Pondering the Precambrian #42

Proterozoic:

NeoProterozoic:

Changes to the carbon cycle and environment  were detected from the Ediacaran/Cambrian boundary in China.

Using boron to detect the salinity of the paleooceans gives some interesting twists.

Evolution of multiple basins from the Neoproterozoic are described.

Is there a biochemical signal dividing metazoan clades into two?

Did animal-like embryos predate actual metazoans (animals).

How NOT to become a metazoan.

A Cryogenian interglacial deposit has been found in China.

When did the Sturtian glaciation begin during the Cryogenian in South China?

Mesoproterozoic:

Where was South China in the Nuna/Columbia and Rodinia supercontinents during the Mesoproterozoic?

The Stenian Copper Harbor and Nonesuch Formations have a marine origin.

PaleoProterozoic:

Evidence of explosive basalt volcanism found from the paleoproterozoic.

In French Guiana, a gold rich Rhyacian Period formation has been found.  It contains sedimentary layers within the volcanic ones.  Are they fossiliferous?

After the Lomagundi-Jatuli Event, the carbon-sulfur cycle seemed to return to strong activity in the aftermath in the Rhyacian/Orosirian Periods.

Archean:

It appears during the GOE, the ocean basins remained anoxic.

Earth's magnetic field may have been in place during the Archean.

Evidence of subduction from the Late Archean comes from Australia.

The Kaapvaal Craton may be a remnant of the Pilbara Craton that split off in the MesoArchean.

Extraterrestrial impacts might have triggered bursts in plate tectonics.

Ancient microbes played an important role in warming the early Earth.

Hadean:

The impact history of Earth should NOT be correlated to other planets in the solar system.

Origin of Life:

There is a hyperacidic, hypersaline geothermal system in Dallol, Ethiopia that is lifeless and that has implications for the origin of life.

DNA is only one of several possible genetic information molecules.

Lifelike chemistry has been created in the lab.

Ribose and other sugars have been found in meteorites.

Some of the earliest animal fossils are possibly chemical 'gardens' that produce pseudofossils.

Pondering the Precambrian #41

Proterozoic:

Glauconite is rare is precambrian deposits of the Bhima Basin.  Why?

NeoProterozoic:

There is evidence of the oxygen and other environmental conditions across the Ediacaran/Cambrian boundary from southern China.

Could the marine waters have been highly alkali at the terminal Ediacaran?

Cloudina acted as an anchor for Ediacaran reefs.

Sulfur and carbon cycles from the Ediacaran formations of the Lower Yangtze block.

Oxygen levels during the NeoProterozoic were transient.

There were two episodes of phosogenesis in the ocean water of the Doushantou formation.

The Doushantou and Datangpo formations from the Ediacaran and Cryogenian have been argued to represent evidence of photosynthetic productivity during the Snowball Earth episodes.  This is probably wrong.

There is evidence from India of biofilms from the Neoproterozoic.

The microfossils of the Katanga Supergroup are probably pseudofossils, alas.

Boxonia bearing stromatolites from Mongolia suggest variation in their formation compared to others.

Ediacaran Biota-like disc fossils have been found in the Cryogenian, 80 million years earlier than ever before.

The Cryogenian basin deposits of south China suggest massive hydrothermal activity during the Sturtian Glaciation (Snowball Earth).

What was the stratosphere of the Snowball Earth like?

The Tonian Callana Group has a carbon anomaly that does not match the one observed at Bitter Springs.

MesoProterozoic:

There is evidence of the breakup of the Columbia Supercontinent from Brazil.

PaleoProterozoic:

Kimberlites from the Siderian suggest there is an isolated mantle reservoir.  And may be fingerprints of the ancient makeup of the world's geology.

There was a very long lived tectono-magmatic event 1.8 billion years ago.

The geochronology of the Hart Dolerite of Australia is documented.

Archean:

There is evidence of oceanic crust subduction from the NeoArchean.

There is also evidence of mountain building from the NeoArchean.

There is evidence of a subduction/accretion/collision event at the end of the NeoArchean, confirming full blown plate tectonics were in progress at that time frame.

Mesoarchean deposits from islands suggest the pelagic environment was rich in boron.

