MERMAIDs Reveal Secrets from Below the Ocean Floor

Seismologists use waves generated by earthquakes to scan the interior of our planet, much like doctors image their patients using medical tomography. Earth imaging has helped us track down the deep origins of volcanic islands such as Hawaii, and identify the source zones of deep earthquakes.

“Imagine a radiologist forced to work with a CAT scanner that is missing two-thirds of its necessary sensors,” said Frederik Simons, a professor of geosciences at Princeton. “Two-thirds is the fraction of the Earth that is covered by oceans and therefore lacking seismic recording stations. Such is the situation faced by seismologists attempting to sharpen their images of the inside of our planet.”

Some 15 years ago, when he was a postdoctoral researcher, Simons partnered with Guust Nolet, now the George J. Magee Professor of Geoscience and Geological Engineering, Emeritus, and they resolved to remediate this situation by building an undersea robot equipped with a hydrophone—an underwater microphone that can pick up the sounds of distant earthquakes whose waves deliver acoustic energy into the oceans through the ocean floor.

This week, Nolet, Simons and an international team of researchers published the first scientific results from the revolutionary seismic floats, dubbed MERMAIDs—Mobile Earthquake Recording in Marine Areas by Independent Divers.

The researchers, from institutions in the United States, France, Ecuador and China, found that the volcanoes on Galápagos are fed by a source 1,200 miles (1,900 km) deep, via a narrow conduit that is bringing hot rock to the surface. Such “mantle plumes” were first proposed in 1971 by one of the fathers of plate tectonics, Princeton geophysicist W. Jason Morgan, but they have resisted attempts at detailed seismic imaging because they are found in the oceans, rarely near any seismic stations.

MERMAIDs drift passively, normally at a depth of 1,500 meters—about a mile below the sea surface—moving 2-3 miles per day. When one detects a possible incoming earthquake, it rises to the surface, usually within 95 minutes, to determine its position with GPS and transmit the seismic data.

By letting their nine robots float freely for two years, the scientists created an artificial network of oceanic seismometers that could fill in one of the blank areas on the global geologic map, where otherwise no seismic information is available.

The unexpectedly high temperature that their model shows in the Galápagos mantle plume “hints at the important role that plumes play in the mechanism that allows the Earth to keep itself warm,” said Nolet.

“Since the 19th century, when Lord Kelvin predicted that Earth should cool to be a dead planet within a hundred million years, geophysicists have struggled with the mystery that the Earth has kept a fairly constant temperature over more than 4.5 billion years,” Nolet explained. “It could have done so only if some of the original heat from its accretion, and that created since by radioactive minerals, could stay locked inside the lower mantle. But most models of the Earth predict that the mantle should be convecting vigorously and releasing this heat much more quickly. These results of the Galápagos experiment point to an alternative explanation: the lower mantle may well resist convection, and instead only bring heat to the surface in the form of mantle plumes such as the ones creating Galápagos and Hawaii.”

To further answer questions on the heat budget of the Earth and the role that mantle plumes play in it, Simons and Nolet have teamed up with seismologists from the Southern University of Science and Technology (SUSTech) in Shenzhen, China, and from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Together, and with vessels provided by the French research fleet, they are in the process of launching some 50 MERMAIDs in the South Pacific to study the mantle plume region under the island of Tahiti.

“Stay tuned! There are many more discoveries to come,” said professor Yongshun (John) Chen, a 1989 Princeton graduate alumnus who is head of the Department of Ocean Science and Engineering at SUSTech, which is leading the next phase of what they and their international team have called EarthScope-Oceans.

Earthquake with Magnitude 7.5 in Indonesia – an Unusual and Steady Speed

An international team of researchers from the French National Research Institute for Sustainable Development (IRD-France), Université Côte d”Azur, University of California Los Angeles and California Institute of Technology has determined the propagation speed of the 7.5 magnitude earthquake which occurred in Indonesia in September 2018: 4.1 km/s along 150 km. The results, which also shed light on the earthquake rupture path, are published on February 4th in Nature Geoscience.

