The Earth’s core is melting … and freezing

The inner core of the Earth is simultaneously melting and freezing due to circulation of heat in the overlying rocky mantle, according to new research from the University of Leeds, UC San Diego and the Indian Institute of Technology.

The findings, published tomorrow in Nature, could help us understand how the inner core formed and how the outer core acts as a ‘geodynamo’, which generates the planet’s magnetic field.

“The origins of Earth’s magnetic field remain a mystery to scientists,” said study co-author Dr Jon Mound from the University of Leeds. “We can’t go and collect samples from the centre of the Earth, so we have to rely on surface measurements and computer models to tell us what’s happening in the core.”

“Our new model provides a fairly simple explanation to some of the measurements that have puzzled scientists for years. It suggests that the whole dynamics of the Earth’s core are in some way linked to plate tectonics, which isn’t at all obvious from surface observations.

“If our model is verified it’s a big step towards understanding how the inner core formed, which in turn helps us understand how the core generates the Earth’s magnetic field.”

The Earth’s inner core is a ball of solid iron about the size of our moon. This ball is surrounded by a highly dynamic outer core of a liquid iron-nickel alloy (and some other, lighter elements), a highly viscous mantle and a solid crust that forms the surface where we live.

Over billions of years, the Earth has cooled from the inside out causing the molten iron core to partly freeze and solidify. The inner core has subsequently been growing at the rate of around 1mm a year as iron crystals freeze and form a solid mass.

The heat given off as the core cools flows from the core to the mantle to the Earth’s crust through a process known as convection. Like a pan of water boiling on a stove, convection currents move warm mantle to the surface and send cool mantle back to the core. This escaping heat powers the geodynamo and coupled with the spinning of the Earth generates the magnetic field.

Scientists have recently begun to realise that the inner core may be melting as well as freezing, but there has been much debate about how this is possible when overall the deep Earth is cooling. Now the research team believes they have solved the mystery.

Using a computer model of convection in the outer core, together with seismology data, they show that heat flow at the core-mantle boundary varies depending on the structure of the overlying mantle. In some regions, this variation is large enough to force heat from the mantle back into the core, causing localised melting.

The model shows that beneath the seismically active regions around the Pacific ‘Ring of Fire’, where tectonic plates are undergoing subduction, the cold remnants of oceanic plates at the bottom of the mantle draw a lot of heat from the core. This extra mantle cooling generates down-streams of cold material that cross the outer core and freeze onto the inner core.

Conversely, in two large regions under Africa and the Pacific where the lowermost mantle is hotter than average, less heat flows out from the core. The outer core below these regions can become warm enough that it will start melting back the solid inner core.

Co-author Dr Binod Sreenivasan from the Indian Institute of Technology said: “If Earth’s inner core is melting in places, it can make the dynamics near the inner core-outer core boundary more complex than previously thought.

“On the one hand, we have blobs of light material being constantly released from the boundary where pure iron crystallizes. On the other hand, melting would produce a layer of dense liquid above the boundary. Therefore, the blobs of light elements will rise through this layer before they stir the overlying outer core.

“Interestingly, not all dynamo models produce heat going into the inner core. So the possibility of inner core melting can also place a powerful constraint on the regime in which the Earth’s dynamo operates.”

Co-author Dr Sebastian Rost from the University of Leeds added: “The standard view has been that the inner core is freezing all over and growing out progressively, but it appears that there are regions where the core is actually melting. The net flow of heat from core to mantle ensures that there’s still overall freezing of outer core material and it’s still growing over time, but by no means is this a uniform process.

“Our model allows us to explain some seismic measurements which have shown that there is a dense layer of liquid surrounding the inner core. The localized melting theory could also explain other seismic observations, for example why seismic waves from earthquakes travel faster through some parts of the core than others.”

