Pondering the Precambrian #8

Proterozoic:

NeoProterozoic:

Some paleosols found in the Baltic were formed during the NeoProterozoic during an extended weathering event.

What came first?  High levels oxygen in the atmosphere or animals?  Animals.

Evidence encrusting became an ability of Metazoans during the Ediacaran.

Were the Beltanelliformis fossils of the Ediacaran just cyanobacteria colonies?

There is evidence of carbon isotopic excursions from the Ediacaran Bilara Group in India.

The Ediacaran carbon isotopic excursions in South China are associated with phosphite upwellings.

There is evidence of the Ediacaran glaciations from Morocco.

Evidence of the transition from anoxic oceanic conditions during the Cryogenian to the oxic in the Ediacaran as well as the great meltwater from the Cryogenian snowball has been found.

Evidence of the mixing of glacial waters with the anoxic ocean has been found in California from during the Cryogenian.

There appears to have been an oxygen spike during the early Tonian.

There are banded iron formations in early NeoProterozoic Hainan, China.

Can Nanosims be useful in examining organic walled microfossils from the NeoProterozoic?

It appears something was burrowing in the late Ediacaran of Mongolia, much earlier than previously thought.


Exploring the effect of oxygen on the formation of haze in the NeoProterozoic atmosphere.

MesoProterozoic:

The Upper Vindhyans formation in India dates from the Late MesoProterozoic.

Evidence from China suggests the oceans of the Ectasian were anoxic to suboxic.

There is also evidence of Ectasian/Calymmian boundary oceans having banded iron formations.

Evidence from China also suggests active hydrothermal systems put a lot of iron into the shallow seas during the break up of the Columbia supercontinent.

Does photosynthesis in modern plants date to the Stenian?

PaleoProterozoic:

It appears the oceans of the Siderian become oxygenated 250 million years BEFORE the atmosphere did.

There is evidence a snowball earth scenario unfolded during the Siderian as well.

It looks like there is evidence of a tsunami or a storm from the PaleoProterozoic.

Organic remains in granular iron formations suggest they might be biological in origin.

Paleoproterozoic stromatolites from South Africa were formed in a restricted basin.

Evidence of the Great Oxygenation Event comes from rock salt from Russia.

Archean:

The Late Heavy Bombardment is being questioned.

By the end of the NeoArchean, the first modern continents had formed.

There may be volcanogenic pseudofossils in the PaleoArchean formation at Pilbara.

There's evidence of an oceanic plateau from the Archean in Nunavut, Canada.

University of Tuebingen in Germany and the Chinese Academy of Sciences in Beijing experimentally attempted to recreate the ocean of the Archean to explain its banded iron formations.

When did life originate?  At the dawn of the Archean?  Or later?

Hadean:

What the nitrogen cycle of a prebiotic Earth might have been like.

META:

Chromium isotopes can give false data for oxygen levels.

Chemists have found a plausible formula for the origin of life in Prebiotic earth.

Argnine might have played an important role in the origin of life.

A new origin of life hypothesis has been forwarded.

Did volcanic eruptions spew enough sulfur dioxide to kick off the origin of life?

Manganese oxides can produce pseudofossils.

Constraining the Early Earth's climate and oceanic acidity.

When was the earliest life could have originated?

Did ice in the earliest Earth's ocean help facilitate the origin of life?

Oceanic tides may greatly decrease when supercontinents form.

Pondering the Precambrian #5

Proterozoic:

Evidence of anoxic and alkaline oceanic conditions in the Vempalle and Tadpatri formations in India from stromatolites.

NeoProterozoic:

The evolution of sponges is reviewed.

Ediacaran:

There was a very negative and unexplained carbon-13 shift during the aftermath of the Snowball Earth episode.

At the dawn of the Ediacaran, during the retreat of the Ghaub Glaciation, the Ediacaran oceans' oxygen levels fluctuated.

Relatives of Cloudina from China are helping our understanding of the Ediacaran biomineralized metazoan.

