Reports on the Annals of the Former World

[Recusant]

Nah, eternity sounds boring. Besides, worshiping for eternity does not seem like paradise to me.

The atheist in a foxhole certain to be over-run by a merciless foe. I salute him, and should have done when he wrote that post.  Really we are all in a similar situation, though the circumstances may be less certainly imminent. Regrets abound; we abide, for now. 

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Sticking their nasty rock hammers in where they don't belong! Everybody knows the Grand Canyon was formed during the time of Noahz Flud.   

Yes, it's from this past summer, but in a geological sense that was a couple of nanoseconds ago. 

"Geologists dig into Grand Canyon's mysterious gap in time" | EurekAlert

A new study led by the University of Colorado Boulder reveals the complex history behind one of the Grand Canyon's most well-known geologic features: A mysterious and missing gap of time in the canyon's rock record that covers hundreds of millions of years.

The research comes closer to solving a puzzle, called the "Great Unconformity," that has perplexed geologists since it was first described nearly 150 years ago.

Think of the red bluffs and cliffs of the Grand Canyon as Earth's history textbook, explained Barra Peak, lead author of the new study and a graduate student in geological sciences at CU Boulder. If you scale down the canyon's rock faces, you can jump back almost 2 billion years into the planet's past. But that textbook is also missing pages: In some areas, more than 1 billion years' worth of rocks have disappeared from the Grand Canyon without a trace.

Geologists want to know why.

"The Great Unconformity is one of the first well-documented geologic features in North America," Peak said. "But until recently, we didn't have a lot of constraints on when or how it occurred."

Now, she and her colleagues think they may be narrowing in on an answer in a paper published this month in the journal Geology. The team reports that a series of small yet violent faulting events may have rocked the region during the breakup of an ancient supercontinent called Rodinia. The resulting havoc likely tore up the earth around the canyon, causing rocks and sediment to wash away and into the ocean.

[Continues . . .]

The paper is open access. A PDF of it is available by clicking through from the link below.

"Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity paleotopography in the Grand Canyon region, USA" | Geology

Abstract:

The Great Unconformity is an iconic geologic feature that coincides with an enigmatic period of Earth's history that spans the assembly and breakup of the supercontinent Rodinia and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history below the Great Unconformity at its classic Grand Canyon locality in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ across both long (∼100 km) and short (tens of kilometers) spatial wavelengths.

The spatially variable thermal histories implied by these data are best explained by Proterozoic syndepositional normal faulting that induced differences in exhumation and burial across the region. The data, geologic relationships, and thermal history models suggest Neoproterozoic rock exhumation and the presence of a basement paleo high at the present-day Lower Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Upper Granite Gorge.

The paleo high created a topographic barrier that may have limited deposition to restricted marine or nonmarine conditions. This paleotopographic evolution reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that induced multiple events that formed unconformities over hundreds of millions of years, all with claim to the title of a "Great Unconformity."

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From whence the highly addictive element oxygen in the atmosphere of our Earth? With a special guest appearance by that particularly interesting crystal, zircon.

"Where did the Earth's oxygen come from? New study hints at an unexpected source" | The Conversation

The amount of oxygen in the Earth's atmosphere makes it a habitable planet.

Twenty-one per cent of the atmosphere consists of this life-giving element. But in the deep past — as far back as the Neoarchean era 2.8 to 2.5 billion years ago — this oxygen was almost absent.

So, how did Earth's atmosphere become oxygenated?

Our research, published in Nature Geoscience, adds a tantalizing new possibility: that at least some of the Earth's early oxygen came from a tectonic source via the movement and destruction of the Earth's crust.

The Archean eon represents one third of our planet's history, from 2.5 billion years ago to four billion years ago.

This alien Earth was a water-world, covered in green oceans, shrouded in a methane haze and completely lacking multi-cellular life. Another alien aspect of this world was the nature of its tectonic activity.

One feature of modern subduction zones is their association with oxidized magmas. These magmas are formed when oxidized sediments and bottom waters — cold, dense water near the ocean floor — are introduced into the Earth's mantle. This produces magmas with high oxygen and water contents.

On modern Earth, the dominant tectonic activity is called plate tectonics, where oceanic crust — the outermost layer of the Earth under the oceans — sinks into the Earth's mantle (the area between the Earth's crust and its core) at points of convergence called subduction zones. However, there is considerable debate over whether plate tectonics operated back in the Archean era.

Our research aimed to test whether the absence of oxidized materials in Archean bottom waters and sediments could prevent the formation of oxidized magmas. The identification of such magmas in Neoarchean magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.

We collected samples of 2750- to 2670-million-year-old granitoid rocks from across the Abitibi-Wawa subprovince of the Superior Province — the largest preserved Archean continent stretching over 2000 km from Winnipeg, Manitoba to far-eastern Quebec. This allowed us to investigate the level of oxidation of magmas generated across the Neoarchean era.