2,300-year climate record suggests severe tropical droughts as northern temperatures rise

The study compared the record in the Pumacocha sediment core (PC) to various geological records from South America -- Cascayunga Cave (CC), the Quelccaya ice Cap (QIC), and the Cariaco Basin (CB) -- as well as the annual position of the Intertropical Convergence Zone (ITCZ). -  U. of Pittsburgh
The study compared the record in the Pumacocha sediment core (PC) to various geological records from South America — Cascayunga Cave (CC), the Quelccaya ice Cap (QIC), and the Cariaco Basin (CB) — as well as the annual position of the Intertropical Convergence Zone (ITCZ). – U. of Pittsburgh

A 2,300-year climate record University of Pittsburgh researchers recovered from an Andes Mountains lake reveals that as temperatures in the Northern Hemisphere rise, the planet’s densely populated tropical regions will most likely experience severe water shortages as the crucial summer monsoons become drier. The Pitt team found that equatorial regions of South America already are receiving less rainfall than at any point in the past millennium.

The researchers report in the Proceedings of the National Academy of Sciences (PNAS) that a nearly 6-foot-long sediment core from Laguna Pumacocha in Peru contains the most detailed geochemical record of tropical climate fluctuations yet uncovered. The core shows pronounced dry and wet phases of the South American summer monsoons and corresponds with existing geological data of precipitation changes in the surrounding regions.

Paired with these sources, the sediment record illustrated that rainfall during the South American summer monsoon has dropped sharply since 1900-exhibiting the greatest shift in precipitation since around 300 BCE-while the Northern Hemisphere has experienced warmer temperatures.

Study coauthor Mark Abbott, a professor of geology and planetary science in Pitt’s School of Arts and Sciences who also codesigned the project, said that he and his colleagues did not anticipate the rapid decrease in 20th-century rainfall that they observed. Abbott worked with lead author and recent Pitt graduate Broxton Bird; Don Rodbell, study codesigner and a geology professor at Union College in Schenectady, N.Y.; recent Pitt graduate Nathan Stansell; Pitt professor of geology and planetary science Mike Rosenmeier; and Mathias Vuille, a professor of atmospheric and environmental science at the State University of New York at Albany. Both Bird and Stansell received their PhD degrees in geology from Pitt in 2009.

“This model suggests that tropical regions are dry to a point we would not have predicted,” Abbott said. “If the monsoons that are so critical to the water supply in tropical areas continue to diminish at this pace, it will have devastating implications for the water resources of a huge swath of the planet.”

The sediment core shows regular fluctuations in rainfall from 300 BCE to 900 CE, with notably heavy precipitation around 550. Beginning in 900, however, a severe drought set in for the next three centuries, with the driest period falling between 1000 and 1040. This period correlates with the well-known demise of regional Native American populations, Abbott explained, including the Tiwanaku and Wari that inhabited present-day Boliva, Chile, and Peru.

After 1300, monsoons increasingly drenched the South American tropics. The wettest period of the past 2,300 years lasted from roughly 1500 to the 1750s during the time span known as the Little Ice Age, a period of cooler global temperatures. Around 1820, a dry cycle crept in briefly, but quickly gave way to a wet phase before the rain began waning again in 1900. By July 2007, when the sediment core was collected, there had been a steep, steady increase in dry conditions to a high point not surpassed since 1000.

To create a climate record from the sediment core, the team analyzed the ratio of the oxygen isotope delta-O-18 in each annual layer of lake-bed mud. This ratio has a negative relationship with rainfall: Levels of delta-O-18 are low during the wetter seasons and high when monsoon rain is light. The team found that the rainfall history suggested by the lake core matched that established by delta-O-18 analyses from Cascayunga Cave in the Peruvian lowlands and the Quelccaya Ice Cap located high in the Andes. The Pumacocha core followed the climatological narrative of these sources between the years 980 and 2006, but provided much more detail, Abbott said.

