Himalaya tectonic dam with a discharge

The Himalaya features some of the most impressive gorges on Earth that have been formed by rivers. The geologic history of the famous Tsangpo Gorge, in the eastern Himalaya, now needs to be rewritten.

A team of German, Chinese, and American geoscientists have namely discovered a canyon, filled with more than 500 m of sediments beneath the bed of the present-day Yarlung Tsangpo River upstream from the gorge. Using drill cores, the scientists were able to reconstruct the former valley floor of this river, which allowed them to reconstruct the geological history of the Tsangpo Gorge (Science, 21.11.2014). They discovered that the gorge obtained its steep form in response to rapid tectonic uplift in the Himalaya, two to three million years ago. “Because of its high gradient, the river incises its bed very rapidly”, explains Dirk Scherler from the GFZ German Research Centre for Geosciences. “The rocks here are eroded at annual rates of up to one centimeter per year which is matched by tectonic uplift of the same rate.” The collision of India with the Eurasian continent has created a tectonic dam here.

This barrier caused a lower flow velocity of the Yarlung Tsangpoupstream. Previously, the river had deeply incised into the Tibetan Plateau. Due to the reduced flow rate the sediments which the Yarlung Tsangpo River and its tributaries eroded from the highlands were deposited in the river bed along hundreds of kilometers.

The scientists estimated that these deposits are up to 1000 m thick. “Five drillings have been conducted over a distance of 300 km upstream of the gorge”, says Dirk Scherler. “One of the drillings encountered bedrock after 540 meters of sediments. From the drill cores, we were able to infer the reduction in stream flow velocity and date the initiation of sedimentation using cosmogenic nuclides. These are rare isotopes that are produced by cosmic rays near the Earth’s surface. Three Million years ago, the river was still incising into Himalayan bedrock.” But today the once huge canyon is buried by sediments.

The Yarlung Tsangpo is the largest high mountain river on Earth. It flows along a distance of 1700 km across the Tibetan Plateau, at an elevation of around 4000 meters and follows the boundary between India and Eurasia. In the eastern Himalaya, the river leaves the high plateau and breaks through the world famous, horseshoe-shaped Tsangpo Gorge for the plains of India.

The new findings show that rapid incision of the Yarlung Tsangpo and the development of the Tsangpo Gorge occurred in response to tectonic uplift, and not, as previously thought, the other way round. In addition, these observations refute existing hypotheses that relate the origin of the Tsangpo Gorge to river capture of the Yarlung Tsangpo by the Brahmaputra River.

Himalaya tectonic dam with a discharge

The Himalaya features some of the most impressive gorges on Earth that have been formed by rivers. The geologic history of the famous Tsangpo Gorge, in the eastern Himalaya, now needs to be rewritten.

A team of German, Chinese, and American geoscientists have namely discovered a canyon, filled with more than 500 m of sediments beneath the bed of the present-day Yarlung Tsangpo River upstream from the gorge. Using drill cores, the scientists were able to reconstruct the former valley floor of this river, which allowed them to reconstruct the geological history of the Tsangpo Gorge (Science, 21.11.2014). They discovered that the gorge obtained its steep form in response to rapid tectonic uplift in the Himalaya, two to three million years ago. “Because of its high gradient, the river incises its bed very rapidly”, explains Dirk Scherler from the GFZ German Research Centre for Geosciences. “The rocks here are eroded at annual rates of up to one centimeter per year which is matched by tectonic uplift of the same rate.” The collision of India with the Eurasian continent has created a tectonic dam here.

This barrier caused a lower flow velocity of the Yarlung Tsangpoupstream. Previously, the river had deeply incised into the Tibetan Plateau. Due to the reduced flow rate the sediments which the Yarlung Tsangpo River and its tributaries eroded from the highlands were deposited in the river bed along hundreds of kilometers.

