze production as the ultraviolet flux from the reddening sun plummets. The duration of such a window exceeds the time necessary for life to have begun on Earth. Similar environments, with approximately 200K water-ammonia oceans warmed by methane greenhouses under red stars, are an alternative to the approximately 30OK water-CO2 environments considered the classic "habitable" planet.

Ammonia↗
PubMed1997

Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact.

A comprehensive analysis of volatiles in the Chicxulub impact strongly supports the hypothesis that impact-generated sulfate aerosols caused over a decade of global cooling, acid rain, and disruption of ocean circulation, which contributed to the mass extinction at the Cretaceous/Tertiary (K/T) boundary. The crater size, meteoritic content of the K/T boundary clay, and impact models indicate that the Chicxulub crater was formed by a short period comet or an asteroid impact that released 0.7-3.4 x 10(31) ergs of energy. Impact models and experiments combined with estimates of volatiles in the projectile and target rocks predict that over 200 gigatons (Gt) each of SO2 and water vapor, and over 500 Gt of CO2, were globally distributed in the stratosphere by the impact. Additional volatiles may have been produced on a global or regional scale that formed sulfate aerosols rapidly in cooler parts of the vapor plume, causing an early, intense pulse of sulfuric acid rain. Estimates of the conversion rate of stratospheric SO2 and water vapor to sulfate aerosol, based on volcanic production of sulfate aerosols, coupled with calculations of diffusion, coagulation, and sedimentation, demonstrate that the 200 Gt stratospheric SO2 and water vapor reservoir would produce sulfate aerosols for 12 years. These sulfate aerosols caused a second pulse of acid rain that was global. Radiative transfer modeling of the aerosol clouds demonstrates (1) that if the initial rapid pulse of sulfate aerosols was global, photosynthesis may have been shut down for 6 months and (2) that for the second prolonged aerosol cloud, solar transmission dropped 80% by the end of first year and remained 50% below normal for 9 years. As a result, global average surface temperatures probably dropped between 5 degrees and 31 degrees K, suggesting that global near-freezing conditions may have been reached. Impact-generated CO2 caused less than 1 degree K greenhouse warming and therefore was insignificant compare to the sulfate cooling. The magnitude of sulfate cooling depends largely upon the rate of ocean mixing as surface waters cool, sink, and are replaced by upwelling of deep ocean water. This upwelling apparently drastically altered ocean stratification and circulation, which may explain the global collapse of the delta 13C gradient between surface and deep ocean waters at the K/T boundary.

Acid Rain↗
PubMed1998

Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite.

High-temperature models for origin of the carbonates in Martian meteorite ALH84001 are implausible. The impact metasomatism model, invoking reaction between CO2 rich fluid and the host orthopyroxenite, requires conversion of olivine into orthopyroxene, yet olivine in ALH84001 shows no depletion in carbonate-rich areas; or else conversion of orthopyroxene into silica, which should have yielded a higher silica/carbonate ratio. The impact melt model implies that the fracture-linked carbonates, as products of melt injection, should appear as continuous planar veins, but in many areas they do not. Both vapor deposition and impact melting seem inconsistent with the zoned poikilotopic texture of many large carbonates. The popular hydrothermal model is inconsistent with the virtual absence of secondary hydrated silicates in ALH84001. Prior brecciation should have facilitated alteration. Hydrothermal fluids would be warm, and rate of hydration of mafic silicates obeys an Arrhenius law, at least up to approximately 100 degrees C. Most important, hydrothermal episodes tend to last for many years. Many areas of the ancient Martian crust show evidence for massive flooding. I propose that the carbonates formed as evaporite deposits from floodwaters that percolated through the fractures of ALH84001, but only briefly, as evaporation and groundwater flow caused the water table to quickly recede beneath the level of this rock during the later stages of the flood episode. The setting might have been a layer of megaregolith beneath a surface catchment of pooled floodwater, analogous to a playa lake. Carbonate precipitation would occur in response to evaporative concentration of the water. To explain the scarcity of sulfates in ALH84001, the water table must be assumed to recede quickly relative to the rate of evaporation. During the period when ALH84001 was above the water table, evaporation would have slowed, as the evaporation front passed beneath the surface of the debris layer, and possibly earlier, if the shrinking pool of surface water developed a porous sulfate crust. Alternatively, ALH84001 may have developed as a Martian form of calcrete, i.e., the evaporating flood(s) may have been entirely below ground as it (they) passed slowly through ALH84001. The greatest advantage of the flood evaporite model is that it exposes ALH84001 to carbonate precipitation without prolonged exposure to aqueous alteration. The model also seems consistent with the heavy and extremely heterogeneous oxygen isotopic compositions of the carbonates. However, this hypothesis seems no more than marginally consistent with the suggestion of McKay et al. [1996] that the carbonates are biogenic.

