Pacific plate shrinking as it cools

A map produced by scientists at the University of Nevada, Reno, and Rice University shows predicted velocities for sectors of the Pacific tectonic plate relative to points near the Pacific-Antarctic ridge, which lies in the South Pacific ocean. The researchers show the Pacific plate is contracting as younger sections of the lithosphere cool. -  Corné Kreemer and Richard Gordon
A map produced by scientists at the University of Nevada, Reno, and Rice University shows predicted velocities for sectors of the Pacific tectonic plate relative to points near the Pacific-Antarctic ridge, which lies in the South Pacific ocean. The researchers show the Pacific plate is contracting as younger sections of the lithosphere cool. – Corné Kreemer and Richard Gordon

The tectonic plate that dominates the Pacific “Ring of Fire” is not as rigid as many scientists assume, according to researchers at Rice University and the University of Nevada.

Rice geophysicist Richard Gordon and his colleague, Corné Kreemer, an associate professor at the University of Nevada, Reno, have determined that cooling of the lithosphere — the outermost layer of Earth — makes some sections of the Pacific plate contract horizontally at faster rates than others and cause the plate to deform.

Gordon said the effect detailed this month in Geology is most pronounced in the youngest parts of the lithosphere — about 2 million years old or less — that make up some the Pacific Ocean’s floor. They predict the rate of contraction to be 10 times faster than older parts of the plate that were created about 20 million years ago and 80 times faster than very old parts of the plate that were created about 160 million years ago.

The tectonic plates that cover Earth’s surface, including both land and seafloor, are in constant motion; they imperceptibly surf the viscous mantle below. Over time, the plates scrape against and collide into each other, forming mountains, trenches and other geological features.

On the local scale, these movements cover only inches per year and are hard to see. The same goes for deformations of the type described in the new paper, but when summed over an area the size of the Pacific plate, they become statistically significant, Gordon said.

The new calculations showed the Pacific plate is pulling away from the North American plate a little more — approximately 2 millimeters a year — than the rigid-plate theory would account for, he said. Overall, the plate is moving northwest about 50 millimeters a year.

“The central assumption in plate tectonics is that the plates are rigid, but the studies that my colleagues and I have been doing for the past few decades show that this central assumption is merely an approximation — that is, the plates are not rigid,” Gordon said. “Our latest contribution is to specify or predict the nature and rate of deformation over the entire Pacific plate.”

The researchers already suspected cooling had a role from their observation that the 25 large and small plates that make up Earth’s shell do not fit together as well as the “rigid model” assumption would have it. They also knew that lithosphere as young as 2 million years was more malleable than hardened lithosphere as old as 170 million years.

“We first showed five years ago that the rate of horizontal contraction is inversely proportional to the age of the seafloor,” he said. “So it’s in the youngest lithosphere (toward the east side of the Pacific plate) where you get the biggest effects.”

The researchers saw hints of deformation in a metric called plate circuit closure, which describes the relative motions where at least three plates meet. If the plates were rigid, their angular velocities at the triple junction would have a sum of zero. But where the Pacific, Nazca and Cocos plates meet west of the Galápagos Islands, the nonclosure velocity is 14 millimeters a year, enough to suggest that all three plates are deforming.

“When we did our first global model in 1990, we said to ourselves that maybe when we get new data, this issue will go away,” Gordon said. “But when we updated our model a few years ago, all the places that didn’t have plate circuit closure 20 years ago still didn’t have it.”

There had to be a reason, and it began to become clear when Gordon and his colleagues looked beneath the seafloor. “It’s long been understood that the ocean floor increases in depth with age due to cooling and thermal contraction. But if something cools, it doesn’t just cool in one direction. It’s going to be at least approximately isotropic. It should shrink the same in all directions, not just vertically,” he said.

