Pondering the Precambrian #39

Proterozoic:

NeoProterozoic:

Global microfossil changes across the Ediacaran/Cambrian are characterized.

A new method of characterizing oxygen availability across the Ediacaran/Cambrian boundary has been found.

The lack of attention to taphonomy can mislead about the late Ediacaran fossils.

Data from Argentina covers the Ediacaran's oxygenation event.

Simulations of Ernettia suggest it was a filter feeder and fed better in groups.

Has there been a case of lateral gene transfer detected in bilaterians?

Ancient paralogies suggest jellyfish are a sister group to all other metazoans.

Green algae transitioned to macroscopic growth multiple times, but only as recently as the NeoProterozoic.

There is evidence of strange magmatism in the Sahara from the Ediacaran 580 million years ago.

An astronomical time scale for the middle/upper Doushantuo Formation has been established.

Braided rivers were not the rule prior to the evolution of plant life despite previous thought.

The Paleo Hunan Ocean was completely closed by 830 million years ago during the Tonian.

Evidence of two carbon excursions are detected in Tonian deposits in Namibia.

MesoProterozoic:

A new model suggests a sluggish, tepid mesoproterozoic ecosystem.

A very large igneous magmatic province has been found in South Africa from Stenian of South Africa.  These provinces are associated with mass extinctions like the Permian and Triassic/Jurassic.

An impact has been found in Scotland dating from the Mesoproterozoic.

Was anaerobic photosynthesis the reason for the delayed rise in oxygen in Earth's atmosphere?

PaleoProterozoic:

Iron stones were laid down by iron oxidizing bacteria during the Statherian.

There is evidence of deep subduction from North China from the Paleoproterozoic.

There is also evidence of a significant back arc continental collision during the Orosirian in the North China Craton.

There is evidence, according to Retallack, of macroscopic terrestrial life from the Orosirian.

The shape of the Dhala Crater in India from the Rhyacian is reconstructed.

Paleoproterozoic dolomites shed some light on the evolution of marine chemistry.

Archean:

The early Earth's oceans may not have been as hot as originally thought.

Compared to later granites, Archean granites had relatively unstable compositions.

Archean sulfur isotopes from Australia's Fraser Zone have unexplained ratios and amounts.

Did the subduction of the oceanic basins take place during the mesoarchean?

Could continents have existed from the dawn of the Archean, during the Eoarchean?

Eoarchean hydrothermal vent boron deposits give some insight to the origin of life.

Origin of Life:

Peptides can form without amino acids.

Could microscopic bubbles (interfaces) have helped kickstart life?

Evidence of Microbial Activity From PaleoArchean Australia

Carbonaceous microstructures from sedimentary laminated chert within the 3.46 Ga Apex Basalt, Chinaman Creek locality, Pilbara, Western Australia

Authors:

Hickman-Lewis et al

Abstract:

Hydrothermal black chert veins intruding the 3.46 Ga Apex Basalt contain some of Earth’s oldest microfossil-like objects, whose biogenicity has been questioned. Whilst these black chert veins have been studied in great detail, relatively little is known about the stratiform, seafloor, sedimentary cherts that are conformably interbedded with volcanic rocks of the Apex Basalt.

Herein, we document and assess the biogenicity of carbonaceous microstructures present in the lowermost of the stratiform chert units (informally known as the ‘Apex chert’), at the Chinaman Creek locality in the Marble Bar greenstone belt, Pilbara Craton, Western Australia. Carbonaceous material mostly occurs within clotted grey-black cherts and microgranular ‘grainstone-like’ cherts within the stratiform unit, the latter being the major focus of this study. In the clotted cherts, carbon occurs as lobate, fluffy grains, rare compressed flakes, and as a grain boundary phase around spherulitic silica. There is no morphological evidence to support the biogenicity of these microstructures. In contrast, the microgranular chert contains fluffy and flaky carbonaceous grains, plus laminated grains comprising multiple non-isopachous wrinkled carbonaceous laminae, with noted thickening towards some ridge crests, as determined by confocal laser scanning microscopy. Roll-up structures provide evidence of an initial plasticity, interpreted to have formed via the tearing-up and current-induced plastic deformation of microbial mat fragments. Geochemical mapping, using laser Raman micro-spectroscopy and NanoSIMS, respectively demonstrates the antiquity of the carbon, and reveals a close correlation between carbon, nitrogen and sometimes sulphur, concentrated within dark brown to black laminae. Adjacent to microgranular zones are zones of more persistent carbonaceous, undulose, filament-like laminae that entrain relict sediment grains. These microstructures are directly comparable to a sub-type of microbially induced sedimentary structure (MISS), widely reported from younger siliciclastic sediments colonised by microbial biofilms.