The scientists estimated that these deposits are up to 1000 m thick. “Five drillings have been conducted over a distance of 300 km upstream of the gorge”, says Dirk Scherler. “One of the drillings encountered bedrock after 540 meters of sediments. From the drill cores, we were able to infer the reduction in stream flow velocity and date the initiation of sedimentation using cosmogenic nuclides. These are rare isotopes that are produced by cosmic rays near the Earth’s surface. Three Million years ago, the river was still incising into Himalayan bedrock.” But today the once huge canyon is buried by sediments.

The Yarlung Tsangpo is the largest high mountain river on Earth. It flows along a distance of 1700 km across the Tibetan Plateau, at an elevation of around 4000 meters and follows the boundary between India and Eurasia. In the eastern Himalaya, the river leaves the high plateau and breaks through the world famous, horseshoe-shaped Tsangpo Gorge for the plains of India.

The new findings show that rapid incision of the Yarlung Tsangpo and the development of the Tsangpo Gorge occurred in response to tectonic uplift, and not, as previously thought, the other way round. In addition, these observations refute existing hypotheses that relate the origin of the Tsangpo Gorge to river capture of the Yarlung Tsangpo by the Brahmaputra River.

Climate capers of the past 600,000 years

The researchers remove samples from a core segment taken from Lake Van at the center for Marine environmental sciences MARUM in Bremen, where all of the cores from the PALEOVAN project are stored. -  Photo: Nadine Pickarski/Uni Bonn
The researchers remove samples from a core segment taken from Lake Van at the center for Marine environmental sciences MARUM in Bremen, where all of the cores from the PALEOVAN project are stored. – Photo: Nadine Pickarski/Uni Bonn

If you want to see into the future, you have to understand the past. An international consortium of researchers under the auspices of the University of Bonn has drilled deposits on the bed of Lake Van (Eastern Turkey) which provide unique insights into the last 600,000 years. The samples reveal that the climate has done its fair share of mischief-making in the past. Furthermore, there have been numerous earthquakes and volcanic eruptions. The results of the drilling project also provide a basis for assessing the risk of how dangerous natural hazards are for today’s population. In a special edition of the highly regarded publication Quaternary Science Reviews, the scientists have now published their findings in a number of journal articles.

In the sediments of Lake Van, the lighter-colored, lime-containing summer layers are clearly distinguishable from the darker, clay-rich winter layers — also called varves. In 2010, from a floating platform an international consortium of researchers drilled a 220 m deep sediment profile from the lake floor at a water depth of 360 m and analyzed the varves. The samples they recovered are a unique scientific treasure because the climate conditions, earthquakes and volcanic eruptions of the past 600,000 years can be read in outstanding quality from the cores.

The team of scientists under the auspices of the University of Bonn has analyzed some 5,000 samples in total. “The results show that the climate over the past hundred thousand years has been a roller coaster. Within just a few decades, the climate could tip from an ice age into a warm period,” says Doctor Thomas Litt of the University of Bonn’s Steinmann Institute and spokesman for the PALEOVAN international consortium of researchers. Unbroken continental climate archives from the ice age which encompass several hundred thousand years are extremely rare on a global scale. “There has never before in all of the Middle East and Central Asia been a continental drilling operation going so far back into the past,” says Doctor Litt. In the northern hemisphere, climate data from ice-cores drilled in Greenland encompass the last 120,000 years. The Lake Van project closes a gap in the scientific climate record.

The sediments reveal six cycles of cold and warm periods


Scientists found evidence for a total of six cycles of warm and cold periods in the sediments of Lake Van. The University of Bonn paleoecologist and his colleagues analyzed the pollen preserved in the sediments. Under a microscope they were able to determine which plants around the eastern Anatolian Lake the pollen came from. “Pollen is amazingly durable and is preserved over very long periods when protected in the sediments,” Doctor Litt explained. Insight into the age of the individual layers was gleaned through radiometric age measurements that use the decay of radioactive elements as a geologic clock. Based on the type of pollen and the age, the scientists were able to determine when oak forests typical of warm periods grew around Lake Van and when ice-age steppe made up of grasses, mugwort and goosefoot surrounded the lake.