A previous study by Gordon and former Rice graduate student Ravi Kumar calculated the effect of thermal contraction on vertical columns of oceanic lithosphere and determined its impact on the horizontal plane, but viewing the plate as a whole demanded a different approach. “We thought about the vertically integrated properties of the lithosphere, but once we did that, we realized Earth’s surface is still a two-dimensional problem,” he said.

For the new study, Gordon and Kreemer started by determining how much the contractions would, on average, strain the horizontal surface. They divided the Pacific plate into a grid and calculated the strain on each of the nearly 198,000 squares based on their age, as determined by the seafloor age model published by the National Geophysical Data Center.

“That we could calculate on a laptop,” Gordon said. “If we tried to do it in three dimensions, it would take a high-powered computer cluster.”

The surface calculations were enough to show likely strain fields across the Pacific plate that, when summed, accounted for the deformation. As further proof, the distribution of recent earthquakes in the Pacific plate, which also relieve the strain, showed a greater number occurring in the plate’s younger lithosphere. “In the Earth, those strains are either accommodated by elastic deformation or by little earthquakes that adjust it,” he said.

“The central assumption of plate tectonics assumes the plates are rigid, and this is what we make predictions from,” said Gordon, who was recently honored by the American Geophysical Union for writing two papers about plate movements that are among the top 40 papers ever to appear in one of the organization’s top journals. “Up until now, it’s worked really well.”

“The big picture is that we now have, subject to experimental and observational tests, the first realistic, quantitative estimate of how the biggest oceanic plate departs from that rigid-plate assumption.”

Severe drought is causing the western US to rise

The severe drought gripping the western United States in recent years is changing the landscape well beyond localized effects of water restrictions and browning lawns. Scientists at Scripps Institution of Oceanography at UC San Diego have now discovered that the growing, broad-scale loss of water is causing the entire western U.S. to rise up like an uncoiled spring.

Investigating ground positioning data from GPS stations throughout the west, Scripps researchers Adrian Borsa, Duncan Agnew, and Dan Cayan found that the water shortage is causing an “uplift” effect up to 15 millimeters (more than half an inch) in California’s mountains and on average four millimeters (0.15 of an inch) across the west. From the GPS data, they estimate the water deficit at nearly 240 gigatons (62 trillion gallons of water), equivalent to a six-inch layer of water spread out over the entire western U.S.

Results of the study, which was supported by the U.S. Geological Survey (USGS), appear in the August 21 online edition of the journal Science.

While poring through various sets of data of ground positions from highly precise GPS stations within the National Science Foundation’s Plate Boundary Observatory and other networks, Borsa, a Scripps assistant research geophysicist, kept noticing the same pattern over the 2003-2014 period: All of the stations moved upwards in the most recent years, coinciding with the timing of the current drought.

Agnew, a Scripps Oceanography geophysics professor who specializes in studying earthquakes and their impact on shaping the earth’s crust, says the GPS data can only be explained by rapid uplift of the tectonic plate upon which the western U.S. rests (Agnew cautions that the uplift has virtually no effect on the San Andreas fault and therefore does not increase the risk of earthquakes).

For Cayan, a research meteorologist with Scripps and USGS, the results paint a new picture of the dire hydrological state of the west.

“These results quantify the amount of water mass lost in the past few years,” said Cayan. “It also represents a powerful new way to track water resources over a very large landscape. We can home in on the Sierra Nevada mountains and critical California snowpack. These results demonstrate that this technique can be used to study changes in fresh water stocks in other regions around the world, if they have a network of GPS sensors.”

Has the puzzle of rapid climate change in the last ice age been solved?

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful 'conveyor belt' in the world's oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. -  Map: Alfred-Wegener-Institut
During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful ‘conveyor belt’ in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. – Map: Alfred-Wegener-Institut

During the last ice age a large part of North America was covered with a massive ice sheet up to 3km thick. The water stored in this ice sheet is part of the reason why the sea level was then about 120 meters lower than today. Young Chinese scientist Xu Zhang, lead author of the study who undertook his PhD at the Alfred Wegener Institute, explains. “The rapid climate changes known in the scientific world as Dansgaard-Oeschger events were limited to a period of time from 110,000 to 23,000 years before present. The abrupt climate changes did not take place at the extreme low sea levels, corresponding to the time of maximum glaciation 20,000 years ago, nor at high sea levels such as those prevailing today – they occurred during periods of intermediate ice volume and intermediate sea levels.” The results presented by the AWI researchers can explain the history of climate changes during glacial periods, comparing simulated model data with that retrieved from ice cores and marine sediments.