Using x-ray tomography, attempts are made to interpret trace fossils made by metazoans during the Ediacaran.

Why did metazoans start growing in size during the Ediacaran?

Could the discovery of certain acanthomorphic acritarchs allow for better correlation of different Ediacaran fossils localities?

A unique fossil of Rangea has been found in ironstone and demonstrates a semi rigid skeleton for the Ediacaran critter.

There was an anoxic ocean volume in Brazil during the Ediacaran.

A transitional fauna from the end Ediacaran was found in Nevada.

Cryogenian:

A very thick layer of sediment from the Cryogenian has been found in Death Valley.  Evidence of glacial activity and rifting is present.

Tonian:

Did animals diverge into different lineages in the Tonian prior to the Snowball Earth episode?

The chemical weathering and Rodinia's continental drift in the Tonian contributed to the lead up to the Cryogenian Snowball Earth episode.  It also had a lot of parallels with the lead up to the Huronian Snowball Earth episode.

The evidence of evolutionary radiation of eukaryotes in Africa at the time of the MesoProterozoic/NeoProterozoic boundary.

Vase shaped microfossils have been found in Arizona, USA from the Tonian.

MesoProterozoic:

Was there an oceanic oxygenation spike during the Ectasian 1.36 billion years ago?

Evidence from Mauritania suggests what the paleoecology of the MesoProterozoic might have been like.

PaleoProterozoic:

Examining a paleoproterozoic gold deposit suggests there were two different sources for its carbon content and some caution about inferring the deposition temperatures from Raman spectroscopy.

Examining a paleosol from the Statherian shows atmospheric carbon dioxide was 4.8x preindustrial levels.

Evidence of continental assembly prior to the supercontinent Columbia from Brazil during the Orosirian.

There is evidence of significant geological turmoil during the Orosirian and is interpreted as a true polar wander.

The iron deposits of the Dongshan in China from the Orosirian show evidence of two different sources for the iron.

Siderian fossils from Western Australia seem to show benthic microbial mats seem to have continued to thrive during the Great Oxidation Event.

The evidence from the first known Snowball Earth episode during the Siderian appears to support the Jormungund scenario (slushball earth).

Archean:

Microbial dissimilatory iron reduction was common across the Archean/Proterozoic Boundary.

Plankton-like micro fossils from Australia have been found in South Africa.

Evidence of anaerobic photosynthetic microbes has been found from the PaleoArchean of Australia.

Did life really start on land?

META:

How did the Earth stay warm enough for life despite the Faint Young Star Paradox?

The importance of methane to prebiotic chemistry.

Plate Tectonics Were Initiated by Mantle Plumes

Plate tectonics on the Earth triggered by plume-induced subduction initiation

Authors:

Gerya et al

Abstract:

Scientific theories of how subduction and plate tectonics began on Earth—and what the tectonic structure of Earth was before this—remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

Lessons From Venus: Impact Origin of Archean Cratons

Impact origin of Archean cratons

Author:

Hansen

Abstract:

Archean cratons consist of crustal granite-greenstone terrains (GGTs) coupled to roots of strong, buoyant cratonic lithospheric mantle (CLM). Although this association is unique to the Archean and formed from ca. 4.0 to 2.5 Ga, the origins of terrestrial cratons are debated. I propose that crustal plateaus, quasi-circular craton-like features (∼1400−2400 km diameter, 0.5−4 km high), on Earth’s sister planet Venus might serve as analogs for Archean cratons. Crustal plateaus, which are isostatically supported by a compositionally controlled low-density root, host a distinctive surface called ribbon-tessera terrain. Ribbon-tessera also occurs as arcuate-shaped inliers in the Venus lowlands, widely interpreted as remnants of rootless crustal plateaus. Within each crustal plateau, surface ribbon-tessera terrain comprises a vast igneous province analogous to terrestrial GGTs, and the plateau root is analogous to CLM. Crustal plateaus and ribbon-tessera terrain collectively represent Venus’ oldest preserved features and surfaces, and they formed during an ancient period of globally thin lithosphere. To explain the linked features of crustal plateaus, a bolide impact hypothesis has been proposed in which a large bolide pierces ancient thin lithosphere, leading to massive partial melting in the sublithospheric mantle. In this model, melt escapes to the surface, forming an enormous lava pond, which evolves to form ribbon-tessera terrain; mantle melt residue forms a strong, resilient buoyant root, leading to plateau support and long-term stability of an individual crustal plateau. Building on the similarity of GGT−CLM and Venus crustal plateaus, I propose an exogenic hypothesis for Archean craton formation in which a large bolide pierces thin Archean lithosphere, causing localized high-temperature, high-fraction partial melting in the sublithospheric mantle; melt rises, forming an igneous province that evolves as a GGT, and melt residue develops a complementary CLM. By this mechanism, Archean cratons may have formed in a spatially and temporally punctuated fashion at a time when large bolides showered Archean Earth.

Short PR.

A Lecture on the Origin of Life

Was Earth's First Free Atmospheric Oxygen Produced by the Sun?

About one-fifth of the Earth's atmosphere is oxygen, pumped out by green plants as a result of photosynthesis and used by most living things on the planet to keep our metabolisms running. But before the first photosynthesizing organisms appeared about 2.4 billion years ago, the atmosphere likely contained mostly carbon dioxide, as is the case today on Mars and Venus.

Over the past 40 years, researchers have thought that there must have been a small amount of oxygen in the early atmosphere. Where did this abiotic ("non-life") oxygen come from? Oxygen reacts quite aggressively with other compounds, so it would not persist for long without some continuous source.

Now UC Davis graduate student Zhou Lu, working with professors in the Departments of Chemistry and of Earth and Planetary Sciences, has shown that oxygen can be formed in one step by using a high energy vacuum ultraviolet laser to excite carbon dioxide. (The work is published Oct. 3 in the journal Science).

"Previously, people believed that the abiotic (no green plants involved) source of molecular oxygen is by CO2 + solar light — > CO + O, then O + O + M — > O2 + M (where M represents a third body carrying off the energy released in forming the oxygen bond)," Zhou said in an email. "Our results indicate that O2 can be formed by carbon dioxide dissociation in a one step process. The same process can be applied in other carbon dioxide dominated atmospheres such as Mars and Venus."

Zhou used a vacuum ultraviolet laser to irradiate CO2 in the laboratory. Vacuum ultraviolet light is so-called because it has a wavelength below 200 nanometers and is typically absorbed by air. The experiments were performed by using a unique ion imaging apparatus developed at UC Davis.

Such one-step oxygen formation could be happening now as carbon dioxide increases in the region of the upper atmosphere, where high energy vacuum ultraviolet light from the Sun hits Earth or other planets. It is the first time that such a reaction has been shown in the laboratory. According to one of the scientists who reviewed the paper for Science, Zhou's work means that models of the evolution of planetary atmospheres will now have to be adjusted to take this into account.

link.

Modeling the Archean World's Climate

Controls on the Archean Climate System Investigated with a Global Climate Model

Authors:

Wolf et al

Abstract:

The most obvious means of resolving the faint young Sun paradox is to invoke large quantities of greenhouse gases, namely, CO2 and CH4. However, numerous changes to the Archean climate system have been suggested that may have yielded additional warming, thus easing the required greenhouse gas burden. Here, we use a three-dimensional climate model to examine some of the factors that controlled Archean climate. We examine changes to Earth's rotation rate, surface albedo, cloud properties, and total atmospheric pressure following proposals from the recent literature. While the effects of increased planetary rotation rate on surface temperature are insignificant, plausible changes to the surface albedo, cloud droplet number concentrations, and atmospheric nitrogen inventory may each impart global mean warming of 3–7 K. While none of these changes present a singular solution to the faint young Sun paradox, a combination can have a large impact on climate. Global mean surface temperatures at or above 288 K could easily have been maintained throughout the entirety of the Archean if plausible changes to clouds, surface albedo, and nitrogen content occurred.

