Geologic Eras determined; the anthropocene? (Introduction)

by David Turell , Sunday, May 29, 2016, 14:43 (3099 days ago)

I've been down he Grand Canyon by raft a number of times. once with a geologic professor whose specialty was the Grand Canyon. I've seen the named layers, formed by deposition and erosion, touched the Great Unconformity were the Canyon misses 750 million years of Earth history. How do we know that? It's found elsewhere. We speak glibly of the Cambrian and the pre-Cambrian, but they are definite time periods that can be demarcated by what is in the ground layers, larger eras, and sub-parts, epochs. We have been in the Holocene epoch, and are we now in a new period affected by humans, called the Anthropocene?:-http://nautil.us/blog/are-we-in-the-anthropocene-yet-"In the early 1990s, a few miles west of El Kef, a town in Tunisia, geologists set a small golden spike in between two layers of clay that remains there to this day. They wanted to mark the tiny yet striking layer of iridium—a hard, dense, silvery-white metal—sandwiched in the middle. It was deposited by debris falling from the atmosphere all over the world, 66 million years ago, after an asteroid hit the planet and wiped out 75 percent of Earth's plant and animals species, including the dinosaurs [Chicxulub]. Geologists now recognize the iridium layer, no matter where it's found, as the boundary between the end of Mesozoic era and the beginning of Cenozoic era that, as you can see below, continues to this day. -***-"It still says we dwell in the Holocene epoch, the most recent stage of the Cenozoic. Colin Waters, the lead author of the report, is a geologist with the British Geological Survey and secretary of the International Commission on Stratigraphy's (ICS) Working Group on the Anthropocene, the “Age of Man.” The term is often used as shorthand for humanity's outsize impact on the natural world. Waters, though, simply argues that since it's possible to make out the geological boundary, we should name it. There are plenty of useful markers, he says—climatic, biological, and geochemical—to delineate the start of the Anthropocene from the end of the Holocene, just as the iridium layer marks the start of the Cenozoic. -***-"Lucy Edwards, though, isn't ready to say goodbye to the Holocene. A geologist with the United States Geological Survey, with a background in paleontology and stratigraphy, she recently coauthored a paper with the geologist Stanley Finney in Geological Society of America Today, called “The ‘Anthropocene' epoch: Scientific decision or political statement?” She thinks it's too soon to tell whether humankind's impact on the planet is producing a physical boundary as distinct as previous ones. -***-"For nearly a century, geologists argued about whether it was valid to have a Holocene epoch, or whether it was just one interglacial of many interglacials over the last million years. There's no criteria that says, “An epoch has to be this long.” If you look at a stratigraphic chart, things become shorter and shorter duration the closer you get to the current day. It's just saying we have more information that we can subdivide on a finer scale. But if you look out your window, most of what we're looking at is from the Holocene. We are now going through a significant change to the planet. So with the Anthropocene, we're saying we have the justification to recognize that change to the sedimentary record that will be permanent. As soon as you can see a change from the state of the planet, irrespective of what's causing it, that's the reason for recognizing the boundary.-***
" It's a committee decision. You get an international group to first figure out what they're looking for, what defines the boundary. Originally it was what you had in the field—fossils of clams, snails, trilobites. Now it's whatever you can use. It might be a geochemical signal, a fossil, or just some criterion that you think you can recognize worldwide or almost worldwide. You find the best sections you can that illustrate the boundary, then you pick a point that's the defining point.-"If you were a geologist looking back at today, it would look like an instantaneous event, but today it's happening too slowly to use as a tool. It's not to say that human evolution didn't have major milestones in the last 10,000 years, but they all start gradually and spread gradually. They're not good as a geological signal. A whole host of new technologies come together after the Second World War, and it's recognizable in the sediments. You could measure it in marine sediments, deep lake sediments, in glacial ice, speleothems—the layers of stalactites and stalagmites in caves—even tree rings."-Comment: Presented just for background interest. Sedimentation continuously covers the Earth in layers. Archeologists in Israel are digging back just 2,500 hundred years ago, and more.