Carbon Dioxide↗
PubMed1998

The biological potential of Mars, the early Earth, and Europa.

The potential biomass that could have existed on Mars is constrained by the total amount of energy available to construct it. From an inventory of the available geochemical sources of energy, we estimate that from the time of the onset of the visible geologic record 4 b.y. ago to the present, as much as 20 g cm-2 of biota could have been created. This is the same amount that could have been constructed on the early Earth in only 100 million years. This indicates that there likely was sufficient energy available to support an origin of life on Mars but not sufficient energy to create a ubiquitous and lush biosphere. Similar calculations for Europa suggest that even less would have been available there.

Biomass↗
PubMed1985

Oxidant abundances in rainwater and the evolution of atmospheric oxygen.

A one-dimensional photochemical model has been used to estimate the flux of dissolved hydrogen peroxide (H2O2) and of other soluble species in rainwater as a function of atmospheric oxygen level. H2O2 should have replaced O2 as the dominant oxidant in rainwater at oxygen levels below 10(-3)-10(-2) times the present atmospheric level (PAL). The exact value of pO2 at which H2O2 becomes more important than O2 depends on the abundance of trace gases such as CO, CH4, and NO. H2O2 was probably an important oxidant even in an O2-free atmosphere, provided that CO2 levels were significant higher than today's. In model atmospheres containing free O2 the concentration of photochemically produced oxidants generally exceeds that of photochemically produced reductants. The oxidizing power of rainwater is therefore greater than that due to dissolved molecular O2 alone. The difference is small at present but becomes important at O2 levels less than 10(-3) PAL. At O2 levels between 10(-4) and 10(-5) PAL the oxidizing power of rainwater is almost independent of pO2. Precambrian soils in which a part or all of the Fe2+ in their source rocks has been oxidized to Fe3+ could therefore have developed in the presence of an atmosphere with very low values of pO2. On the other hand, the upper limit for pO2 during early and mid-Precambrian time suggested by the incomplete oxidation of FeO in soils developed on basaltic rocks is affected only slightly by the presence of photochemical products in rainwater.

Atmosphere↗
PubMed1988

Long-term climate change and the geochemical cycle of carbon.

We study the interactions between the geochemical cycles of carbon and long-term changes in climate. Climate change is studied with a simple, zonally averaged energy balance climate model that includes the greenhouse effect of carbon dioxide explicitly. The geochemical model balances the rate of consumption of carbon dioxide in silicate weathering against its release by volcanic and metamorphic processes. The silicate weathering rate is expressed locally as a function of temperature, carbon dioxide partial pressure, and runoff. The global weathering rate is calculated by integrating these quantities over the land area as a function of latitude. Carbon dioxide feedback stabilizes the climate system against a reduction in solar luminosity and may contribute to the preservation of equable climate on the early Earth, when solar luminosity was low. The system responds to reduced land area by increasing carbon dioxide partial pressure and warming the globe. Our model makes it possible to study the response of the system to changing latitudinal distribution of the continents. A concentration of land area at high latitudes leads to high carbon dioxide partial pressures and high global average temperature because weathering of high-latitude continents is slow. Conversely, concentration of the continents at low latitudes yields a cold globe and ice at low latitudes, a situation that appears to be representative of the late Precambrian glacial episode. This model is stable against ice albedo catastrophe even when the ice line occurs at low latitudes. In this it differs from energy balance models that lack the coupling to the geochemical cycle of carbon.

Atmosphere↗
PubMed1989

The effect on Earth's surface temperature from variations in rotation rate, continent formation, solar luminosity, and carbon dioxide.