Understanding Oxygen's Rise in the PaleoAtmosphere

The rise of oxygen in Earth’s early ocean and atmosphere

Authors:

Lyons et al

Abstract:

The rapid increase of carbon dioxide concentration in Earth’s modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth’s history.

Another Model of the Archean Eon's Crust

Earth's mantle temperatures during the Archean eon, which commenced some 4 billion years ago, were significantly higher than they are today. According to recent model calculations, the Archean crust that formed under these conditions was so dense that large portions of it were recycled back into the mantle. This is the conclusion reached by Dr. Tim Johnson who is currently studying the evolution of the Earth's crust as a member of the research team led by Professor Richard White of the Institute of Geosciences at Johannes Gutenberg University Mainz (JGU). According to the calculations, this dense primary crust would have descended vertically in drip form. In contrast, the movements of today's tectonic plates involve largely lateral movements with oceanic lithosphere recycled in subduction zones. The findings add to our understanding of how cratons and plate tectonics, and thus also the Earth's current continents, came into being.

Because mantle temperatures were higher during the Archean eon, the Earth's primary crust that formed at the time must have been very thick and also very rich in magnesium. However, as Johnson and his co-authors explain in their article recently published in Nature Geoscience, very little of this original crust is preserved, indicating that most must have been recycled into the Earth's mantle. Moreover, the Archean crust that has survived in some areas such as, for example, Northwest Scotland and Greenland, is largely made of tonalite–trondhjemite–granodiorite complexes and these are likely to have originated from a hydrated, low-magnesium basalt source. The conclusion is that these pieces of crust cannot be the direct products of an originally magnesium-rich primary crust. These TTG complexes are among the oldest features of our Earth's crust. They are most commonly present in cratons, the oldest and most stable cores of the current continents.

With the help of thermodynamic calculations, Dr. Tim Johnson and his collaborators at the US-American universities of Maryland, Southern California, and Yale have established that the mineral assemblages that formed at the base of a 45-kilometer-thick magnesium-rich crust were denser than the underlying mantle layer. In order to better explore the physics of this process, Professor Boris Kaus of the Geophysics work group at Mainz University developed new computer models that simulate the conditions when the Earth was still relatively young and take into account Johnson's calculations.

These geodynamic computer models show that the base of a magmatically over-thickened and magnesium-rich crust would have been gravitationally unstable at mantle temperatures greater than 1,500 to 1,550 degrees Celsius and this would have caused it to sink in a process called 'delamination'. The dense crust would have dripped down into the mantle, triggering a return flow of mantle material from the asthenosphere that would have melted to form new primary crust. Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to provide a source of the tonalite–trondhjemite–granodiorite complexes. The dense residues of these processes, which would have a high content of mafic minerals, must now reside in the mantle.

link.

Evidence of a Microcontinent and Modern Plate Tectonics in MesoArchean India

An exotic Mesoarchean microcontinent: The Coorg Block, southern India

Authors:

M. Santosh, Qiong-Yan Yang, E. Shaji, T. Tsunogae, M. Ram Mohan and M. Satyanarayanan

Abstract:

Sandwiched between the Dharwar Craton in the north and the Neoarchean – Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite-trondhjemite-granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes of all the major lithologies from this block. Computation of metamorphic P-T conditions from mineral chemical data shows consistent granulite-facies P-T conditions of 820-870 °C and up to 6 kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean 207Pb/206Pb ages of 3153.4 ± 9 to 3184.0 ± 5.5 Ma for syenogranites, 3170.3 ± 6.8 Ma for biotite granite, 3275 ± 5.1 Ma for trondhjemite, 3133 ± 12 to 3163.8 ± 6.9 Ma for charnockites, 3156 ± 10 to 3158.3 ± 8.2 for mafic enclaves, 3161 ± 16 Ma for diorite and 3173 ± 16 Ma for felsic volcanic tuff. An upper intercept age of 3363 ± 59 Ma and a lower intercept age of 2896 ± 130 Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective 207Pb/206Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from greater than 3.3 to 3.1 Ga was immediately followed by metamorphism at ca. 3.0 to 2.9 Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1 Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5 Ga, 2.0 Ga, 0.8 Ga and 0.55 Ga associated with various subduction-accretion-collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5 Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.