How rapid temperature changes might have occurred during times when the Northern Hemisphere ice sheets were at intermediate sizes (see schematic depictions on http://bit.ly/1uQoI70).

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong westerly winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful “conveyor belt” in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions.

During the extended cold phases the ice sheets continued to thicken. When higher ice sheets prevailed over North America, typical in periods of intermediate sea levels, the prevailing westerly winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north. Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean. Consequently, a thicker Laurentide Ice Sheet over North America resulted in increased ocean circulation and therefore greater transportation of heat to the north. The climate in the Northern Hemisphere became dramatically warmer within a few decades until, due to the retreat of the glaciers over North America and the renewed change in wind conditions, it began to cool off again.

“Using the simulations performed with our climate model, we were able to demonstrate that the climate system can respond to small changes with abrupt climate swings,” explains Professor Gerrit Lohmann, leader of the Paleoclimate Dynamics group at the Alfred Wegener Institute, Germany. In doing so he illustrates the new study’s significance with regards to contemporary climate change. “At medium sea levels, powerful forces, such as the dramatic acceleration of polar ice cap melting, are not necessary to result in abrupt climate shifts and associated drastic temperature changes.”

At present, the extent of Arctic sea ice is far less than during the last glacial period. The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Climate changes following the pattern of the last ice age are therefore not to be anticipated under today’s conditions.

“There are apparently some situations in which the climate system is more resistant to change while in others the system tends toward strong fluctuations,” summarises Gerrit Lohmann. “In terms of the Earth’s history, we are currently in one of the climate system’s more stable phases. The preconditions, which gave rise to rapid temperature changes during the last ice age do not exist today. But this does not mean that sudden climate changes can be excluded in the future.”

Induced quakes rattle less than tectonic quakes, except near epicenter

Induced earthquakes generate significantly lower shaking than tectonic earthquakes with comparable magnitudes, except within 10 km of the epicenter, according to a study to be published online August 19 in the Bulletin of the Seismological Society of America (BSSA). Within 10 km of the epicenter, the reduced intensity of shaking is likely offset by the increased intensity of shaking due to the shallow source depths of injection-induced earthquakes.

Using data from the USGS “Did You Feel It?” system, Seismologist Susan Hough explored the shaking intensities of 11 earthquakes in the central and eastern United States (CEUS) considered likely caused by fluid injection.

“Although moderate injection-induced earthquakes in the CEUS will be widely felt due to low regional attenuation,” writes Hough, “the damage from earthquakes induced by injection will be more concentrated in proximity to the event epicenters than shaking from tectonic earthquakes.”

Gorges are eradicated by downstream sweep erosion

Local surface uplift can block rivers, particularly in mountainous regions. The impounded water, however, always finds its way downstream, often cutting a narrow gorge into the rocks. Subsequent erosion of the rocks can lead to a complete eradication of this initial incision, until not a trace is left of the original breakthrough. In extreme cases the whole gorge disappears, leaving behind a broad valley with a flat floodplain. Previously, the assumption was that this transition from a narrow gorge to a wide valley was driven by gorge widening and the erosion of the walls of the gorges.

