Pondering the Precambrian #4

NeoProterozoic:

Ediacaran:

From Brazil, Ediacaran vase shaped fossils have been found.  is this the earliest known protist fossils yet?

How are the various frond critters of the Ediacaran related to each other?

How did the Ediacaran critter Dickinsonia grow?

A mixed Cloudinia-Corumbella-Namacalathus assemblage shows increasing ecological complexity over the course of the Ediacaran.

Cryogenian:

Did a freshwater layer exist on the world's oceans and persist after the Snowball Earth episodes like the one in the Cryogenian?

As the Snowball Earth ended, how much oxygen weathering took place and what were the biotic impacts?

Tonian:

There was a huge Andes-like mountain range on the northwest of Rodinia that may have lasted for 100 million years during the Tonian 800 million years ago.

MesoProterozoic:

Wind patterns have been inferred from dunes from Calymmian Brazil.

The diversity of Eukaryote microfossils of Calymmian China is impressive.

PaleoProterozoic:

In Statherian China, there is evidence of a sillicified microbiota from the Dahongyu Formation.

The Sudbury Impact appears to have caused long lived volcanic eruptions during the Orosirian PaleoProterozoic.  Since it was one of 150 impacts within a relatively short period, combined with this above volcanism, it should be no surprise life didn't recover to take a second stab at complexity for a billion years.

The Sudbury Basin continued to have geothermal heat during the Huronian snowball earth.

Can the Rhyacian/Siderian Glaciations (huronian snowball earth) be dated based on subglacial hydrothermal activity?

Beginning in the Siderian, ancient carbon was subsumed into the Earth's mantle.

Did anaerobic oxygenic photosynthesis (read the paper) come about in cyanobacteria prior to modern aerobic photosynthesis?

During the Siderian, Earth had a hazy, methane filled atmosphere.

Did eukaryotes arise during the Siderian?

Archean:

Lenticular organisms from South Africa are related to the Pilbara forms.

Did life arise during the EoArchean WITHOUT using phosphate?

Fossils were found from the Eoarchean 3.77 billion years ago in Quebec, Canada.

How did the crust form?

Hadean:

The Earth probably began with a solid shell for a crust, like Mars.

META:

New branches have been found in Archaea.

Iron eating, methanogen organisms probably kept the Earth warm for its first 2 billion years.

Mineral self assembly was common in the early years of the Earth.

Evidence of "El Nino" Like Cycles From the Siderian PaleoProterozoic Huronian Glaciations

Climatic cycles recorded in glacially influenced rhythmites of the Gowganda Formation, Huronian Supergroup

Authors:

Howe et al

Abstract:

The Gowganda Formation of the 2.45-2.2 Ga Huronian Supergroup contains glacially-induced, varve-like rhythmites that potentially preserve a detailed record of climatic conditions during the Paleoproterozoic Era. Four rhythmic couplet thickness records were measured at two outcrops near Wharncliffe, Ontario for the purpose of time-series analysis. The couplets, which range from 1 – 32 mm thick, are composed of alternating layers of siltstone and claystone. Time-series analysis of the couplet thickness records using the MTM Toolkit of Mann and Lees (1996) consistently revealed periodicities in the range of 2.2-2.9 couplets per cycle, which is consistent with climatic cycles such as the quasi-biennial oscillation (QBO) and the El Niño Southern Oscillation (ENSO) observed in modern times. This periodicity suggests that the rhythmic couplets represent annual deposits (i.e. varves). Evidence for the presence of cycles at 3.0-4.9 couplets, 6.6-6.9 couplets, 8.8-9.2 couplets, 22.8 couplets, and 30.1-31.0 couplets were also observed in some couplet thickness records; however, the presence of these longer term cycles was inconsistent from site to site.

Tracking Copper Through the Great Oxidation Event

Cu isotopes in marine black shales record the Great Oxidation Event

Authors:

Fru et al

Abstract:

The oxygenation of the atmosphere ∼2.45–2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth’s redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ65CuERM-AE633) in organic carbon-rich shales spanning the period 2.66–2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in 65Cu, along with the preferential removal of 65Cu by iron oxides, left seawater and marine biomass depleted in 65Cu but enriched in 63Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ65Cu values coincides with a shift to negative sedimentary δ56Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ65Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Did the Oceans get Arsenic Poisoning at the end of the Huronian Glaciations/Siderian PaleoProterozoic Snowball Earth?

By examining rocks at the bottom of ancient oceans, an international group of researchers have revealed that arsenic concentrations in the oceans have varied greatly over time. But also that in the very early oceans, arsenic co-varied with the rise of atmospheric oxygen and coincided with the coming and going of global glaciations. The study was recently published in the Nature Group Journal, Scientific Reports.

"In the article we argue that when we first see the appearance of complex life on Earth, is when life have developed mechanisms to resist catastrophic chemical changes forced by global glaciations. And that this enabled the expansion of complex life in oceans, and paved the way for our own evolution", says Dr Ernest Chi Fru of Stockholm University, who has led the research group.

The first appearance of oxygen in the atmosphere occurred at a time when marine arsenic concentrations were dramatically low, at about after 2.45 billion years ago. This is also a period when Earth experienced its first known global glaciation. At the end of these glaciations, considerable rise in marine arsenic concentrations concurred with rapid demise of atmospheric oxygen.

