Potential methane reservoirs beneath Antarctica

The new study demonstrates that old organic matter in sedimentary basins located beneath the Antarctic Ice Sheet may have been converted to methane by micro-organisms living under oxygen-deprived conditions. The methane could be released to the atmosphere if the ice sheet shrinks and exposes these old sedimentary basins.

The researchers estimate that 50 per cent of the West Antarctic Ice Sheet (1 million km2) and 25 per cent of the East Antarctic Ice Sheet (2.5 million km2) overlies preglacial sedimentary basins, containing about 21,000 billion tonnes of organic carbon.

Team leader, Professor Wadham said: “This is an immense amount of organic carbon, more than ten times the size of carbon stocks in northern permafrost regions. Our laboratory experiments tell us that these sub-ice environments are also biologically active, meaning that this organic carbon is probably being metabolised to carbon dioxide and methane gas by microbes.”

The researchers then numerically simulated the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model. They found that sub-ice conditions favour the accumulation of methane hydrate (that is, methane trapped within a structure of water molecules, forming a solid similar to regular ice).

They also calculated that the potential amount of methane hydrate and free methane gas beneath the Antarctic Ice Sheet could be up to 400 billion tonnes (that is, 400 Pg of carbon), a similar order of magnitude to some estimates made for Arctic permafrost. The predicted shallow depth of these potential reserves also makes them more susceptible to climate forcing than other methane hydrate reserves on Earth.

Dr Sandra Arndt, a NERC fellow at the University of Bristol who conducted the numerical modelling, said: “It’s not surprising that you might expect to find significant amounts of methane hydrate trapped beneath the ice sheet. Just like in sub-seafloor sediments, it is cold and pressures are high which are important conditions for methane hydrate formation.”

If substantial methane hydrate and gas are present beneath the Antarctic Ice Sheet, methane release during episodes of ice-sheet collapse could act as a positive feedback on global climate change during past and future ice-sheet retreat.

Professor Slawek Tulaczyk, glaciologist from the University of California, Santa Cruz, said: “Our study highlights the need for continued scientific exploration of remote sub-ice environments in Antarctica, because they may have far greater impact on Earth’s climate system than we have appreciated in the past.”

Potential methane reservoirs beneath Antarctica

The new study demonstrates that old organic matter in sedimentary basins located beneath the Antarctic Ice Sheet may have been converted to methane by micro-organisms living under oxygen-deprived conditions. The methane could be released to the atmosphere if the ice sheet shrinks and exposes these old sedimentary basins.

The researchers estimate that 50 per cent of the West Antarctic Ice Sheet (1 million km2) and 25 per cent of the East Antarctic Ice Sheet (2.5 million km2) overlies preglacial sedimentary basins, containing about 21,000 billion tonnes of organic carbon.

Team leader, Professor Wadham said: “This is an immense amount of organic carbon, more than ten times the size of carbon stocks in northern permafrost regions. Our laboratory experiments tell us that these sub-ice environments are also biologically active, meaning that this organic carbon is probably being metabolised to carbon dioxide and methane gas by microbes.”

The researchers then numerically simulated the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model. They found that sub-ice conditions favour the accumulation of methane hydrate (that is, methane trapped within a structure of water molecules, forming a solid similar to regular ice).

They also calculated that the potential amount of methane hydrate and free methane gas beneath the Antarctic Ice Sheet could be up to 400 billion tonnes (that is, 400 Pg of carbon), a similar order of magnitude to some estimates made for Arctic permafrost. The predicted shallow depth of these potential reserves also makes them more susceptible to climate forcing than other methane hydrate reserves on Earth.

Dr Sandra Arndt, a NERC fellow at the University of Bristol who conducted the numerical modelling, said: “It’s not surprising that you might expect to find significant amounts of methane hydrate trapped beneath the ice sheet. Just like in sub-seafloor sediments, it is cold and pressures are high which are important conditions for methane hydrate formation.”

If substantial methane hydrate and gas are present beneath the Antarctic Ice Sheet, methane release during episodes of ice-sheet collapse could act as a positive feedback on global climate change during past and future ice-sheet retreat.

