Hexbyte Glen Cove New research strengthens link between glaciers and Earth’s ‘Great Unconformity’

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Researchers used thermochronometric data from four North American locations to determine the cause of the “Great Unconformity”—a massive loss of rock about 700 million years ago. Credit: Kalin McDannell

New research provides further evidence that rocks representing up to a billion years of geological time were carved away by ancient glaciers during the planet’s “Snowball Earth” period, according to a study published in Proceedings of the National Academy of Sciences.

The research presents the latest findings in a debate over what caused the Earth’s “Great Unconformity”—a time gap in the geological record associated with the erosion of up to 3 miles thick in areas across the globe.

“The fact that so many places are missing the from this has been one of the most puzzling features of the rock record,” said C. Brenhin Keller, an assistant professor of earth sciences and senior researcher on the study. “With these results, the pattern is starting to make a lot more sense.”

The massive amount of missing rock that has come to be known as the Great Unconformity was first named in the Grand Canyon in the late 1800s. The conspicuous geological feature is visible where rock layers from distant time periods are sandwiched together, and it is often identified where rocks with fossils sit directly above those that do not contain fossils.

“This was a fascinating time in Earth’s history,” said Kalin McDannell, a postdoctoral researcher at Dartmouth and the lead author of the paper. “The Great Unconformity sets the stage for the Cambrian explosion of life, which has always been puzzling since it is so abrupt in the fossil record—geological and evolutionary processes are usually gradual.”

For over a century, researchers have sought to explain the cause of the missing geological time.

In the last five years, two opposing theories have come into focus: One explains that the rock was carved away by ancient glaciers during the Snowball Earth period about 700 to 635 million years ago. The other focuses on a series of plate tectonic events over a much longer period during the assembly and breakup of the supercontinent Rodinia from about 1 billion to 550 million years ago.

Research led by Keller in 2019 first proposed that widespread erosion by continental ice sheets during the Cryogenian glacial interval caused the loss of rock. This was based on geochemical proxies that suggested that large amounts of mass erosion matched with the Snowball Earth period.

“The new research verifies and advances the findings in the earlier study,” said Keller. “Here we are providing independent evidence of rock cooling and miles of exhumation in the Cryogenian period across a large area of North America.”

The study relies on a detailed interpretation of thermochronology to make the assessment.

Thermochronology allows researchers to estimate the temperature that mineral crystals experience over time as well as their position in the continental crust given a particular thermal structure. Those histories can provide evidence of when missing rock was removed and when rocks currently exposed at the surface may have been exhumed.

In Colorado’s Ladder Canyon, rocks that differ in age by about a billion years sit together across the Great Unconformity. Credit: C. Brenhin Keller

The researchers used multiple measurements from previously published thermochronometric data taken across four North American locations. The areas, known as cratons, are parts of the continent that are chemically and physically stable, and where plate tectonic activity would not have been common during that time.

By running simulations that searched for the time-temperature path the rocks experienced, the research recorded a widespread signal of rapid, high magnitude cooling that is consistent with about 2-3 miles of erosion during Snowball Earth glaciations across the interior of North America.

“While other studies have used thermochronology to question the glacial origin, a like the Great Unconformity requires a global assessment,” said McDannell. “Glaciation is the simplest explanation for erosion across a vast area during the Snowball Earth period since ice sheets were believed to cover most of North America at that time and can be efficient excavators of rock.”

According to the research team, the competing theory that tectonic activity carved out the missing rock was put forth in 2020 when a separate research group questioned whether ancient glaciers were erosive enough to cause the massive loss of rock. While that research also used thermochronology, it applied an alternate technique at only a single tectonically active location and suggested that the erosion occurred prior to Snowball Earth.

“The underlying concept is pretty simple: Something removed a whole lot of rock, resulting in a whole lot of missing time,” said Keller. “Our research demonstrates that only glacial erosion could be responsible at this scale.”

According to the researchers, the new findings also help explain links between the erosion of rock and the emergence of complex organisms about 530 million years ago during the Cambrian explosion. It is believed that erosion during the Snowball Earth period deposited nutrient-rich sediment in the ocean that could have provided a fertile environment for the building blocks of complex life.

The study notes that the two hypotheses of how the rock eroded are not mutually exclusive—it is possible that both tectonics and glaciation contributed to global Earth system disruption during the formation of the Great Unconformity. It appears, however, that only glaciation can explain erosion in the center of the continent, far from the tectonic margins.

“Ultimately with respect to the Great Unconformity, it may be that the generally accepted reconstruction(s) of more concentrated equatorial packing of the Rodinian continents along with the unique environmental conditions of the Neoproterozoic, proved to be a time of geologic serendipity unlike most any other in Earth history,” the research paper says.

According to the team, this is the first research that uses their thermochronology modeling approach to study a period that extends well beyond a billion years. In the future, the team will repeat their work on other continents, where they hope to further test these hypotheses about how the Great Unconformity was created and preserved.

According to the team, resolving differences in the research is critical to understanding early Earth history and the interconnection of climatic, tectonic and biogeochemical processes.

“The fact that there may have been tectonic erosion along the craton margins does not rule out glaciation,” said McDannell. “Unconformities are composite features, and our work suggests Cryogenian erosion was a key contributor, but it is possible that both earlier and later were involved in forming the unconformity surface in different places. A global examination will tell us more.”