Once they determine the composition of the vegetation present and the requirements of the plants, the scientists can reconstruct with a high degree of accuracy the temperature and amount of rainfall during different epochs. These analyses enable the team of researchers to read the varves of Lake Van like thousands of pages of an archive. With these data, the team was able to demonstrate that fluctuations in climate were due in large part to periodic changes in the Earth’s orbit parameters and the commensurate changes in solar insolation levels. However, the influence of North Atlantic currents was also evident. “The analysis of the Lake Van sediments has presented us with an image of how an ecosystem reacts to abrupt changes in climate. This fundamental data will help us to develop potential scenarios of future climate effects,” says Doctor Litt.

Risks of earthquakes and volcanic eruptions in the region of Van

Such risk assessments can also be made for other natural forces. “Deposits of volcanic ash with thicknesses of up to 10 m in the Lake Van sediments show us that approximately 270,000 years ago there was a massive eruption,” the University of Bonn paleoecologist said. The team struck some 300 different volcanic events in its drillings. Statistically, that corresponds to one explosive volcanic eruption in the region every 2000 years. Deformations in the sediment layers show that the area is subject to frequent, strong earthquakes. “The area around Lake Van is very densely populated. The data from the core samples show that volcanic activity and earthquakes present a relatively high risk for the region,” Doctor Litt says. According to media reports, in 2011 a 7.2 magnitude earthquake in the Van province claimed the lives of more than 500 people and injured more than 2,500.

Publication: “Results from the PALEOVAN drilling project: A 600,000 year long continental archive in the Near East”, Quaternary Science Reviews, Volume 104, online publication: (http://dx.doi.org/10.1016/j.quascirev.2014.09.026)

Climate capers of the past 600,000 years

The researchers remove samples from a core segment taken from Lake Van at the center for Marine environmental sciences MARUM in Bremen, where all of the cores from the PALEOVAN project are stored. -  Photo: Nadine Pickarski/Uni Bonn
The researchers remove samples from a core segment taken from Lake Van at the center for Marine environmental sciences MARUM in Bremen, where all of the cores from the PALEOVAN project are stored. – Photo: Nadine Pickarski/Uni Bonn

If you want to see into the future, you have to understand the past. An international consortium of researchers under the auspices of the University of Bonn has drilled deposits on the bed of Lake Van (Eastern Turkey) which provide unique insights into the last 600,000 years. The samples reveal that the climate has done its fair share of mischief-making in the past. Furthermore, there have been numerous earthquakes and volcanic eruptions. The results of the drilling project also provide a basis for assessing the risk of how dangerous natural hazards are for today’s population. In a special edition of the highly regarded publication Quaternary Science Reviews, the scientists have now published their findings in a number of journal articles.

In the sediments of Lake Van, the lighter-colored, lime-containing summer layers are clearly distinguishable from the darker, clay-rich winter layers — also called varves. In 2010, from a floating platform an international consortium of researchers drilled a 220 m deep sediment profile from the lake floor at a water depth of 360 m and analyzed the varves. The samples they recovered are a unique scientific treasure because the climate conditions, earthquakes and volcanic eruptions of the past 600,000 years can be read in outstanding quality from the cores.

The team of scientists under the auspices of the University of Bonn has analyzed some 5,000 samples in total. “The results show that the climate over the past hundred thousand years has been a roller coaster. Within just a few decades, the climate could tip from an ice age into a warm period,” says Doctor Thomas Litt of the University of Bonn’s Steinmann Institute and spokesman for the PALEOVAN international consortium of researchers. Unbroken continental climate archives from the ice age which encompass several hundred thousand years are extremely rare on a global scale. “There has never before in all of the Middle East and Central Asia been a continental drilling operation going so far back into the past,” says Doctor Litt. In the northern hemisphere, climate data from ice-cores drilled in Greenland encompass the last 120,000 years. The Lake Van project closes a gap in the scientific climate record.