Professor Slawek Tulaczyk, glaciologist from the University of California, Santa Cruz, said: “Our study highlights the need for continued scientific exploration of remote sub-ice environments in Antarctica, because they may have far greater impact on Earth’s climate system than we have appreciated in the past.”

Past tropical climate change linked to ocean circulation

A new record of past temperature change in the tropical Atlantic Ocean’s subsurface provides clues as to why the Earth’s climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M University.

Geological oceanographer Matthew Schmidt and two of his graduate students teamed up with Ping Chang, a physical oceanographer and climate modeler, to help uncover an important climate connection between the tropics and the high latitude North Atlantic. Their new findings are in the current issue of PNAS (Proceedings of the National Academy of Sciences).

The researchers used geochemical clues in fossils called foraminifera, tiny sea creatures with a hard shell, collected from a sediment core located off the northern coast of Venezuela, to generate a 22,000-year record of past ocean temperature and salinity changes in the upper 1,500 feet of water in the western tropical Atlantic. They also conducted global climate model simulations under the past climate condition to interpret this new observational record in the context of changes in the strength of the global ocean conveyor-belt circulation.

“What we found was that subsurface temperatures in the western tropical Atlantic rapidly warmed during cold periods in Earth’s past,” Schmidt explains.

“Together with our new modeling experiments, we think this is evidence that when the global conveyor slowed down during cold periods in the past, warm subsurface waters that are normally trapped in the subtropical North Atlantic flowed southward and rapidly warmed the deep tropics. When the tropics warmed, it altered climate patterns around the globe.”

He notes that as an example, if ocean temperatures were to warm along the west coast of Africa, the monsoon rainfall in that region would be dramatically reduced, affecting millions of people living in sub-Saharan Africa. The researchers also point out that the southward flow of ocean heat during cold periods in the North Atlantic also causes the band of rainfall in the tropics known as the Intertropical Convergence Zone to migrate southward, resulting in much drier conditions in northern South American countries and a wetter South Atlantic.

“Evidence is mounting that the Earth’s climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate,” Schmidt said.

“What we found in our subsurface reconstruction was that the onset of warmer temperatures, thought to reflect the opening of this ‘gateway’ mechanism, occurred in less than a few centuries. It also tells us that it might be a good idea to monitor subsurface temperatures in the western tropical Atlantic to assess how the strength of the ocean conveyor might be changing over the next few decades as Earth’s climate continues to warm.”

“One way to prepare for future climate change is to increase our understanding of how it has operated in the recent past.

Past tropical climate change linked to ocean circulation

A new record of past temperature change in the tropical Atlantic Ocean’s subsurface provides clues as to why the Earth’s climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M University.

Geological oceanographer Matthew Schmidt and two of his graduate students teamed up with Ping Chang, a physical oceanographer and climate modeler, to help uncover an important climate connection between the tropics and the high latitude North Atlantic. Their new findings are in the current issue of PNAS (Proceedings of the National Academy of Sciences).

The researchers used geochemical clues in fossils called foraminifera, tiny sea creatures with a hard shell, collected from a sediment core located off the northern coast of Venezuela, to generate a 22,000-year record of past ocean temperature and salinity changes in the upper 1,500 feet of water in the western tropical Atlantic. They also conducted global climate model simulations under the past climate condition to interpret this new observational record in the context of changes in the strength of the global ocean conveyor-belt circulation.

“What we found was that subsurface temperatures in the western tropical Atlantic rapidly warmed during cold periods in Earth’s past,” Schmidt explains.

“Together with our new modeling experiments, we think this is evidence that when the global conveyor slowed down during cold periods in the past, warm subsurface waters that are normally trapped in the subtropical North Atlantic flowed southward and rapidly warmed the deep tropics. When the tropics warmed, it altered climate patterns around the globe.”

He notes that as an example, if ocean temperatures were to warm along the west coast of Africa, the monsoon rainfall in that region would be dramatically reduced, affecting millions of people living in sub-Saharan Africa. The researchers also point out that the southward flow of ocean heat during cold periods in the North Atlantic also causes the band of rainfall in the tropics known as the Intertropical Convergence Zone to migrate southward, resulting in much drier conditions in northern South American countries and a wetter South Atlantic.