William Guenthner, from the University of Illinois at Urbana-Champaign; Peter Zeitler from Lehigh University; and David Shuster from the University of California, Berkeley and the Berkeley Geochronology Center served as co-authors of the paper.



More information:
Kalin T. McDannell et al, Thermochronologic constraints on the origin of the Great Unconformity, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2118682119

Citation:
New research strengthens link between glaciers and Earth’s ‘Great Unconformity’ (2022, January 25)

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Hexbyte Glen Cove Electrons falling flat: Germanium falls into a 2-D arrangement on zirconium diboride thumbnail

Hexbyte Glen Cove Electrons falling flat: Germanium falls into a 2-D arrangement on zirconium diboride

Hexbyte Glen Cove

Figure 1. Ball-and-stick model for bitriangular Ge lattice on zirconium diboride   Germanium atoms (light and dark blue) spontaneously crystallize into a two-dimensional (2D) “bitriangular” lattice on zirconium diboride thin films grown on germanium single crystals (green: Zr atoms, orange: B atoms). Credit: Japan Advanced Institute of Science and Technology

Scientists have recently revealed, both theoretically and experimentally, that germanium atoms can arrange themselves into a 2-D “bi-triangular” lattice on zirconium diboride thin films grown on germanium single crystals to form a “flat band material” with an embedded “kagome” lattice. The result provides experimental support to a theoretical prediction of flat bands emerging from trivial atomic geometry and indicates the possibility of their existence in many more materials.

The human mind is naturally drawn to objects that possess symmetry; in fact, the notion of beauty is often conflated with symmetry. In nature, nothing epitomizes symmetry more than crystals. Since their discovery, crystals have attracted a great deal of attention not only by their unique “symmetrical” aesthetic appeal but also by their unique properties. One of these properties is the behavior of electrons inside a crystal. From a physical point of view, an electron within a crystal can be fully characterized by its energy and a quantity called “crystal momentum,” which relates to how fast the electron moves in a crystal. The relationship between the energy and crystal momentum of electrons is what scientists refer to as “band ,” which, put simply, is the allowed energy levels for the electrons within the crystal.

Recently, materials scientists have turned their attention towards what are called “flat band materials”—a class of materials possessing a in which the energy does not vary with the crystal momentum and hence resembles a flat line when plotted as a function of crystal momentum—owing to their ability to give rise to exotic states of matter, such as ferromagnetism (iron-like spontaneous magnetism) and superconductivity (zero resistance to electricity flow). Generally, these “flat bands” are observed in special 2-D structures that go by names like “checkerboard ,” “dice lattice,” “kagome lattice,” etc. and are typically observed either within the crystal or at the surface of layered materials. A pertinent question thus presents itself—is it possible to embed such lattices into completely new 2-D structures? Efforts to design 2-D materials have focused on answering this question, and a recent finding suggests that the answer is a “yes.”

Now, in a study published in Physical Review B as a Rapid Communication, an international team of scientists from the Japan Advanced Institute of Science and Technology (JAIST), the University of Tokyo, the Japan Atomic Energy Agency, and Institute for Molecular Science in Japan and Tamkang University in Taiwan, led by Dr. Antoine Fleurence and Prof. Yukiko Yamada-Takamura, has reported a possible new flat band material obtained from germanium (Ge) atoms arranging themselves into a 2-D bi-triangular lattice on zirconium diboride thin films grown on germanium . While the team had already grown this 2-D material years ago, they were only recently able to unveil its structure.

Last year, a part of the team published a theoretical paper in the same journal underlining the conditions under which a 2-D bi-triangular lattice can form a flat band. They found that this is related to a “kagome” (meaning weaved basket pattern in Japanese) lattice—a term originally coined by Japanese physicists in the ’50s to study magnetism. “I was really excited when I found out that the electronic structure of kagome lattice can be embedded into a very different-looking 2-D structure,” recalls Prof. Chi-Cheng Lee, a physicist at Tamkang University, Taiwan, involved in the study, who predicted the presence of flat bands in the “bitriangular” lattice.

The prediction was finally confirmed after the team, in their current study, characterized the prepared 2-D material using various techniques such as scanning tunneling microscopy, positron diffraction, and core-level and angle-resolved photoelectron emission; and backed up the experimental data with theoretical calculations to reveal the underlying bi-triangular lattice.

“The result is really exciting as it shows that flat bands can emerge even from trivial structures and can possibly be realized in many more materials. Our next step is to see what happens at low temperature, and how it is related to the flat bands of the Ge bi-triangular lattice,” says Dr. Fleurence, who is also the first author of this paper.

Indeed, who would’ve thought that a typical, run-of-the-mill semiconductor like germanium could offer such exotic and unprecedented possibilities? The 2-D world might have more surprises up its sleeve than we imagine.



More information:
A. Fleurence et al. Emergence of nearly flat bands through a kagome lattice embedded in an epitaxial two-dimensional Ge layer with a bitriangular structure, Physical Review B (2020). DOI: 10.1103/PhysRevB.102.201102

Provided by
Japan Advanced Institute of Science and Technology

Citation:
Electrons falling flat: Germanium falls into a 2-D arrangement on zirconium diboride (2020, December 4)
retrieved 4 December 2020
from https://phys.org/news/2020-12-electrons-falling-flat-germanium-falls.html

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