The Earth Story

Impact

This is a complicated figure from a recently published paper featuring a really neat geologic site. This is an outcrop of the debris from the asteroid impact at the end of South America after the impact.

Start with the map in the upper right. The map shows the modern day locations of the Chicxulub impact crater on the Yucatan Peninsula and the tiny island of Gorgonilla off the coast of Colombia, where this outcrop was found.

The vertical column at the far left shows the sedimentary sequence. These rocks are off the coast of South America, near the Andes Mountains today. 65 million years ago, there were volcanoes nearby just like we find along this coastline today. This sequence of rocks therefore includes a basaltic lava flow and tuffs that have mixed with sediments. Tuffs are ash that exploded out of volcanoes, so volcanoes were feeding this site and the type of sediment deposited was alternating between sandy and muddy.

Then, right in the middle of this sequence, a whole bunch of spheres of glass appear. These are debris from the impact itself and they’re shown at the top in the figure labeled A. When the asteroid hit the Yucatan Peninsula, it vaporized and melted some of the target rocks. That molten rock splashed out of the crater like water bursting from a balloon and it rained down around the entire surrounding area. It cooled off rapidly as it flew through the air, creating spheres of hot glass that pelted the landscape. That spherule layer is the direct debris of the impact.

The final 2 frames show what has happened to the other sedimentary layers. The spherule layer is colored green in the center image – note that it’s a little bit bent. The zoomed in image below it shows that the once simple sedimentary beds have been bent and disturbed. When the impact took place, it caused geologic disruption throughout the entire region. It is thought that earthquakes continued shaking the land for tens of minutes after the impact, long enough for blast debris to rain down and still be disrupted. These bent sedimentary layers were shaken up by the energy released after the initial blast.

The scientists at this site characterized pollen samples in the sediments before and after the impact to analyze the types of plants living in these areas. They found that the plant populations shifted dramatically after the impact and one group of ferns that has been thought to occupy areas that are hit with disasters shows up just after the impact layer.

-JBB

Image credit and original paper: https://bit.ly/2rGLjqn

Heart Eyes from a Meteorite The image below is an impact spherule, seen through a microscope in crossed polarized light. An impact spherule forms when a meteorite hits the Earth; on impact, some of the rock on the ground is vaporized, and this vapor expands into a plume of rock particles which cool and condense into molten droplets called spherules. Spherules preserved in the rock layer can offer new information about a meteor impact, even if the crater from the impact has disappeared due to erosion. This particularly happy looking spherule is found in rock from the Barberton Greenstone Belt, one of the oldest and best-preserved examples of Archean continental crust; the spherule records a part of Earth's meteorite impact history as far back as 3.2-3.5 billion years ago. -CEL Sources: http://bit.ly/2ulrrZ4 http://go.nature.com/2foSBvs http://bit.ly/2vBcq9p Image: Seda Özdemir (distributed via imaggeo.egu.eu) http://bit.ly/2ulrrZ4

Heart Eyes from a Meteorite The image below is an impact spherule, seen through a microscope in crossed polarized light. An impact spherule forms when a meteorite hits the Earth; on impact, some of the rock on the ground is vaporized, and this vapor expands into a plume of rock particles which cool and condense into molten droplets called spherules. Spherules preserved in the rock layer can offer new information about a meteor impact, even if the crater from the impact has disappeared due to erosion. This particularly happy looking spherule is found in rock from the Barberton Greenstone Belt, one of the oldest and best-preserved examples of Archean continental crust; the spherule records a part of Earth's meteorite impact history as far back as 3.2-3.5 billion years ago. -CEL Sources: http://bit.ly/2ulrrZ4 http://go.nature.com/2foSBvs http://bit.ly/2vBcq9p Image: Seda Özdemir (distributed via imaggeo.egu.eu) http://bit.ly/2ulrrZ4

Both of these things can’t be right: Part 2

In my last post, I discussed evidence that oxygen first appeared in the atmosphere almost exactly 2.33 billion years ago (http://bit.ly/1sMwXTK). This tiny, just discovered grain, was used to argue that oxygen must have been present in Earth’s upper atmosphere over 2.7 billion years ago…and both of these studies can’t be right.