A team of scientists from the GFZ German Research Centre for Geosciences in Potsdam has now revealed a new mechanism that drives this process of fluvial erosion (Nature Geoscience, 17.08.2014). The geoscientists analyzed the development of a gorge on the Da’an Chi river in Taiwan over a period of almost ten years. There, uplift that was caused by the Jiji earthquake of 1999 (magnitude 7.6), and that runs transverse to the river, had formed a blockage. Earthquakes of that size occur there every 300 to 500 years. “Before the quake there was no sign of a gorge at all in this riverbed, which is one and a half kilometers wide”, explains Kristen Cook of the GFZ. “We have here the world’s first real-time observation of the evolution of gorge width by fluvial erosion over the course of several years.” Currently the gorge is roughly a kilometer long, 25 meters wide and up to 17 meters deep. Initially, the gorge walls were eroded at a rate of five meters per year, and today are still retreating one and a half meters per year.

The scientists identified a hitherto unknown mechanism by which the gorge is destroyed. “Downstream sweep erosion” they termed this process. “A wide braided channel upstream of the gorge is necessary,” explains co-author Jens Turowski (GFZ). “The course of this channel changes regularly and it has to flow in sharp bends to run into the gorge. In these bends, the bed-load material that is transported by the river hits the upper edge of the gorge causing rapid erosion.” This mechanism gradually washes away all of the bedrock surrounding the gorge and, therefore, is the cause for the planation of the riverbed over the complete width of the valley. Assuming the current erosion rate of 17 meters per year, it will take here at the Da’an Chi River only 50 to 100 years until again a flat beveled channel again fills the valley. In contrast, lateral erosion in the gorge would be too slow to eradicate the gorge in the time of one earthquake cycle. The newly discovered downstream sweep erosion is far more effective.

Foreshock series controls earthquake rupture

A long lasting foreshock series controlled the rupture process of this year’s great earthquake near Iquique in northern Chile. The earthquake was heralded by a three quarter year long foreshock series of ever increasing magnitudes culminating in a Mw 6.7 event two weeks before the mainshock. The mainshock (magnitude 8.1) finally broke on April 1st a central piece out of the most important seismic gap along the South American subduction zone. An international research team under leadership of the GFZ German Research Centre for Geosciences now revealed that the Iquique earthquake occurred in a region where the two colliding tectonic plates where only partly locked.

The Pacific Nazca plate and the South American plate are colliding along South America’s western coast. While the Pacific sea floor submerges in an oceanic trench under the South American coast the plates get stressed until occasionally relieved by earthquakes. In about 150 years time the entire plate margin from Patagonia in the south to Panama in the north breaks once completely through in great earthquakes. This cycle is almost complete with the exception of a last segment – the seismic gap near Iquique in northern Chile. The last great earthquake in this gap occurred back in 1877. On initiative of the GFZ this gap was monitored in an international cooperation (GFZ, Institut de Physique du Globe Paris, Centro Sismologico National – Universidad de Chile, Universidad de Catolica del Norte, Antofagasta, Chile) by the Integrated Plate Boundary Observatory Chile (IPOC), with among other instruments seismographs and cont. GPS. This long and continuous monitoring effort makes the Iquique earthquake the best recorded subduction megathrust earthquake globally. The fact that data of IPOC is distributed to the scientific community in near real time, allowed this timely analysis.

Ruptures in Detail

The mainshock of magnitude 8.1 broke the 150 km long central piece of the seismic gap, leaving, however, two large segments north and south intact. GFZ scientist Bernd Schurr headed the newly published study that appeared in the lastest issue of Nature Advance Online Publication: “The foreshocks skirted around the central rupture patch of the mainshock, forming several clusters that propagated from south to north.” The long-term earthquake catalogue derived from IPOC data revealed that stresses were increasing along the plate boundary in the years before the earthquake. Hence, the plate boundary started to gradually unlock through the foreshock series under increasing stresses, until it finally broke in the Iquique earthquake. Schurr further states: “If we use the from GPS data derived locking map to calculate the convergence deficit assuming the ~6.7 cm/yr convergence rate and subtract the earthquakes known since 1877, this still adds up to a possible M 8.9 earthquake.” This applies if the entire seismic gap would break at once. However, the region of the Iquique earthquake might now form a barrier that makes it more likely that the unbroken regions north and south break in separate, smaller earthquakes.