Tracking Mantle Plume Activitiy Through Deep Time

Accretionary complexes in the Asia-Pacific region: Tracing archives of ocean plate stratigraphy and tracking mantle plumes

Authors:

I.Yu. Safonova and M. Santosh

Abstract:

The accretionary complexes of Central and East Asia (Russia, Kazakhstan, Kyrgyzstan, Tajikistan, Mongolia, and China) and the Western Pacific (China, Japan, Russia) preserve valuable records of ocean plate stratigraphy (OPS). From a comprehensive synthesis of the nature of occurrence, geochemical characteristics and geochronological features of the oceanic island basalts (OIB) and ophiolite units in the complexes, we track extensive plume-related magmatism in the Paleo-Asian and Paleo-Pacific Oceans. We address the question of continuous versus episodic intraplate magmatism and its contribution to continental growth. An evaluation of the processes of subduction erosion and accretion illustrates continental growth at the active margins of the Siberian, Kazakhstan, Tarim and North China blocks, the collision of which led to the construction of the Central Asian Orogenic Belt (CAOB). Most of the OIB-bearing OPS units of the CAOB and the Western Pacific formed in relation to two superplumes: the Asian (Late Neoproterozoic) and the Pacific (Cretaceous), with a continuing hot mantle upwelling in the Pacific region that contributes to the formation of modern OIBs. Our study provides further insights into the processes of continental construction because the accreted seamounts play an important role in the growth of convergent margins and enhance the accumulation of fore-arc sediments.

The Wilson Cycle: The Rise and Fall of Supercontinents Through Deep Time

The supercontinent cycle: A retrospective essay

Authors:

R. Damian Nance, J. Brendan Murphy and M. Santosh

Abstract:

The recognition that Earth history has been punctuated by supercontinents, the assembly and breakup of which have profoundly influenced the evolution of the geosphere, hydrosphere, atmosphere and biosphere, is arguably the most important development in Earth Science since the advent of plate tectonics. But whereas the widespread recognition of the importance of supercontinents is quite recent, the concept of a supercontinent cycle is not new and advocacy of episodicity in tectonic processes predates plate tectonics. In order to give current deliberations on the supercontinent cycle some historical perspective, we trace the development of ideas concerning long-term episodicity in tectonic processes from early views on episodic orogeny and continental crust formation, such as those embodied in the chelogenic cycle, through the first realization that such episodicity was the manifestation of the cyclic assembly and breakup of supercontinents, to the surge in interest in supercontinent reconstructions. We then chronicle some of the key contributions that led to the cycle's widespread recognition and the rapidly expanding developments of the past ten years.

A World Without Life is a World With Smaller Continents

Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage

Authors:

Höning et al

Abstract:

The origin and evolution of life has undoubtedly had a major impact on the evolution of Earth's oceans and atmosphere. Recent studies have suggested that bioactivity may have had an even deeper impact and may have caused a change in the redox-state of the mantle and provided a path for the formation of continents. We here present a numerical model that assumes that bioactivity increases the continental weathering rate and that relates the sedimentation rate to the growth of continents and to the hydration of the mantle using elements of plate tectonics and mantle convection. The link between these factors is provided by assuming that an increase of the thickness of the sedimentary layer of low permeability on top of a subducting oceanic slab will reduce its dewatering upon subduction. This in turn leads to a greater availability of water in the source region of andesitic partial melt, resulting in an enhanced rate of continental crust production, and to an increased regassing rate of the mantle. The mantle in turn responds by reducing the mantle viscosity and increasing the convective circulation rate, degassing rate and plate speed. We use parameters that are observed for the present Earth and gauge uncertain parameters such that the present day continental surface area and mantle water concentration can be obtained. Our steady state results show two stable fixed points in a phase plane defined by the fractional continental surface area and the water concentration in the mantle, one of them pertaining to a wet mantle and the continental surface area of the present day Earth, and the other to a dry mantle and a small continental surface area. When the sedimentation rate is reduced, both fixed points move and the area of attraction of the latter fixed point increases systematically. We conclude that if the presence of life has increased the continental weathering rate, as is widely believed, and led to the observables of a wet mantle and a continental surface coverage of roughly 40%, an abiotic Earth would likely have evolved towards a dry mantle with a small continental surface area instead.

M Class Stars Can Produce Abiotic Oxygen, Ozone Rich Atmospheres

The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars

Authors:

France et al.

Abstract:

The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both FUV and NUV wavelengths. The combined FUV+NUV spectra are publically available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; > 10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3 for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four stars with moderate signal-to-noise COS time-resolved spectra, we find UV emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s timescales. Finally, we observe relatively bright H2 fluorescent emission from four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832). Additional modeling work is needed to differentiate between a stellar photospheric or possible exoplanetary origin for the hot (T(H2) \approx 2000-4000 K) molecular gas observed in these objects.

There was a paleo atmosphere paper which indicated the presence of oxygen prior to the great oxygenation event.  I have to wonder if UV flux from a younger star might not be the reason.

Archean Basalts Indicate Paleo Atmosphere was Cooler, Different Composition

Conditions of subaerial weathering of basalts in the Neoarchean and Paleoproterozoic

Authors:

Nadezhda A. Alfimova, Alexey A. Novoselov, Vjacheslav A. Matrenichev and Carlos Roberto de Souza Filho

Abstract:

Precambrian weathering profiles (or regoliths) were formed on the Earth's surface in contact with meteoritic water. This signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering. The aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of Precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation. We have demonstrated that, in a wide range of modeling conditions, weathering profiles of basalts consist mostly of clay minerals (smectites and illite). Carbonate minerals may also be deposited within the profile at the initial stage of weathering, which are later dissolved during a further process. The time, necessary for complete dissolution of primary minerals and crystallization of authigenic minerals, is between 100-140 Kyr. Modeling results indicate that Neoarchean and Paleoproterozoic (2.8 - 2.0 Ga) weathering profiles were formed under an atmosphere with pCO2 not much higher than 25 PAL and climatic conditions close to those found in modern times. Weathering under CO2-rich (pCO2 = 1 and 10 bar) and CH4-rich atmospheres as well as high temperature (50 °C and 75 °C) conditions do not produce compounds corresponding to the analyzed samples.

Hadean and Early Archean Earth Was More Like Jupiter's Moon Io With an Atmosphere Than Modern Earth

Heat-pipe Earth

Authors:

William B. Moore & A. Alexander G. Webb

Abstract:

The heat transport and lithospheric dynamics of early Earth are currently explained by plate tectonic and vertical tectonic models, but these do not offer a global synthesis consistent with the geologic record. Here we use numerical simulations and comparison with the geologic record to explore a heat-pipe model in which volcanism dominates surface heat transport. These simulations indicate that a cold and thick lithosphere developed as a result of frequent volcanic eruptions that advected surface materials downwards. Declining heat sources over time led to an abrupt transition to plate tectonics. Consistent with model predictions, the geologic record shows rapid volcanic resurfacing, contractional deformation, a low geothermal gradient across the bulk of the lithosphere and a rapid decrease in heat-pipe volcanism after initiation of plate tectonics. The heat-pipe Earth model therefore offers a coherent geodynamic framework in which to explore the evolution of our planet before the onset of plate tectonics.

pop sci link.

