scholarly journals Atmospheric loss since the onset of the Martian geologic record: Combined role of impact erosion and sputtering

1998 ◽  
Vol 103 (E10) ◽  
pp. 22689-22694 ◽  
Author(s):  
David A. Brain ◽  
Bruce M. Jakosky
Keyword(s):  
Geologic Record ◽  
Atmospheric Loss ◽  
Impact Erosion

scholarly journals Role of microorganisms in the production of lime mud and implications for interpretation of ancient micrite deposits

1992 ◽  
Vol 6 ◽  
pp. 248-248
Author(s):  
L.L. Robbins ◽  
K. Yates
Keyword(s):  
Calcium Carbonate ◽  
Green Algae ◽  
Close Association ◽  
Carbonate Platforms ◽  
Blue Green Algae ◽  
Marine Deposits ◽  
Lime Mud ◽  
Transmission Electron ◽  
Geologic Record

Geologists have long been plagued by the lack of evidence for the origin of ancient micrite deposits: any paleontological evidence is typically obscured or lacking altogether. The role of modern marine picoplankton gives insight into the origin of one mode of lime mud formation and may aid in the interpretation of ancient marine deposits.Whitings, patches of floating lime mud in supersaturated seawater on carbonate platforms, obtained from the Bahama Bank were analyzed utilizing biochemical techniques and Transmission Electron Microscopy. All Whitings indicated a close association between picoplankton cellular material and calcium carbonate crystals. Culture experiments indicated the presence of at least ten different picoplankton species in Whitings water. Two major genera found were the blue-green algae, Synechococcus and Synechocystis. Field and laboratory experimental data indicated that these cells and cellular organics play a major role in Whitings formation. The cells may undergo epicellular precipitation of calcium carbonate induced by photosynthesis. Environmental conditions necessary for this process have been delineated through field data and laboratory experiments.While picoplankton organics are rarely preserved over geologic time, the product of their life habit, namely lime mud, is preserved as micrite. Thick occurrences of micrite deposited in marine environments are widespread throughout the geologic record, ranging in age from Precambrian to Recent. Although rare, fossilized blue-green algae have been observed in Archean rocks and may be the only evidence that implicates these organisms in lime mud formation. The Whitings phenomenon serves as an excellent example in which a specific type of organism may be a prolific contributor to the rock record, and yet leaves no direct paleontological evidence of its involvement.

scholarly journals Limited role for methane in the mid-Proterozoic greenhouse

2016 ◽  
Vol 113 (41) ◽  
pp. 11447-11452 ◽  
Author(s):  
Stephanie L. Olson ◽  
Christopher T. Reinhard ◽  
Timothy W. Lyons
Keyword(s):  
Radiative Forcing ◽  
System Model ◽  
Low Latitude ◽  
Earth History ◽  
Subsurface Ocean ◽  
Low Levels ◽  
Earth’S Climate ◽  
Geologic Record ◽  
Marine Biosphere

Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH4to the atmosphere, enhancing the climatic significance of CH4early in Earth’s history. Indeed, the assumption of elevatedpCH4during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH4levels and attendant radiative forcing. Here, we revisit the role of CH4in Earth’s climate system during Proterozoic time. We use an Earth system model to quantify CH4fluxes from the marine biosphere and to examine the capacity of biogenic CH4to compensate for the faint young Sun during the “boring billion” years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH4coupled to SO42−reduction is a highly effective obstacle to CH4accumulation in the atmosphere, possibly limiting atmosphericpCH4to less than 10 ppm by volume for the second half of Earth history regardless of atmosphericpO2. If recentpO2constraints from Cr isotopes are correct, we predict that reduced UV shielding by O3should further limitpCH4to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH4for the majority of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH4greenhouse.

scholarly journals Reaction-infiltration instability in a compacting porous medium

2018 ◽  
Vol 852 ◽  
pp. 5-36 ◽  
Author(s):  
David W. Rees Jones ◽  
Richard F. Katz
Keyword(s):  
Porous Medium ◽  
Host Rock ◽  
Numerical Solutions ◽  
Critical Conditions ◽  
Porous Flow ◽  
Magma Flow ◽  
Horizontal Length ◽  
Geological Features ◽  
Geologic Record

Certain geological features have been interpreted as evidence of channelized magma flow in the mantle, which is a compacting porous medium. Aharonov et al. (J. Geophys. Res., vol. 100 (B10), 1995, pp. 20433–20450) developed a simple model of reactive porous flow and numerically analysed its instability to channels. The instability relies on magma advection against a chemical solubility gradient and the porosity-dependent permeability of the porous host rock. We extend the previous analysis by systematically mapping out the parameter space. Crucially, we augment numerical solutions with asymptotic analysis to better understand the physical controls on the instability. We derive scalings for the critical conditions of the instability and analyse the associated bifurcation structure. We also determine scalings for the wavelengths and growth rates of the channel structures that emerge. We obtain quantitative theories for and a physical understanding of, first, how advection or diffusion over the reactive time scale sets the horizontal length scale of channels and, second, the role of viscous compaction of the host rock, which also affects the vertical extent of channelized flow. These scalings allow us to derive estimates of the dimensions of emergent channels that are consistent with the geologic record.