The team then established a connection between rainfall and Northern Hemisphere temperatures by comparing their core to the movement of the Intertropical Convergence Zone (ITCZ), a balmy strip of thunderstorms near the equator where winds from the Northern and Southern Hemispheres meet. Abbott and his colleagues concluded that warm Northern temperatures such as those currently recorded lure the ITCZ-the main source of monsoons-north and ultimately reduce the rainfall on which tropical areas rely.

The historical presence of the ITCZ has been gauged by measuring the titanium concentrations of sea sediment, according to the PNAS report. High levels of titanium in the Cariaco Basin north of Venezuela show that the ITCZ lingered in the upper climes at the same time the South American monsoon was at its driest, between 900 and 1100. On the other hand, the wettest period at Pumacocha-between 1400 and 1820, which coincided with the Little Ice Age-correlates with the ITCZ’s sojourn to far south of the equator as Northern Hemisphere temperatures cooled.

‘Fool’s Gold’ from the deep is fertilizer for ocean life

This is a black smoker from the Mariner vent site in the Pacific Ocean's Eastern Lau Spreading Center. -  University of Delaware
This is a black smoker from the Mariner vent site in the Pacific Ocean’s Eastern Lau Spreading Center. – University of Delaware

Similar to humans, the bacteria and tiny plants living in the ocean need iron for energy and growth. But their situation is quite different from ours–for one, they can’t turn to natural iron sources like leafy greens or red meat for a pick-me-up.

So, from where does their iron come?

New research results published in the current issue of the journal Nature Geoscience point to a source on the seafloor: minute particles of pyrite, or fool’s gold, from hydrothermal vents at the bottom of the ocean.

Scientists already knew the vents’ cloudy plumes, which spew forth from the earth’s interior, include pyrite particles, but thought they were solids that settled back on the ocean bottom.

Now, scientists at the University of Delaware and other institutions have shown the vents emit a significant amount of microscopic pyrite particles that have a diameter 1,000 times smaller than that of a human hair.

Because the nanoparticles are so small, they are dispersed into the ocean rather than falling to the sea floor.

Barbara Ransom, program director in the National Science Foundation’s (NSF) Division of Ocean Sciences, which funded the research, called the discovery “very exciting.”

“These particles have long residence times in the ocean and can travel long distances from their sources, forming a potentially important food source for life in the deep sea,” she said.

The project also received support from another NSF program, the Experimental Program to Stimulate Competitive Research, or EPSCOR.

The mineral pyrite, or iron pyrite, has a metallic luster and brass-yellow color that led to its nickname: fool’s gold. In fact, pyrite is sometimes found in association with small quantities of gold.

Scientist George Luther of the University of Delaware explained the importance of the lengthy amount of time pyrite exists suspended in its current form in the sea, also known as its residence time.

Pyrite, which consists of iron and sulfur as iron disulfide, does not rapidly react with oxygen in seawater to form oxidized iron, or “rust,” allowing it to stay intact and move throughout the ocean better than other forms of iron.

“As pyrite travels from the vents to the ocean interior and toward the surface ocean, it oxidizes gradually to release iron, which becomes available in areas where iron is depleted so that organisms can assimilate it, then grow,” Luther said.

“It’s an ongoing iron supplement for the ocean–much as multivitamins are for humans.”

Growth of tiny plants known as phytoplankton can affect atmospheric oxygen and carbon dioxide levels.

Much of the research was performed by scientist and lead author Mustafa Yucel of the Universite Pierre et Marie Curie in France, conducted while Yucel worked on a doctorate at the University of Delaware.

It involved scientific cruises to the South Pacific and East Pacific Rise using the manned deep-sea submersible Alvin and the remotely operated vehicle Jason, both operated by the Woods Hole Oceanographic Institution.

Methane levels 17 times higher in water wells near hydrofracking sites

A study by Duke University researchers has found high levels of leaked methane in well water collected near shale-gas drilling and hydrofracking sites. The scientists collected and analyzed water samples from 68 private groundwater wells across five counties in northeastern Pennsylvania and New York.