Late Permian Milankovitch Cycles Time Calibrated, Indicate a 22 Hour Day

Time-calibrated Milankovitch cycles for the late Permian

Authors:

1. Huaichun Wu (a,b)
2. Shihong Zhang (a)
3. Linda A. Hinnov (c)
4. Ganqing Jiang (d)
5. Qinglai Feng (e)
6. Haiyan Li (a)
7. Tianshui Yang (a)

Affiliations:

a. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China

b. School of Ocean Sciences, China University of Geosciences, Beijing 100083, China

c. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA

d. Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA

e. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

Abstract:

An important innovation in the geosciences is the astronomical time scale. The astronomical time scale is based on the Milankovitch-forced stratigraphy that has been calibrated to astronomical models of paleoclimate forcing; it is defined for much of Cenozoic–Mesozoic. For the Palaeozoic era, however, astronomical forcing has not been widely explored because of lack of high-precision geochronology or astronomical modelling. Here we report Milankovitch cycles from late Permian (Lopingian) strata at Meishan and Shangsi, South China, time calibrated by recent high-precision U–Pb dating. The evidence extends empirical knowledge of Earth’s astronomical parameters before 250 million years ago. Observed obliquity and precession terms support a 22-h length-of-day. The reconstructed astronomical time scale indicates a 7.793-million year duration for the Lopingian epoch, when strong 405-kyr cycles constrain astronomical modelling. This is the first significant advance in defining the Palaeozoic astronomical time scale, anchored to absolute time, bridging the Palaeozoic–Mesozoic transition.

Ancient Mantle Keels Formed the Kernels for Continents

New research led by a University of Calgary geophysicist provides strong evidence against continent formation above a hot mantle plume, similar to an environment that presently exists beneath the Hawaiian Islands.

The analysis, published this month in Nature Geoscience, indicates that the nuclei of Earth's continents formed as a byproduct of mountain-building processes, by stacking up slabs of relatively cold oceanic crust. This process created thick, strong 'keels' in the Earth's mantle that supported the overlying crust and enabled continents to form.

The scientific clues leading to this conclusion derived from computer simulations of the slow cooling process of continents, combined with analysis of the distribution of diamonds in the deep Earth.

The Department of Geoscience's Professor David Eaton developed computer software to enable numerical simulation of the slow diffusive cooling of Earth's mantle over a time span of billions of years.

Working in collaboration with former graduate student, Assistant Professor Claire Perry from the Universite du Quebec a Montreal, Eaton relied on the geological record of diamonds found in Africa to validate his innovative computer simulations.

"For the first time, we are able to quantify the thermal evolution of a realistic 3D Earth model spanning billions of years from the time continents were formed," states Perry.

Mantle plumes consist of an upwelling of hot material within Earth's mantle. Plumes are thought to be the cause of some volcanic centres, especially those that form a linear volcanic chain like Hawaii. Diamonds, which are generally limited to the deepest and oldest parts of the continental mantle, provide a wealth of information on how the host mantle region may have formed.

"Ancient mantle keels are relatively strong, cold and sometimes diamond-bearing material. They are known to extend to depths of 200 kilometres or more beneath the ancient core regions of continents," explains Professor David Eaton. "These mantle keels resisted tectonic recycling into the deep mantle, allowing the preservation of continents over geological time and providing suitable environments for the development of the terrestrial biosphere."

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Modeling the Erosion of Hot, Dense Atmospheres by Impacts

Impact induced erosion of hot and dense atmospheres

Authors:

1. Valery Shuvalov (a)
2. Ekkehard Kührt (b)
3. Detlef de Niem (b)
4. Kai Wünnemann (c)

Affiliations:

a. Institute of Geospheres Dynamics, Russian Academy of Sciences, LeninskiiProspekt 38-1, 119334 Moscow, Russia

b. German Aerospace Center, Institute of Planetary Research, Rutherfordstraße 2, D-12489 Berlin, Germany

c. Museum für Naturkunde, Invalidenstraße 43, D-10115 Berlin, Germany

Abstract:

Previous investigations of impact-induced atmospheric erosion considered mainly crater-forming impacts. Simple estimates show that in dense primary planetary atmospheres, considerable erosion could be induced by aerial bursts resulting from impacts of 1 to 10 km sized projectiles. Numerical simulations of cometary and asteroidal impacts (striking unmodified and crater-forming, impacting as fragmented meteorites, or causing aerial bursts) into dense (200 bar) atmospheres of different temperatureshave been performed to obtain the amount of atmospheric erosion. The results have been approximated by simple analytical formulae.

Evidence of NeoArchean Volcanic Islands Arcs in India

Zircon U-Pb geochronology and Hf isotope of felsic volcanics from Attappadi, southern India: Implications for Neoarchean convergent margin tectonics

Authors:

1. M.N. Praveen (a)
2. M. Santosh (b)
3. Q.Y. Yang (b)
4. Z.C. Zhang (b)
5. H. Huang (b)
6. S. Singanenjam (c)
7. K.S. Sajinkumar (d)

Affiliations:

a. Geological Survey of India, Dharani Bhawan, Manikanteshwaram PO, Thiruvananthapuram 695 013, India

b. School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China

c. Geological Survey of India, Rajaji Bhavan, Besant Nagar, Chennai 600 090, India

d. Department of Geology, University of Kerala, Thiruvananthapuram 695 581

Abstract:

The Attappadi area on the south-western flanks of the Archean Dharwar Craton in southern India is located along the E-W trending Bhavani Shear Zone which marks the trace of a Neoarchean suture zone. The dominant rock types in the area include meta-ultramafics, amphibolites, TTG (tonalite-trondhjemite-granodiorite) gneisses, metapelites, and sulphidic banded iron formation (BIF). Here we report the occurrence of felsic volcanic rocks preserving primary textures from the Anaikatti area in eastern Attappadi. The felsic volcanics are interbanded with the BIF, amphibolite, metapelite, metapyroxenite and hornblende gneisses. The felsic volcanics are divided into two types based on their textures. The Type-1 rock is medium grained with gneissic texture and lack unequivocal primary volcanic textures. Type-2 felsic volcanics are thinly laminated, fine grained and at places preserve relict soft-sediment deformation structures. They also contain relict volcanic clasts or lapilli and are interpreted as felsic tuff. Geochemically, the Attappadi felsic volcanics are rhyolitic in composition and have arc-related trace element signatures. They also possess a calc-alkaline volcanic affinity.

We report LA-ICPMS U-Pb ages from zircons in four samples of the felsic tuffs which show weighted mean ages of 2567 ± 18 Ma and (MSWD = 1.4), 2499 ± 19 Ma (MSWD = 0.57), 2555 ± 24 Ma (MSWD = 1.7) and 2576 ± 64 Ma (MSWD = 5.8). The late Neoarchean – early Paleoproterozoic ages obtained in our study correlate well with the zircon U-Pb ages reported in recent studies from ophiolites and other suprasubduction suites from Attappadi and surrounding regions. The zircon εHf values range from -11.1 to 7.6 suggesting heterogeneous source material involving both juvenile and older reworked components. We build a tectonic model for the SW margin of the Dharwar Craton with an oceanic realm characterized by island arcs and widespread submarine tholeiitic as well as komatiitic ultramafic and mafic volcanism during the Neoarchean. This predominantly mafic volcanism on the ocean floor is represented by primitive komatiitic lavas, oxide and sulphide facies BIF. The birth of volcanic arc at the convergent margin is marked by felsic volcanism and the deposition of felsic volcanics and volcano-sedimentary successions. In the final stage of ocean closure, the ocean-plate and continental arc assemblages were brought in juxtaposition including the accretion of the ophiolitic fragments. Our study confirms the recent models of arc-arc and arc-continental accretion to the southern margin of the Dharwar Craton and major continental growth at the end of the Archean.