The sediments reveal six cycles of cold and warm periods


Scientists found evidence for a total of six cycles of warm and cold periods in the sediments of Lake Van. The University of Bonn paleoecologist and his colleagues analyzed the pollen preserved in the sediments. Under a microscope they were able to determine which plants around the eastern Anatolian Lake the pollen came from. “Pollen is amazingly durable and is preserved over very long periods when protected in the sediments,” Doctor Litt explained. Insight into the age of the individual layers was gleaned through radiometric age measurements that use the decay of radioactive elements as a geologic clock. Based on the type of pollen and the age, the scientists were able to determine when oak forests typical of warm periods grew around Lake Van and when ice-age steppe made up of grasses, mugwort and goosefoot surrounded the lake.

Once they determine the composition of the vegetation present and the requirements of the plants, the scientists can reconstruct with a high degree of accuracy the temperature and amount of rainfall during different epochs. These analyses enable the team of researchers to read the varves of Lake Van like thousands of pages of an archive. With these data, the team was able to demonstrate that fluctuations in climate were due in large part to periodic changes in the Earth’s orbit parameters and the commensurate changes in solar insolation levels. However, the influence of North Atlantic currents was also evident. “The analysis of the Lake Van sediments has presented us with an image of how an ecosystem reacts to abrupt changes in climate. This fundamental data will help us to develop potential scenarios of future climate effects,” says Doctor Litt.

Risks of earthquakes and volcanic eruptions in the region of Van

Such risk assessments can also be made for other natural forces. “Deposits of volcanic ash with thicknesses of up to 10 m in the Lake Van sediments show us that approximately 270,000 years ago there was a massive eruption,” the University of Bonn paleoecologist said. The team struck some 300 different volcanic events in its drillings. Statistically, that corresponds to one explosive volcanic eruption in the region every 2000 years. Deformations in the sediment layers show that the area is subject to frequent, strong earthquakes. “The area around Lake Van is very densely populated. The data from the core samples show that volcanic activity and earthquakes present a relatively high risk for the region,” Doctor Litt says. According to media reports, in 2011 a 7.2 magnitude earthquake in the Van province claimed the lives of more than 500 people and injured more than 2,500.

Publication: “Results from the PALEOVAN drilling project: A 600,000 year long continental archive in the Near East”, Quaternary Science Reviews, Volume 104, online publication: (http://dx.doi.org/10.1016/j.quascirev.2014.09.026)

Lack of oxygen delayed the rise of animals on Earth

Christopher Reinhard and Noah Planavsky conduct research for the study in China. -  Yale University
Christopher Reinhard and Noah Planavsky conduct research for the study in China. – Yale University

Geologists are letting the air out of a nagging mystery about the development of animal life on Earth.

Scientists have long speculated as to why animal species didn’t flourish sooner, once sufficient oxygen covered the Earth’s surface. Animals began to prosper at the end of the Proterozoic period, about 800 million years ago — but what about the billion-year stretch before that, when most researchers think there also was plenty of oxygen?

Well, it seems the air wasn’t so great then, after all.

In a study published Oct. 30 in Science, Yale researcher Noah Planavsky and his colleagues found that oxygen levels during the “boring billion” period were only 0.1% of what they are today. In other words, Earth’s atmosphere couldn’t have supported a diversity of creatures, no matter what genetic advancements were poised to occur.

“There is no question that genetic and ecological innovation must ultimately be behind the rise of animals, but it is equally unavoidable that animals need a certain level of oxygen,” said Planavsky, co-lead author of the research along with Christopher Reinhard of the Georgia Institute of Technology. “We’re providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals.”

The scientists found their evidence by analyzing chromium (Cr) isotopes in ancient sediments from China, Australia, Canada, and the United States. Chromium is found in the Earth’s continental crust, and chromium oxidation is directly linked to the presence of free oxygen in the atmosphere.

Specifically, the team studied samples deposited in shallow, iron-rich ocean areas, near the shore. They compared their data with other samples taken from younger locales known to have higher levels of oxygen.

Oxygen’s role in controlling the first appearance of animals has long vexed scientists. “We were missing the right approach until now,” Planavsky said. “Chromium gave us the proxy.” Previous estimates put the oxygen level at 40% of today’s conditions during pre-animal times, leaving open the possibility that oxygen was already plentiful enough to support animal life.