International Field Campaign

Despite the fact that the IPOC instruments delivered continuous data before, during and after the earthquake, the GFZ HART (Hazard And Risk Team) group went into the field to meet with international colleagues to conduct additional investigations. More than a dozen researchers continue to measure on site deformation and record aftershocks in the aftermath of this great rupture. Because the seismic gap is still not closed, IPOC gets further developed. So far 20 multi-parameter stations have been deployed. These consist of seismic broadband and strong-motion sensors, continuous GPS receivers, magneto-telluric and climate sensors, as well as creepmeters, which transmit data in near real-time to Potsdam. The European Southern astronomical Observatory has also been integrated into the observation network.

Study of Chilean quake shows potential for future earthquake

Near real-time analysis of the April 1 earthquake in Iquique, Chile, showed that the 8.2 event occurred in a gap on the fault unruptured since 1877 and that the April event was not what the scientists had expected, according to an international team of geologists.

“We assumed that the area of the 1877 earthquake would eventually rupture, but all indications are that this 8.2 event was not the 8.8 event we were looking for,” said Kevin P. Furlong, professor of geophysics, Penn State. “We looked at it to see if this was the big one.”

But according to the researchers, it was not. Seismologists expect that areas of faults will react the same way over and over. However, the April earthquake was about nine times less energetic than the one in 1877 and was incapable of releasing all the stress on the fault, leaving open the possibility of another earthquake.

The Iquique earthquake took place on the northern portion of the subduction zone formed when the Nazca tectonic plate slides under the South American plate. This is one of the longest uninterrupted plate boundaries on the planet and the site of many earthquakes and volcanos. The 8.2 earthquake was foreshadowed by a systematic sequence of foreshocks recorded at 6.0, 6.5, 6.7 and 6.2 with each foreshock triggering the next until the main earthquake occurred.

These earthquakes relieved the stresses on some parts of the fault. Then the 8.2 earthquake relieved more stress, followed by a series of aftershocks in the range of 7.7. While the aftershocks did fill in some of the gaps left by the 8.2 earthquake, the large earthquake and aftershocks could not fill in the entire gap where the fault had not ruptured in a very long time. That area is unruptured and still under stress.

The foreshocks eased some of the built up stress on 60 to 100 miles of fault, and the main shock released stress on about 155 miles, but about 155 miles of fault remain unchanged, the researchers report today (Aug. 13) in Nature.

“There can still be a big earthquake there,” said Furlong. “It didn’t release the total hazard, but it told us something about this large earthquake area. That an 8.8 rupture doesn’t always happen.”

The researchers were able to do this analysis in near real time because of the availability of large computing power and previously laid groundwork.

The computing power allowed researchers to model the fault more accurately. In the past, subduction zones were modeled as if they were on a plane, but the plate that is subducting curves underneath the other plate creating a 3-dimensional fault line. The researchers used a model that accounted for this curving and so more accurately recreated the stresses on the real geology at the fault.

“One of the things the U.S. Geological Survey and we have been doing is characterizing the major tectonic settings,” said Furlong. “So when an earthquake is imminent, we don’t need a lot of time for the background.”

In essence, they are creating a library of information about earthquake faults and have completed the first level, a general set of information on areas such as Japan, South America and the Caribbean. Now they are creating the levels of north and south Japan or Chile, Peru and Ecuador.

Knowing where the old earthquake occurred, how large it was and how long ago it happened, the researchers could look at the foreshocks, see how much stress they relieved and anticipate, at least in a small way, what would happen.

“This is what we need to do in the future in near real time for decision makers,” said Furl.