Paleoarchean microfossils show a continuous cell wall amongst other characteristics.

Can Xenon help determine if the carbon traces in ancient rocks is biogenic or not?

The fingerprints of Ur, the earliest supercontinent, are in the Bastar Craton in India.

Hadean:

The energy budget and structure of the earth get modeled during and after the Thea impact.

Origin of Life:

Bacterial gene duplicates are more common in eukaryotes than expected.

Is there a relation between hydrothermal chemistry and the origin of cellular life?

META:

Archaea's biochemistry may give hints as to ancient ocean temperature.

A large virus may have helped make the first eukaryote allow endosymbiosis allowing for the evolution of photosynthesis in eukaryotes.

Pondering the Precambrian #40

Proterozoic:

NeoProterozoic:

Palaeopascichnus jiumenensis can be used to determine the Ediacaran/Cambrian boundary.

There's a new way to determine oxygen levels during paleo time periods.  It seems to check out in the Neoproterozoic-Cambrian transition based on comparison to other methods.

A bilateralan metazoan (animal) fossil and trackway was found in China dating from the Ediacaran.

The rise of siliceous sponges helped shift the Ediacaran ecosystems to the modern Cambrian. 

The differences between the Weng'an and Kuanchuanpu biotas is largely due to how they were preserved.

Acritarchs from the Ediacaran Doushantuo Formation allow for biostratigraphic correlation with the Tanarium conoideum–Cavaspina basiconica Assemblage Zone.

There appears to have been long term denudation of the continents from the middle Ediarcaran based on Nd isotope traces in carbonates.

A new formation has been found dating from the Cryogenian with evidence of bacterial and algae life in South China.

How much hydrothermal activity was there during the Marinoan Glaciation (snowball earth episode) during the Cryogenian?

There is evidence of a large igneous province in the Tonian.

MesoProterozoic:

Horodyski moniliformis, regarded by some as a eukaryote fossil from the Calymmian, has been declared a pseudofossil.

PaleoProterozoic:

Plate tectonics have evolved over the last 2.5 billion years.

Is there evidence for the oxygen overshoot hypothesis from the Orosirian?

There was a paleocontinent named 'Atlantica' proposed.  Recent research suggests it was configured rather differently than proposed.

Evidence has arisen supporting a snowball earth scenario during the Huronian glaciation.

An asteroid impact has been IDed from the Rhyacian in Australia.

Did the supercontinental cycle begin in the PaleoProterozoic?

Archean:

There is evidence of a PaleoArchean continent.

Origin of Life:

Prebiotic amino acids can bind to lipid membranes and stabilize them.

When did eukaryotes originate?  Do we have a way to understand that?

Did the lack of nitrogenase limit early life?

Pondering the Precambrian #28

Proterozoic:

NeoProterozoic:

Ediacaran:

A cyanobacteria normally associated with the Phanerozoic has been found in Ediacaran deposits in China.

There was a significant shift in what the limiting nutrients were across the Ediacaran/Cambrian boundary.

How did the black shales form across the Ediacaran/Cambrian boundary?

Was the Sao Francisco Basin a restricted basin during the Ediacaran?

A section in China appears to be a record of the late Ediacaran glaciations, showing a cold, dry climate.

The Cathayasian block was attached to Gondwana since 630 million years ago.

The fate of an inland sea from during the Neoproterozoic gets discussed.

Evidence of the Milkanovich cycles is preserved in Ediacaran deposits.

Tubular fossils from the Weng'an Biota are algae, not metazoans.

Weng'an Biota also provides examples of preserved encysting of eukaryotes.

Cryogenian:

Could the banded iron formations of the Cryogenian be related to ocean acidification?

There is evidence of a less than complete snowball earth from China during the Marinoan Glaciation.

Cryogenian Period deposits of Datangpo Formation of China from the interglacial period between the Sturtian and Marinoan glaciations show a very stratified ocean.

The majority of the time, the NeoProterozoic oceans were anoxic and were supersaturated with dolomite.

Tonian:

Fungus fossils appear to have been found in Tonian (neoproterozoic) or Stenian (mesoproterozoic) deposits in Canada.

There appears to have been a paleorifting event during the Tonian of the Sao Francisco-Congo continent.