“At least some of the homeowners who claim that their wells were contaminated by shale-gas extraction appear to be right,” says Robert B. Jackson, Nicholas Professor of Global Environmental Change and director of Duke’s Center on Global Change.

Hydraulic fracturing, also called hydrofracking or fracking, involves pumping water, sand and chemicals deep underground into horizontal gas wells at high pressure to crack open hydrocarbon-rich shale and extract natural gas.

The study found no evidence of contamination from chemical-laden fracking fluids, which are injected into gas wells to help break up shale deposits, or from “produced water,” wastewater that is extracted back out of the wells after the shale has been fractured.

The peer-reviewed study of well-water contamination from shale-gas drilling and hydrofracking appears this week in the online Early Edition of the Proceedings of the National Academy of Sciences.

“We found measurable amounts of methane in 85 percent of the samples, but levels were 17 times higher on average in wells located within a kilometer of active hydrofracking sites,” says Stephen Osborn, postdoctoral research associate at Duke’s Nicholas School of the Environment. The contamination was observed primarily in Bradford and Susquehanna counties in Pennsylvania.

Water wells farther from the gas wells contained lower levels of methane and had a different isotopic fingerprint.

“Methane is CH4. By using carbon and hydrogen isotope tracers we could distinguish between thermogenic methane, which is formed at high temperatures deep underground and is captured in gas wells during hydrofracking, and biogenic methane, which is produced at shallower depths and lower temperatures,” says Avner Vengosh, professor of geochemistry and water quality. Biogenic methane is not associated with hydrofracking.

“Methane in water wells within a kilometer had an isotopic composition similar to thermogenic methane,” Vengosh says. “Outside this active zone, it was mostly a mixture of the two.”

The scientists confirmed their finding by comparing the dissolved gas chemistry of water samples to the gas chemistry profiles of shale-gas wells in the region, using data from the Pennsylvania Department of Environmental Protection. “Deep gas has a distinctive chemical signature in its isotopes,” Jackson says. “When we compared the dissolved gas chemistry in well water to methane from local gas wells, the signatures matched.”

Methane is flammable and poses a risk of explosion. In very high concentrations, it can cause asphyxiation. Little research has been conducted on the health effects of drinking methane-contaminated water and methane isn’t regulated as a contaminant in public water systems under the EPA’s National Primary Drinking Water Regulations.

The Duke team collected samples from counties overlying the Marcellus shale formation. Accelerated gas drilling and hydrofracking in the region in recent years has fueled concerns about well-water contamination by methane, produced water and fracking fluids, which contain a proprietary mix of chemicals that companies often don’t disclose.

Tree rings tell a 1,100-year history of El Niño

This graph shows El Niño amplitude derived from North American tree rings (blue) and instrumental measurements (red). The green curve represents the long-term trend in El Nino strength. (Individual El Niño events occur typically at intervals of 2-7 years.) Periods of strong El Niño activity are indicated by amplitudes above 1.0. Superimposed on a general rising trend, cycles of strong activity occurred about every 50-90 years. -  International Pacific Research Center
This graph shows El Niño amplitude derived from North American tree rings (blue) and instrumental measurements (red). The green curve represents the long-term trend in El Nino strength. (Individual El Niño events occur typically at intervals of 2-7 years.) Periods of strong El Niño activity are indicated by amplitudes above 1.0. Superimposed on a general rising trend, cycles of strong activity occurred about every 50-90 years. – International Pacific Research Center

El Niño and its partner La Niña, the warm and cold phases in the eastern half of the tropical Pacific, play havoc with climate worldwide. Predicting El Niño events more than several months ahead is now routine, but predicting how it will change in a warming world has been hampered by the short instrumental record. An international team of climate scientists has now shown that annually resolved tree-ring records from North America, particularly from the US Southwest, give a continuous representation of the intensity of El Niño events over the past 1100 years and can be used to improve El Niño prediction in climate models. The study, spearheaded by Jinbao Li, International Pacific Research Center, University of Hawai’i at Manoa, is published in the May 6 issue of Nature Climate Change.