A new look at what’s in ‘fracking’ fluids raises red flags

Scientists are getting to the bottom of what's in fracking fluids — with some troubling results. -  Doug Duncan/U.S. Geological Survey
Scientists are getting to the bottom of what’s in fracking fluids — with some troubling results. – Doug Duncan/U.S. Geological Survey

As the oil and gas drilling technique called hydraulic fracturing (or “fracking”) proliferates, a new study on the contents of the fluids involved in the process raises concerns about several ingredients. The scientists presenting the work today at the 248th National Meeting & Exposition of the American Chemical Society (ACS) say that out of nearly 200 commonly used compounds, there’s very little known about the potential health risks of about one-third, and eight are toxic to mammals.

The meeting features nearly 12,000 presentations on a wide range of science topics and is being held here through Thursday by ACS, the world’s largest scientific society.

William Stringfellow, Ph.D., says he conducted the review of fracking contents to help resolve the public debate over the controversial drilling practice. Fracking involves injecting water with a mix of chemical additives into rock formations deep underground to promote the release of oil and gas. It has led to a natural gas boom in the U.S., but it has also stimulated major opposition and troubling reports of contaminated well water, as well as increased air pollution near drill sites.

“The industrial side was saying, ‘We’re just using food additives, basically making ice cream here,'” Stringfellow says. “On the other side, there’s talk about the injection of thousands of toxic chemicals. As scientists, we looked at the debate and asked, ‘What’s the real story?'”

To find out, Stringfellow’s team at Lawrence Berkeley National Laboratory and University of the Pacific scoured databases and reports to compile a list of substances commonly used in fracking. They include gelling agents to thicken the fluids, biocides to keep microbes from growing, sand to prop open tiny cracks in the rocks and compounds to prevent pipe corrosion.

What their analysis revealed was a little truth to both sides’ stories – with big caveats. Fracking fluids do contain many nontoxic and food-grade materials, as the industry asserts. But if something is edible or biodegradable, it doesn’t automatically mean it can be easily disposed of, Stringfellow notes.

“You can’t take a truckload of ice cream and dump it down the storm drain,” he says, building on the industry’s analogy. “Even ice cream manufacturers have to treat dairy wastes, which are natural and biodegradable. They must break them down rather than releasing them directly into the environment.”

His team found that most fracking compounds will require treatment before being released. And, although not in the thousands as some critics suggest, the scientists identified eight substances, including biocides, that raised red flags. These eight compounds were identified as being particularly toxic to mammals.

“There are a number of chemicals, like corrosion inhibitors and biocides in particular, that are being used in reasonably high concentrations that potentially could have adverse effects,” Stringfellow says. “Biocides, for example, are designed to kill bacteria – it’s not a benign material.”

They’re also looking at the environmental impact of the fracking fluids, and they are finding that some have toxic effects on aquatic life.

In addition, for about one-third of the approximately 190 compounds the scientists identified as ingredients in various fracking formulas, the scientists found very little information about toxicity and physical and chemical properties.

“It should be a priority to try to close that data gap,” Stringfellow says.

New tools reveal mysteries of an ancient Arctic terrane

This is a satellite image of northern and western Alaska, Bering Strait, and the Chukotka Peninsula of Russia. -  NASA Worldview satellite image, https://earthdata.nasa.gov/labs/worldview/.
This is a satellite image of northern and western Alaska, Bering Strait, and the Chukotka Peninsula of Russia. – NASA Worldview satellite image, https://earthdata.nasa.gov/labs/worldview/.

The evolution and origin of Earth’s Arctic realm and the nature, location, and age of its major tectonic boundaries remain subjects of considerable uncertainty. This new compilation of studies from The Geological Society of America demonstrates the power of modern research tools to penetrate the effects of orogenesis and reconstruct the area’s pre-deformational tectonic and paleogeographic history.

The largest piece of continental crust that plays a role in Arctic tectonics is the Arctic Alaska-Chukotka terrane or microplate. This microplate includes northern Alaska and northeastern-most Russia, along with the adjacent continental shelves. Because of its size, understanding its origin and movements during the Paleozoic and Mesozoic are critical components of tectonic and paleogeographic models.