In the new study, the researchers acknowledged that oxygen levels were “highly dynamic” in the early atmosphere, with the potential for occasional spikes. However, they said, “It seems clear that there is a first-order difference in the nature of Earth surface Cr cycling” before and after the rise of animals.

“If we are right, our results will really change how people view the origins of animals and other complex life, and their relationships to the co-evolving environment,” said co-author Tim Lyons of the University of California-Riverside. “This could be a game changer.”

“There’s a lot of interest right now in a broader discussion surrounding the role that environmental stability played in the evolution of complex life, and we think our results are a significant contribution to that,” Reinhard said.

Lack of oxygen delayed the rise of animals on Earth

Christopher Reinhard and Noah Planavsky conduct research for the study in China. -  Yale University
Christopher Reinhard and Noah Planavsky conduct research for the study in China. – Yale University

Geologists are letting the air out of a nagging mystery about the development of animal life on Earth.

Scientists have long speculated as to why animal species didn’t flourish sooner, once sufficient oxygen covered the Earth’s surface. Animals began to prosper at the end of the Proterozoic period, about 800 million years ago — but what about the billion-year stretch before that, when most researchers think there also was plenty of oxygen?

Well, it seems the air wasn’t so great then, after all.

In a study published Oct. 30 in Science, Yale researcher Noah Planavsky and his colleagues found that oxygen levels during the “boring billion” period were only 0.1% of what they are today. In other words, Earth’s atmosphere couldn’t have supported a diversity of creatures, no matter what genetic advancements were poised to occur.

“There is no question that genetic and ecological innovation must ultimately be behind the rise of animals, but it is equally unavoidable that animals need a certain level of oxygen,” said Planavsky, co-lead author of the research along with Christopher Reinhard of the Georgia Institute of Technology. “We’re providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals.”

The scientists found their evidence by analyzing chromium (Cr) isotopes in ancient sediments from China, Australia, Canada, and the United States. Chromium is found in the Earth’s continental crust, and chromium oxidation is directly linked to the presence of free oxygen in the atmosphere.

Specifically, the team studied samples deposited in shallow, iron-rich ocean areas, near the shore. They compared their data with other samples taken from younger locales known to have higher levels of oxygen.

Oxygen’s role in controlling the first appearance of animals has long vexed scientists. “We were missing the right approach until now,” Planavsky said. “Chromium gave us the proxy.” Previous estimates put the oxygen level at 40% of today’s conditions during pre-animal times, leaving open the possibility that oxygen was already plentiful enough to support animal life.

In the new study, the researchers acknowledged that oxygen levels were “highly dynamic” in the early atmosphere, with the potential for occasional spikes. However, they said, “It seems clear that there is a first-order difference in the nature of Earth surface Cr cycling” before and after the rise of animals.

“If we are right, our results will really change how people view the origins of animals and other complex life, and their relationships to the co-evolving environment,” said co-author Tim Lyons of the University of California-Riverside. “This could be a game changer.”

“There’s a lot of interest right now in a broader discussion surrounding the role that environmental stability played in the evolution of complex life, and we think our results are a significant contribution to that,” Reinhard said.

The breathing sand

An Eddy Correlation Lander analyzes the strength of the oxygen fluxes at the bottom of the North Sea. -  Photo: ROV-Team, GEOMAR
An Eddy Correlation Lander analyzes the strength of the oxygen fluxes at the bottom of the North Sea. – Photo: ROV-Team, GEOMAR

A desert at the bottom of the sea? Although the waters of the North Sea exchange about every two to three years, there is evidence of decreasing oxygen content. If lower amounts of this gas are dissolved in seawater, organisms on and in the seabed produce less energy – with implications for larger creatures and the biogeochemical cycling in the marine ecosystem. Since nutrients, carbon and oxygen circulate very well and are processed quickly in the permeable, sandy sediments that make up two-thirds of the North Sea, measurements of metabolic rates are especially difficult here. Using the new Aquatic Eddy Correlation technique, scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel, Leibniz Institute of Freshwater Ecology and Inland Fisheries, the University of Southern Denmark, the University of Koblenz-Landau, the Scottish Marine Institute and Aarhus University were able to demonstrate how oxygen flows at the ground of the North Sea. Their methods and results are presented in the Journal of Geophysical Research: Oceans.