The authors infer -- from the way modern photosynthetic organisms react to changing marine arsenic concentrations -- that this event was due to widespread ocean toxicity resulting from the release of toxic elements into the oceans when the ice melted.

link.

Great Oxygenation Event had a Precedessor 100 Million Years Earlier

Earth scientists from the University of Alberta, University of Waterloo, Arizona State University, University of California Riverside, and Georgia Institute of Technology have found evidence that Earth's transition to a permanently oxygenated atmosphere was anything but smooth.

Their paper, published this month in Science Advances, uses geochemical data from sedimentary rocks in Western Australia to show that a burst of O2 production by photosynthetic cyanobacteria temporarily increased O2 concentrations in Earth's atmosphere and shallow oceans roughly 2.5 billion years ago.

"We are tracking atmospheric changes through time to understand how oxygen increased to the level needed to support complex life," says Rob Creaser, professor of earth and atmospheric sciences at the University of Alberta. "When the Earth first formed, there was no oxygen in the atmosphere. Our analytical facilities here at the U of A allowed us to conduct precise analyses of this rock sample to understand the tempo at which that oxygen built up through photosynthesis.

Creaser's lab at the University of Alberta in the Canadian Centre for Isotopic Microanalysis is one of only a few in the world with the ability to take the precise measurements of osmium needed to conduct this type of analysis. "Without this type of facility, we wouldn't be able to write this paper or investigate this process."

The new data suggest that O2 levels in the Earth's atmosphere fluctuated until enough O2 finally accumulated to create a permanently oxygenated atmosphere around 2.4 billion years ago, a transition widely known as the "Great Oxidation Event." "The onset of Earth's surface oxygenation may have been a complex process characterized by multiple 'whiffs' of O2 until a tipping point was crossed," says Creaser's former PhD student and UAlberta alumnus Brian Kendall, a professor of Earth and Environmental Sciences at the University of Waterloo and lead author on the paper.

link.

2nd link.

Siderian/Rhyacian PaleoProterozoic Environment of Western Australia Across the Great Oxygenation Event Through Huronian Glaciations

Sedimentology, chemostratigraphy, and stromatolites of lower Paleoproterozoic carbonates, Turee Creek Group, Western Australia

Authors:

Martindale et al

Abstract:

The ca. 2.45–2.22 Ga Turee Creek Group, Western Australia, contains carbonate-rich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2–2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation.

Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn- and post-depositional remineralization of organic matter.

In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate–siliciclastic shelf. These carbonates exhibit moderate δ13C values of −2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

The Paleo Recipe for Complex Life on Earth

Let's step back through deep time to consider an intriguing event. One which has been written about, but far from sufficiently and its existence could have some pretty profound implications about Earth and life's history.

 

Looking back to the Precambrian.  

The Earth is racked by global glaciations.  Ones which have been called the Snowball Earth.  They stretch over the entire planet from the poles through the equator.  There is some dispute whether or not the oceans were completely frozen over (a slushball earth vs snowball), but the glaciations are acknowledged as real.

The monstrous glaciation is understood to have been triggered by a sudden drop in the atmospheric carbon dioxide levels.  This, in turn, is believed to have been caused by photosynthetic organisms drawing down the CO2 levels while spiking the oxygen levels (relatively speaking).  Yes, the snowball earth events are, like in the Eocene's Azolla Event, examples of biogenic climate change.

Within 100 million years of the end of the Snowball Earth complex life arose.  Except it would vanish from the fossil record, probably having gone extinct and changing the biological fate of the Earth.

Wait.  You thought I was talking about the Cryogenian and Ediacaran?

After all, the Cryogenian's Marinoan Glaciation (or maybe Sturtian or Kaigas), NeoProterozoic Oxygenation Event, Ediacaran with its biota and then Cambrian do parallel all of the above.  Except that complex life is obviously still around and the Phanerozoic is quite biologically diverse, to say the least.

But, no, despite the parallels, I am not talking about the rise of modern complex life.  Rather I am talking about events which took place more than a billion years earlier, during the PaleoProterozoic.

Oxygen, Glaciers and Complex Life, oh my!

There has been up talk of the Great Oxygenation Event.  This is when life puped out oxygen in sufficiently large quantities oxygen were poured into the atmosphere by photosynthetic life, namely something like the cyanobacteria.  This may have been one of the first mass extinctions because obligate anaerobes cannot survive in the presence of oxygen, for one, and for the second, it radically changed the climate.

By pulling down the carbon dioxide and releasing free oxygen, the cyanobacteria tipped the earth from a greenhouse climate to an icehouse climate.  This in turn, without the 'modern' feedback loops, turned into what is suspected to be the earliest Snowball Earth, during the Siderian and Rhyacian.  This glaciation, called the Huronian, lasted (as far as we can tell) for 300 million years, from the mid Siderian to about 50 million years before the end of the Rhyacian. 