The morphology and chemical composition of both the non-isopachous laminated grains and the filament-like laminae are consistent with a biological interpretation, suggesting microscopic MISS were present in the microgranular stratiform ‘Apex chert’. However, the fact that neither macroscopic MISS nor bona fide microfossils have yet been reported from this unit, coupled with the proximity of these structures to active hydrothermal vents, potentially discharging hot carbon-rich fluids, urges caution in such an interpretation. The Chinaman Creek ‘Apex chert’ investigated here is one of at least five sedimentary, laminated cherts within the Apex Basalt. These horizons are promising targets in the search for biological activity within a dominantly volcanic Archaean environment.

How Far Back in Deep Time did Tectonic Plate Subduction Begin?

Heading down early on? Start of subduction on Earth

Authors:

Turner et al

Abstract:

How the Earth's earliest crust was formed and when present-day plate tectonics (i.e., subduction) and life commenced remain fundamental questions in Earth sciences. Whereas the bulk composition of the crust is similar to that of rocks generated in subduction settings, it does not necessarily follow that melting and crust formation require subduction. Many workers suggest that subduction may have only commenced toward the end of the Archean or later. Here we observe that both the stratigraphy and geochemistry of rocks found in Quebec, Canada, that have been variously argued to be 4.4 or 3.8 Ga in age, closely match those from the modern-day Izu-Bonin-Mariana forearc. We suggest that this geochemical stratigraphy might provide a more robust test of ancient tectonic setting than individual chemical or isotopic signatures in rocks or detrital minerals. If correct, the match suggests that at least some form of subduction may have been operating as early as the Hadean or Eoarchean. This could have provided an ideal location for the development of first life.

A New Model for how the Newly Formed Earth's Crust, Mantle and Core Differentiated

Early differentiation of the bulk silicate Earth as recorded by the oldest mantle reservoir

Authors:

Xuan-Ce Wang, Zheng-Xiang Li, Xian-Hua Li

Abstract:

An emerging challenge for understanding the Earth system is to determine the relative roles of early planetary processes versus progressive differentiation in shaping the Earth's chemical architecture. An enduring tenet of modern chemical geodynamics is that the Earth started as a well-mixed and homogeneous body which evolved progressively over the geologic time to several chemically distinct domains. As a consequence, the observable chemical heterogeneity in mantle-derived rocks has generally been attributed to the Earth's dynamic evolution over the past 4.5 Ga. However, the identification of chemical heterogeneity formed during the period 4.53–4.45 Ga in the ca. 60 Ma Baffin Bay high-magnesium lavas provides strong evidence that chemical effects of early differentiation can persist in mantle reservoirs to the present day. Here, we demonstrate that such an ancient mantle reservoir is likely composed of enriched and depleted dense melts, and propose a model for early global differentiation of the bulk silicate Earth that would produce two types of dense melts with distinctive chemical compositions in the deep Earth. These dense melts ultimately became parts of the thermo-chemical piles near the core-mantle boundary that have been protected from complete entrainment by subsequent mantle convection currents. We argue that although such dense melts likely exhibit some ‘primordial’ geochemical signatures, they are not representative of the bulk silicate Earth. Our work provides a strong case for the mantle chemical heterogeneity being formed by a major differentiation event shortly after planet accretion rather than through the subsequent geodynamic evolution

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.

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.

NeoArchean Tumbiana Formation Conical Stromatolites Were From Phototrophic Microorganisms?

Sedimentology, stratigraphy and geochemistry of a stromatolite biofacies in the 2.72 Ga Tumbiana Formation, Fortescue Group, Western Australia

Authors:

1. J.M. Coffey (a)
2. D.T. Flannery (a)
3. M.R. Walter (a)
4. S.C. George (a, b)

Affiliations:

a. Australian Centre for Astrobiology, School of Biotechnology and Biomolecular Sciences, University of New South Wales, New South Wales 2052, Australia

b. Department of Earth and Planetary Sciences, Macquarie University, New South Wales, 2109, Australia

Abstract:

The 2.72 Ga Tumbiana Formation is a succession of clastic and carbonate rocks outcropping along the southern margin of the Pilbara Craton in Western Australia. It hosts abundant, diverse and exceptionally well-preserved stromatolites and has provided the setting for numerous investigations focussing on the Archean biosphere. Despite its palaeobiological significance, the overall depositional setting of the Tumbiana Formation remains unclear. Here we present the results of stratigraphic, sedimentological and geochemical investigation of the Tumbiana Formation in the well-known Redmont/Knossos area and at several localities in the northwestern Pilbara sub-basin. We suggest these data are best explained by deposition in fluvial and lacustrine environments of an inward-draining continental basin. δ13Corg values vary from -49.9‰ to -15.0‰. Conical stromatolite morphologies, commonly attributed to cyanobacteria, are anomalously little depleted in 13Corg, implying a higher relative contribution of organic matter from phototrophic versus methane cycling metabolisms.

Evidence of Modern Subduction Zone Vulcanism in MesoArchean Greenland

Archaean andesite petrogenesis: insights from the Grædefjord Supracrustal Belt, southern West Greenland

Authors:

1. Kristoffer Szilas (a)
2. J. Elis Hoffmann (b, c)
3. Anders Scherstén (d)
4. Thomas F. Kokfelt (e)
5. Carsten Münker (b)

Affiliations:

a. Lamont-Doherty Earth Observatory, PO Box 1000, Palisades, NY 10964-8000, USA

b. Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, 50674 Köln, Germany

c. Steinmann-Institute, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn

d. Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden

e. Geological Survey of Denmark and Greenland - GEUS, Øster Voldgade 10, 1350 Copenhagen K, Denmark

Abstract:

We present new whole-rock major, trace and platinum-group element data, as well as Sm-Nd and Lu-Hf isotope data for meta-volcanic rocks from the Mesoarchaean Grædefjord Supracrustal Belt (GSB), located within the Tasiusarsuaq terrane, southern West Greenland. We also present new in-situ zircon U-Pb isotope data (by LA-ICP-MS) for associated felsic rocks. This region has experienced amphibolite to lower granulite facies metamorphism, causing re-equilibration of most mineral phases (including zircon).

An intrusive tonalite sheet with a zircon U-Pb age of 2888 ±6.8 Ma, yields a minimum age for the GSB. The Sm-Nd and Lu-Hf isotope data do not provide meaningful isochron ages, but the isotope compositions of the mafic rocks are consistent with the ca. 2970 Ma regional volcanic event, which is documented in previous studies of the Tasiusarsuaq terrane. The major and trace element data suggest a significant crustal contribution in the petrogenesis of andesitic volcanic rocks in the GSB. The trace element variation of these andesitic leucoamphibolites cannot be explained by bulk assimilation-fractional-crystallisation (AFC) processes involving local basement. Rather, the observed patterns require binary mixing between basaltic and felsic end-member magmas with between 50-80% contributions from the latter (depending on the assumed felsic composition). Hf-isotope constraints point to contamination with pre-existing continental crust with an age of ca. 3250 Ma. Basement gneisses of this age were previously described at two localities in the Tasiusarsuaq terrane, which supports the mixing hypothesis. Thus the felsic end-member likely represents melts derived from the local basement.

Ultramafic rocks (18.35-22.80 wt.% MgO) in GSB have platinum-group element (PGE) patterns that are similar to magmas derived from high-degree melting of mantle, but they have relatively enriched trace element patterns. We propose that the ultramafic rocks represent arc-related picrites or alternatively were derived by melting of metasomatised sub-continental lithospheric mantle.

Overall these new geochemical data from the Mesoarchaean Grædefjord Supracrustal Belt and the petrogenetic mixing model in particular, are similar to observations from modern continental subduction zone environments, which also require large degrees of mixing with felsic basement melts. Therefore, we propose that the metavolcanic rocks formed in a modern-style subduction zone geodynamic setting, which due to the hotter Archaean mantle conditions allowed for substantial amounts of partial melting and magma mixing, rather than assimilating pre-existing continental crust

Carbon Dioxide Depleted in Late Archean Sea Water

Decrease of seawater CO2 concentration in the Late Archean: An implication from 2.6 Ga seafloor hydrothermal alteration

Authors:

1. Takazo Shibuya (a, b, c)
2. Miyuki Tahata (d)
3. Yuichiro Ueno (d)
4. Tsuyoshi Komiya (e)
5. Ken Takai (a, b, f)
6. Naohiro Yoshida (g, h)
7. Shigenori Maruyama (e)
8. Michael J. Russell (c)