“Evidence is mounting that the Earth’s climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate,” Schmidt said.

“What we found in our subsurface reconstruction was that the onset of warmer temperatures, thought to reflect the opening of this ‘gateway’ mechanism, occurred in less than a few centuries. It also tells us that it might be a good idea to monitor subsurface temperatures in the western tropical Atlantic to assess how the strength of the ocean conveyor might be changing over the next few decades as Earth’s climate continues to warm.”

“One way to prepare for future climate change is to increase our understanding of how it has operated in the recent past.

Antarctic ice sheet quakes shed light on ice movement and earthquakes

Analysis of small, repeating earthquakes in an Antarctic ice sheet may not only lead to an understanding of glacial movement, but may also shed light on stick slip earthquakes like those on the San Andreas fault or in Haiti, according to Penn State geoscientists.

“No one has ever seen anything with such regularity,” said Lucas K. Zoet, recent Penn State Ph. D. recipient, now a postdoctoral fellow at Iowa State University. “An earthquake every 25 minutes for a year.”

The researchers looked at seismic activity recorded during the Transantarctic Mountains Seismic Experiment from 2002 to 2003 on the David Glacier in Antarctica, coupled with data from the Global Seismic Network station Vanda. They found that the local earthquakes on the David Glacier, about 20,000 identified, were predominantly the same and occurred every 25 minutes give or take five minutes.

The researchers note in the current Nature Geoscience that, “The remarkable similarity of the waveforms ? indicates that they share the same source location and source mechanisms.” They suggest that “the same subglacial asperity repeatedly ruptures in response to steady loading from the overlying ice, which is modulated by stress from the tide at the glacier front.”

“Our leading idea is that part of the bedrock is poking through the ductile till layer beneath the glacier,” said Zoet.

The researchers have determined that the asperity — or hill — is about a half mile in diameter.

The glacier, passing over the hill, creates a stick slip situation much like that on the San Andreas fault. The ice sticks on the hill and stress gradually builds until the energy behind the obstruction is high enough to move the ice forward. The ice moves in a step-by-step manner rather than smoothly.

But motion toward the sea is not the only thing acting on the ice streaming from David glacier. Like most glaciers near oceans, the edge of the ice floats out over the water and the floating ice is subject to the action of tides.

“When the tide comes in it pushes back on the ice, making the time between slips slightly longer,” said Sridhar Anandakrishnan, professor of geoscience. “When the tide goes out, the time between slips decreases.”

However, the researchers note that the tides are acting at the ground line, a long way from the location of the asperity and therefore the effects that shorten or lengthen the stick slip cycle are delayed.

“This was something we didn’t expect to see,” said Richard B. Alley, Evan Pugh Professor of Geosciences. “Seeing it is making us reevaluate the basics.”

He also noted that these glacial earthquakes, besides helping glaciologists understand the way ice moves, can provide a simple model for the stick slip earthquakes that occur between landmasses.

“We have not completely explained how ice sheets flow unless we can reproduce this effect,” said Alley. “We can use this as a probe and look into the physics so we better understand how glaciers move.”

Before 2002, this area of the David glacier flowed smoothly, but then for nearly a year the 20-minute earthquake intervals occurred and then stopped. Something occurred at the base of the ice to start and then stop these earthquakes.

“The best idea we have is that during those 300 days, a dirty patch of ice was in contact with the mount, changing the way stress was transferred,” said Zoet. “The glacier is experiencing earthquakes again, although at a different rate. It would be nice to study that.”

Unfortunately, the seismographic instruments that were on the glacier in 2002 no longer exist, and information is coming from only one source at the moment.

Antarctic ice sheet quakes shed light on ice movement and earthquakes

Analysis of small, repeating earthquakes in an Antarctic ice sheet may not only lead to an understanding of glacial movement, but may also shed light on stick slip earthquakes like those on the San Andreas fault or in Haiti, according to Penn State geoscientists.