In 2010, a remarkable find was brought to light from Gabon, Africa.  There appeared to be colonial, or even possibly true complex life in the Rhyacian.  Further study of the fossils strongly suggested they were some form of complex life.  However, strangely, not like our complex life which seems to have arisen piecemeal during the Cryogenian into the Ediacaran and exploded during the Cambrian.  The timing of the appearance of the Francevillian Biota

Reflections in the Mirror: Parallel Events, Evolution? | ?noitulovE, stnevE lellaraP

The Francevillian Biota has several biotic members.  Its easy to see some physical parallels between the Ediacaran Biota members and those above.  There are forms which are reminicent but not the same as the Edaicaran discoid, Dickinsonia and rangeomorph fossils.  They are not the same, by any means, but the suggested parallels are interesting.  No, I am not saying the Ediacaran forms evolved form the Rhyacian!  Quite the contrary!  Rather, I am suggesting there is an event of parallel evolution taking place.  Independently.  Form follows function.  However, this is more suggestively so than closely (unlike the classic of sharks, ichthyosaurs and dolphins), but what's even better of a parallel is the events which led up to the both the evolution of the Francevillian and Ediacaran Biotas.

The events of the two different time frames, Siderian/Rhyacian & Cryogenian/Ediacaran (global glaciations, O2 spike and the seeming rise of complex life) is pretty exciting.  Is this what it would take to have Earth-evolved life to produce complex life forms?  Perhaps.  Its worth looking into.  However, the huge question becomes, if complex life evolved 2.05 billion years why is there a hiatus for over a billion years.  Those years sometimes being called the Boring Billion (more properly, the MesoProterozoic).  During that time frame, we have evidence of microbial life, but nothing like an animal or plant.  So?  What happened?  It must have gone extinct.  There are two possible explanations, assuming this is in fact complex life in the Francevillian Biota.

One is the Francevillian Biota didn't produce a proper Gaia-like feedback system and they tipped over into a feedback loop which caused them to ruin their own environment.  Or rather they ran out of their nutrients.  Or failed to develop a self sustaining ecosystem.  Its a biological failure then.  Producing Gaia is hard.  At best its 50-50 proposition of true.

The second possible reason is even more compelling.  Why?  Because of the time and energy involved.  27 million years after the Francevillian fossils were deposited, one of the biggest asteroid impacts the Earth has seen since the Late Heavy Bombardment took place.  And 150 million years after that, a comparable asteroid smashed into the world.  Both of these impacts dwarf the Chicxulub Crater.

Say Hello to my Little Friend.  Ahem, BFG.  Twice.

It has already been established giant impacts can cause mass extinctions.  The impact in Chicxulub, Mexico 66 million years ago, wiped out 50% of life on the planet.  The KT or K-Pg Extinction made way for our modern ecology and our own evolution.  The Chicxulub impact was the equivalent of over 100 teratons of explosives, roughly over 1000 times the combined nuclear arsenals on Earth.  There are two impacts which followed the Francevillian Biota which were significantly more powerful.

The Vredefort Impact took place 'first.'   The astrobleme for Vredefort is in southern Africa and dates to 2.023 billion years ago.  The crater it left was 300 km in diameter: Chicxulub is 'only' 180 km in diameter.  The impact would have been significantly more energetic than Chicxulub.  This is guaranteed mass extinction.  In fact, its a significantly more devastating mass extinction because the Francevillian Biota is almost assuredly younger and less robust than when our lineage dating from the Cryogenian took it in the shorts from Chicxulub.  Had Chicxulub happened before the Cambrian Exoplosion, its probable there wouldn't be anything remotely complex on the planet.  Vredefort was worse.

As if Vredefort was not enough, 150 million years later, circa 1.849 billion years ago, came the Sudbury Impact.  It was of a comparable energy and bollide size. If any thing complex survived the Vredefort, which would be doubtful, the Sudbury almost assuredly finished them off.  The two impacts, btw, bracket the Orosirian Period.

Interestingly, a paper which just came out supports the scenario.  Although the authors are studying about two different impacts at an earlier time, they found geological evidence of the upper sea boiling from the devastation wrought.  The Sudbury and the Vredefort Impacts both have the potential to done the exact same thing.  Given the Francevillian Biota was a shallow water assemblage, its entirely likely they were wiped out in one or both of the events.

The surviving largely microbial life would trundle on in a relatively stable environment for another billion years thence.

Every Earth Deserves a Second Chance

Assuming the Francevillian Biota was, in fact, complex life, Earth would need another billion years until the Ediacaran to take another stab at complex life.  This time it would be successful and continue into the present, despite mass extinctions.  Fortunately, there have been no bollides of comparable mass or energy as the Vredefort or Sudbury since.  Their descendents are us.

The hypothesis we had extremely distant cousins makes for some interesting conjectures.

• Under the right stimulus, it seems complex life arises when working with earth evolved organisms.  However, it does seem to require those proper chain of events.  

• Additionally, as soon as life had a chance, it also appears to (inadvertently) triggered the events which led up to complex life.  This, in turn, suggests complex life may be common where it is possible to evolve.  

• However, it is also possible to wipe out complex life without requiring too crazy of events (or intelligent intervention).