Affiliations:

a. Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

b. Submarine Hydrothermal System Research Group, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

c. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

d. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

e. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

f. Subsurface Geobiology Advanced Research (SUGAR) project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

g. Department of Environmental Science and Technology, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan

h. Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan

Abstract:

Before continents attained a critical aerial dimension on the early Earth, hydrothermal carbonation of subseafloor crust is considered to have played the dominant role in fixing CO2 from the CO2-rich ocean. However, it is uncertain how and when the seawater CO2 level decreased and the strong carbonation of oceanic crust ceased. Here we report the depth profiles of the volume concentration and the carbon isotopes of calcites in the Late Archean/Paleoproterozoic volcanic rocks (Fortescue and Hamersley groups), exposed in the southwestern Pilbara Craton, Western Australia. The depth profiles indicate that 2.6 Ga seafloor hydrothermal carbonation is well preserved in the study area and that the CO2 content of subseafloor crust per seafloor unit area is estimated to be clearly lower than those in the Early and Middle Archean and similar to the Phanerozoic equivalents. This suggests that the CO2 concentration in seawater decreased from the Middle Archean to the Late Archean. This period broadly corresponds to the time of the first appearance of supercontinent on Earth. The amalgamation of continents has the potential to decrease seawater CO2 concentration due to the removal of platform carbonate to continental interior. Subsequent fragmentation of supercontinent likely cause the carbonate deposition around newly created continental shelves. It is therefore implied that seawater CO2 concentration in the early Earth was lowered by not only the hydrothermal carbonation of subseafloor crust but also through the formation and breakup of supercontinent in the Late Archean.

Young Faint Sun Paradox May Partially Be Artifact of Bad Modeling

Solving the "faint young sun paradox" -- explaining how early Earth was warm and habitable for life beginning more than 3 billion years ago even though the sun was 20 percent dimmer than today -- may not be as difficult as believed, says a new University of Colorado Boulder study.

In fact, two CU-Boulder researchers say all that may have been required to sustain liquid water and primitive life on Earth during the Archean eon 2.8 billion years ago were reasonable atmospheric carbon dioxide amounts believed to be present at the time and perhaps a dash of methane. The key to the solution was the use of sophisticated three-dimensional climate models that were run for thousands of hours on CU's Janus supercomputer, rather than crude, one-dimensional models used by almost all scientists attempting to solve the paradox, said doctoral student Eric Wolf, lead study author.

"It's really not that hard in a three-dimensional climate model to get average surface temperatures during the Archean that are in fact moderate," said Wolf, a doctoral student in CU-Boulder's atmospheric and oceanic sciences department. "Our models indicate the Archean climate may have been similar to our present climate, perhaps a little cooler. Even if Earth was sliding in and out of glacial periods back then, there still would have been a large amount of liquid water in equatorial regions, just like today."

[...]

"In our opinion, the one-dimensional models of early Earth created by scientists to solve this paradox are too simple -- they are essentially taking the early Earth and reducing it to a single column atmospheric profile," said Toon. "One-dimensional models are simply too crude to give an accurate picture."

Wolf and Toon used a general circulation model known as the Community Atmospheric Model version 3.0 developed by the National Center for Atmospheric Research in Boulder and which contains 3-D atmosphere, ocean, land, cloud and sea ice components. The two researchers also "tuned up" the model with a sophisticated radiative transfer component that allowed for the absorption, emission and scattering of solar energy and an accurate calculation of the greenhouse effect for the unusual atmosphere of early Earth, where there was no oxygen and no ozone, but lots of CO2 and possibly methane.

The simplest solution to the faint sun paradox, which duplicates Earth's present climate, involves maintaining roughly 20,000 parts per million of the greenhouse gas CO2 and 1,000 ppm of methane in the ancient atmosphere some 2.8 billion years ago, said Wolf. While that may seem like a lot compared to today's 400 ppm of CO2 in the atmosphere, geological studies of ancient soil samples support the idea that CO2 likely could have been that high during that time period. Methane is considered to be at least 20 times more powerful as a greenhouse gas than CO2 and could have played a significant role in warming the early Earth as well, said the CU researchers.

There are other reasons to believe that CO2 was much higher in the Archean, said Toon, who along with Wolf is associated with CU's Laboratory for Atmospheric and Space Physics. The continental area of Earth was smaller back then so there was less weathering of the land and a lower release of minerals to the oceans. As a result there was a smaller conversion of CO2 to limestone in the ocean. Likewise, there were no "rooted" land plants in the Archean, which could have accelerated the weathering of the soils and indirectly lowered the atmospheric abundance of CO2, Toon said.