Measuring the oxidation-state of these magmatic rocks — formed through the cooling and crystalization of magma or lava — is challenging. Post-crystallization events may have modified these rocks through later deformation, burial or heating.

So, we decided to look at the mineral apatite which is present in the zircon crystals in these rocks. Zircon crystals can withstand the intense temperatures and pressures of the post-crystallization events. They retain clues about the environments in which they were originally formed and provide precise ages for the rocks themselves.

Small apatite crystals that are less than 30 microns wide — the size of a human skin cell — are trapped in the zircon crystals. They contain sulfur. By measuring the amount of sulfur in apatite, we can establish whether the apatite grew from an oxidized magma.

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The paper is behind a paywall.

Abstract:

Oxidized, sulfur-rich arc magmas are ubiquitous in modern subduction-zone environments. These magmas are thought to form when the fluids released during prograde metamorphism of subducting oceanic crust and overlying sediments oxidize and hydrate the asthenospheric mantle. In contrast, Archaean arc-type magmas are thought to be relatively reduced and sulfur poor, owing to the lower concentrations of marine sulfate and limited oxidative seafloor alteration in the anoxic ocean before the Great Oxidation Event some 2.4 billion years ago (Ga).

Here we measure the total sulfur concentration and relative abundances of S6+, S4+ and S2− in zircon-hosted apatite grains from sodic and potassic intrusive rocks from the ~2.7 Ga southeastern Superior Province, Canada. We find that, rather than being reduced and sulfur poor, the sulfur budget of the Neoarchaean magmas was dominated by S6+ and abruptly increased to concentrations comparable to Phanerozoic arc magmas following the interpreted onset of subduction at approximately 2.7 Ga, coincident with the first global pulse of crust generation. These findings indicate that oxidized, sulfur-rich magmas formed in subduction zones independent of ocean redox state and could have influenced oceanic–atmospheric and metallogenic evolution in the Neoarchaean.

[Recusant]

Seems entirely appropriate for this thread. 

"Is some of the body that collided with Earth to form the Moon still recognisable inside our planet?" | The Conversation

Scientists have dated the birth of the Solar System to about 4.57 billion years ago. About 60 million years later a "giant impact" collision between the infant Earth and a Mars-sized body called Theia created the Moon.

Now, new research suggests that the remains of the large object that collided with the young Earth to form the Moon are still identifiable deep within the planet as two large lumps. These lumps make up about 8% of the volume of the Earth's mantle, which is the rocky zone between the Earth's iron core and its crust.

The new study, led by Qian Yuan of Arizona State University and Caltech, argues that the heat generated by this collision was not enough to melt the whole of the Earth's mantle, so the innermost mantle remained solid.

Consequently, the researchers say, the melted mantle of Theia didn't completely mix with Earth's mantle. That would have made the Theia remnants indistinguishable from Earth's mantle as a whole. Instead, a lot of Theia's mantle ended up as two continent-sized lumps that now sit on top of the Earth's core-mantle boundary.

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The paper is behind a paywall.

Abstract:

Seismic images of Earth's interior have revealed two continent-sized anomalies with low seismic velocities, known as the large low-velocity provinces (LLVPs), in the lowermost mantle1. The LLVPs are often interpreted as intrinsically dense heterogeneities that are compositionally distinct from the surrounding mantle2.

Here we show that LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth's mantle after the Moon-forming giant impact3. Our canonical giant-impact simulations show that a fraction of Theia's mantle could have been delivered to proto-Earth's solid lower mantle.

We find that TMM is intrinsically 2.0–3.5% denser than proto-Earth's mantle based on models of Theia's mantle and the observed higher FeO content of the Moon. Our mantle convection models show that dense TMM blobs with a size of tens of kilometres after the impact can later sink and accumulate into LLVP-like thermochemical piles atop Earth's core and survive to the present day.

The LLVPs may, thus, be a natural consequence of the Moon-forming giant impact. Because giant impacts are common at the end stages of planet accretion, similar mantle heterogeneities caused by impacts may also exist in the interiors of other planetary bodies.

[Recusant]

This is intriguing. The climate didn't "recover" for approximately five million years after the massive volcanic injection of CO₂ into the atmosphere that occurred at the end of the Permian period. Models suggest that it should have taken much less time (approximately 100,000 years). The paper described below hypothesizes that the longer time scale of the recovery was due to "reverse weathering," which I take to mean a massive proliferation of certain microscopic organisms in the oceans. However I think my understanding is oversimplified and possibly incorrect. 

"Study uncovers key to delayed climate recovery following mass extinction event" | Phys.org

study led by a University of Waikato Ph.D. student has shed light on the cause of delayed climate recovery following Earth's most severe extinction event 251 million years ago—a discovery that will contribute to our understanding of the global climate system.