The Sognefjel complex appears to have formed in the Asgardian Sea during the early Tonian.

Did the Paleo-South China Ocean close during the Tonian?

MesoProterozoic:

The Eastern European Craton has a Mesoproterozoic signal.

The southern Grenville formation dates from Mesoproterozoic and is from purely Laurentian sources.

Reported stromatolites from a Stenian lake cannot be proven to be biogenic.

Paleoproterozoic:

600 million years of sedimentation is examined from the Paleoproterozoic of Lapland.

Statherian Period evolution of the Oolongbuluke terraine is explained.

In Brazil, the Sobreiro Formation appears to be from the Orosirian/Statherian boundary.

Archean:

There is evidence of retreating oceanic slab subduction from the NeoArchean of China.

Evidence from India suggests there was significant local variation in Archean ocean conditions.

The Caozhuang basin appears to be a case of sagduction.

Fossils from paleoarchean South Africa show bacteria dividing and they appear to be very similar to cyanobacteria Pleurocapsales.

Eoarchean stromatolites get examined.

Hadean:

Was the Earth covered in a lava ocean before Theia impacted and created the moon?

META:

What are the implications of a hotter mantle, but colder subduction during the Precambrian?

Pondering the Precambrian #26

Proterozoic:

NeoProterozoic:

The Earth's magnetic field was 10% of what it is today during the Ediacaran and the Earth's solid core may date from that period.

Evidence from Murmansk supports the weak magnetic field hypothesis during the Proterozoic.

Trace fossils from the Ediacaran have been found in Brittany, France.

Ediacaran environmental changes are recorded in Brazil.

The iodine content of the the Doushanto deposits of the Ediacaran.

Microorganisms are the source of organic carbon found in Sichuan from the Ediacaran into the Cambrian. 

It appears Cloudina and other tubular organisms from the terminal Ediacaran appear to have reproduced asexually.

There is evidence of hydrothermal activity altering various deposits during the Ediacaran.

Evidence of the breakup of Rodinia to the accretion of Gondwana in the Ediacaran from Paraguay.

A new biomarker has been found from the Cryogenian that hints at how complex life evolved after the Snowball Earth.

There was a deep marine organic reservoir in the Cryogenian.

Starting in the Cryogenian until the start of the Carboniferous, there was significant lack of impacts.  There might be evidence of the Snowball Earth via a global wiping of the impact craters from before that point, too.

Could the Great Noncomformity be due to the Snowball Earth?

The Great Noncomformity represents a 200to 300 million year gap in the depositional history of the world according to evidence from the North China Craton.

There are graphite particles in Cryogenian deposits of Nantuo.

Manganese ore deposits in South China were formed in the interglacial between the Sturtian and Marinoan glacials by microbial activity.

Is hydrothermal activity from the Tonian of the Western Australian Craton evidence of the breakup of Rodinia?

Mesoproterozoic:

Eukaryotes diversified earlier than previously thought, starting in the Ectasian.

Paleoproterozoic:

There was a 30 degree shift in the mafic dyke swarms during the Paleoproterozoic.

The surface conditions of at the start of the Great Oxygenation Event were anoxic.

There is evidence from the Yangtze Block that contradicts the hypothesis that there was a shutdown of plate tectonics during the PaleoProterozoic.

Archean:

A coupled crust/mantle formed before 2.5 billion years ago.

Evidence from the MesoArchean to the Paleoproterozoic of Norway show how the continents were built up.

More evidence of episodic crust growth starting in the MesoArchean.

There may be EoArchean deposits in the North China craton.

Sarmatia, Pilbara, and Kaapvaal Cratons were all part of the single supercontinent Vaalbara.

There's no evidence of pre 3.95 billion year old fossils.

Hadean:

Did the impact with Theia provide the volatiles the Earth needed for life?

Did asteroid impacts have a central role in the formation of the original continents?

META:

There is a 600 million year superocean cycle modulating a longer supercontinent.

Could sulfur dioxide have helped with the start of prebiotic carbohydrates?

There are biochemical hints that the last common universal ancestor - the last life form from which everything alive is descended - was not a hyperthermophile.