Another solution to achieving a habitable but slightly cooler climate under the faint sun conditions is for the Archean atmosphere to have contained roughly 15,000 to 20,000 ppm of CO2 and no methane, said Wolf. "Our results indicate that a weak version of the faint young sun paradox, requiring only that some portion of the planet's surface maintain liquid water, may be resolved with moderate greenhouse gas inventories," the authors wrote in Astrobiology.

"Even if half of Earth's surface was below freezing back in the Archean and half was above freezing, it still would have constituted a habitable planet since at least 50 percent of the ocean would have remained open," said Wolf. "Most scientists have not considered that there might have been a middle ground for the climate of the Archean.

"The leap from one-dimensional to three-dimensional models is an important step," said Wolf. "Clouds and sea ice are critical factors in determining climate, but the one-dimensional models completely ignore them."

Profound, Chronologically Correllated Deep Time Events on the Surface of the Earth

Secular changes at the Earth's surface; evidence from palaeosols, some sedimentary rocks, and palaeoclimatic perturbations of the Proterozoic Eon

Author:

1. Grant M. Young (a)

Affiliation:

a. Department of Earth Sciences, Western University, London, Ontario, Canada N6A 5B7

Abstract:

Secular changes in surficial processes and products are closely linked to plate tectonics, atmospheric composition, solar evolution and climate. Most siliciclastic sediments and rocks are derived from weathering profiles rather than directly from older rocks. Many palaeosols older than ~ 2.2 Ga show depletion in Fe(T), whereas in younger palaeosols, and modern soils, Fe is virtually immobile. This is thought to reflect the accumulation of free oxygen in the Earth's atmosphere, as is the ‘disappearance’ of iron formations after about 1.8 Ga. The temporal distribution of mature siliciclastic sedimentary rocks containing detrital pyrite and uraninite, and the subsequent appearance of fluvial and shallow marine red beds provide compelling evidence for oxygenation in early Palaeoproterozoic times. During periods of supercontinentality, especially at low palaeolatitudes, enhanced weathering of exposed rock surfaces led to production of thick, extensive quartzarenites, some of which are associated with glaciogenic rocks. Intense weathering during periods of supercontinentality would have decreased atmospheric CO2, leading to extensive glaciations near the beginning and end of the Proterozoic Eon, and initiating a feedback loop that resulted in strong climatic oscillations until continental break-up brought stability to the climatic regime. It has been proposed that atmospheric oxygenation took place during warm climatic episodes between these glaciations when abundant nutrients were flushed into the oceans, stimulating unprecedented cyanobacterial blooms. Although many other supercontinents have been proposed, these unusual climatic conditions probably reflect rare critical relationships among solar luminosity, atmospheric composition and palaeolatitudinal distribution of continental lithosphere. Reappearance of iron formations associated with some Neoproterozoic glaciations may be explained by hydrothermal activity in semi-isolated rift basins.

Plankton Fossils from 3.4 Billion Years Ago Archean

Spindle-shaped inclusions in 3 billion-year-old rocks are microfossils of plankton that probably inhabited the oceans around the globe during that time, according to an international team of researchers.

"It is surprising to have large, potentially complex fossils that far back," said Christopher H. House, professor of geosciences, Penn State, and lead author.

However, the researchers not only showed that these inclusions in the rocks were biological in origin, but also that they were likely planktonic autotrophs -- free-floating, tiny ocean organisms that produce energy from their environment.

The researchers looked at marine sediment rocks from the Farrel Quartzite in Western Australia. Isotopic analysis using secondary ion mass spectrometry was carried out at UCLA. "Ken (Kenichiro Sugitani, professor, Graduate School of Environmental Studies, Nagoya University, Japan, and a co-author) discovered these unusually shaped microfossils embedded in really old rock," said House.

To determine if these inclusions were actually biological in origin, the researchers looked at 15 different samples of Farrel Quartzite and determined their stable carbon isotope ratios. The percentage of carbon 13 in the microfossils was indicative of material produced by biological processes. They found that the carbon 13 percentage in the background organic matter in the surrounding rock was different from that of the microstructures.

"When considered along with published morphological and chemical studies, these results indicate that the Farrel Quartzite microstructures are bona fide microfossils, and support the interpretation that the spindles were planktonic," the researchers report in the current issue of Geology. The morphological and chemical studies were done by Sugitani and Dorothy Oehler, who is also a co-author and research scientist, Astromaterials Research and Exploration Science Directorate, NASA -- Johnson Space Center.