Earthquakes happen when rocks on either side of a tectonic fault shift suddenly in opposite directions. Two main seismic waves that carry out shaking of a breaking fault are S-waves, which shear rocks and propagate at about 3.5 km/s, and P-waves, which compress rocks and propagate faster, at about 5 km/s.

Geophysical observations show that the speed at which an earthquake ruptures along the fault is either slower than S-waves or almost as fast as P-waves. The latter, so-called supershear earthquakes, occur very rarely and can produce very strong shaking. Only a few have been observed, and they happen on faults that are remarkably straight, geological “superhighways” that present little obstacle to speeding earthquakes.

“Forbidden” speed range

In this study, the international team coordinated by Jean-Paul Ampuero, seismologist at IRD and Université Côte d”Azur, analysed the 7.5 magnitude earthquake that rocked the Sulawesi island in Indonesia on September 28th, devastating Palu’s region.

The impact of the event—more than 2,000 deaths—was aggravated by a devastating sequence of secondary effects, involving soil liquefaction, landslides and a tsunami.

Thanks to a high-resolution analysis of seismological data, researchers identified the propagation speed of the earthquake: 4.1 km/s, an unusual speed, between the speed of S- and P-waves. “This is the first time we observed this speed so steadily,” underlines Jean-Paul Ampuero. “This earthquake ran in the ‘forbidden’ speed range, and can be considered as a supershear event, even if it’s not as fast as previous ones.”

By analyzing optical and radar images recorded by satellites especially re-tasked to observe the earthquake aftermath, the researchers determined the path of the fault rupture. They found that the fault was not straight, but had at least two major bends, and left more than five meters of ground offset across the city of Palu. ” This path has major obstacles, which should have reduced the earthquake’s speed, but it stayed at 4.1 km/s along 150 km,” says Jean-Paul Ampuero.

Toward a better anticipation of future earthquakes

The findings challenge current views of earthquakes in ways that could help researchers and public authorities prepare better for future events. “In classical earthquake models, faults live in idealized intact rocks “, says Ampuero, ” but real faults are wrapped in a layer of rocks that have been fractured and softened by previous earthquakes. Steady rupture at speeds that are unexpected on intact rocks can actually happen on damaged rocks, simply because they have slower seismic wave speeds.”

The Palu earthquake may offer the first clear test of such recent models if followed up by studies of the structure of the fault and its zone of damaged rocks. Because the impact of an earthquake depends strongly on its speed, such studies on other faults around the world could anticipate earthquake effects better.

Future work may also determine if the speed of the Palu earthquake enhanced its cascading effects, by promoting coastal and submarine landslides that in turn contributed to the tsunami.

Researchers Unearth an Ice Age in the African Desert

A field trip to Namibia to study volcanic rocks led to an unexpected discovery by West Virginia University geologists Graham Andrews and Sarah Brown.

While exploring the desert country in southern Africa, they stumbled upon a peculiar land formation—flat desert scattered with hundreds of long, steep hills. They quickly realized the bumpy landscape was shaped by drumlins, a type of hill often found in places once covered in glaciers, an abnormal characteristic for desert landscapes.

“We quickly realized what we were looking at because we both grew up in areas of the world that had been under glaciers, me in Northern Ireland and Sarah in northern Illinois,” said Andrews, an assistant professor of geology. “It’s not like anything we see in West Virginia where we’re used to flat areas and then gorges and steep-sided valleys down into hollows.”

After returning home from the trip, Andrews began researching the origins of the Namibian drumlins, only to learn they had never been studied.

“The last rocks we were shown on the trip are from a time period when southern Africa was covered by ice,” Andrews said. “People obviously knew that part of the world had been covered in ice at one time, but no one had ever mentioned anything about how the drumlins formed or that they were even there at all.”

WVU researcher unearths an ice age in the African desert
Andrew McGrady. Credit: WVU

Andrews teamed up with WVU geology senior Andy McGrady to use morphometrics, or measurements of shapes, to determine if the drumlins showed any patterns that would reflect regular behaviors as the ice carved them.