Tree rings in the US Southwest, the team found, agree well with the 150-year instrumental sea surface temperature records in the tropical Pacific. During El Niño, the unusually warm surface temperatures in the eastern Pacific lead to changes in the atmospheric circulation, causing unusually wetter winters in the US Southwest, and thus wider tree rings; unusually cold eastern Pacific temperatures during La Niña lead to drought and narrower rings. The tree-ring records, furthermore, match well existing reconstructions of the El Niño-Southern Oscillation and correlate highly, for instance, with δ18O isotope concentrations of both living corals and corals that lived hundreds of years ago around Palmyra in the central Pacific.

“Our work revealed that the towering trees on the mountain slopes of the US Southwest and the colorful corals in the tropical Pacific both listen to the music of El Niño, which shows its signature in their yearly growth rings,” explains Li. “The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1100 years.”

The tree rings reveal that the intensity of El Niño has been highly variable, with decades of strong El Niño events and decades of little activity. The weakest El Niño activity happened during the Medieval Climate Anomaly in the 11th century, whereas the strongest activity has been since the 18th century.

These different periods of El Niño activity are related to long-term changes in Pacific climate. Cores taken from lake sediments in the Galapagos Islands, northern Yucatan, and the Pacific Northwest reveal that the eastern-central tropical Pacific climate swings between warm and cool phases, each lasting from 50 to 90 years. During warm phases, El Niño and La Niña events were more intense than usual. During cool phases, they deviated little from the long-term average as, for instance, during the Medieval Climate Anomaly when the eastern tropical Pacific was cool.

“Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming,” says co-author Shang-Ping Xie. “Current models diverge in their projections of its future behavior, with some showing an increase in amplitude, some no change, and some even a decrease. Our tree-ring data offer key observational benchmarks for evaluating and perfecting climate models and their predictions of the El Niño-Southern Oscillation under global warming.”

Does the central Andean backarc have the potential for a great earthquake?

Ben Brooks, 'O. Ozcacha and Todd Ericksen stand next to one of the GPS stations that was used in the study. -  Image courtesy Ben Brooks, SOEST/UHM
Ben Brooks, ‘O. Ozcacha and Todd Ericksen stand next to one of the GPS stations that was used in the study. – Image courtesy Ben Brooks, SOEST/UHM

The region east of the central Andes Mountains has the potential for larger scale earthquakes than previously expected, according to a new study posted online in the May 8th edition of Nature Geoscience. Previous research had set the maximum expected earthquake size to be magnitude 7.5, based on the relatively quiet history of seismicity in that area. This new study by researchers from the University of Hawaii at Manoa (UHM) and colleagues contradicts that limit and instead suggests that the region could see quakes with magnitudes 8.7 to 8.9.

Benjamin Brooks, Associate Researcher in the Hawaii Institute of Geophysics and Planetology in the School of Ocean and Earth Science and Technology at UHM and colleagues used GPS data to map movement of the Earth’s surface in the Subandean margin, along the eastern flank of the Andes Mountains. They report a sharp decrease in surface velocity from west to east. “We relate GPS surface movements to the subsurface via deformation models”, says Brooks. “In this case, we use a simple elastic model of slip on a buried dislocation (fault) and do millions of Monte Carlo simulations to determine probability distributions for the model parameters (like slip, width, depth, dip, etc.).” From these data, the researchers conclude that the shallow section in the east of the region is currently locked in place over a length of about 100 km, allowing stress to build up as the tectonic plates in the region slowly move against each other. Rupture of the entire locked section by one earthquake could result in shaking of magnitudes up to 8.9, they estimate.

This project is a long-term collaborative effort between UHM, Ohio State University, Arizona State University, the Bolivian Instituto Geografico Militar (IGM), the Bolivian Seismological Observatory (Observatorio San Calixto), the Universidad Nacional de Cuyo (Argentina), and University of Memphis. The project’s general name is the Central and Southern Andes Project (CAP).