The spindle-shaped microfossils are from 20 to 60 microns in length, about the size of fine sand and within the size range of today's microplankton.

Stable carbon isotope analysis can determine the biological origin of these microfossils because they used carbon dioxide to create energy and incorporated the carbon into themselves. During this process, the organisms selectively incorporate more carbon 12 than carbon 13 from the available carbon, producing a signature of biological origin.

Oehler notes that the spindles appear to be the same as those found in rocks from the Strelly Pool Formation in Western Australia and the Onverwacht Group in South Africa and Swaziland that are both 3.4 billion years old.

"The existence of these microfossils in diverse locations as far back as 3.4 billion years ago suggests that the oceans probably had life in them for a very extended period of time," said Oehler. "Moreover, this has implications beyond what we have done here, suggesting the evolution of diverse life proceeded quickly."

Supercontinent Cycle in Place at Archean/Proterozoic Boundary

Episodic crustal growth in the southern segment of the Trans-North China Orogen across the Archean-Proterozoic boundary

Authors:

1. Xiao-Long Huang (a)
2. Simon A. Wilde (b)
3. Jun-Wei Zhong (a, c)

Affiliations:

a. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

b. Department of Applied Geology, Curtin University, PO Box U1987, Perth, Western Australia 6845, Australia

c. Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming, 650093, China

Abstract:

The Dengfeng and Taihua complexes are well-exposed Neoarchean to Paleoproterozoic units in the southern segment of the Trans-North China Orogen (TNCO). Zircon U-Pb dating shows that the Dengfeng Complex records two episodes (2568 ± 11 Ma and 2306 ± 6 Ma) of tonalite–trondhjemite–granodiorite (TTG) magmatism. All of the TTG rocks are characterized by high SiO2 (66.7 − 75.4 wt%), Na2O (3.20 − 5.06 wt%) and relatively low MgO (0.40 − 1.78 wt%). The Late Neoarchean TTG gneisses have very low contents of HREE (YbN = 0.69 − 2.75) and Y (1.73 − 7.07 ppm), with moderate [La/Yb]N (24.1 − 53.8) and high Sr/Y (65.1 − 291.3) ratios. The Early Paleoproterozoic TTG gneisses have low contents of HREE (YbN = 2.93 − 6.37) and Y (6.7 − 11.0 ppm), with moderate [La/Yb]N (10.1 − 27.0) and Sr/Y (10.6 − 52.1) ratios. Both suites show pronounced negative Nb-Ta, P and Ti anomalies but positive Sr and Pb anomalies. The Late Neoarchean TTG gneisses all have similar bulk-rock Nd and zircon Hf model ages with mainly positive ɛNd(t), and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole. The early Paleoproterozoic TTG gneisses have extremely variable ɛNd(t) (-6.23 to + 4.23) and heterogeneous zircon ɛHf(t) (-3.3 to + 3.1), which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet. The Taihua Complex in the Xiaoqinling area records three episodes of Early Paleoproterozoic TTG magmatism (2.48 Ga at Caotan, 2.31 Ga at Houjiacun and 2.16 Ga at Bayuan), younger than the Taihua Complex in the Lushan area (2.85 − 2.72 Ga). All rocks have relatively low contents of HREE (YbN = 1.03 − 8.32) and Y (2.84 − 24 ppm), with moderate [La/Yb]N (8.7 − 88.4) and Sr/Y (19.8 − 125.8) ratios, and show negative Ta-Nb and Ti anomalies and positive Sr and Pb anomalies. The Caotan gneisses at 2.48 Ga and the Houjiacun TTG gneisses at 2.31 Ga have low Mg# (0.14 − 0.45), low Cr (< 42 ppm) and Ni contents (1 − 21 ppm), with variable but overall positive ɛNd(t) and ɛHf(t) values, and were derived from the partial melting of thickened lower crust with residual garnet and amphibole. The younger Bayuan TTG gneisses at 2.16 Ga have low SiO2 (57.11 − 64.89 wt%), high MgO (2.64 − 4.62 wt%), Cr (100 − 247 ppm) and Ni (32 − 80 ppm), with negative whole rock ɛNd(t) and zircon ɛHf(t) values, resulted from the partial melting of delaminated lower crust that interacted with peridotitic mantle. The geochronology of the Dengfeng Complex (in the Dengfeng area) and the Taihua Complex (in the Lushan, Xiong’er and Xiaoqinling areas) reveals at least four magmatic episodes in the southern segment of the TNCO from the Late Mesoarchean to Early Paleoproterozoic (2.85 − 2.72 Ga, 2.57 − 2.48 Ga, 2.34 − 2.30 Ga and 2.20 − 2.07 Ga). The rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock ɛNd(t) and zircon ɛHf(t) values, suggesting two episodes of crustal growth formed in a subduction tectonic setting. The magmatic rocks of the third episode consist of both the juvenile and pre-existing crustal materials with variable whole rock ɛNd(t) and zircon ɛHf(t) values, which were generated in a subduction zone during the initial assembly of the NCC within the Columbia supercontinent cycle. The final episode of magmatism lacks juvenile materials with whole rock ɛNd(t) and zircon ɛHf(t) values being consistently negative. These may have resulted from the orogenic collapse. The episodic continental growth recorded in the southern segment of the TNCO was caused by subduction and consequent orogeny, consistent with global supercontinent cycles within the Late Archaean and Early Paleoproterozoic.