• It seems likely Mars will definitely not have any traces of complex life.  If the Earth was still getting battered at 1.85 billion years ago, that is well past the point when Mars' atmosphere thinned.  Additionally, the Hellas Basin is an impact crater which is 2,300 km in diameter.  If Vredefort was tough, then Hellas was hell and the likelihood of anything surviving through that impact is almost nil.  The Hellas Impact took place ~3.8 billion years ago.  For that matter, the Argyre Planitia is also another impact basin and it is 1,800 km in diameter, if not as deep as the Hellas.  Its highly unlikely any life survived past these events.  There's a slim chance Earth could have reintroduced life, but by the time the Francevillian Biota arose, Mars had something akin to its present atmosphere.

The story outlined above, the hypothesis which I've hung out there in my first paleo post in some time, hangs together quite well in view of the science.  However, it needs to be tested.  We can only test the hypothesis by finding other locales similar to Franceville.  Likewise, a lot of detailed examination of the Francevillian Biota as known is needed. Likewise, deposits from after Vredefort and in similar paleoenvironmental conditions are needed from the Orosirian.  

Then what's needed is to understand why the MesoProterozoic didn't have any glaciations and, thus, did not produce the complex life as we (squintingly) know it earlier.

Oxygen-17 Evidence of the PaleoClimate of Snowball Earths Past

Revealing the climate of snowball Earth from Δ17O systematics of hydrothermal rocks

Authors:

Herawartz et al

Abstract:

The oxygen isotopic composition of hydrothermally altered rocks partly originates from the interacting fluid. We use the triple oxygen isotope composition (17O/16O, 18O/16O) of Proterozoic rocks to reconstruct the 18O/16O ratio of ancient meteoric waters. Some of these waters have originated from snowball Earth glaciers and thus give insight into the climate and hydrology of these critical intervals in Earth history. For a Paleoproterozoic [∼2.3–2.4 gigayears ago (Ga)] snowball Earth, δ18O = −43 ± 3‰ is estimated for pristine meteoric waters that precipitated at low paleo-latitudes (≤35°N). Today, such low 18O/16O values are only observed in central Antarctica, where long distillation trajectories in combination with low condensation temperatures promote extreme 18O depletion. For a Neoproterozoic (∼0.6–0.7 Ga) snowball Earth, higher meltwater δ18O estimates of −21 ± 3‰ imply less extreme climate conditions at similar paleo-latitudes (≤35°N). Both estimates are single snapshots of ancient water samples and may not represent peak snowball Earth conditions. We demonstrate how 17O/16O measurements provide information beyond traditional 18O/16O measurements, even though all fractionation processes are purely mass dependent.

How Were the Siderian PaleoProterozoic Banded Iron Formations Deposited?

Seafloor silicification and hardground development during deposition of 2.5 Ga banded iron formations

Authors:

Rasmussen et al

Abstract:

Banded iron formations (BIFs) are important archives of the ancient oceans, atmosphere, and biosphere, but fundamental questions remain about their origin. It is widely assumed that BIFs were derived from layers of ferric oxyhydroxides and silica that precipitated directly from a water column that was enriched in dissolved iron and silica. The reported lack of current-generated structures and clastic particles beyond mud grade, and the perceived basin-scale extent of laminae, is regarded as evidence for uninterrupted pelagic settling with no sedimentary reworking. New sedimentological and petrographic results show that laminated cherts in the 2.5 Ga Dales Gorge Member of the Brockman Iron Formation, Western Australia, preserve textures indicative of in situ brecciation immediately below the seafloor and the deposition of intraformational sandstones composed of chert clasts in a chert matrix. Chert intraclasts have two sedimentary components: silt-sized microgranules and submicron-sized particles, indicating that the original sediment comprised iron-rich silicate muds that were cemented on or just below the seafloor by pore-filling silica. Silicified muds were episodically eroded by density currents, and the resulting detritus was transported as sand-sized clasts and locally deposited in a matrix of microgranules and mud. Our results support the hypothesis that high concentrations of silica in early Precambrian seawater favored episodic silica cementation of sediments on the seafloor. We suggest that competition between sediment accumulation and seafloor silica cementation, with subsequent differential compaction, explains primary layering in BIFs between beds of relatively thickly laminated chert and beds of thinly laminated, iron-rich minerals. The thickest laminated chert beds are interpreted to represent intervals when seafloor silicification outpaced deposition of hydrothermal muds, forming the equivalent of Phanerozoic hardgrounds at sequence boundaries.

Evidence of a Large Asteroid/Comet Impact During the Siderian PaleoProterozoic

First detection of extraterrestrial material in ca. 2.49 Ga impact spherule layer in Kuruman Iron Formation, South Africa

Authors:

Simonson et al

Abstract:

Thin layers rich in formerly molten spherules interpreted as distal ejecta from large impacts by extraterrestrial bodies have been found in 8 stratigraphic units deposited between ca. 2.63 Ga and 2.49 Ga and attributed to a minimum of 4 separate impacts. Here we report geochemical evidence of extraterrestrial material in the only one of these spherule layers where it has not been previously reported, the Kuruman spherule layer (KSL) in the Kuruman Iron Formation, a banded iron formation (BIF) in the Griqualand West Basin (South Africa). We identified the KSL in 3 drill cores separated by as much as ~350 km and analyzed 2 core samples that have a mean Ir concentration of ~12.9 ppb and nearly chondritic interelement ratios of platinum group elements Ir, Ru, Pt, and Rh. This suggests that the samples contain ~1%–3% by mass extraterrestrial material even though the spherules are highly diluted by ambient sediment. Our geochemical data strongly support the correlation of the KSL with the Dales Gorge spherule layer (DGSL) in a penecontemporaneous BIF in the Hamersley Basin (Western Australia). The KSL and DGSL are close matches in terms of major and various trace element contents and the DGSL has a comparable Ir content of ~11.5 ppb. Therefore it is very likely the KSL and DGSL are distal ejecta from a ca. 2.49 Ga impact by a single extraterrestrial object greater than 10 km across. The lack of any significant changes in the stratigraphic succession in either basin also implies that large impacts alone are not sufficient to cause long-term changes in Earth's surface environments.

Evidence of Tectonic Uplift From Siderian/Rhyacian PaleoProterozoic China

Palaeopedogenesis of red palaeosols in Yunnan Plateau, southwestern China: Pedogenical, geochemical and mineralogical evidences and palaeoenvironmental implication

Authors:

Lu et al

Abstract:

Red palaeosol, as an important archive of ancient pedoenvironments, effectively reflects the palaeoenvironmental change and tectonic uplift history of the plateau. Four red palaeosol profiles were collected at 2200 to 2400 m elevations from the Yunnan Plateau (YP) to investigate the pedogenic features, chemical weathering process and mineral assemblages of soils. Pedological, geochemical and mineralogical techniques were used to understand the palaeopedogenic, palaeoenvironmental change and landscape evolution of the Plateau. Pedologically, the red palaeosols were characterized by the dark red color (a hue of 5YR or redder), strong acidity (pH < 5.4), and high free iron oxide (> 50 g kg− 1) and clay (> 50%) contents. The red palaeosols were enriched with Fe2O3, Al2O3, and TiO2 with very low contents of CaO, MgO, Na2O, and K2O and slightly low contents of Si2O and MnO. Chemical weathering indices, such as CIA (CIA = [Al2O3 / (Al2O3 + CaO + Na2O + K2O)] × 100), Sa (Sa = SiO2 / Al2O3), Saf (Saf = SiO2 / Al2O3 + Fe2O3), and A–CN–K diagram (Al2O3–CaO + Na2O–K2O), indicated that the red palaeosols had experienced strong chemical weathering processes. Scanning electron microscopy analyses revealed that quartz grains of the red palaeosols were characterized by a number of deep dissolution pits and cracks on their surfaces. The clay minerals of the red palaeosols were composed of kaolinite, gibbsite, hematite, and vermiculite in the order of abundance. Pedogenical, geochemical and clay mineralogical evidences revealed that the red palaeosols were developed by palaeopedogenic processes under tropical climatic environments. The presence of highly weathered soils at an altitude of 2200–2400 m indicated the influence of tectonic uplift on the soil vertical distribution. It is estimated that the red palaeosols had uplifted about 1600–2000 m since their initial formation. Palaeopedogenesis of red palaeosols provides new insight into the palaeoclimatic and palaeoenvironmental reconstruct and tectonic movement of YP.

A Continguous (!?!!!) Section Across the Great Oxidation Event From Siderian/Rhyacian PaleoProterozoic Australia

Sedimentology of the Paleoproterozoic Kungarra Formation, Turee Creek Group, Western Australia: A conformable record of the transition from early to modern Earth

Authors:

Van Kranendonk et al

Abstract:

This paper presents the first, detailed sedimentological analysis of the Paleoproterozoic Kungarra Formation, the lowermost of three formations comprising the Turee Creek Group in Western Australia, which was deposited across the rise in atmospheric oxygen (the Great Oxidation Event, or GOE) and the transition from early to modern Earth.

The data show that the Kungarra Formation has a gradational, conformable lower contact with underlying banded iron-formation of the Hamersley Group and predominantly comprises an upward-shallowing succession from deepwater shales and siltstones, through rippled fine-grained sandstones and stromatolitic carbonates, to tidal flat deposits that immediately underlie coastal–fluvial deposits of the overlying Koolbye Formation.

At the base of the Kungarra Formation is a gradual transition from alternating units of magnetic green shale and thin units of banded iron-formation that pass upsection to units of non-magnetic shale and ferruginous chert and grey chert, reflecting a gradual loss of iron from the world's oceans accompanying the rise of atmospheric oxygen. A falling stage systems tract is recognised above this transition in the Hardey Syncline area, capped by stromatolitic carbonates and a period of exposure marked by an erosional unconformity and carbonate beachrock. Two glacio-eustatic cycles are recognised within the middle to upper parts of the Kungarra Formation, each of which is marked by the rapid onset of falling systems tracts and characterised by falling systems tracts during and following diamictite deposition.

Stratigraphic data are used to infer a depobasin filled by a sediment wedge prograding from southeast to northwest, in contrast to previous models of a north-northeastward deepening foreland basin. The lack of seismites or internal unconformities within the formation precludes a foredeep setting. Rather, deposition is interpreted as having occurred within an intracratonic basin, with detritus sourced from erosion of uplifted bedrock to the southeast.