This is a tiny sphere of iron, about a tenth the diameter of a human hair. It was one of a handful discovered by dissolving a 2.7 billion year old limestone found in the Pilbara region of western Australia. Some of these tiny bits of iron contain crystals of iron-nickel metal at their core, a signature of them being meteoritic in origin. These are therefore the oldest micrometeorites humans have yet found on this planet.

When a micrometeorite hits Earth’s atmosphere, it heats up rapidly and melts, often producing a shooting star. That grain melts quickly and then cools off within seconds to form a solid sphere, allowing a tiny bit of time for chemistry to occur.

Iron metal, when exposed to oxygen, will react rapidly and rust, forming minerals like magnetite and hematite. These reactions can take years at Earth’s surface temperature, but in the atmosphere when the sample is heated to thousands of degrees they can complete in seconds. This reaction is happening all the time today with lots of oxygen in the atmosphere, but there’s no reason why this reaction should happen at all if there was no oxygen in the atmosphere.

These grains, therefore, suggest that there must have been oxygen, in fact a lot of oxygen, in the atmosphere 2.7 billion years ago. There is good evidence that life figured out photosynthesis before this time, so it makes sense that oxygen could be present that early. The only problem is…this makes no sense with the data presented in the last post.

In the last post, I outlined that sulfur isotope changes require a lack of oxygen in the atmosphere until 2.33 billion years ago, 400 million years after these grains formed. How can these two data sets be put together?

The scientists who discovered these micrometeorites suggested one hypothesis and it has been outlined in a number of press reports. Their idea is that the Earth had oxygen in its upper atmosphere and a methane-rich haze in the lower atmosphere. The oxygen in the upper atmosphere could react with these meteorites to rust them, the lower atmosphere could still have very little oxygen, and a temperature boundary created by the haze layer could keep the two parts of the atmosphere from mixing much.

That hypothesis was presented in their paper, but there’s one thing that hasn’t been published yet in the press reports; this mechanism actually can’t work with the sulfur isotope signature!

The sulfur isotope changes I talked about in the last post don’t happen today because oxygen in the atmosphere absorbs light at the same wavelengths required to ionize sulfur. If there is oxygen in the upper atmosphere, above the sulfur, it will actually block the light from getting to the lower atmosphere.

It might well be possible to have a layered atmosphere, with oxygen in the upper atmosphere and methane in the lower atmosphere, as these scientists suggest. However, this mechanism would still shut off the sulfur isotope signal – if the oxygen is higher up in the atmosphere than the sulfur, it will block the light that is thought to cause the sulfur isotope signal!

This is complicated geology. We’re interpreting isotopes in one case and small grains in another case, both of them over 2 billion years old. Both of them tell interesting stories about the early Earth’s atmosphere. The problem is…the stories don’t add together. Both stories cannot be true at the same time - the mechanism for sulfur changes that I talked about in the last post requires there to be no oxygen in the way, while these meteorites were used to argue for oxygen in the way.

There’s something fundamentally missing from our interpretation of this puzzle. One proposal by researchers evaluating this new work is that it’s actually all the sun; the sun could be ionizing both sulfate and water in the upper atmosphere, creating free oxygen that could bond with these meteorites as they enter, but that may not be enough oxygen. This is now an interesting puzzle for future geologists and atmospheric scientists to understand.

-JBB

Image credit: Tomkins et al. http://rdcu.be/ioti

Reference: http://bit.ly/1Tsy6Ih http://bit.ly/27e8FBt

Earth’s early impacts

Today, asteroids large enough to cause global effects only hit the planet Earth once during a period of tens or hundreds of millions of years. However, when the solar system was younger, there was much more debris around and it was likely stirred up by occasional big motions of the giant planets. Although geologic processes have removed many of the craters from these early impacts, scientists are gradually finding evidence like these rocks for their existence.