These findings came as a surprise to Brooks. “No one suspected the previous estimates were too low, it was a discovery that came out of my broader interest which is studying the way in which mountains (in this case the Andes) actively grow and deform.”

Major Arturo Echalar of the Bolivian IGM says “The findings here are critical in helping us to continue to provide the most up-to-date and accurate information regarding geological hazards in Bolivia.”

The researchers are quick to report that the findings only demonstrate the potential for an earthquake of such a size. “It is not yet known if one of that size has ever happened in the Bolivian Subandes,” adds Brooks. “Nonetheless we hope that this information will be widely disseminated and considered in Bolivia by the people ( including the general population, engineers, planners, emergency mitigators, policy makers, etc.) who may be most affected by a potential event here.”

There are two important steps that the researchers are now undertaking simultaneously to confirm these findings. They are performing paleoseismolgic research to determine dates and sizes of past earthquakes, and they will continue to monitor the earthquake zone to see if some of the accumulated strain can be released aseismically, potentially slowing down the time until the next big event. “As we state in the paper, we believe that the Mandeyapecua thrust fault at the mountain front exhibits evidence for past earthquake ruptures”, says Brooks. “So by applying techniques like digging trenches and identifying and dating offset layers we’ll be able to quantify the seismic past of the region.”

Caves and their dripstones tell us about the uplift of mountains

<IMG SRC="/Images/410329163.jpg" WIDTH="350" HEIGHT="262" BORDER="0" ALT="In a recent Geology paper, geologists from the universities of Innsbruck and Leeds report on ancient cave systems discovered near the summits of the Allgäu Mountains that preserved the oldest radiometrically dated dripstones currently known from the European Alps. – Michael Meyer”>
In a recent Geology paper, geologists from the universities of Innsbruck and Leeds report on ancient cave systems discovered near the summits of the Allgäu Mountains that preserved the oldest radiometrically dated dripstones currently known from the European Alps. – Michael Meyer

In one of his songs Bob Dylan asks “How many years can a mountain exist before it is washed to the sea?”, and thus poses an intriguing geological question for which an accurate answer is not easily provided. Mountain ranges are in a constant interplay between climatically controlled weathering processes on the one hand and the tectonic forces that cause folding and thrusting and thus thickening of the Earth’s crust on the other hand. While erosion eventually erases any geological obstacles, tectonic forces are responsible for piling- and lifting-up rocks and thus for forming spectacular mountain landscapes such as the European Alps. In reality, climate, weathering and mountain uplift interact in a complex manner and quantifying rates for erosion and uplift, especially for the last couple of millions of years, remains a challenging task.

In a recent Geology paper Michael Meyer (University of Innsbruck) et al. report on ancient cave systems discovered near the summits of the Allgäu Mountains (Austria) that preserved the oldest radiometrically dated dripstones currently known from the European Alps. “These cave deposits formed ca. 2 million years ago and their geochemical signature and biological inclusions are vastly different from other cave calcites in the Alps” says Meyer, who works at the Institute of Geology and Paleontology at the University of Innsbruck, Austria. By carefully analysing these dripstones and using an isotopic modelling approach the authors were able to back-calculate both, the depth of the cave and the altitude of the corresponding summit area at the time of calcite formation. Meyer et al. thus derived erosion and uplift rates for the northern rim of the Alps and – most critically – for a geological time period that is characterized by reoccurring ice ages and hence by intensive glacial erosion. “Our results suggest that 2 million years ago the cave was situated ~1500 meters below its present altitude and the mountains were probably up to 500 meters lower compared to today”, states Meyer. These altitudinal changes were significant and much of this uplift can probably be attributed to the gradual unloading of the Alps due to glacial erosion.