“No one has ever seen anything with such regularity,” said Lucas K. Zoet, recent Penn State Ph. D. recipient, now a postdoctoral fellow at Iowa State University. “An earthquake every 25 minutes for a year.”

The researchers looked at seismic activity recorded during the Transantarctic Mountains Seismic Experiment from 2002 to 2003 on the David Glacier in Antarctica, coupled with data from the Global Seismic Network station Vanda. They found that the local earthquakes on the David Glacier, about 20,000 identified, were predominantly the same and occurred every 25 minutes give or take five minutes.

The researchers note in the current Nature Geoscience that, “The remarkable similarity of the waveforms ? indicates that they share the same source location and source mechanisms.” They suggest that “the same subglacial asperity repeatedly ruptures in response to steady loading from the overlying ice, which is modulated by stress from the tide at the glacier front.”

“Our leading idea is that part of the bedrock is poking through the ductile till layer beneath the glacier,” said Zoet.

The researchers have determined that the asperity — or hill — is about a half mile in diameter.

The glacier, passing over the hill, creates a stick slip situation much like that on the San Andreas fault. The ice sticks on the hill and stress gradually builds until the energy behind the obstruction is high enough to move the ice forward. The ice moves in a step-by-step manner rather than smoothly.

But motion toward the sea is not the only thing acting on the ice streaming from David glacier. Like most glaciers near oceans, the edge of the ice floats out over the water and the floating ice is subject to the action of tides.

“When the tide comes in it pushes back on the ice, making the time between slips slightly longer,” said Sridhar Anandakrishnan, professor of geoscience. “When the tide goes out, the time between slips decreases.”

However, the researchers note that the tides are acting at the ground line, a long way from the location of the asperity and therefore the effects that shorten or lengthen the stick slip cycle are delayed.

“This was something we didn’t expect to see,” said Richard B. Alley, Evan Pugh Professor of Geosciences. “Seeing it is making us reevaluate the basics.”

He also noted that these glacial earthquakes, besides helping glaciologists understand the way ice moves, can provide a simple model for the stick slip earthquakes that occur between landmasses.

“We have not completely explained how ice sheets flow unless we can reproduce this effect,” said Alley. “We can use this as a probe and look into the physics so we better understand how glaciers move.”

Before 2002, this area of the David glacier flowed smoothly, but then for nearly a year the 20-minute earthquake intervals occurred and then stopped. Something occurred at the base of the ice to start and then stop these earthquakes.

“The best idea we have is that during those 300 days, a dirty patch of ice was in contact with the mount, changing the way stress was transferred,” said Zoet. “The glacier is experiencing earthquakes again, although at a different rate. It would be nice to study that.”

Unfortunately, the seismographic instruments that were on the glacier in 2002 no longer exist, and information is coming from only one source at the moment.

Drastic desertification

The Dead Sea, a salt sea without an outlet, lies over 400 meters below sea level. Tourists like its high salt content because it increases their buoyancy. “For scientists, however, the Dead Sea is a popular archive that provides a diachronic view of its climate past,” says Prof. Dr. Thomas Litt from the Steinmann-Institute for Geology, Mineralogy and Paleontology at the University of Bonn.

Using drilling cores from riparian lake sediments, paleontologists and meteorologists from the University of Bonn deduced the climate conditions of the past 10,000 years. This became possible because the Dead Sea level has sunk drastically over the past years, mostly because of increasing water withdrawals lowering the water supply.

Oldest pollen analysis

In collaboration with the GeoForschungsZentrum Potsdam (German Research Centre for Geosciences) and Israel’s Geological Service, the researchers took a 21 m long sediment sample in the oasis Ein Gedi at the west bank of the Dead Sea. They then matched the fossil pollen to indicator plants for different levels of precipitation and temperature. Radiocarbon-dating was used to determine the age of the layers. “This allowed us to reconstruct the climate of the entire postglacial era,” Prof. Litt reports. “This is the oldest pollen analysis that has been done on the Dead Sea to date.”