Reconstruction the Last Eukaryote Common Ancestor's genome to understand the evolution from the First Eukaryote Common Ancestor to the LECA.

Chunks of RNA can be formed prebioticly.

Studying algae suggest eukaryotes have received numerous DNA additions from bacteria.

Pondering the Precambrian #7

Proterozoic:

NeoProterzoic:

The Tarim Craton may have been connected to the North India Block rather than Australia within the supercontinent of Rodinia during the Tonian.

Has evidence of mountain building during the Tonian been found in Scotland?

The Tonian/Cryogenian transition seems to have been located in Svalbard.

How complete is the evidence of glaciations during the Cryogenian in Scotland?

How oxygenated was the ocean between the Snowball Earth episodes?

Caution with carbon-13 shifts are urged in Cryogenian deposits as they are deposit dependent.

Evidence of hydrothermal modification of a dolostone during the Marinoan Glaciations.

Was there a delayed oxygenation of the oceans from the Cryogenian to the Ediacaran?

Evidence from Mongolia's 540 million year old Khesen formation documents the rise of animals.

Evidence that Ediacaran Parvancorina was motile and could sense benthic currents.

Were there constant anoxic bottom waters during the Ediacaran?

Cyanobacterial-algal crusts have been found from Ediacaran paleosols.

MesoProterozoic:

Cyanobacterial microfossils are examined from Calymmian China.

Were tellurium and selenium weathered from sandstones into the Mesoproterozoic oceans?

PaleoProterozoic:

Biosignatures have been found for eukaryotes from Statherian China.

There is evidence of an impact in India during the Siderian.

Was there a subduction cessation during the Siderian in China?

More evidence of warm subduction during the Orosirian from Cameroon.

Was the Great Oxygenation Event really an abiotic process?!

There appears to have been gradual oxygenation in the oceans (in some cases) just prior to the GOE.

A large igneous province is detected from India from the Rhyacian.

Was the early biosphere nutrient limited?

Archean:

Evidence from China, specifically the North China Craton, about the composition of NeoArchean sea water.

Inland lakes appear to have fostered the diversification of microbial life during the MesoArchean.

The oldest known paleosols (fossil soil) date from the PaleoArchean (3.46 billion years ago) and originate in Australia and may have formed from sulfuric acid weathering.

Could there be a problem with the cherts being used from the PaleoArchean to claim microbial life?

Or is there a definitive example of fossil life from the PaleoArchean?  Does that mean life is common in the universe?

Is some of Earth's magma ocean preserved in PaleoArchean rock?

META:

Exploring the Precambrian lithosphere of Zambia and Malawi.

The effect of paleoseawater chemistry on hydrothermal ridges.

Origin of Life:

The origin and diversification of the endosymbiont known as the mitochondria.

A new lineage of eukaryotes has given information on mitochondrial genome reduction.

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.

Using Zinc and Cadmium Isotopes to Determine Post Snowball Earth Environmental Conditions

Zinc and cadmium stable isotopes in the geological record: A case study from the post-snowball Earth Nuccaleena cap dolostone

Authors:

John et al

Abstract:

Zinc and cadmium are important biologically cycled nutrients in the modern ocean. Recent analytical advances and sampling efforts such as GEOTRACES have led to measurements of the global distribution of Zn and Cd and their stable isotope ratios (δ66Zn and δ114Cd), providing an understanding of how biological processes cycle these elements and their isotopes in the modern ocean. This opens the door to better application of δ66Zn and δ114Cd as tracers of biogeochemical processes archived in the geological record. Here we present new measurements of [Cd] and δ114Cd on samples from the Nuccaleena cap dolostone in South Australia, which was deposited during deglaciation of the ca. 635 Ma old Marinoan snowball Earth. Our new data are combined with previously measured δ66Zn data from the same samples. The combined record of δ114Cd and δ66Zn is used to explore various hypotheses about the sources and sinks of Cd and Zn in the post-Marinoan ocean. Key features of this record are that [Zn] and [Cd] remain relatively similar throughout, that δ66Zn and δ114Cd change at the same stratigraphic level and in the same direction, and that δ66Zn changes are about four times larger than δ114Cd changes even though both quantities reflect a similar fractional mass difference. We suggest that both Zn and Cd concentration and isotopic data are consistent with a change in the sink by which Zn and Cd are removed from the oceans. We propose that no isotope fractionation is expressed during removal of Zn and Cd as sulfides, while Zn and Cd removed with biological material are about 0.6‰ and 0.2‰ lighter than deep ocean δ66Zn and δ114Cd, respectively. Many previous studies attribute the burial of isotopically light biological Zn to preferential assimilation of isotopically lighter Zn by phytoplankton. In contrast, we propose that lighter biological δ66Zn occurs mostly because isotopically heavy Zn was scavenged by organic matter, which causes dissolved δ66Zn in the surface ocean to be lighter than deep ocean δ66Zn. The organic matter originated from primary producers that thrived in the surface ocean during deglaciation. For Cd, whose isotope cycling in the modern ocean is dominated by preferential assimilation of isotopically lighter Cd by phytoplankton, much less biological fractionation is expressed. The combined analyses of δ66Zn and δ114Cd provide insight into the biogeochemical processes which occurred after the Marinoan snowball Earth glaciation, and may be a useful new tool to explore the history of life on Earth in other geological records as well.

How to Determine the Origin of Potential Cryogenian Neoproterozoic Dropstones From Death Valley

A diamictite dichotomy: Glacial conveyor belts and olistostromes in the Neoproterozoic of Death Valley, California, USA

Authors:

Le Heron et al

Abstract:

Multiple intercalations of glacially derived and slope-derived diamictites testify to the drawbacks of correlating Neoproterozoic diamictites more widely, but shed new light on the close interrelationship of these processes in the Cryogenian world. In the Neoproterozoic of Death Valley, California (USA), rifting of Rodinia occurred concomitantly with a major glacial event that deposited the Kingston Peak Formation. A new sedimentologic investigation of this formation in the Silurian Hills demonstrates, for the first time, that some diamictites are ultimately of glacial origin. Abundant dropstone textures occur in interstratified heterolithic deposits, with clasts of identical composition (gneiss, schist, granite, metabasite, quartzite) to those of boulder-bearing diamictites suggesting a common source (the glacial conveyor belt). In stark contrast, megaclast-bearing diamictites, yielding clasts of carbonate and siliciclastic preglacial strata as much as 100 m across, are interpreted as olistostromes. The occurrence of syn-sedimentary faults within the succession allows glacial versus slope-derived material to be distinguished for the first time.

There was Significantly Less Sediment Laid Down During the Cryogenian NeoProterozoic (Snowball Earth) Glaciations

Slow net sediment accumulation sets snowball Earth apart from all younger glacial episodes

Authors:

Partin et al

Abstract:

During the Sturtian and Marinoan "snowball Earth" episodes, ice cover is thought to have extended from polar to tropical latitudes. We test the supposition that such an extreme glacial climate, not repeated in the subsequent ∼635 m.y. of Earth history, would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans. With >500 sediment accumulation rates to characterize Sturtian and Marinoan deposits, we find median accumulation rates at least four to 15 times slower than expected for Phanerozoic glaciomarine deposits as characterized by >10,000 rates. Our comparison is conservative with respect to time span, latitude, and distance from the ice margin. Phanerozoic accumulation rates decrease systematically when averaged over longer time spans. Comparisons were drawn, therefore, at 5 and 57 m.y. time spans to match minimum Marinoan and Sturtian durations, respectively. Cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin. After accounting for time span, snowball Earth deposits at low latitude are found to be thinner than would be expected either for high-latitude Cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice-rafted debris. The rate discrepancy is not readily attributed to overestimates of the total Marinoan or Sturtian durations. If sediment fluxes during warm melt intervals did approach Phanerozoic rates, these intervals must have occupied a much smaller proportion of snowball Earth episodes than in younger glacial climates.

