Rising carbon dioxide levels at end of last ice age not tied to Pacific Ocean, as had been suspected

Scientists sampling a deep-sea sediment core. -  Image courtesy of Alan Mix
Scientists sampling a deep-sea sediment core. – Image courtesy of Alan Mix

After the last ice age peaked about 18,000 years ago, levels of atmospheric carbon dioxide rose about 30 percent. Scientists believe that the additional carbon dioxide — a heat-trapping greenhouse gas — played a key role in warming the planet and melting the continental ice sheets. They have long hypothesized that the source of the gas was the deep ocean.

But a new study by a University of Michigan paleoclimatologist and two colleagues suggests that the deep ocean was not an important source of carbon during glacial times. The finding will force researchers to reassess their ideas about the fundamental mechanisms that regulate atmospheric carbon dioxide over long time scales.

“We’re going back to the drawing board. It’s certainly fair to say that we need to have some other working hypotheses at this point,” said U-M paleoclimatologist David Lund, lead author of a paper in this week’s edition of the journal Nature Geoscience.

“If we can improve our understanding of the carbon cycle in the past, we will be better positioned moving forward as CO2 levels rise due to anthropogenic causes,” said Lund, an assistant professor in the U-M Department of Earth and Environmental Sciences. Lund’s co-authors are Alan Mix of Oregon State University and John Southon of the University of California, Irvine.

The study, which involved radiocarbon-dating of sediments from a core collected at a deep-ocean site (water depth 8,943 feet) off the coast of southwestern Oregon, was supported by the National Science Foundation and the University of Michigan.

The work involved radiocarbon dating dozens of sediment samples that contained microscopic shells created by plankton. The samples were collected from various locations in the core, spanning the period from 8,000 to 22,000 years ago. Over thousands of years, ocean water circulates from the surface to the bottom, then back to the surface. The radiocarbon results revealed the basin’s circulation or “ventilation” rate, the amount of time that had passed since the various deep-water samples were last in contact with the atmosphere.

The scientists expected to find that the ventilation rate in the basin slowed during glacial times, allowing carbon dioxide to accumulate in the abyss and depleting atmospheric levels of the gas.

Surprisingly, they found that the ventilation rate during glacial times was roughly the same as it is today, suggesting that the Pacific was not an important reservoir of carbon during glacial times.

“Frankly, we’re kind of baffled by the whole thing,” said Oregon State University paleo-oceanographer Alan Mix, one of the co-authors. “The North Pacific was such an obvious source for the carbon, but it just doesn’t match up.”

“At least we’ve shown where the carbon wasn’t,” Mix said. “Now we just have to find where it was.”

Tenerife geology discovery is among ‘world’s best’

Pablo Dávila-Harris looks at part of the huge landslide deposit discovered on Tenerife, showing the chaotic  and shattered rubble from the collapsed volcano. (The central dark debris-block is about 15 meters in diameter and must weigh many tons). -  Pablo Dávila-Harris
Pablo Dávila-Harris looks at part of the huge landslide deposit discovered on Tenerife, showing the chaotic and shattered rubble from the collapsed volcano. (The central dark debris-block is about 15 meters in diameter and must weigh many tons). – Pablo Dávila-Harris

Volcanologists from the University of Leicester have uncovered one of the world’s best-preserved accessible examples of a monstrous landslide that followed a huge volcanic eruption on the Canarian island of Tenerife.

Seven hundred and thirty-three thousand years ago, the southeast slopes of Tenerife collapsed into the sea, during the volcanic eruption. The onshore remains of this landslide have just been discovered amid the canyons and ravines of Tenerife’s desert landscape by volcanologists Pablo Dávila-Harris and Mike Branney of the University of Leicester’s Department of Geology.

The findings have been published in this October’s edition of the international journal Geology. The research was funded by CONACYT, Mexico.

Dr Branney said: “It is one of the world’s best-preserved accessible examples of such an awesome phenomenon, because the debris from such landslides mostly spreads far across the deep ocean floor, inaccessible for close study.

“The beautifully-displayed Tenerife rubble includes blocks of rapidly chilled lava, added as the volcano erupted. Radioactive minerals within them enabled the researchers’ colleague, Michael Storey at Roskilde University, Denmark, to provide such a precise date for this natural catastrophe.

“Climate change is often invoked as a trigger for ocean-island landslides, but in this case it seems that a growing dome of hot lava triggered the landslide by pushing the side of the volcano outwards.