In total, there were three different formations of vegetation around this salt sea. In moist phases, a lush, sclerophyll vegetation thrived as can be found today around the Mediterranean Sea. When the climate turned drier, steppe vegetation took over. Drier episodes yet were characterized by desert plants. The researchers found some rapid changes between moist and dry phases.

Transforming pollen data into climate information

The pollen data allows inferring what kinds of plants were growing at the corresponding times. Meteorologists from the University of Bonn took this paleontological data and converted it into climate information. Using statistical methods, they matched plant species with statistical parameters regarding temperature and precipitation that determine whether a certain plant can occur. “This allows us to make statements on the probable climate that prevailed during a certain period of time within the catchment area of the Dead Sea,” reports Prof. Dr. Andreas Hense from the University of Bonn’s Meteorological Institute.

The resilience of the resulting climate information was tested using the data on Dead Sea level fluctuations collected by their Israeli colleagues around Prof. Dr. Mordechai Stein from the Geological Services in Jerusalem. “The two independent data records corresponded very closely,” explains Prof. Litt. “In the moist phases that were determined based on pollen analysis, our Israeli colleagues found that water levels were indeed rising in the Dead Sea, while they fell during dry episodes.” This is plausible since the water level of a terminal lake without an outlet is exclusively determined by precipitation and evaporation.

Droughts led to the biblical exodus

According to the Bonn researchers’ data, there were distinct dry phases particularly during the pottery Neolithic (about 7,500 to 6,500 years ago), as well as at the transition from the late Bronze Age to the early Iron Age (about 3,200 years ago). “Humans were also strongly affected by these climate changes,” Prof. Litt summarizes the effects. The dry phases might have resulted in the Canaanites’ urban culture collapsing while nomads invaded their area.
“At least, this is what the Old Testament refers to as the exodus of the Israelites to the Promised Land.”

Dramatic results

In addition, this look back allows developing scenarios for potential future trends. “Our results are dramatic; they indicate how vulnerable the Dead Sea ecosystems are,” says Prof. Litt. “They clearly show how surprisingly fast lush Mediterranean sclerophyll vegetation can morph into steppe or even desert vegetation within a few decades if it becomes drier.” Back then, the consequences in terms of agriculture and feeding the population were most likely devastating. The researchers want to probe even further back into the climate past of the region around the Dead Sea by drilling even deeper.

Drastic desertification

The Dead Sea, a salt sea without an outlet, lies over 400 meters below sea level. Tourists like its high salt content because it increases their buoyancy. “For scientists, however, the Dead Sea is a popular archive that provides a diachronic view of its climate past,” says Prof. Dr. Thomas Litt from the Steinmann-Institute for Geology, Mineralogy and Paleontology at the University of Bonn.

Using drilling cores from riparian lake sediments, paleontologists and meteorologists from the University of Bonn deduced the climate conditions of the past 10,000 years. This became possible because the Dead Sea level has sunk drastically over the past years, mostly because of increasing water withdrawals lowering the water supply.

Oldest pollen analysis

In collaboration with the GeoForschungsZentrum Potsdam (German Research Centre for Geosciences) and Israel’s Geological Service, the researchers took a 21 m long sediment sample in the oasis Ein Gedi at the west bank of the Dead Sea. They then matched the fossil pollen to indicator plants for different levels of precipitation and temperature. Radiocarbon-dating was used to determine the age of the layers. “This allowed us to reconstruct the climate of the entire postglacial era,” Prof. Litt reports. “This is the oldest pollen analysis that has been done on the Dead Sea to date.”

In total, there were three different formations of vegetation around this salt sea. In moist phases, a lush, sclerophyll vegetation thrived as can be found today around the Mediterranean Sea. When the climate turned drier, steppe vegetation took over. Drier episodes yet were characterized by desert plants. The researchers found some rapid changes between moist and dry phases.

Transforming pollen data into climate information

The pollen data allows inferring what kinds of plants were growing at the corresponding times. Meteorologists from the University of Bonn took this paleontological data and converted it into climate information. Using statistical methods, they matched plant species with statistical parameters regarding temperature and precipitation that determine whether a certain plant can occur. “This allows us to make statements on the probable climate that prevailed during a certain period of time within the catchment area of the Dead Sea,” reports Prof. Dr. Andreas Hense from the University of Bonn’s Meteorological Institute.

