Longest known continuous record of the Paleozoic discovered in Yukon wilderness
Date:
July 8, 2021
Source:
Stanford University
Summary:
Expeditions to a remote area of Yukon, Canada, have uncovered
a 120- million-year-long geological record of a time when land
plants and complex animals first evolved and ocean oxygen levels
began to approach those in the modern world.
FULL STORY ========================================================================== Hundreds of millions of years ago, in the middle of what would eventually become Canada's Yukon Territory, an ocean swirled with armored trilobites, clam-like brachiopods and soft, squishy creatures akin to slugs and squid.
==========================================================================
A trove of fossils and rock layers formed on that ancient ocean floor
have now been unearthed by an international team of scientists along the
banks of the Peel River a few hundred miles south of the Arctic's Beaufort
Sea. The discovery reveals oxygen changes at the seafloor across nearly
120 million years of the early Paleozoic era, a time that fostered the
most rapid development and diversification of complex, multi-cellular
life in Earth's history.
"It's unheard of to have that much of Earth's history in one place," said Stanford University geological scientist Erik Sperling, lead author of
a July 7 study detailing the team's findings in Science Advances. Most
rock formations from the Paleozoic Era have been broken up by tectonic
forces or eroded over time. "There's nowhere else in the world that I
know of where you can study that long a record of Earth history, where
there's basically no change in things like water depth or basin type."
Oxygen was scarce in the deep water of this and other oceans at the
dawn of the Paleozoic, roughly 541 million years ago. It stayed scarce
until the Devonian, roughly 405 million years ago, when, in a geological
blink -- no more than a few million years -- oxygen likely rocketed
to levels close to those in modern oceans and the diversity of life
on Earth exploded. Big, predatory fish appeared. Primitive ferns and
conifers marched across continents previously ruled by bacteria and
algae. Dragonflies took flight. And all of this after nearly four billion
years of Earth's landscapes being virtually barren.
Scientists have long debated what might have caused the dramatic shift
from a low oxygen world to a more oxygenated one that could support
a diverse web of animal life. But until now, it has been difficult to
pin down the timing of global oxygenation or the long-term, background
state of the world's oceans and atmosphere during the era that witnessed
both the so-called Cambrian explosion of life and the first of Earth's
"Big Five" mass extinctions, about 445 million years ago at the end of
the Ordovician.
"In order to make comparisons throughout these huge swaths of our history
and understand long-term trends, you need a continuous record," said
Sperling, an assistant professor of geological sciences at Stanford's
School of Earth, Energy & Environmental Sciences (Stanford Earth).
========================================================================== Context for past life With permission from the Na Cho Nyak Dun and
Tetlit Gwitch'in communities in Yukon, Sperling's team, which included researchers from Dartmouth College and the Yukon Geological Survey,
spent three summers at the Peel River site.
Arriving by helicopter, the research team hacked through brush with
machetes beside Class VI rapids to collect hundreds of fist-sized samples
of rock from more than a mile of interbedded layers of shale, chert and
lime mudstone.
Back at Sperling's lab at Stanford, a small army of summer undergraduates
and graduate students worked over five summers to help analyze the
fossils and chemicals entombed in the rocks. "We spent a lot of time
splitting open rocks and looking at graptolite fossils," Sperling
said. Because graptolites evolved a vast array of recognizable body
shapes relatively quickly, the pencil-like markings left by the fossils
of these colony-dwelling sea creatures give geologists a way to date
the rocks in which they're found.
Once the researchers had finished identifying and dating graptolite
fossils, they ground the rocks in a mill, then measured iron, carbon, phosphorus and other elements in the resulting powder to assess the
ocean conditions at the time and place where the layers formed. They
analyzed 837 new samples from the Peel River site, as well as 106 new
samples from other parts of Canada and 178 samples from around the world
for comparison.
Winners and losers The data show low oxygen levels, or anoxia, likely
persisted in the world's oceans for millions of years longer than
previously thought -- well into the Phanerozoic, when land plants and
early animals began to diversify. "The early animals were still living in
a low oxygen world," Sperling said. Contrary to long-held assumptions,
the scientists found Paleozoic oceans were also surprisingly free of
hydrogen sulfide, a respiratory toxin often found in the anoxic regions
of modern oceans.
When oxygen eventually did tick upward in marine environments, it came
about just as larger, more complex plant life took off. "There's a
ton of debate about how plants impacted the Earth system," Sperling
said. "Our results are consistent with a hypothesis that as plants
evolved and covered the Earth, they increased nutrients to the ocean,
driving oxygenation." In this hypothesis, the influx of nutrients to
the sea would have given a boost to primary productivity, a measure of
how quickly plants and algae take carbon dioxide and sunlight, turn them
into new biomass -- and release oxygen in the process.
The change probably killed off graptolites. "Although more
oxygen is really good for a lot of organisms, graptolites
lost the low oxygen habitat that was their refuge,"
Sperling said. "Any environmental change is going to have
winners and losers. Graptolites might have been the losers." ========================================================================== Story Source: Materials provided by Stanford_University. Original written
by Josie Garthwaite. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Erik A. Sperling, Michael J. Melchin, Tiffani Fraser, Richard
G. Stockey,
Una C. Farrell, Liam Bhajan, Tessa N. Brunoir, Devon B. Cole,
Benjamin C.
Gill, Alfred Lenz, David K. Loydell, Joseph Malinowski, Austin
J. Miller, Stephanie Plaza-Torres, Beatrice Bock, Alan D. Rooney,
Sabrina A.
Tecklenburg, Jacqueline M. Vogel, Noah J. Planavsky, Justin
V. Strauss. A long-term record of early to mid-Paleozoic marine
redox change. Science Advances, 2021; 7 (28): eabf4382 DOI:
10.1126/sciadv.abf4382 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2021/07/210708185945.htm
--- up 8 weeks, 6 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)