While normal glaciers have sequential patterns of growing and melting, they do not move much, Andrews explained. However, they determined that the drumlins featured large grooves, which showed that the ice had to be moving at a fast pace to carve the grooves.

These grooves demonstrated the first evidence of an ice stream in southern Africa in the late Paleozoic Age, which occurred about 300 million years ago.

“The ice carved big, long grooves in the rock as it moved,” Andrews said. “It wasn’t just that there was ice there, but there was an ice stream. It was an area where the ice was really moving fast.”

McGrady used freely available information from Google Earth and Google Maps to measure their length, width and height.

WVU researcher unearths an ice age in the African desert

“This work is very important because not much has been published on these glacial features in Namibia,” said McGrady, a senior geology student from Hamlin. “It’s interesting to think that this was pioneer work in a sense, that this is one of the first papers to cover the characteristics of these features and gives some insight into how they were formed.”

Their findings also confirm that southern Africa was located over the South Pole during this period.

“These features provide yet another tie between southern Africa and south America to show they were once joined,” Andrews said.

The study, “First description of subglacial megalineations from the late Paleozoic ice age in southern Africa” is published in the Public Library of Science’s PLOS ONE journal.

“This is a great example of a fundamental discovery and new insights into the climatic history of our world that remain to be discovered,” said Tim Carr, chair of the Department of Geology and Geography.

European Waters Drive Ocean Overturning, Key For Regulating Climate

A new international study finds that the Atlantic meridional overturning circulation (MOC), a deep-ocean process that plays a key role in regulating Earth’s climate, is primarily driven by cooling waters west of Europe.

In a departure from the prevailing scientific view, the study shows that most of the overturning and variability is occurring not in the Labrador Sea off Canada, as past modeling studies have suggested, but in regions between Greenland and Scotland. There, warm, salty, shallow waters carried northward from the tropics by currents and wind, sink and convert into colder, fresher, deep waters moving southward through the Irminger and Iceland basins.

Overturning variability in this eastern section of the ocean was seven times greater than in the Labrador Sea, and it accounted for 88 percent of the total variance documented across the entire North Atlantic over the 21-month study period.

These findings, unexpected as they may be, can help scientists better predict what changes might occur to the MOC and what the climate impacts of those changes will be, said Susan Lozier, the Ronie-Rochele Garcia-Johnson Professor of Earth and Ocean Sciences at Duke University’s Nicholas School of the Environment.

“To aid predictions of climate in the years and decades ahead, we need to know where this deep overturning is currently taking place and what is causing it to vary,” said Lozier, who led the international observational study that produced the new data.

“Overturning carries vast amounts of anthropogenic carbon deep into the ocean, helping to slow global warming,” said co-author Penny Holliday of the National Oceanography Center in Southampton, U.K. “The largest reservoir of this anthropogenic carbon is in the North Atlantic.”

“Overturning also transports tropical heat northward,” Holliday said, “meaning any changes to it could have an impact on glaciers and Arctic sea ice. Understanding what is happening, and what may happen in the years to come, is vital.”

Scientists from 16 research institutions from seven countries collaborated on the new study. They published their peer-reviewed findings Feb. 1 in Science.

“I cannot say enough about the importance of this international collaboration to the success of this project,” Lozier said. “Measuring the circulation in the subpolar North Atlantic is incredibly challenging so we definitely needed an ‘all hands on deck’ approach.”

This paper is the first from the $32 million, five-year initial phase of the OSNAP (Overturning in the Subpolar North Atlantic Program) research project, in which scientists have deployed moored instruments and sub-surface floats across the North Atlantic to measure the ocean’s overturning circulation and shed light on the factors that cause it to vary. Lozier is lead investigator of the project, which began in 2014.

“As scientists, it is exciting to learn that there are more pieces to the overturning puzzle than we first thought,” said co-author Johannes Karstensen of the GEOMAR Helmholtz Centre for Ocean Research Kiel, in Germany.