The resilience of the resulting climate information was tested using the data on Dead Sea level fluctuations collected by their Israeli colleagues around Prof. Dr. Mordechai Stein from the Geological Services in Jerusalem. “The two independent data records corresponded very closely,” explains Prof. Litt. “In the moist phases that were determined based on pollen analysis, our Israeli colleagues found that water levels were indeed rising in the Dead Sea, while they fell during dry episodes.” This is plausible since the water level of a terminal lake without an outlet is exclusively determined by precipitation and evaporation.

Droughts led to the biblical exodus

According to the Bonn researchers’ data, there were distinct dry phases particularly during the pottery Neolithic (about 7,500 to 6,500 years ago), as well as at the transition from the late Bronze Age to the early Iron Age (about 3,200 years ago). “Humans were also strongly affected by these climate changes,” Prof. Litt summarizes the effects. The dry phases might have resulted in the Canaanites’ urban culture collapsing while nomads invaded their area.
“At least, this is what the Old Testament refers to as the exodus of the Israelites to the Promised Land.”

Dramatic results

In addition, this look back allows developing scenarios for potential future trends. “Our results are dramatic; they indicate how vulnerable the Dead Sea ecosystems are,” says Prof. Litt. “They clearly show how surprisingly fast lush Mediterranean sclerophyll vegetation can morph into steppe or even desert vegetation within a few decades if it becomes drier.” Back then, the consequences in terms of agriculture and feeding the population were most likely devastating. The researchers want to probe even further back into the climate past of the region around the Dead Sea by drilling even deeper.

Why do the Caribbean Islands arc?

This shows USC geophysicists Meghan Miller and Thorsten Becker in Mexico. -  Courtesy of Meghan Miller and Thorsten Becker
This shows USC geophysicists Meghan Miller and Thorsten Becker in Mexico. – Courtesy of Meghan Miller and Thorsten Becker

The Caribbean islands have been pushed east over the last 50 million years, driven by the movement of the Earth’s viscous mantle against the more rooted South American continent, reveals new research by geophysicists from USC.

The results, published today in Nature Geoscience, give us a better understanding of how continents resist the constant movement of the Earth’s plates – and what effect the continental plates have on reshaping the surface of the Earth.

“Studying the deep earth interior provides insights into how the Earth has evolved into its present form,” said Meghan S. Miller, assistant professor of earth sciences in the USC Dornsife College of Letters, Arts and Sciences, and lead author of the paper. “We’re interested in plate tectonics, and the southeastern Caribbean is interesting because it’s right near a complex plate boundary.”

Miller and Thorsten W. Becker, associate professor of earth sciences at USC Dornsife College, studied the margin between the Caribbean plate and the South American plate, ringed by Haiti, the Dominican Republic, Puerto Rico and a crescent of smaller islands including Barbados and St. Lucia.

But just like the First Law of Ecology (and time travel), when it comes to the earth, everything really is connected. So to study the motion of the South American continent and Caribbean plate, the researchers had to first model the entire planet – 176 models to be exact, so large that they took several weeks to compute even at the USC High Performance Computing Center.

For most of us, earthquakes are something to be dreaded. But for Miller and Becker they are a necessary source of data, providing seismic waves that can be tracked all over the world to provide an image of the Earth’s deep interior like a CAT scan. The earthquake waves move slower or more quickly depending on the temperature and composition of the rock, and also depending on how the crystals within the rocks align after millions of years of being pushed around in mantle convection.

“If you can, you want to solve the whole system and then zoom in,” Becker said. “What’s cool about this paper is that we didn’t just run one or two models. We ran a lot, and it allowed us to explore different possibilities for how mantle flow might work.”

Miller and Becker reconstructed the movement of the Earth’s mantle to a depth of almost 3,000 kilometers, upending previous hypotheses of the seismic activity beneath the Caribbean Sea and providing an important new look at the unique tectonic interactions that are causing the Caribbean plate to tear away from South America.