Evidence of Glaciations, Possibly Huronian, From PaleoProtoerozoic India

Carbon and oxygen isotope systematics of a Paleoproterozoic cap-carbonate sequence from the Sausar Group, Central India

Authors:

Mohanty et al

Abstract:

Well-preserved sedimentological features of the Paleoproterozoic age Sausar Group of central India indicate glaciogenic origin for the diamictite in the lower part of the unit. Deglaciation was responsible for a marine transgression and the deposition of proximal glaciomarine facies and a cap-carbonate, followed by fine clastics with manganese ore. Analyses of δ13C and δ18O contents in the carbonate unit overlying the diamictite, together with Sr and Ba contents and REE data, indicate the preservation of primary geochemical signatures. The average δ13Ccarb content ranges between − 3.1‰ and + 0.1‰ V-PDB, having peak negative δ13C excursions of up to − 7.4‰ V-PDB, similar to Paleoproterozoic and Neoproterozoic cap-carbonates elsewhere. High Sr and Ba contents indicate the presence of aragonite and barite precursors, similar to well-studied Neoproterozoic cap-carbonates. A single continuous carbonate unit shows δ13C excursions from − 4.4 to + 2.6‰ V-PDB. The lithological association and chemostratigraphy are comparable with the carbonate–tillite association of the Huronian Supergroup of Canada, the Snowy Pass Supergroup of the USA, the Transvaal Supergroup of South Africa, and the Turee Creek Group of Australia.

Evidence of Minas Accretionary Orogeny From Siderian PaleoProterozoic

A juvenile accretion episode (2.35-2.32 Ga) in the Mineiro belt and its role to the Minas accretionary orogeny: zircon U-Pb-Hf and geochemical evidences

Authors:

Teixeira et al

Abstract:

Zircon U-Pb-Hf and geochemical data provide new clues for a juvenile TTG segment in the Mineiro belt, outlined by published information about the nearby 2.35 Ga Lagoa Dourada suite. The Resende Costa Orthogneiss and coeval rocks yield U-Pb crystallization ages from 2351 ± 48 to 2317 ± 16 Ma and coherent Sm-Nd TDM ages between 2.3 and 2.5 Ga. These rocks show ɛNd(2.35Ga) and ɛSr(2.35Ga) values that suggest derivation from a short-lived, slightly depleted source akin to products of island arcs, whereas the geochemistry is consistent with a tholeiitic source with minor crustal assimilation. The positive to negative zircon ɛHf(2.35Ga) suggest the subordinate involvement of crustal components during the hypothesized c. 2.35 Ga slab subduction event. In a similar manner, metabasalts (lower unit) of the adjoining Congonhas-Itaverava belt show Nd-Sr isotopic and chemical constraints indicating derivation from an early Paleoproterozoic tholeiitic source subjected to crustal assimilation. A coeval metagraywacke from the upper unit yields U-Pb zircon detrital ages as young as 2349 + 14 Ma suggesting the maximum depositional age for the precursor basin (foreland setting?) whilst most of the grains indicate contribution from nearby Archean sources. The Resende Costa-Lagoa Dourada rocks characterize the precocious arc magmatism of the Minas accretionary orogeny. This composite orogeny created the Mineiro belt and the adjoining Mantiqueira and Juiz de Fora belts in the 2.35-2.00 Ga interval. Due to such a polycyclic framework the Resende Costa Orthogneiss and coeval rocks show multiple metamorphic overprints dated at 2140 and 2050 Ma. The new and compiled data and regional geologic correlations provide evidence for a growing Paleoproterozoic landmass in the South America continent.

Fossil Evidence of Iron Oxidizing Bacteria From the Great Oxidation Event

Fossil evidence of iron-oxidizing chemolithotrophy linked to phosphogenesis in the wake of the Great Oxidation Event

Authors:

Crosby et al

Abstract:

The oxygenation of Earth's atmosphere allowed for the diversification of metabolisms to include those that rely on oxygen and its derivatives. For example, chemolithotrophic oxidation of sulfide and iron both require oxygen or nitrate as terminal electron acceptors. A growing number of oxygen-utilizing chemolithotrophs are known to accumulate intracellular polyphosphate as an energy reserve that allows them to adapt to the fluctuating redox conditions in their distinctive-gradient habitats. Polyphosphate is also thought to play an important role in the formation of phosphatic mineral deposits. Here we present fossil evidence of iron-oxidizing bacteria preserved as filamentous iron oxides within phosphatic Paleoproterozoic stromatolites. The filaments include twisted stalks similar to those produced by modern iron-oxidizing bacteria that are known to metabolize polyphosphate and inhabit steep redox gradients. Fossil iron-oxidizing bacteria preserved within some of the oldest known phosphorites serve as indicators of O2-Fe(II) gradients that may have supported microbially mediated phosphogenesis via polyphosphate metabolism and/or an active iron redox pump.