These rocks come from one of many drill cores taken in Western Australia. Australia preserves some of the Earth’s oldest crust in an area called the Pilbara Craton. This craton is filled with large granitic plutons and metamorphosed sedimentary rocks stretched around them, a remnant of the formation of Earth’s earliest continental crust. The sedimentary rocks are over 3 billion years old and therefore also preserve chemical remnants of the environment when life was first evolving and diversifying. To investigate these processes, scientists have taken a series of drill cores through the sediments that give them intact stratigraphic sequences through these Archean aged rocks.

This rock comes from one of those drill cores. Its age is constrained by dating of other rocks in this drill core to be 3.46 billion years old. The small grains you see are spherules, small bits of molten rock splashed out by an impact. They are found in a drill core through different layers of chert, a silica-rich rock that is common in Archean sediments; the different colors come from different parts of the surrounding unit.

To establish that these were produced when an asteroid hit Earth, a team of researchers led by a scientist at Geoscience Australia characterized their textures and chemistry. They found elevated levels of platinum group elements, things like platinum, palladium, gold, and iridium, in addition to other elements like nickel and sulfur that are rare on Earth (they’re locked in the core) but abundant in meteorites. They also found angular particles (impact debris) and occasionally spherules that were broken and injected with quartz, illustrating the violence of their formation.

This spherule layer is the 17th impact spherule layer known from either South Africa or Australia. Those impacts occurred over hundreds of millions of years, so life would have had time to recover from them, but discovering so many of these layers shows that Earth was truly much more of a shooting gallery in the Archaean and surviving this environment would have been difficult for many species.

Interpreting this history is also complicated for geologists; because the rocks are metamorphosed and difficult to date precisely, some of those impacts could be correlated between the continents, but that’s hard to tell. For example, a few meters above the main spherule-bearing layer there is a second layer containing spherules. Because the rocks are metamorphosed, the scientists can’t tell if that layer actually represents a second impact or instead just a sedimentary reworking of spherules from the lower layer.

-JBB

Image credit and reference: Glickson et al., 2016 (Precambrian Research) http://bit.ly/25aGpBm

References: http://www.psu.edu/dept/spacegrant/ABDP/ http://bit.ly/23VUQmW

Spherule Layers

This layer is found in a sequence of sedimentary rocks in the Barberton Greenstone Belt of South Africa – some of the oldest rocks exposed on continents in the world. This layer is estimated to be 3.24 billion years old. It is a geologic record of what I would call…a very bad day.

These are impact-generated spherules. This layer formed when an asteroid, at least several kilometers in diameter, slammed into the Earth. The energy of the impact superheated the target rocks and the impactor, vaporizing part of it and also splashing much of the flash-melted rock outward as small droplets. These droplets would have then splashed down on the earth and cooled as they traveled through the atmosphere, creating layers rich in spherules. Many of these layers contain elements that are abundant in asteroids but are rare at Earth’s surface such as iridium; a chemical fingerprint of these as impact spherules.

Large impacts capable of producing these spherules were much more common early in the Earth’s history when there were more asteroids around to hit planets and when gravitational stirring in the giant planets may have thrown asteroids into the inner part of the solar system.

Many of the exact processes that led to the formation of the terrestrial planets are still not understood, including the frequency of impacts in the early solar system. We have some evidence for a pulse of asteroid impacts about 4 billion years ago from the ages of rocks picked up on the moon, but this proposed “Late Heavy Bombardment” concept remains controversial as we don’t have exact ages for craters, just ages when some rocks were reheated.

By counting the number of these layers found in early Archean sequences on Earth and measuring their thicknesses, scientists are able to estimate the number and size of impacts in the early solar system. A recent effort to do just that led to an estimate of cratering frequency similar to that observed on the moon. The distribution of impact sizes also suggests that the objects hitting the Earth (and the Moon) early in the planet’s history represented a population of objects similar to those observed in the asteroid belt today. In other words, it looks like the objects being thrown at the Earth came from a population similar to those found in the solar system today.

-JBB

Image credit: http://bit.ly/1QyjczX

References: http://bit.ly/1HHhYzR http://bit.ly/1QB6eOH http://bit.ly/1p7gLdK http://bit.ly/1QoZHfa