Proposed evolutionary histories of solar luminosity, atmospheric carbon dioxide amounts, Earth rotation rate, and continent formation have been used to generate a time evolution of Earth's surface temperature. While speculative because of uncertainties in the input parameters, such a study does help to prioritize the areas of most concern to paleoclimatic research while illustrating the relationships and mutual dependencies. The mean temperature averages about 5 K higher than today over most of geologic time; the overall variation is less than 15 K. The evolution of Earth's rotation rate makes a significant contribution to the surface temperature distribution as late as 0.5 b.y. ago. While there is little change in equatorial temperatures, polar temperatures decrease, being some 15 K lower 3.5 b.y. ago than with present day rotation. The effect of continent growth on albedo is of secondary importance.

Atmosphere↗
PubMed1999

Groundwater formation of martian valleys.

The martian surface shows large outflow channels, widely accepted as having been formed by gigantic floods that could have occurred under climatic conditions like those seen today. Also present are branching valley networks that commonly have tributaries. These valleys are much smaller than the outflow channels and their origins and ages have been controversial. For example, they might have formed through slow erosion by water running across the surface, either early or late in Mars' history, possibly protected from harsh conditions by ice cover. Alternatively, they might have formed through groundwater or ground-ice processes that undermine the surface and cause collapse, again either early or late in Mars' history. Long-duration surface runoff would imply climatic conditions quite different from the present environment. Here we present high-resolution images of martian valleys that support the view that ground water played an important role in their formation, although we are unable as yet to establish when this occurred.

Evolution, Planetary↗
PubMed1999

Periodicity in marine phosphorus burial rate.

There have been arguments both for and against a periodicity of 26-33 million years (Myr) in terrestrial and extraterrestrial records. The best way to identify such periodicity is the analysis of geomarine evolutionary records. We have analysed the marine sedimentary phosphorus burial rate (PBR), as fluctuations in this rate are strong indicators of the coupling of climate, continental weathering and ocean primary productivity. We find a statistically significant harmonic component of 33 +/- 3 Myr against the estimated robust background noise spectrum, supporting the idea that geomarine processes are cyclic.

Evolution, Planetary↗
PubMed1999

Dual modes of the carbon cycle since the Last Glacial Maximum.

The most conspicuous feature of the record of past climate contained in polar ice is the rapid warming which occurs after long intervals of gradual cooling. During the last four transitions from glacial to interglacial conditions, over which such abrupt warmings occur, ice records indicate that the CO2 concentration of the atmosphere increased by roughly 80 to 100 parts per million by volume. But the causes of the atmospheric CO2 concentration increases are unclear. Here we present the stable-carbon-isotope composition (delta 13 CO2) of CO2 extracted from air trapped in ice at Taylor Dome, Antarctica, from the Last Glacial Maximum to the onset of Holocene times. The global carbon cycle is shown to have operated in two distinct primary modes on the timescale of thousands of years, one when climate was changing relatively slowly and another when warming was rapid, each with a characteristic average stable-carbon-isotope composition of the net CO2 exchanged by the atmosphere with the land and oceans. delta 13 CO2 increased between 16.5 and 9 thousand years ago by slightly more than would be estimated to be caused by the physical effects of a 5 degrees C rise in global average sea surface temperature driving a CO2 efflux from the ocean, but our data do not allow specific causes to be constrained.

Antarctic Regions↗
PubMed1998

Low-latitude glaciation and rapid changes in the Earth's obliquity explained by obliquity-oblateness feedback.

Palaeomagnetic data suggest that the Earth was glaciated at low latitudes during the Palaeoproterozoic (about 2.4-2.2 Gyr ago) and Neoproterozoic (about 820-550 Myr ago) eras, although some of the Neoproterozoic data are disputed. If the Earth's magnetic field was aligned more or less with its spin axis, as it is today, then either the polar ice caps must have extended well down into the tropics-the 'snowball Earth' hypothesis-or the present zonation of climate with respect to latitude must have been reversed. Williams has suggested that the Earth's obliquity may have been greater than 54 degrees during most of its history, which would have made the Equator the coldest part of the planet. But this would require a mechanism to bring the obliquity down to its present value of 23.5 degrees. Here we propose that obliquity-oblateness feedback could have reduced the Earth's obliquity by tens of degrees in less than 100 Myr if the continents were situated so as to promote the formation of large polar ice sheets. A high obliquity for the early Earth may also provide a natural explanation for the present inclination of the lunar orbit with respect to the ecliptic (5 degrees), which is otherwise difficult to explain.

Atmosphere↗