“In the shattered landscape that remained, lakes formed as rivers were dammed by debris, and the change to the shape of the island altered the course of explosive volcanic eruptions for hundreds of thousands of years afterwards.”

The researchers state that such phenomena are common but infrequent, and understanding them is vital, for their effects go far beyond a single ocean island. Tsunamis generated from such events may travel to devastate coastlines thousands of miles away.

“Understanding the Earth’s more violent events will help us be prepared, should repeat performances threaten,” they state.

New research findings impact Seattle, Sierra Nevada

Improved ground motion prediction for Seattle Basin

Seattle, Washington sits atop a deep, complex sedimentary basin that is known to amplify seismic shaking. Researchers at University of Washington have developed a new model to evaluate the velocity of seismic waves at shallow depth, offering new detail of the top 3.5 kilometers (~2.1 miles) that will lead to refined seismic hazard assessments for the area.

Many existing velocity models are sufficient for predicting grounds motions on rock sites, but not in the more complex crustal and sedimentary basins found in the Seattle area. Developed using passive recording of noise, this model reveals the internal structure of the sedimentary basin beneath Seattle including areas with lower velocities than previous models. There is a pronounced low velocity zone just north of the Seattle Fault in the vicinity of Elliott Bay as well as more subtle variations in other parts of the city. Velocity variations within the basin suggest diverse geological features, such as sub-basins, and will change predictions of amplified ground motion.

“Basin Shear-Wave Velocities beneath Seattle, Washington, from Noise-Correlation Rayleigh Waves,” by Andrew A. Delorey and John E. Vidale, University of Washington.

Contact: Andrew Delorey, adelorey@u.washington.edu or 206-335-6170.

Two faults exposed in eastern Sierra Nevada

Excavated trenches reveal two faults that bound the eastern flank of the Sierra Nevada in Antelope Valley, California and the Carson Range in Reno, Nevada. Observations by researchers at University of Nevada, Reno, suggest new details about the active faulting of the area.

Fault scarps that vertically offset young alluvial fan deposits along the Antelope Valley fault suggest the most recent surface rupture was at least 14 miles (23 km) long. Radiocarbon dating of bulk soil samples suggests the most recent earthquake occurred approximately 1350 years ago and the penultimate earthquake almost 5000 years earlier (or

6250 years before present day).
The trench along the Carson Range provides insufficient data to quantify an earthquake event history, though large offsets appear to happen infrequently. The fault dips at a very low angle, which could have significant meaning for the behavior of the fault and the severity of related ground motion.

“Paleoseismic Trenches across the Sierra Nevada and Carson Range Fronts in Antelope Valley, California and Reno, Nevada,” by Alexandra Sarmiento, Steven Wesnousky and Jayne Bormann of University of Nevada, Reno.

Contact: Alexandra Sarmiento, sarmiento.alexandrac@gmail.com or 702-526-8722.

Assessing California earthquake forecasts

In the study, UC Davis researchers compare seven different earthquake forecasts (including their own) that were submitted to a competition organized by the Southern California Earthquake Center.

The findings should help researchers both develop better earthquake forecasts and improve their tools for assessing those forecasts, said Donald Turcotte, a distinguished professor of geology at UC Davis and co-author of the paper.

The center launched the competition in 2005 based on a previous forecast published by the UC Davis group in 2001. Teams were invited to forecast the probability of an earthquake of magnitude 4.95 or greater, from Jan. 1, 2006, to Dec. 31, 2010, in almost 8,000 grid squares covering California and bordering areas.

During this time, 31 earthquakes struck in 22 grid squares, with the largest being the magnitude 7.2 earthquake just south of the U.S.-Mexican border in April 2010. All seven forecasts showed some utility in forecasting the locations of likely earthquakes: The best forecasts were about 10 times better than a random forecast.

The forecast submitted by the UC Davis group was the most accurate in picking the locations of the earthquakes, correctly labeling 17 of 22 grids and giving the highest probability of an earthquake in eight of these 17. Using a different forecasting method, Agnes Helmstetter of UCLA and colleagues gave the highest average probability of an earthquake for all 22 affected grids, although it did less well at assigning a higher probability to grid squares where an earthquake occurred.

“Just as there are alternative ways to forecast earthquakes, there are also alternative ways to evaluate the success of the forecasts,” Turcotte said, noting that other publications evaluating the forecasts are expected.