Month: September 2022

UPTICK IN SEDIMENTATION. “Then there’s the huge uptick in sedimentation rates at the start of the Cambrian. All the new sediment required plenty of space to fall into, something that would have only been possible if massive levels of erosion took place beforehand.” https://lnkd.in/ds7GcBF View in LinkedIn

CRATER EROSION. “Around 700 million years ago, they found, the Earth's impact craters were wiped nearly clean. Only two huge craters, the Sudbury basin in Canada and the Vredefort crater in South Africa, predate Snowball Earth — and those craters were staggeringly enormous, originally measuring 93 miles (150 km) and 185 miles (300 km) across, respectively. They've been eroded to a fraction of their original size. [Crash! 10 Biggest Impact Craters on Earth].” https://lnkd.in/ds7GcBF View in LinkedIn

GLOBALLY VARIABLE. “Keller and his team think that the glaciers of Snowball Earth wiped clean all other impact craters, scraping a bit off the top of Sudbury and Vredefort, too. By their calculations, an average of between 1.8 and 3 vertical miles (3 and 5 km) of crust were scraped away by Snowball Earth's ice sheets over 64 million years. In some spots, Keller said, the loss was greater, and in others, no crust was lost at all.” https://lnkd.in/ds7GcBF View in LinkedIn

GLACIAL EROSION THEORY. “The ice would have had to shave only an average of 0.002 inches (0.0625 millimeters) of dirt and rock off the crust each year to accomplish this feat, Keller said. That's a breeze even for modern-day glaciers, he said. Today, erosion rates for continental ice sheets range from 0.004 to 0.19 inches (0.1 to 4.8 mm), with steep mountain glaciers moving nearly 4 inches (100 mm) of rock and dirt annually.” https://lnkd.in/ds7GcBF View in LinkedIn

STUBBORN RESISTANCE TO IDEAS. “Scientists had considered glaciers as a possible cause of the Great Unconformity before, but the idea had been largely abandoned. A 1973 paper on the idea by University of North Carolina geologist William White failed to garner a single citation by other researchers. Other theories include the impossible (giant tides that wiped the land clean, but would have required the moon to exist hundreds of millions of years earlier than it actually did) and the more reasonable (the uplift and subsequent weathering of a massive supercontinent).” https://lnkd.in/ds7GcBF View in LinkedIn

GLOBAL CO2. “It's possible that both uplift and glaciers played a role in clearing out kilometers of crust. In 2013, researchers found that rocks of the Snowball Earth era had captured and stored carbon dioxide from the atmosphere, perhaps because extreme weathering had made the rocks particularly porous. This capture of carbon dioxide could have triggered global cooling, the flip side of the global warming occurring in modern times due to the burning of fossil fuels. The cooling could have led to a global icy climate, and the resulting glaciers could have then sped up erosion even more.” https://lnkd.in/ds7GcBF View in LinkedIn

DARWIN CONFOUNDED. “Darwin's dilemma” regarding the origin and early evolution of Metazoa arguably persists, because incontrovertible fossil evidence for animals remains largely, or some might say completely, absent from Neoproterozoic rocks.” https://lnkd.in/d8SyHX7 View in LinkedIn

THE DOGMA. “The conventional interpretation of the metazoan body fossil record is that it documents an abrupt (in geological terms) appearance of animals around the base of the Cambrian Period 541 million years ago (Ma) – the “Cambrian Explosion” hypothesis. By around 520 Ma, virtually all of the animal phyla with recalcitrant tissues that one might expect to become fossilized are represented in the fossil record, as revealed by Lagerstätten such as the exceptionally preserved Chengjiang biota 4 (Fig. 1 summarizes major Ediacaran and early Cambrian fossil assemblages). However, this body fossil record is at odds with molecular clock analyses, which consistently indicate that Metazoa originated somewhere between 850 and 650 Ma in the Tonian or Cryogenian intervals, before diversifying through the Cryogenian and the Ediacaran.” https://lnkd.in/d8SyHX7 View in LinkedIn

NEW THINKING. “Although quantitative studies have yet to be undertaken in the Precambrian, it is clear that the rock record from the critical interval around the Precambrian‐Cambrian boundary is less complete than we might like. In many localities around the world, shallow‐marine Cambrian or Ordovician rocks lie unconformably above Precambrian rocks. This “Great Unconformity” records a prolonged period of continental denudation in the latest Neoproterozoic followed by a major marine transgression, suggesting that many latest Ediacaran shallow‐marine rock sequences – reflecting environments that may be expected to contain the highest diversity of animal life – have been eroded away.” https://lnkd.in/d8SyHX7 View in LinkedIn

SUBDUCTION. “Moreover, plate tectonic reconstructions suggest that much of the remaining sediment deposited during the latest Precambrian and Early Cambrian would have been subducted or accreted and destroyed at the Delamerian‐Ross subduction zone, which initiated in the Cambrian. The rock record, and therefore the fossil record, is probably particularly poor during the precise interval over which the first animals are predicted to have diversified and radiated, despite the apparently excellent potential for soft tissue preservation at this time.” https://lnkd.in/d8SyHX7 View in LinkedIn