“Though the overturning in the Labrador Sea is smaller than we expected, we have learned that this basin plays a large role in transporting freshwater from the Arctic,” Karstensen said. “Continued measurements in that basin will be increasingly important,” as the Arctic changes unexpectedly.

The new paper contains data collected over a 21-month period from August 2014 to April 2016.

Ancient Crystals Offer Evidence Of The Start of Earth’s Core Solidifying

A quartet of researchers from the University of Rochester and the University of California has found evidence of the starting period for the solidification of Earth’s core. In their paper published in the journal Nature Geoscience, Richard Bono, John Tarduno, Francis Nimmo and Rory Cottrell describe their analysis of ancient crystals found in eastern Canada, what they found, and why they believe their results offer clues about the formation of Earth’s inner core. Peter Driscoll, with the Carnegie Institution for Science, has written a News and Views piece on the study in the same journal issue.

Planetary scientists have found strong evidence that suggests the Earth has an inner and an outer core. The inner core is believed to be solid, while the outer core is made up of molten material. Prior evidence has also indicated that the entire core was once liquid, but as the interior cooled, the innermost part began to crystallize. It is at this point that scientists disagree—some suggest the start of solidification began as far back as 2.5 billion years ago. Others believe it was much more recent—perhaps as recent as just 500 million years ago. In this new effort, the researchers have found evidence that supports the latter theory.

The work by the researchers involved carefully analyzing plagioclase and clinopyroxene crystals, which have been dated to approximately 565 million years ago. The crystals are important because they contain bits of metal called inclusions. The inclusions are very small and needle-shaped and aligned themselves with the Earth’s magnetic field as they became embedded in the crystal. Since the Earth’s magnetic field is generated by activity in the inner core, the inclusions are a means of determining the state of the core during the time when the crystals formed. The researchers report that their analysis showed that the magnetic field was significantly weaker than it is today, suggesting that solidification of the core must have occurred soon thereafter or the magnetic field would have collapsed altogether. The reason it did not, theory suggests, is because as the inner core solidified, he magnetic field became stronger.

How Climate Change Caused the World’s First Empire to Collapse

Not one smoke stack, vehicle, or petroleum of any form was mentioned in this scientific article. However, there does appear to be an assumption of rhythmic cycles.

Gol-e-Zard Cave lies in the shadow of Mount Damavand, which at more than 5,000 meters dominates the landscape of northern Iran. In this cave, stalagmites and stalactites are growing slowly over millennia and preserve in them clues about past climate events. Changes in stalagmite chemistry from this cave have now linked the collapse of the Akkadian Empire to climate changes more than 4,000 years ago.

Akkadia was the world’s first empire. It was established in Mesopotamia around 4,300 years ago after its ruler, Sargon of Akkad, united a series of independent city states. Akkadian influence spanned along the Tigris and Euphrates rivers from what is now southern Iraq, through to Syria and Turkey. The north-south extent of the empire meant that it covered regions with different climates, ranging from fertile lands in the north which were highly dependent on rainfall (one of Asia’s “bread baskets”), to the irrigation-fed alluvial plains to the south.

It appears that the empire became increasingly dependent on the productivity of the northern lands and used the grains sourced from this region to feed the army and redistribute the food supplies to key supporters. Then, about a century after its formation, the Akkadian Empire suddenly collapsed, followed by mass migration and conflicts. The anguish of the era is perfectly captured in the ancient Curse of Akkad text, which describes a period of turmoil with water and food shortages: “… the large arable tracts yielded no grain, the inundated fields yielded no fish, the irrigated orchards yielded no syrup or wine, the thick clouds did not rain.”

Drought and dust

The reason for this collapse is still debated by historians, archaeologists and scientists. One of the most prominent views, championed by Yale archaeologist Harvey Weiss (who built on earlier ideas by Ellsworth Huntington), is that it was caused by an abrupt onset of drought conditions which severely affected the productive northern regions of the empire.