Evidence of Fluctuating Paleoatmospheric Oxygen Levels Before and After the Cryogenian Marinoan (Snowball Earth) Glaciations

Chromium isotope, REE and redox-sensitive trace element chemostratigraphy across the late Neoproterozoic Ghaub glaciation, Otavi Group, Namibia

Authors:

Rodler et al

Abstract:

Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmospheric oxygenation and continental weathering thus facilitating the reconstruction of the redox state of ancient seawater. We use the δ53Cr signature coupled with REE+Y patterns and redox-sensitive trace elements to monitor environmental changes recorded by marine carbonates of the Otavi Group, Namibia. These carbonates were deposited in a platform and foreslope setting in subtropical latitudes during the Neoproterozoic and comprise the transition from a marine depositional setting through glaciation into a postglacial environment in four stages. Preglacial carbonates (Stage 1) yield positively fractionated δ53Cr values, increased U and Mn concentrations, indicative of mobilization during oxidative terrestrial weathering and stabilization in oxic surface waters. Carbonates deposited just before the Ghaub diamictites (Stage 2) record δ53Cr values (>+0.4 ‰) comparable to modern seawater and negative Ce anomalies (∼0.7) characteristic for oxygenated seawater. We interpret this as a pulse of intense oxidative weathering shortly before the advance of the glaciers. Marginal shale contamination persists in carbonates of both sections and is slightly elevated during the glacial aftermath; Cr is vulnerable towards detrital contamination. Early postglacial cap dolostones (Stage 3) were influenced by enhanced detrital contamination potentially supplied by freshwater particulate load, which was then drastically reduced in the overlying postglacial limestones in the upper Maieberg Fm (Stage 4) where near-preglacial δ53Cr values are reached again. REE+Y patterns along with Eu and Ce anomalies record a transformation from a marine, slightly anoxic and stratified water column with distal hydrothermal influence to a freshwater-influenced depositional environment with decreased hydrothermal activity and fluctuating oxic surface water conditions after glacial retreat. Here, we demonstrate that carbonate δ53Cr signatures are sensitive to changes in continental weathering balanced between detrital contamination and oxidative weathering on land and are capable of tracing fluctuating redox conditions prior and after one of the major syn-Marinoan glaciations.

Were There Anoxic Oceans During the Cryogenian NeoProterozoic?

Integrated geochemical-petrographic insights from component-selective δ238U of Cryogenian marine carbonates

Authors:

Hood et al

Abstract:

Emerging geochemical proxies have improved our understanding of the broad-scale history of Earth's oxygenation. However, paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting. This is particularly an issue with marine carbonates, which although being potentially ideal ocean redox archives, are commonly altered during diagenesis. Here we provide new insight into the robustness of uranium isotopes (238U/235U ratios: δ238U values) as paleoredox tracers by determining texture-specific δ238U values from a well-described Cryogenian (Balcanoona) reef complex in South Australia. We found high variability in δ238U values between different carbonate components, even within a single sample. Petrographically, the best-preserved components from the Balcanoona reef are marine cements, which have a mean δ238U value of –0.23‰, essentially unfractionated from riverine inputs. These values are interpreted as reflecting a marine system with widespread anoxic and iron-rich settings. Less-well-preserved phases have δ238U values spanning almost the entire extent of the documented isotopic range. This integrated petrographic-geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case-by-case basis in future carbonate metal isotope geochemistry.

Explaining the Iron Drop Stones of the Sturtian Glaciation (Snowball Earth)

Sub–ice shelf ironstone deposition during the Neoproterozoic Sturtian glaciation

Authors:

Lechte et al

Abstract:

The Neoproterozoic Sturtian glaciation is considered to be among the most severe glaciations in Earth history, possibly encompassing the entire planet and lasting for more than 50 m.y. Iron formations are globally associated with Sturtian glacial successions, although the influence of glaciation on the genesis of these iron formations remains contentious. Here we examine the Sturtian iron formations of Namibia and Australia that feature finely laminated ironstones containing up to 55% total iron. These ironstones are repeatedly interbedded with massive diamictites, yet dropstones and other clastic input are nearly absent in the laminated ironstone facies. Intercalated diamictites are variably ferruginous and characterized by a strong glacial influence with evidence of glaciotectonism. The ferruginous facies are laterally discontinuous and commonly occupy paleobathymetric depressions. Rare earth element signatures from these iron formations are similar to those from modern seawater but lack cerium anomalies. The paradox of dropstone-free, laminated sediments intimately interlaminated with massive ice-proximal diamictites can be resolved by deposition under an ice shelf. Polynya activity and the mixing of cold, oxygenated glacial fluids with ferruginous seawater via an ice pump mechanism may explain the deposition of these iron formations and their restriction to Sturtian glacial successions globally