Dripstones have been used to reconstruct past climate and environmental change in a variety of ways. The study of Meyer et al. is novel, however, as it highlights the potential of caves and their deposits to quantitatively constrain mountain evolution on a timescale of millions of years and further shows how the interplay of tectonic and climatic processes can be understood. Key to success is an accurate age control provided by Uranium-Lead dating. This method is commonly used to constrain the age of much older rocks and minerals but has only rarely be applied to dripstones – i.e. only those with high Uranium concentrations – and luckily this is the case for the samples from the Allgäu Mountains.

What lies beneath the seafloor?

An international team of scientists report on the first observatory experiment to study the dynamic microbial life of an ever-changing environment inside Earth’s crust. University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science professor Keir Becker contributed the deep-sea technology required to make long-term scientific observations of life beneath the seafloor.

During the four-year subsurface experiment, the research team deployed the first in situ experimental microbial observatory systems below the flank of the Juan de Fuca Ridge, which is located off the coast of Washington (U.S.) and British Columbia (Canada).

Becker and UM Rosenstiel alumnus Andrew Fisher installed the sub-surface observatory technology known as CORK (Circulation Obviation Retrofit Kit), which seals the sub-surface borehole for undisturbed observations of the natural hydrogeological state and microbial ecosystem inside Earth’s crust.

“Similar to a cork in a wine bottle, our technology stops fluids from moving in and out of the drilling hole,” said Becker, a UM Rosenstiel School professor of marine geology and geophysics. “Ocean water is blocked from entering the hole and flushing out the natural system.”

These natural laboratories allow scientists to investigate the hydrogeology, geochemistry, and microbiology of ocean crust.

A large reservoir of seawater exists in Earth’s crust, which is thought to be the largest habitat on Earth. This seawater aquifer supports a dynamic microbial ecosystem that is known to eat hydrocarbons and natural gas, and may have the genetic potential to store carbon. Scientists are interested in better understanding the natural processes taking place below the seafloor, which also give rise to economically important ores along the seafloor and may play a role in earthquakes.

“The paper is important since it is the first in-situ experiment to study subsurface microbiology,” said Becker, a co-author of the paper.

Eddies found to be deep, powerful modes of ocean transport

Researchers from Woods Hole Oceanographic Institution (WHOI) and their colleagues have discovered that massive, swirling ocean eddies-known to be up to 500 kilometers across at the surface-can reach all the way to the ocean bottom at mid-ocean ridges, some 2,500 meters deep, transporting tiny sea creatures, chemicals, and heat from hydrothermal vents over large distances.

The previously unknown deep-sea phenomenon, reported in the April 28 issue of the journal Science, helps explain how some larvae travel huge distances from one vent area to another, said Diane K. Adams, lead author at WHOI and now at the National Institutes of Health.

“We knew these eddies existed,” said Adams, a biologist. “But nobody realized they can affect processes on the bottom of the ocean. Previous studies had looked at the upper ocean.”

Using deep-sea moorings, current meters and sediment traps over a six-month period, along with computer models, Adams and her colleagues studied the eddies at the underwater mountain range known as the East Pacific Rise. That site experienced a well-documented eruption in 2006 that led to a discovery reported last year that larvae from as far away as 350 km somehow traveled that distance to settle in the aftermath of the eruption.

The newly discovered depth of the powerful eddies helps explain that phenomenon but also opens up a host of other scientific possibilities in oceans around the world. “This atmospherically generated mechanism is affecting the deep sea and how larvae, chemical and heat are transported over large distances,” Adams said.

The eddies are generated at the surface by atmospheric events, such as wind jets, which can be strengthened during an El Niño, and “are known to have a strong influence on surface ocean dynamics and production,” say Adams and Dennis J. McGillicuddy from WHOI, along with colleagues from Florida State University, Lamont Doherty Earth Observatory, and the University of Brest in France. But this “atmospheric forcing?is typically not considered in studies of the deep sea,” they report.