Weiss and his colleagues discovered evidence in northern Syria that this once prosperous region was suddenly abandoned around 4,200 years ago, as indicated by a lack of pottery and other archaeological remains. Instead, the rich soils of earlier periods were replaced by large amounts of wind-blown dust and sand, suggesting the onset of drought conditions. Subsequently, marine cores from the Gulf of Oman and the Red Sea which linked the input of dust into the sea to distant sources in Mesopotamia, provided further evidence of a regional drought at the time.

Many other researchers viewed Weiss’s interpretation with skepticism, however. Some argued, for example, that the archaeological and marine evidence was not accurate enough to demonstrate a robust correlation between drought and societal change in Mesopotamia.

A new detailed climate record

Now, stalagmite data from Iran sheds new light on the controversy. In a study published in the journal PNAS, led by Oxford palaeoclimatologist Stacy Carolin, colleagues and I provide a very well dated and high resolution record of dust activity between 5,200 and 3,700 years ago. And cave dust from Iran can tell us a surprising amount about climate history elsewhere.

Gol-e-Zard Cave might be several hundred miles to the east of the former Akkadian Empire, but it is directly downwind. As a result, around 90% of the region’s dust originates in the deserts of Syria and Iraq.

That desert dust has a higher concentration of magnesium than the local limestone which forms most of Gol-e-Zard’s stalagmites (the ones which grow upwards from the cave floor). Therefore, the amount of magnesium in the Gol-e-Zard stalagmites can be used as an indicator of dustiness at the surface, with higher magnesium concentrations indicating dustier periods, and by extension drier conditions.

The stalagmites have the additional advantage that they can be dated very precisely using uranium-thorium chronology. Combining these methods, our new study provides a detailed history of dustiness in the area, and identifies two major drought periods which started 4,510 and 4,260 years ago, and lasted 110 and 290 years respectively. The latter event occurs precisely at the time of the Akkadian Empire’s collapse and provides a strong argument that climate change was at least in part responsible.

The collapse was followed by mass migration from north to south which was met with resistance by the local populations. A 180km wall – the “Repeller of the Amorites” – was even built between the Tigris and Euphrates in an effort to control immigration, not unlike some strategies proposed today. The stories of abrupt climate change in the Middle East therefore echo over millennia to the present day.

JUST IN: Researchers Find Deep Ocean Getting Colder

A pair of researchers, one with the Woods Hole Oceanographic Institution, the other Harvard University, has found evidence of deep ocean cooling that is likely due to the Little Ice Age. In their paper published in the journal Science, Jake Gebbie and Peter Huybers describe their study of Pacific Ocean temperatures over the past 150 years and what they found.

The model showed that the Pacific Ocean cooled over the course of the 20th century at depths of 1.8 to 2.6 kilometers. The amount is still not precise, but the researchers suggest it is most likely between 0.02 and 0.08° C. That cooling, the researchers suggest, is likely due to the Little Ice Age, which ran from approximately 1300 until approximately 1870. Prior to that, there was a time known as the Medieval Warm Period, which had caused the deep waters of the Pacific to warm just prior to the cooling it is now experiencing.

Prior research has suggested that it takes a very long time for water in the Pacific Ocean to circulate down to its lowest depths. This is because it is replenished only from the south, which means it takes a very long time for water on the surface to make its way to the bottom – perhaps as long as several hundred years. That is what Gebbie and Huber found back in 2012. That got them to thinking that water temperature at the bottom of the Pacific could offer a hint of what surface temperatures were like hundreds of years ago.

To find out if that truly was the case, the researchers obtained data from an international consortium called the Argo Program – a group of people who together have been taking ocean measurements down to depths of approximately two kilometers. As a comparative reference, the researchers also obtained data gathered by the crew of the HMS Challenger – they had taken Pacific Ocean temperatures down to a depth of two kilometers during the years 1872 to 1876. The researchers used the data from both projects to build a computer model meant to mimic the circulation of water in the Pacific Ocean over the past century and a half.