Evidence of Glacial Meltwater/Volcanic Interaction From Rifting of Supercontinent Rodinia During Tonian/Crogenian NeoProterozoic

High temperature glacial meltwater-rock reaction in the Neoproterozoic: evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

Authors:

He et al

Abstract:

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U-Pb dating for granitic gneisses from the northeastern Sulu orogen, east-central China. Zircon U-Pb dating yields protolith ages of 753±15 Ma to 780±13 Ma and metamorphic ages of 209±3 to 244±7 Ma. The Neoproterozoic cores with concordant U-Pb ages exhibit a wide δ18O range from -11.0 to 5.8‰, which is nearly the same as those for cores with discordant U-Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around -10‰ whereas the rims of the other samples show a wider range from -9.8 to 5.0‰. The δ18O values as negative as -11.0‰ for zircons with concordant Neoproterozoic U-Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than -9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water-rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

Was Venus Habitable Until the the Start of Earth's Sturtian Glaciations (Snowball Earth)

Was Venus the First Habitable World of our Solar System?

Authors:

Way et al

Abstract:

Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere over 90 times as thick as present day Earth's. Billions of years ago the picture may have been very different. We have created a suite of 3D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present day Venus orbital parameters, an ocean volume consistent with current theory and measurements, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than about 16 Earth days, despite an incident solar flux 46-70% higher than modern Earth receives. At its current rotation period of 243 days, Venus's climate could have remained habitable until at least 715 million years ago if it hosted a shallow primordial ocean. These results demonstrate the vital role that rotation and topography play in understanding the climatic history of exoplanetary Venus-like worlds being discovered in the present epoch.

A new way of Determining Paleoatmospheric Oxygen Content

Paradigm shift in determining Neoproterozoic atmospheric oxygen

Authors:

Blamey et al

Abstract:

We present a new and innovative way of determining the oxygen level of Earth's past atmosphere by directly measuring inclusion gases trapped in halite. After intensive screening using multiple depositional, textural/fabric, and geochemical parameters, we determined that tectonically undisturbed cumulate, chevron, and cornet halite inclusions may retain atmospheric gas during crystallization from shallow saline, lagoonal, and/or saltpan brine. These are the first measurements of inclusion gas for the Neoproterozoic obtained from 815 ± 15–m.y.–old Browne Formation chevron halite of the Officer Basin, southwest Australia. The 31 gas measurements afford us a direct glimpse of the composition of the mid- to late Neoproterozoic atmosphere and register an average oxygen content of 10.9%. The measured pO2 puts oxygenation of Earth's paleoatmosphere ∼100–200 m.y. ahead of current models and proxy studies. It also puts oxygenation of the Neoproterozoic atmosphere in agreement with time of diversification of eukaryotes and in advance of the emergence of marine animal life.

Was the Cryogenian Marinoan Glaciation REALLY a Snowball Earth?

Duration and nature of the end-Cryogenian (Marinoan) glaciation

Authors:

Prave et al

Abstract:

The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.

There was an 8 Million Year Warm Period Between Cryogenian Glaciations

The Late Cryogenian Warm Interval, NE Svalbard: Chemostratigraphy and genesis

Authors:

Fairchild et al

Abstract:

The Late Cryogenian Warm Interval (LCWI) refers to a non-glacial interval that separates presumed representatives of the Sturtian and Marinoan panglaciations. Its duration is poorly constrained radiometrically and its deposits are relatively poorly known in most geographic regions. This paper aims to constrain the duration, palaeoenvironments and petrogenesis of such deposits in the classic region of NE Spitsbergen, Svalbard. The succession comprises a 200–205 m dolomitic shale (Macdonaldryggen Member, known as E3, of the Elbobreen Formation) overlain by oolitic dolomite Slangen Member (E4), 15–25 m thick, with limestone developed at top and base of E3 in the south of the area. The assumed age context of the succession has been confirmed by the presence of a typical Sturtian cap carbonate profile of negative to positive δ13C, and primary Sr isotope compositions of basal E3 limestones  <0 .7072="" 0.7076.="" and="" br="" e3="" limestones="" of="" upper="">