Moreover, the eddies appear to form seasonally, suggesting repeated interactions with undersea ridges such as the Eastern Pacific Rise. The models “predict a train of eddies across the ocean,” Adams said. “There may be two to three eddies per year at this location,” Adams said. Each one, she says, “could connect the site of the eruption to other sites hundreds of miles away.” Elsewhere, she adds, “there are numerous places around the globe where they could be interacting with the deep sea.”

In her 2010 report on larvae traveling great distances to settle at the eruption site, WHOI Senior Scientist Lauren S. Mullineaux , along with Adams and others, suggested the larvae traveled along something like an undersea superhighway, ocean-bottom “jets” travelling up to 10 centimeters a second. But conceding that even those would not be enough to carry the larvae all that distance in such a short time, the researchers speculated that large eddies may be propelling the migrating larvae even faster.

Adams’s current work follows up on that possibility. “The mechanism we found helps explain what we saw in the first paper,” Adams said.

It is the larger picture, over longer periods of time, however, that Adams and her colleagues find particularly intriguing. “Transport [of ocean products] could occur wherever?eddies interact with ridges-including the Mid-Atlantic Ridge, the Southwest Indian Ridge, and the East Scotia Ridge-and the surrounding deep ocean,” the researchers say.

And because the eddies appear to form repeatedly, the high-speed, long-distance transport can last for months. “Although the deep sea and hydrothermal vents in particular are often naively thought of as being isolated from the surface ocean and atmosphere, the interaction of the surface-generated eddies with the deep sea offers a conduit for seasonality and longer-period atmospheric phenomena to influence the ‘seasonless’ deep sea,” Adams and her colleagues write.

“Thus, although hydrothermal sources of heat, chemical and larval fluxes do not exhibit seasonality there is potential for long-distance transport and dispersal to have seasonal to interannual variability.”

Water currents of South Africa could stabilize climate in Europe

One of the ocean currents which particularly interests oceanographers and climatologists is the Gulf Stream. This current, originating in the Gulf of Mexico, transports enormous amounts of warm tropical waters to the North Atlantic and is the cause of Europe’s habitable climate. Climate predictions point to the fact that this will change in the future and affect especially the climate in countries of the Mediterranean region, with more dry spells. As global warming progresses, the North Atlantic will receive more precipitation and a greater amount of water from the melting of glaciers in Greenland, thus reducing the salinity of ocean water and weakening the Gulf Stream’s effects.

The article published in Nature describes an alternative approach which suggests that flows from the Indian Ocean to the South Atlantic, near the tip of Africa, also are important in relation to future current systems in the North Atlantic.

The Agulhas Current, located in the southwest of the Indian Ocean, transports high density salt water to the southern tip of Africa, where part of it escapes to the South Atlantic, contributing to the strength of the global circulation of this ocean. The study describes how this inflow of salt water from the Indian Ocean can compensate the decrease in salinity in the North Atlantic and therefore stabilise the Gulf Stream and the climate in Europe. These processes have been simulated using computational climate models.

The article reviews information available until now and enumerates the steps which must be taken with the aim of carrying out a better assessment of the processes involved in this current system. To demonstrate the dynamics of the Agulhas Current, its sensitivity to climate change and the way it transmits its signals to the North Atlantic, researchers point out the need to combine long-term studies on temperature variation and salinity of the Agulhas Current, analyses on climate changes in the past and detailed computer simulation models.

The existence of connections between the Agulhas Current and Europe’s climate has been the focus of study these past six years of the research group directed by Dr Rainer Zahn.

The authors of the research article are members of a consortium of marine scientists from United States, Germany, The Netherlands, United Kingdom and Spain working together with the objective of studying the effects of the Agulhas Current on regional and global climates. This group forms part of the Scientific Committee on Oceanic Research (SCOR), member of the International Council for Science. Other member institutions include the US National Science Foundation, the World Climate Research Programme (WCRP), the International Association for the Physical Sciences of the Oceans (IAPSO) and the International Marine Global Change Study (IMAGES).