Modeling the Archean Ocean Surface Enviroment

Production, preservation, and biological processing of mass-independent sulfur isotope fractionation in the Archean surface environment

Authors:

1. Itay Halevy (a)

Affiliations:

a. Department of Environmental Sciences, Weizmann Institute of Science, Rehovot 76100, Israel

Abstract:

Mass-independent fractionation of sulfur isotopes (S MIF) in Archean and Paleoproterozoic rocks provides strong evidence for an anoxic atmosphere before ∼2,400 Ma. However, the origin of this isotopic anomaly remains unclear, as does the identity of the molecules that carried it from the atmosphere to Earth’s surface. Irrespective of the origin of S MIF, processes in the biogeochemical sulfur cycle modify the primary signal and strongly influence the S MIF preserved and observed in the geological record. Here, a detailed model of the marine sulfur cycle is used to propagate and distribute atmospherically derived S MIF from its delivery to the ocean to its preservation in the sediment. Bulk pyrite in most sediments carries weak S MIF because of microbial reduction of most sulfur compounds to form isotopically homogeneous sulfide. Locally, differential incorporation of sulfur compounds into pyrite leads to preservation of S MIF, which is predicted to be most highly variable in nonmarine and shallow-water settings. The Archean ocean is efficient in diluting primary atmospheric S MIF in the marine pools of sulfate and elemental sulfur with inputs from SO2 and H2S, respectively. Preservation of S MIF with the observed range of magnitudes requires the S MIF production mechanism to be moderately fractionating (±20–40‰). Constraints from the marine sulfur cycle allow that either elemental sulfur or organosulfur compounds (or both) carried S MIF to the surface, with opposite sign to S MIF in SO2 and H2SO4. Optimal progress requires observations from nonmarine and shallow-water environments and experimental constraints on the reaction of photoexcited SO2 with atmospheric hydrocarbons.

Plate Tectonics Active 3.8 Billion Years Ago

Researchers still have much to learn about the volcanism that shaped our planet's early history. New evidence from a team led by Carnegie's Frances Jenner demonstrates that some of the tectonic processes driving volcanic activity, such as those taking place today, were occurring as early as 3.8 billion years ago. Their work is published in Geology.

Upwelling and melting of the Earth's mantle at mid-ocean ridges, as well as the eruption of new magmas on the seafloor, drive the continual production of the oceanic crust. As the oceanic crust moves away from the mid-ocean ridges and cools it becomes denser than the underlying mantle. Over time the majority of this oceanic crust sinks back into the mantle, which can trigger further volcanic eruptions. This process is known as subduction and it takes place at plate boundaries.

Volcanic eruptions that are triggered by subduction of oceanic crust are chemically distinct from those erupting at mid-ocean ridges and oceanic island chains, such as Hawaii. The differences between the chemistry of magmas produced at each of these tectonic settings provide 'geochemical fingerprints' that can be used to try to identify the types of tectonic activity taking place early in the Earth's history.

Previous geochemical studies have used similarities between modern subduction zone magmas and those erupted about 3.8 billion years ago, during the Eoarchean era, to argue that subduction-style tectonic activity was taking place early in the Earth's history. But no one was able to locate any suites of volcanic rocks with compositions comparable to modern mid-ocean ridge or oceanic island magmas that were older than 3 billion years and were also free from contamination by continental crust.

