Paleontologists' view of the past is usually just a series of snapshots, brief moments in time when conditions were right at a particular location for animals or plants to fossilize. Now, however, they have discovered a single site that records the development of life over an astonishing 120 million years. Better yet, it spans the Paleozoic, a particularly important but poorly preserved era. The only problem is this treasure trove lies in one of the most inaccessible locations on Earth.
On the banks of the Peel River, which further north joins the Mackenzie Delta into the Arctic Sea, Science Advances reports the discovery of rocks recording life on the ocean floor from 490 to 370 million years ago.
"It's unheard of to have that much of Earth's history in one place,” said Dr Erik Sperling of Stanford University in a statement. "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."
Even when conditions are initially suitable for fossilization to occur, geological processes usually disrupt what is left behind, leaving us with only fragmented records.
The Peel River depots start in the Upper Cambrian, a time when oxygen was too scarce to allow much animal life, and end in the Middle Devonian, when fishes had taken over the seas. Aside from some short interruptions, the entire Ordovician and Silurian eras are included. Further study should tell us a great deal about the species then living in these seas, which were not on the edge of the Arctic Circle at the time as they are now. Such an uninterrupted record can also act as a calibration tool for other deposits, helping to provide a more accurate indication of their timing. “In order to make comparisons throughout these huge swaths of our history and understand long-term trends, you need a continuous record," Sperling said.
Stirling's paper focuses on what the site reveals about the rise of oxygen. The early Earth had little to no oxygen in its atmosphere or oceans. The Great Oxidation Event 2.5-2.2 billion years ago changed this, but there was still not enough oxygen to support today's fast-moving, active life. The timing of the second big change, when oxygen concentrations came to approach those today, is also a matter of considerable uncertainty. It may have occurred as early as 800 million years ago, or possibly as little as half that time ago. Resolving this question will tell us a lot about the capacity of different types of life to survive in low-oxygen environments.
The paper concludes the atmosphere did not approach its current state until later than many scientists have previously thought. "The early animals were still living in a low oxygen world," Sperling said.
Valuable as it is, this information was not won easily. The location is so inaccessible Sperling and colleagues had to fly to the site by helicopter and fight their way through thick brush with machetes. Once there, fieldwork was only possible for a short time before winter set in.