Published in Proceedings of the National Academy of Sciences by lead researcher Sofia Rauzi, this research reveals the role marine clay formation had in slowing the Earth's return to baseline temperatures after the end-Permian mass extinction.

Earth's climate system is generally viewed to recover in the order of 100,000 years following a significant carbon injection event such as the volcanic eruption that triggered the end-Permian mass extinction.

"Climate recovery following the end-Permian took over five million years, so we set out to investigate why temperatures stayed warm for so long," says Ms Rauzi. Analysis of the chemical composition of rocks from New Zealand, Japan, and Norway indicates that increased marine clay formation, also known as reverse weathering, contributed to the sustained high temperatures.

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The paper is behind a paywall.

Abstract:

Elevated temperatures persisted for an anomalously protracted interval following pulsed volcanic carbon release associated with the end-Permian mass extinction, deviating from the expected timescale of climate recovery following a carbon injection event.

Here, we present evidence for enhanced reverse weathering—a CO2 source—following the end-Permian mass extinction based on the lithium isotopic composition of marine shales and cherts. We find that the average lithium isotopic composition of Lower Triassic marine shales is significantly elevated relative to that of all other previously measured Phanerozoic marine shales.

Notably, the record generated here conflicts with carbonate-based interpretations of the lithium isotopic composition of Early Triassic seawater, forcing a re-evaluation of the existing framework used to interpret lithium isotopes in sedimentary archives.

Using a stochastic forward lithium cycle model, we demonstrate that elevated reverse weathering is required to reproduce the lithium isotopic values and trends observed in Lower Triassic marine shales and cherts. Collectively, this work provides direct geochemical evidence for enhanced reverse weathering in the aftermath of Earth's most severe mass extinction.

[Recusant]

I've come across a few examples of karst but probably the most striking karst landscape I've seen is the limestone pavement of the Burren in Ireland. There are more spectacular examples (like the one in Western Australia described below) but I haven't been lucky enough to visit them.

"Limestone and iron reveal puzzling extreme rain in Western Australia 100,000 years ago" | The Conversation

Almost one-sixth of Earth's land surface is covered in otherworldly landscapes with a name that may also be unfamiliar: karst. These landscapes are like natural sculpture parks, with dramatic terrain dotted with caves and towers of bedrock slowly sculpted by water over thousands of years.

Karst landscapes are beautiful and ecologically important. They also represent a record of Earth's past temperature and moisture levels.

However, it can be quite challenging to figure out exactly when karst landscapes formed. In our new work published today in Science Advances, we show a new way to find the age of these enigmatic landscapes, which will help us understand our planet's past in more detail.

[. . .]

In our study, we found a way to measure the age of pebble-sized iron nodules that formed at the same time as a karst landscape.

This method has the technical name of (U/Th)-He geochronology. In it, we measure how much helium is produced by the natural radioactive decay of tiny amounts of the elements uranium and thorium in the iron nodules. By comparing the amounts of uranium, thorium and helium in a sample, we can very accurately calculate the age of the nodules.

We dated microscopic fragments of iron-rich nodules from the iconic Pinnacles Desert in Nambung National Park, Western Australia.

This world-famous site is renowned for its otherworldly karst landscape of acres of limestone pillars towering metres above a sandy desert plain. The Pinnacles form part of the most extensive belt of wind-blown carbonate rock in the world, stretching more than 1,000km along coastal southwestern WA.

[. . .]

We consistently found an age of around 100,000 years for the growth of the iron nodules. This date is supported by known ages from the rocks above and beneath the karst surface, proving the reliability of our new approach.

At the same time as chemical reactions caused growth of the iron-rich nodules within the ancient soil, limestone bedrock was rapidly and extensively dissolved to leave only remnant limestone pinnacles seen today.

[Continues . . .]

The paper is open access:

"Ironing out complexities in karst chronology: (U-Th)/He ferricrete ages reveal wet MIS 5c" | Science Advances

Abstract:

Karst landforms provide insights into landscape evolution and paleoclimate but are inherently challenging to date. An ancient interval of particularly intense weathering of Western Australian Pleistocene aeolianites is recorded in a spectacular pinnacle karst landscape with associated ferricrete nodules. (U-Th)/He dating of the ferricrete nodules revealed an age of 102.8 + 10.6/−11.4 thousand years, corresponding to marine isotope stage 5c.

The (U-Th)/He age thus directly dates the wettest interglacial period in the region over the last 500 thousand years, which was responsible for the dissolution that formed the pinnacles. The reliability of the ferricrete (U-Th)/He age is supported by bounding optically stimulated luminescence and U-Th dates on associated aeolianites and carbonate precipitates, respectively.

A (U-Th)/He approach is globally applicable to aeolianites with associated ferricretes, allowing more accurate dating of the environmental changes affecting these lithologies, and temporally constraining rapid Pleistocene climatic oscillations to better contextualize the associated evolution of the biosphere.