In particular, Miller and Becker point to a part of the South American plate – known as a “cratonic keel” – that is roughly three times thicker than normal lithosphere and much stronger than typical mantle. The keel deflects and channels mantle flow, and provides an important snapshot of the strength of the continents compared to the rest of the Earth’s outer layers.

“Oceanic plates are relatively simple, but if we want to understand how the Earth works as a system – and how faults evolved and where the flow is going over millions of years – we also have to understand continental plates,” Becker said.

In the southeastern Caribbean, the interaction of the subducted plate beneath the Antilles island arc with the stronger continental keel has created the El Pilar-San Sebastian Fault, and the researchers believe a similar series of interactions may have formed the San Andreas Fault.

“We’re studying the Caribbean, but our models are run for the entire globe,” Miller said. “We can look at similar features in Japan, Southern California and the Mediterranean, anywhere we have instruments to record earthquakes.”

Why do the Caribbean Islands arc?

This shows USC geophysicists Meghan Miller and Thorsten Becker in Mexico. -  Courtesy of Meghan Miller and Thorsten Becker
This shows USC geophysicists Meghan Miller and Thorsten Becker in Mexico. – Courtesy of Meghan Miller and Thorsten Becker

The Caribbean islands have been pushed east over the last 50 million years, driven by the movement of the Earth’s viscous mantle against the more rooted South American continent, reveals new research by geophysicists from USC.

The results, published today in Nature Geoscience, give us a better understanding of how continents resist the constant movement of the Earth’s plates – and what effect the continental plates have on reshaping the surface of the Earth.

“Studying the deep earth interior provides insights into how the Earth has evolved into its present form,” said Meghan S. Miller, assistant professor of earth sciences in the USC Dornsife College of Letters, Arts and Sciences, and lead author of the paper. “We’re interested in plate tectonics, and the southeastern Caribbean is interesting because it’s right near a complex plate boundary.”

Miller and Thorsten W. Becker, associate professor of earth sciences at USC Dornsife College, studied the margin between the Caribbean plate and the South American plate, ringed by Haiti, the Dominican Republic, Puerto Rico and a crescent of smaller islands including Barbados and St. Lucia.

But just like the First Law of Ecology (and time travel), when it comes to the earth, everything really is connected. So to study the motion of the South American continent and Caribbean plate, the researchers had to first model the entire planet – 176 models to be exact, so large that they took several weeks to compute even at the USC High Performance Computing Center.

For most of us, earthquakes are something to be dreaded. But for Miller and Becker they are a necessary source of data, providing seismic waves that can be tracked all over the world to provide an image of the Earth’s deep interior like a CAT scan. The earthquake waves move slower or more quickly depending on the temperature and composition of the rock, and also depending on how the crystals within the rocks align after millions of years of being pushed around in mantle convection.

“If you can, you want to solve the whole system and then zoom in,” Becker said. “What’s cool about this paper is that we didn’t just run one or two models. We ran a lot, and it allowed us to explore different possibilities for how mantle flow might work.”

Miller and Becker reconstructed the movement of the Earth’s mantle to a depth of almost 3,000 kilometers, upending previous hypotheses of the seismic activity beneath the Caribbean Sea and providing an important new look at the unique tectonic interactions that are causing the Caribbean plate to tear away from South America.

In particular, Miller and Becker point to a part of the South American plate – known as a “cratonic keel” – that is roughly three times thicker than normal lithosphere and much stronger than typical mantle. The keel deflects and channels mantle flow, and provides an important snapshot of the strength of the continents compared to the rest of the Earth’s outer layers.

“Oceanic plates are relatively simple, but if we want to understand how the Earth works as a system – and how faults evolved and where the flow is going over millions of years – we also have to understand continental plates,” Becker said.

In the southeastern Caribbean, the interaction of the subducted plate beneath the Antilles island arc with the stronger continental keel has created the El Pilar-San Sebastian Fault, and the researchers believe a similar series of interactions may have formed the San Andreas Fault.

“We’re studying the Caribbean, but our models are run for the entire globe,” Miller said. “We can look at similar features in Japan, Southern California and the Mediterranean, anywhere we have instruments to record earthquakes.”