This new GSA Special Paper, edited by Julie A. Dumoulin and Allison B. Till of the U.S. Geological Survey, examines the Arctic Alaska-Chukotka microplate from the Late Proterozoic (about 240 million years ago) to the Devonian (from 360 to 410 million years ago), and includes the first compelling evidence for a rift event that may have detached the Arctic Alaska-Chukotka microplate from the Timanide margin of Baltica.

Ancient shellfish remains rewrite 10,000-year history of El Nino cycles

The middens are ancient dumping sites that typically contain a mix of mollusk shells, fish and bird bones, ceramics, cloth, charcoal, maize and other plants. -  M. Carré / Univ. of Montpellier
The middens are ancient dumping sites that typically contain a mix of mollusk shells, fish and bird bones, ceramics, cloth, charcoal, maize and other plants. – M. Carré / Univ. of Montpellier

The planet’s largest and most powerful driver of climate changes from one year to the next, the El Niño Southern Oscillation in the tropical Pacific Ocean, was widely thought to have been weaker in ancient times because of a different configuration of the Earth’s orbit. But scientists analyzing 25-foot piles of ancient shells have found that the El Niños 10,000 years ago were as strong and frequent as the ones we experience today.

The results, from the University of Washington and University of Montpellier, question how well computer models can reproduce historical El Niño cycles, or predict how they could change under future climates. The paper is now online and will appear in an upcoming issue of Science.

“We thought we understood what influences the El Niño mode of climate variation, and we’ve been able to show that we actually don’t understand it very well,” said Julian Sachs, a UW professor of oceanography.

The ancient shellfish feasts also upend a widely held interpretation of past climate.

“Our data contradicts the hypothesis that El Niño activity was very reduced 10,000 years ago, and then slowly increased since then,” said first author Matthieu Carré, who did the research as a UW postdoctoral researcher and now holds a faculty position at the University of Montpellier in France.

In 2007, while at the UW-based Joint Institute for the Study of the Atmosphere and Ocean, Carré accompanied archaeologists to seven sites in coastal Peru. Together they sampled 25-foot-tall piles of shells from Mesodesma donacium clams eaten and then discarded over centuries into piles that archaeologists call middens.

While in graduate school, Carré had developed a technique to analyze shell layers to get ocean temperatures, using carbon dating of charcoal from fires to get the year, and the ratio of oxygen isotopes in the growth layers to get the water temperatures as the shell was forming.

The shells provide 1- to 3-year-long records of monthly temperature of the Pacific Ocean along the coast of Peru. Combining layers of shells from each site gives water temperatures for intervals spanning 100 to 1,000 years during the past 10,000 years.

The new record shows that 10,000 years ago the El Niño cycles were strong, contradicting the current leading interpretations. Roughly 7,000 years ago the shells show a shift to the central Pacific of the most severe El Niño impacts, followed by a lull in the strength and occurrence of El Niño from about 6,000 to 4,000 years ago.

One possible explanation for the surprising finding of a strong El Niño 10,000 years ago was that some other factor was compensating for the dampening effect expected from cyclical changes in Earth’s orbit around the sun during that period.

“The best candidate is the polar ice sheet, which was melting very fast in this period and may have increased El Niño activity by changing ocean currents,” Carré said.

Around 6,000 years ago most of the ice age floes would have finished melting, so the effect of Earth’s orbital geometry might have taken over then to cause the period of weak El Niños.

In previous studies, warm-water shells and evidence of flooding in Andean lakes had been interpreted as signs of a much weaker El Niño around 10,000 years ago.

The new data is more reliable, Carré said, for three reasons: the Peruvian coast is strongly affected by El Niño; the shells record ocean temperature, which is the most important parameter for the El Niño cycles; and the ability to record seasonal changes, the timescale at which El Niño can be observed.

“Climate models and a variety of datasets had concluded that El Niños were essentially nonexistent, did not occur, before 6,000 to 8,000 years ago,” Sachs said. “Our results very clearly show that this is not the case, and suggest that current understanding of the El Niño system is incomplete.