Buried oxygen rose to the occasion as Earth's early atmosphere formed

Research by geoscientists at Yale, Arizona State University, and Bayerisches Geoinstitut in Germany suggests that convection in Earth's mantle�the slow movement of rocks circulating beneath the surface, caused by heat from inside the Earth�is affected by the distribution of oxygen in those minerals.

Oxygen buried deep underground in minerals may have prompted the churning of Earth's rocky mantle 
billions of years ago and helped transform the planet's early atmosphere, according to a new study 
[Credit: Yale University]

"When there's less oxygen present in the rock, it's denser than when there is more oxygen present, even though the rest of the elements are the same. The more oxidized rock preferentially rises over the reduced rock," said Kanani Lee, the study's principal investigator and an associate professor of geology and geophysics at Yale.

This process had consequences both above and below the surface. Deep below the surface, the more oxygen-depleted rocks sank to the bottom of the rocky mantle, leading to the creation of massive, dense piles just above the Earth's core such as those found deep beneath the Pacific and Atlantic oceans.

"This is the first time anyone has shown that the relative amount of oxygen deep in the Earth influences the minerals that rocks are made of and how it changes their densities," Lee said.

Tingting Gu, a former Yale postdoctoral associate and the paper's lead author, added, "The mantle is not entirely isolated from the surface. For example, gases from volcanic eruptions connect the mantle with the atmosphere. Our model predicts that early in Earth's history, the shallow mantle was less oxidized and thus released gases such as methane that would consume oxygen produced by photosynthesis. But as time progressed and the less dense oxidized material rose in the mantle, biotic oxygen could be preserved and accumulate in the atmosphere. This process could be unique among the terrestrial planets because of their different compositions."

The findings appear online in the journal Nature Geoscience.

Author: Jim Shelton | Source: Yale University [August 02, 2016]

Buried oxygen rose to the occasion as Earth's early atmosphere formed

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Lack of water likely caused extinction of isolated Alaska mammoths

A remnant population of woolly mammoths on a remote Alaska island was likely pushed to extinction by rising sea levels and a lack of access to fresh water, according to a newly published study.

A remnant population of woolly mammoths on a remote Alaska island was likely pushed to extinction 
by a lack of access to fresh water [Credit: Shuttershock]

By analyzing layers of a dated sediment core from a lake on St. Paul Island, researchers determined that mammoths went extinct on the island roughly 5,600 years ago, thousands of years after remnant mainland populations died off. The study also indicated that the Bering Sea island experienced a phase of dry conditions and declining water quality at about the same time the mammoths vanished.

Matthew Wooller, director of the Alaska Stable Isotope Facility at the University of Alaska Fairbanks and a co-author of the study, said past events on St. Paul Island provided a unique opportunity for research. Mammoths were trapped there when rising sea levels submerged the Bering Sea land bridge, and survived about 5,000 years longer than isolated mainland populations. There is no evidence of people having lived on the island during the era.

In 2013, a team of researchers collected a sediment core from the bed of one of the few freshwater lakes on St. Paul Island. Wooller and fellow UAF researcher Kyungcheol Choy measured the stable oxygen isotope ratios of the prehistoric remains of aquatic insects preserved in the sediment from before, during and after the extinction of mammoths from the island.

A research team sets up on a frozen lake on St. Paul Island 
[Credit: Matthew Wooller]

The remains of aquatic organisms living in lakes retain water isotope signatures within their bodies, which allowed researchers studying their exoskeletons to determine that lake levels had diminished. The remains of diatoms and aquatic invertebrates from the core also changed over time, indicating decreasing lake levels and water quality leading up to the mammoth extinction.

Nitrogen isotope analyses of dated mammoth bones and teeth also signaled progressively drier conditions leading up to the extinction event. Wooller said these "multiple lines of evidence" of decreasing lake levels provide a strong case for what led to the animals' extinction.

"It paints a dire picture of the situation for these mammoths," Wooller said. "Freshwater resources look like the smoking gun for what pushed them into this untenable situation."

The study not only determined one of the best-dated prehistoric extinctions, using state-of-the-art techniques on ancient mammoth DNA preserved in the lake core from St. Paul Island, it also showed the vulnerability of small island populations to environmental change.

St. Paul Island gradually shrank to its current size of 110 square kilometers as sea levels rose, reducing the opportunities for mammoths to find new areas with water. Conditions incrementally changed for about 2,000 years before mammoths went extinct.

Modern climate change could shift conditions more rapidly, which could make the story of prehistoric St. Paul Island relevant today, Wooller said.

The results were published today in the Proceedings of the National Academy of Sciences.

Source: University of Alaska Fairbanks [August 01, 2016]

Lack of water likely caused extinction of isolated Alaska mammoths

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