“The so-called ‘Eddy Correlation’ technique detects the flow of oxygen through these small turbulences over an area of several square meters. It considers both the mixing of sediments by organisms living in it and the hydrodynamics of the water above the rough sea floor”, Dr. Peter Linke, a marine biologist at GEOMAR, explains. “Previous methods overlooked only short periods or disregarded important parameters. Now we can create a more realistic picture.” The new method also takes into account the fact that even small objects such as shells or ripples shaped by wave action or currents are able to impact the oxygen exchange in permeable sediments.

On the expedition CE0913 with the Irish research vessel CELTIC EXPLORER, scientists used the underwater robot ROV KIEL 6000 to place three different instruments within the “Tommeliten” area belonging to Norway: Two “Eddy Correlation Landers” recorded the strength of oxygen fluxes over three tidal cycles. Information about the distribution of oxygen in the sediment was collected with a “Profiler Lander”, a seafloor observatory with oxygen sensors and flow meters. A “Benthic chamber” isolated 314 square centimetres of sediment and took samples from the overlying water over a period of 24 hours to determine the oxygen consumption of the sediment.

“The combination of traditional tools with the ‘Eddy Correlation’ technique has given us new insights into the dynamics of the exchange of substances between the sea water and the underlying sediment. A variety of factors determine the timing and amount of oxygen available. Currents that provide the sandy sediment with oxygen, but also the small-scale morphology of the seafloor, ensure that small benthic organisms are able to process carbon or other nutrients. The dependencies are so complex that they can be decrypted only by using special methods”, Dr. Linke summarizes. Therefore, detailed measurements in the water column and at the boundary to the seafloor as well as model calculations are absolutely necessary to understand basic functions and better estimate future changes in the cycle of materials. “With conventional methods, for example, we would never have been able to find that the loose sandy sediment stores oxygen brought in by the currents for periods of less water movement and less oxygen introduction.”

Original publication:
McGinnis, D. F., S. Sommer, A. Lorke, R. N. Glud, P. Linke (2014): Quantifying tidally driven benthic oxygen exchange across permeable sediments: An aquatic eddy correlation study. Journal of Geophysical Research: Oceans, doi:10.1002/2014JC010303.

Links:

GEOMAR Helmholtz Centre for Ocean Research Kiel

Eddy correlation information page

Leibniz Institute of Freshwater Ecology and Inland Fisheries, IGB

University of Southern Denmark

University of Koblenz-Landau

Scottish Marine Institute

Aarhus University

Images:
High resolution images can be downloaded at http://www.geomar.de/n2110-e.

Video footage is available on request.

Contact:
Dr. Peter Linke (GEOMAR FB2-MG), Tel. 0431 600-2115, plinke@geomar.de

Maike Nicolai (GEOMAR, Kommunikation & Medien), Tel. 0431 600-2807, mnicolai@geomar.de

The breathing sand

An Eddy Correlation Lander analyzes the strength of the oxygen fluxes at the bottom of the North Sea. -  Photo: ROV-Team, GEOMAR
An Eddy Correlation Lander analyzes the strength of the oxygen fluxes at the bottom of the North Sea. – Photo: ROV-Team, GEOMAR

A desert at the bottom of the sea? Although the waters of the North Sea exchange about every two to three years, there is evidence of decreasing oxygen content. If lower amounts of this gas are dissolved in seawater, organisms on and in the seabed produce less energy – with implications for larger creatures and the biogeochemical cycling in the marine ecosystem. Since nutrients, carbon and oxygen circulate very well and are processed quickly in the permeable, sandy sediments that make up two-thirds of the North Sea, measurements of metabolic rates are especially difficult here. Using the new Aquatic Eddy Correlation technique, scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel, Leibniz Institute of Freshwater Ecology and Inland Fisheries, the University of Southern Denmark, the University of Koblenz-Landau, the Scottish Marine Institute and Aarhus University were able to demonstrate how oxygen flows at the ground of the North Sea. Their methods and results are presented in the Journal of Geophysical Research: Oceans.