Model simulations of Cretaceous climates: the role of geography and carbon dioxide

1993 ◽  
Vol 341 (1297) ◽  
pp. 307-316 ◽  
Keyword(s):  
Carbon Dioxide ◽  
General Circulation ◽  
Atmospheric Carbon Dioxide ◽  
Circulation Model ◽  
Accurate Estimate ◽  
Early Model ◽  
Solar Insolation ◽  
Model Studies ◽  
Geologic Record

A general circulation model (GENESIS) with seasonally varying solar insolation and a mixed layer ocean is applied to assess the role of continental geometry and increased levels of carbon dioxide to explain the warmth of the Cretaceous period. Model experiments suggest that the role of geography is negligible, in contrast to early model studies with mean annual solar insolation and a simple energy balance ocean. Higher atmospheric carbon dioxide (4 times present) resulted in a 5.5°C globally averaged surface temperature increase, close to the lower limit required to explain the geologic record. Mid-Cretaceous carbon dioxide concentrations of 4-6 times the present day concentrations are a reasonable explanation of Cretaceous warmth if the GENESIS model provides an accurate estimate of climate sensitivity to geography and carbon dioxide.

scholarly journals Vegetation–atmosphere interactions and their role in global warming during the latest Cretaceous

1998 ◽  
Vol 353 (1365) ◽  
pp. 97-112 ◽  
Author(s):  
Garland R. Upchurch ◽  
Bette L. Otto-Bliesner ◽  
Christopher Scotese
Keyword(s):  
High Latitude ◽  
Land Surface ◽  
Bare Soil ◽  
Forest Vegetation ◽  
Geologic History ◽  
Positive Feedbacks ◽  
Global Database ◽  
Recent Climate ◽  
Geologic Record

Forest vegetation has the ability to warm Recent climate by its effects on albedo and atmospheric water vapour, but the role of vegetation in warming climates of the geologic past is poorly understood. This study evaluates the role of forest vegetation in maintaining warm climates of the Late Cretaceous by (1) reconstructing global palaeovegetation for the latest Cretaceous (Maastrichtian); (2) modelling latest Cretaceous climate under unvegetated conditions and different distributions of palaeovegetation; and (3) comparing model output with a global database of palaeoclimatic indicators. Simulation of Maastrichtian climate with the land surface coded as bare soil produces high–latitude temperatures that are too cold to explain the documented palaeogeographic distribution of forest and woodland vegetation. In contrast, simulations that include forest vegetation at high latitudes show significantly warmer temperatures that are sufficient to explain the widespread geographic distribution of high–latitude deciduous forests. These warmer temperatures result from decreased albedo and feedbacks between the land surface and adjacent oceans. Prescribing a realistic distribution of palaeovegetation in model simulations produces the best agreement between simulated climate and the geologic record of palaeoclimatic indicators. Positive feedbacks between high–latitude forests, the atmosphere, and ocean contributed significantly to high–latitude warming during the latest Cretaceous, and imply that high–latitude forest vegetation was an important source of polar warmth during other warm periods of geologic history.

scholarly journals Role of plasma waves in Mars' atmospheric loss

2006 ◽  
Vol 33 (14) ◽  
Author(s):  
R. E. Ergun ◽  
L. Andersson ◽  
W. K. Peterson ◽  
D. Brain ◽  
G. T. Delory ◽  
...  
Keyword(s):  
Plasma Waves ◽  
Atmospheric Loss

Occurrence and biogeographic significance of Heliaster (Echinodermata: Asteroidea) from the Pliocene of southwest Florida

1988 ◽  
Vol 62 (01) ◽  
pp. 126-132 ◽  
Author(s):  
Douglas S. Jones ◽  
Roger W. Portell
Keyword(s):  
Central American ◽  
Eastern Pacific ◽  
Whole Body ◽  
Marine Fauna ◽  
Western Atlantic ◽  
Southwest Florida ◽  
Extant Species ◽  
Geologic Record ◽  
Central American Isthmus

Whole body asteroid fossils are rare in the geologic record and previously unreported from the Cenozoic of Florida. However, specimens of the extant species,Heliaster microbrachiusXantus, were recently discovered in upper Pliocene deposits. This marks the first reported fossil occurrence of the monogeneric Heliasteridae, a group today confined to the eastern Pacific. This discovery provides further non-molluscan evidence of the close similarities between the Neogene marine fauna of Florida and the modern fauna of the eastern Pacific. The extinction of the heliasters in the western Atlantic is consistent with the pattern of many other marine groups in the region which suffered impoverishment following uplift of the Central American isthmus.

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