<0 .7072="" 0.7076.="" and="" br="" e3="" limestones="" of="" upper=""> At the base of E3, interstratification of cap carbonate with ice-rafted and redeposited glacial sediments occurs. Early diagenetic stabilization of carbonate mineralogy from a precursor, possibly ikaite, to calcite or dolomite is inferred. E3 is predominantly dolomitic silt-shale, with sub-millimetre lamination, lacking sand or current-related sedimentary structures. Thin fine laminae are partly pyritized and interpreted as microbial mats. Dolomite content is 25–50%, with δ13C values consistently around +4‰, a value attributed to buffering by dissolution of a precursor metastable carbonate phase. Local calcite cement associates with low δ13C values. The carbonates form silt-sized, chemically zoned rhombic crystals from an environment with dynamically changing Fe and Mn. Three-dimensional reconstructions of cm-scale disturbance structures indicate that they represent horizontally directed sock-like folds, developed by release of overpressure into thin surficial sediment overlying an early-cemented layer.

A shoaling upwards unit near the top of E3 displays calcium sulphate pseudomorphs in dolomite in the north, but storm-dominated limestones in the south, both being overlain by peritidal oolitic dolomites, exposed under the succeeding Wilsonbreen glacial deposits. There is no Trezona δ13C anomaly, possibly implying top-truncation of the succession.

Regular 0.5 m-scale sedimentary rhythms, reflecting subtle variations in sediment texture or composition occur throughout E3 and are interpreted as allocyclic. They are thought to be mainly primary in origin, locally modified slightly during early diagenetic cementation. Rhythms are proposed to represent ca. 18 kyr precession cycles, implying 6–8 Myr deposition between glaciations.

New Evidence of the Rodinia Supercontinent Breakup During the Cryogenian NeoProterozoic

Cryogenian intraplate magmatism along the buried southern Laurentian margin: Evidence from volcanic clasts in Ordovician strata, Marathon uplift, west Texas

Authors:

Hanson et al

Abstract:

Critical evidence bearing on the breakup history of the supercontinent Rodinia near the end of the Proterozoic comes from widespread Cryogenian–Cambrian intraplate igneous assemblages present along the margins of cratonic blocks released during Rodinia fragmentation and now distributed around the globe. This magmatism occurred over a long time span (780–540 Ma) prior to and during final stages of Rodinia breakup along the eastern, western, and northern margins of the Laurentia craton, which forms the centerpiece of Rodinia in many reconstructions. Whether similar protracted magmatism occurred prior to the rift-drift transition along the southern Laurentian margin has remained uncertain because of deep burial beneath younger strata. We present geochemical and geochronological data from volcanic clasts within shelf-derived Ordovician turbidites and debris-flow deposits now exposed in allochthonous thrust slices in the Marathon uplift, west Texas (USA), that document one or more episodes of intraplate magmatism extending back at least to 706 Ma along this part of the ancient margin. These data raise the possibility that Laurentia may have been completely encircled by intraplate igneous activity prior to Rodinia breakup, with implications for the driving forces leading to supercontinent fragmentation and factors controlling the sites of ocean-basin formation during that process.

A Record of low Atmospheric Oxygen From the Proterozoic

A shale-hosted Cr isotope record of low atmospheric oxygen during the Proterozoic

Authors:

Cole et al

Abstract:

The emergence and expansion of animal life on Earth represents a dramatic shift in the structure and complexity of the biosphere. A lack of firm constraints on surface oxygen levels during the mid-Proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or, instead, simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger. Here, we present chromium (Cr) isotope data from marine black shales that provide evidence for minimal Cr oxidation throughout the mid-Proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late Neoproterozoic. This observation requires very low background oxygen levels (less than 1% of present atmospheric levels). Accepting previously proposed estimates of pO2 levels needed to induce Cr isotope fractionation, our data provide support for the persistence of an Earth system in which baseline atmospheric pO2 would have been low enough to inhibit the diversification of animals until ca. 800 Ma. More generally, evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on Earth.