“The so-called ‘Eddy Correlation’ technique detects the flow of oxygen through these small turbulences over an area of several square meters. It considers both the mixing of sediments by organisms living in it and the hydrodynamics of the water above the rough sea floor”, Dr. Peter Linke, a marine biologist at GEOMAR, explains. “Previous methods overlooked only short periods or disregarded important parameters. Now we can create a more realistic picture.” The new method also takes into account the fact that even small objects such as shells or ripples shaped by wave action or currents are able to impact the oxygen exchange in permeable sediments.

On the expedition CE0913 with the Irish research vessel CELTIC EXPLORER, scientists used the underwater robot ROV KIEL 6000 to place three different instruments within the “Tommeliten” area belonging to Norway: Two “Eddy Correlation Landers” recorded the strength of oxygen fluxes over three tidal cycles. Information about the distribution of oxygen in the sediment was collected with a “Profiler Lander”, a seafloor observatory with oxygen sensors and flow meters. A “Benthic chamber” isolated 314 square centimetres of sediment and took samples from the overlying water over a period of 24 hours to determine the oxygen consumption of the sediment.

“The combination of traditional tools with the ‘Eddy Correlation’ technique has given us new insights into the dynamics of the exchange of substances between the sea water and the underlying sediment. A variety of factors determine the timing and amount of oxygen available. Currents that provide the sandy sediment with oxygen, but also the small-scale morphology of the seafloor, ensure that small benthic organisms are able to process carbon or other nutrients. The dependencies are so complex that they can be decrypted only by using special methods”, Dr. Linke summarizes. Therefore, detailed measurements in the water column and at the boundary to the seafloor as well as model calculations are absolutely necessary to understand basic functions and better estimate future changes in the cycle of materials. “With conventional methods, for example, we would never have been able to find that the loose sandy sediment stores oxygen brought in by the currents for periods of less water movement and less oxygen introduction.”

Original publication:
McGinnis, D. F., S. Sommer, A. Lorke, R. N. Glud, P. Linke (2014): Quantifying tidally driven benthic oxygen exchange across permeable sediments: An aquatic eddy correlation study. Journal of Geophysical Research: Oceans, doi:10.1002/2014JC010303.

Links:

GEOMAR Helmholtz Centre for Ocean Research Kiel

Eddy correlation information page

Leibniz Institute of Freshwater Ecology and Inland Fisheries, IGB

University of Southern Denmark

University of Koblenz-Landau

Scottish Marine Institute

Aarhus University

Images:
High resolution images can be downloaded at http://www.geomar.de/n2110-e.

Video footage is available on request.

Contact:
Dr. Peter Linke (GEOMAR FB2-MG), Tel. 0431 600-2115, plinke@geomar.de

Maike Nicolai (GEOMAR, Kommunikation & Medien), Tel. 0431 600-2807, mnicolai@geomar.de

Rivers flow differently over gravel beds, study finds

River researchers used a specially constructed model to study how water flows over gravel river beds. Illinois postdoctoral researcher Gianluca Blois (left) and professor Jim Best also developed a technique to measure the water flow between the pore spaces in the river bed. -  L. Brian Stauffer
River researchers used a specially constructed model to study how water flows over gravel river beds. Illinois postdoctoral researcher Gianluca Blois (left) and professor Jim Best also developed a technique to measure the water flow between the pore spaces in the river bed. – L. Brian Stauffer

River beds, where flowing water meets silt, sand and gravel, are critical ecological zones. Yet how water flows in a river with a gravel bed is very different from the traditional model of a sandy river bed, according to a new study that compares their fluid dynamics.

The findings establish new parameters for river modeling that better represent reality, with implications for field researchers and water resource managers.