Plate Tectonics did NOT Shutdown During the Siderian PaleoProterozoic

Paleoproterozoic arc magmatism in the North China Craton: No Siderian global plate tectonics shutdown

Authors:

Yang et al

Abstract:

Arc magmatism in convergent plate margins has been a major contributor to continental growth. Following arc-arc and arc-continent collision in the Archean leading to the amalgamation of micro-blocks, the North China Craton (NCC) witnessed major pulses of continental arc magmatism during the Paleoproterozoic. In this study, we present geochemistry, zircon U-Pb geochronology and Lu-Hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major Paleoproterozoic suture zones in the NCC – the Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO). Our zircon U-Pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207Pb/206Pb weighted mean ages of 2410 ± 41 Ma for metagranite, 2480 ± 12 Ma, 2125 ± 18 Ma, 1946 ± 8 Ma, 1900 ± 15 Ma and 1879 ± 12 Ma from metagabbros, 2446 ± 11 Ma from charnockite, and 1904 ± 6 Ma and 1901 ± 9 Ma from metatuffs. The 207Pb/206Pb upper intercept age of zircons in the khondalite shows 2102 ± 76 Ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros. The khondalites also carry a group of concordant metamorphic zircons with 207Pb/206Pb mean age of 1881 ± 20 Ma. Metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890 ± 14 Ma and 1852 ± 19 Ma respectively. The age data suggest prolonged arc magmatism in a convergent margin setting during ca. 2.48 to 1.9 Ga, followed by metamorphism at ca. 1.89-1.85 Ga associated with the final collision. Lu-Hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by dominantly positive εHf values (-1.9 to 6.8; mean 1.8). The εHf and TDMC data suggest that the magmas were mostly derived from Neoarchean and Paleoproterozoic juvenile components. The salient geochemical features of these rocks attest to magma generation from heterogeneous source involving subduction–derived arc components with minor input from continental crust. The results presented in this study, together with those from previous investigations in different domains of the IMSZ and TNCO suggest major Paleoproterozoic arc magmatic events in the NCC lasting for nearly 600 million years associated with the final assembly of the crustal blocks into a coherent craton. Construction of the final cratonic architecture of the NCC thus witnessed not only the arc-continent amalgamations at 2.7-2.5 Ga, but also major crust building events in the Paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along active convergent margin, followed by intense deformation and metamorphism during the final collision at 1.85–1.80 Ga. The prominent Paleoproterozoic magmatic records in the NCC do not support the proposal of global plate tectonics shut down in the Siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the Paleoproterozoic.

Evidence From China PaleoProterozoic Plate Tectonics did NOT Shutdown

Early Paleoproterozoic (2.45–2.20 Ga) magmatic activity during the period of global magmatic shutdown: Implications for the crustal evolution of the southern North China Craton

Authors:

Diwu et al

Abstract:

A global database of zircon ages from both granitoids and detrital sediments shows an exceptionally and robust ages gap between 2.45 and 2.20 Ga. The early Paleoproterozoic magmatism dramatically decreased on the Earth, which was proposed to relate to the global plate tectonic shutdown. However, the available data indicate that 2.45–2.20 Ga magmatic rocks are widespread in the Taihua Complex during the quiet interval. The Taihua Complex is located in both the Lushan and Xiaoqinling areas along the southern segment North China Craton (NCC). The Complex is composed mainly of a gneiss series and the Khondalite-dominated supracrustal rock. The latter, occurs as a linear structural belt and was named the Khondalite Belt. The gneiss series in the Lushan area has an age of 2.85–2.72 Ga, whereas the Xiaoqinling region contains widespread tonalitic–trondhjemitic–granodioritic gneisses with ages of 2.45–2.20 Ga. Significant variation of Hf isotopes in zircons and whole-rock ɛNd(t) values suggest that these rocks were produced by variable mixing of a juvenile materials with older crust in an Andean-type continental margin arc or island arc setting. The depositional age of the sedimentary protoliths of the khondalite series can roughly constrained to between 2300 and 1970 Ma. It has been considered that they were deposited on a stable continental margin environment. The U–Pb dating of metamorphic zircons from the Taihua Complex suggest that the southern NCC underwent a polyphase tectonic evolution during the period 1.97–1.82 Ga, the peak metamorphism coeval with crustal thickening has occurred at ∼1.94 Ga.

Crustal Formation in the NeoArchean and PaleoProterozoic of Tanzania

Neoarchean and Paleoproterozoic crust formation in the Ubendian Belt of Tanzania: Insights from zircon geochronology and geochemistry

Authors:

Kazimoto et al

Abstract:

LA-ICP-MS U-Pb zircon geochronological and geochemical data of meta-igneous and metasedimentary rock types of the Katuma Block of the Paleoproterozoic Ubendian Belt in Tanzania are used to unravel the crustal evolution of this metalliferous terrain. The protoliths of the metabasites and orthogneisses previously considered to be Paleoproterozoic are in fact mostly Neoarchean in age (2713 ± 11 Ma to 2638 ± 5 Ma), from which the oldest rocks experienced their first metamorphism during the same Neoarchean orogenic cycle at ca. 2650 Ma. A second event of mafic magmatism (2021 ± 11 Ma) was concomitant with the migmatization of the Neoarchean orthogneisses and was succeeded by granitic intrusions at 1990–1940 Ma. All rocks of the Katuma Block experienced their main metamorphic reworking during several Paleoproterozoic orogenic events, which were recognized by dating of various metamorphic zircon growth zones and the age of magmatic events dated at ca. 2050, 1960 and 1880 Ma. The detritus of the high-grade metasedimentary rocks derived from Neoarchean (Katuma Block or Tanzania Craton?) and Paleoproterozoic provenances and the minimum age for the deposition is constrained by its first metamorphism at ca. 1960 Ma. The Neoarchean and Paleoproterozoic metabasites, gabbronorites and orthogneisses are sub-alkaline in composition displaying a REE and trace element geochemistry akin to those of rocks formed in modern-arc settings. On the basis of the geochemical data, the presence of eclogites, deformation and metamorphic ages, we suggest that in Paleoproterozoic time the Katuma Block was again at an active continental margin, below which a Paleoproterozoic oceanic lithosphere was subducting.