Because of this missing piece of the puzzle, it has been ambiguous whether the subduction-like compositions of volcanic rocks erupted 3.8 billion years ago really were generated at subduction zones, or whether this magmatism should be attributed to other processes taking place early in the Earth's history. Consequently, evidence for subduction-related tectonics earlier than 3 billion years ago has been highly debated in scientific literature.

Jenner and her team collected 3.8 billion-year-old volcanic rocks from Innersuartuut, an island in southwest Greenland, and found the samples have compositions comparable to modern oceanic islands, such as Hawaii.

3.4 Billion Year Old Bacteria Fossils Found in Pilbara, Australia

According to a report in The Washington Post yesterday, scientists analysing Australian rocks have discovered traces of bacteria that lived a record-breaking 3.5 billion years ago – a billion years after the Earth was formed.

Old Dominion University biogeochemist Nora Noffke said the traces of bacteria were the oldest fossils ever described.

“Those are our oldest ancestors," Dr Noffke told a meeting of the Geological Society of America. The Washington Post that unlike dinosaur bones, the newly identified fossils were not petrified body parts.

They are textures on the surfaces of sandstone thought to be sculpted by once-living organisms, Dr Noffke said.

Similar patterns decorate parts of Tunisia’s coast, created by thick mats of bacteria that trap and glue together sand particles. Sand that is stuck to the land beneath the mats and thus protected from erosion can over time turn into rock that can long outlast the living organisms above it, according to The Washington Post article.

The ancient Pilbara region was once shoreline and rocks made from sediment piled up billions of years ago are now exposed and available for examination.

Maud Walsh, a biogeologist at Louisiana State University in Baton Rouge, told The Washington Post that while there were older rocks on Earth, the Pilbara find was the “best-preserved sedimentary rocks we know of”.

“They are the ones most likely to preserve the really tiny structures and chemicals that provide evidence for life," she said.

Last year scientists published the discovery of 3.4 billion-year-old fossils in the Pilbara’s Strelley Pool.

"It’s not just finding this stuff that’s interesting," Alan Decho, a geobiologist at the University of South Carolina’s Arnold School of Public Health, told The Washington Post. "It’s showing that the life had some organisation to it."

Ridges that crisscross the rocks like strands in a spider web hint that primitive bacteria linked up in sprawling networks. Like their modern counterparts, they may have lived in the equivalent of microbial cities that hosted thousands of kinds of bacteria, each specialised for a different task and communicating with the others via chemical signals.

New Neo-Archaen Microbial Evidence

An analysis of sulfide ore deposits from one of the world's richest base-metal mines confirms that oxygen levels were extremely low on Earth 2.7 billion years ago, but also shows that microbes were actively feeding on sulfate in the ocean and influencing seawater chemistry during that geological time period.

The research, reported by a team of Canadian and U.S. scientists in Nature Geoscience, provides new insight into how ancient metal-ore deposits can be used to better understand the chemistry of the ancient oceans – and the early evolution of life.

Sulfate is the second most abundant dissolved ion in the oceans today. It comes from the "rusting" of rocks by atmospheric oxygen, which creates sulfate through chemical reactions with pyrite, the iron sulfide material known as "fool's gold."

The researchers, led by PhD student John Jamieson of the University of Ottawa and Prof. Boswell Wing of McGill, measured the "weight" of sulfur in samples of massive sulfide ore from the Kidd Creek copper-zinc mine in Timmins, Ontario, using a highly sensitive instrument known as a mass spectrometer. The weight is determined by the different amounts of isotopes of sulfur in a sample, and the abundance of different isotopes indicates how much seawater sulfate was incorporated into the massive sulfide ore that formed at the bottom of ancient oceans. That ancient ore is now found on the Earth's surface, and is particularly common in the Canadian shield.

The scientists found that much less sulfate was incorporated into the 2.7 billion-year-old ore at Kidd Creek than is incorporated into similar ore forming at the bottom of oceans today. From these measurements, the researchers were able to model how much sulfate must have been present in the ancient seawater. Their conclusion: sulfate levels were about 350 times lower than in today's ocean. Though they were extremely low, sulfate levels in the ancient ocean still supported an active global population of microbes that use sulfate to gain energy from organic carbon.

"The sulfide ore deposits that we looked at are widespread on Earth, with Canada and Quebec holding the majority of them," says Wing, an associate professor in McGill's Department of Earth and Planetary Science. "We now have a tool for probing when and where these microbes actually came into global prominence."