Evidence of Crustal Growth During Archean/Proterozoic Transition India

Convergent margin processes during Archean–Proterozoic transition in southern India: Geochemistry and zircon U–Pb geochronology of gold-bearing amphibolites, associated metagabbros, and TTG gneisses from Nilambur

Authors:

Shaji et al

Abstract:

The northern domain of the Southern Granulite Terrane (SGT) in India comprises a collage of Mesoarchean to Neoarchean crustal blocks which preserve important imprints of crustal growth during the early history of the Earth. Here we investigate the zircon U–Pb geochronology and geochemistry of a suite of amphibolites, metagabbros, TTG gneisses and charnockites from the Nilambur region, which forms part of the Wynad Gold Belt in the SGT. Magmatic zircons from three amphibolites yield weighted mean 207Pb/206Pb ages of 2661 ± 59 Ma, 2499 ± 19 Ma, 2570 ± 19 Ma and 2542 ± 49 Ma, closely followed by metamorphism at ca. 2.45 Ga. Zircons from the TTG gneisses show protolith emplacement ages of 2619 ± 21 Ma and the overgrowth rims define an age of 2524 ± 6 Ma. Zircons from the metagabbro show spot ages between 2576 Ma and 2717 Ma and a weighted mean 206Pb/207Pb age of 2644 ± 30 Ma, with metamorphism at 2503 ± 28 Ma. The U–Pb data suggest prominent magmatic and metamorphic events during the Archean–Proterozoic transition.

Geochemical features of the Nilambur rock suite suggest that the amphibolites, metagabbros, and TTG gneisses could be related to a common basaltic protolith. Their data are consistent with formation in primitive arc magmatic settings with MORB-like components in the source followed by magmatic differentiation. The amphibolites and metagabbros show negative Nb–Ta, Zr–Hf and Ti anomalies, typical of subduction-related intraoceanic tholeiitic arc basalt. Their low Th/Ce ratios (less than 0 .1 ) preclude any significant contribution from subducted sediments. The rocks are characterized by marked enrichment in LILE and LREE, and show relative depletion of HFSE. Most of the samples show a flat or slightly LREE enriched patterns, with P, Ti, Th and Nb depletion. The results are consistent with magma derivation from MORB-like mantle wedge, without metasomatism by LILE–LREE-rich fluids derived through the dehydration of the subducted slab. The data obtained in this study, and those from recent studies suggest that the crustal blocks adjacent to the southern margin of the Dharwar Carton in Peninsular India preserve important evidence for active convergent margin tectonics during Archean–Proterozoic transition associated with the generation and emplacement of subduction-related arc magmas and continental growth.

Evidence of the Breakup of Supercontinent Kenorland From Siderian PaleoProterozoic Quebec

The geochemistry and petrogenesis of the Paleoproterozoic du Chef dyke swarm, Québec, Canada

Authors:

Ciborowski et al

Abstract:

The du Chef dyke swarm in southern Québec, Canada is composed of numerous northeast trending, greenschist-amphibolite facies, gabbronoritic dykes that crop out either side of the Grenville Front. The age of the du Chef swarm (2408 ± 3 Ga) has led previous authors to suggest a genetic link between the du Chef dykes and coeval swarms (including the Ringvassøy, Scourie, Widgemooltha and Sebanga) preserved on other Archean cratons. These now disparate dyke swarms are proposed to have formed in response to mantle plume-induced continental breakup during the early Proterozoic. This work represents the first geochemical study of the du Chef dykes and shows that the swarm evolved through fractional crystallisation of a tholeiitic parent magma that remained largely uncontaminated during its residence in, and ascent through, the crust. We also show that the primary magma for the du Chef swarm was derived through partial melting of an enriched region of the mantle, with a similar trace element composition to the modern-day HIMU reservoir and that the magma produced was significantly hotter than the ambient mantle at the time. We contend that the du Chef dykes are the product of early Proterozoic mantle plume magmatism and may help pinpoint an ancient hotspot centre that initiated continental break up along the margin of the Superior Craton at ∼2.4 Ga. Other dyke swarms proposed to be genetically linked with the du Chef dykes record a distinctly different petrogenetic history to that of the du Chef dykes, as evidenced by their more volcanic arc-like geochemical signature. These contrasting geochemical signatures in supposedly cogenetic continental tholeiitic rocks may be evidence of early Proterozoic mantle heterogeneity sampled by the rising du Chef mantle plume.