“The shallow zones where water in rivers interacts with the subsurface are critical environmentally, and how we have modeled those in the past may be radically different from reality,” said Jim Best, a professor of geology, geography and geographic information science at the University of Illinois. “If you’re a river engineer or a geomorphologist or a freshwater biologist, predicting where and when sediment transport is going to occur is very important. This study provides us with a very different set of conditions to look at those environments and potentially manage them.”

Best and postdoctoral researcher Gianluca Blois led the study at the U. of I., in collaboration with colleagues in the United Kingdom. The team published its findings in the journal Geophysical Research Letters.

The researchers used a specially constructed flume in the Ven Te Chow Hydrosystems Laboratory at Illinois to experimentally compare scenarios ranging from the traditional model of an impermeable river bottom to a completely permeable river bed – a collection of spheres that simulate gravel.

The researchers used a technique called particle image velocimetry (PIV), a widely used method for quantifying how water flows over a model river bed, pioneered at the U. of I. in the 1980s. Best and Blois developed a method to use PIV endoscopically to study, for the first time, fluid flow within the small spaces between the gravel. This allowed them to quantify flow within the river bed and link it to the stream flow above.

They found that, in the scenario that simulates a gravel bed, the patterns of flow velocity above the bed and the distribution of forces on the river bed were dramatically different from the models on which all previous work has been based. Their experimental scenarios also disproved one popular theory that explained the difference between classic models and field observations for the formation of bed topography, such as dunes.

“Bedforms formed in fine sediments are known to be substantially different from those formed in gravel beds, but we just didn’t know why,” Blois said. “People before us suggested that those differences were due to the roughness of the grains. But we introduced the bed permeability, just like real rivers. This, with our new measurement technique, allowed us to demonstrate that most of the stress variation is actually coming from fluid emerging from the permeable bed, rather than roughness.”

The maps of water flow that the experiments produced could lead to better predictive models, so that researchers can more accurately predict and study how nutrients and pollutants travel and accumulate in rivers. These new models also could provide insight into the growth and behavior of organisms that thrive in the narrow zone where river flow meets the river bed.

“For example, when salmon spawn in gravel-bed rivers, they basically make a depression in the gravel, into which they lay their eggs,” Best said. “By doing that, not only do they protect the eggs, but they create a bump in the sediment that creates a pressure distribution that would keep fine grains from going into the bed, which would be detrimental to the eggs. It’s fascinating that fish actually take advantage of these flow dynamics.”

The researchers are working with collaborators around the world to study how permeability affects turbulence above the bed, how this affects the organisms that grow in the pore spaces, and how tiny particles and dissolved substances accumulate in porous riverbeds.

“It’s going to change the way we conceptualize these systems and model them,” Blois said. “We’re trying to raise awareness of the fact that we are now able to measure the complex flow dynamics in these challenging environments, and that’s going to open up a new paradigm for river research.”

Scientists find ancient mountains that fed early life

This image shows the ancient mountain site, Brazil. -  Carlos Ganade de Araujo
This image shows the ancient mountain site, Brazil. – Carlos Ganade de Araujo

Scientists have found evidence for a huge mountain range that sustained an explosion of life on Earth 600 million years ago.

The mountain range was similar in scale to the Himalayas and spanned at least 2,500 kilometres of modern west Africa and northeast Brazil, which at that time were part of the supercontinent Gondwana.

“Just like the Himalayas, this range was eroded intensely because it was so huge. As the sediments washed into the oceans they provided the perfect nutrients for life to flourish,” said Professor Daniela Rubatto of the Research School of Earth Sciences at The Australian National University (ANU).

“Scientists have speculated that such a large mountain range must have been feeding the oceans because of the way life thrived and ocean chemistry changed at this time, and finally we have found it.”

The discovery is earliest evidence of Himalayan-scale mountains on Earth.

“Although the mountains have long since washed away, rocks from their roots told the story of the ancient mountain range’s grandeur,” said co-researcher Professor Joerg Hermann.

“The range was formed by two continents colliding. During this collision, rocks from the crust were pushed around 100 kilometres deep into the mantle, where the high temperatures and pressures formed new minerals.”

As the mountains eroded, the roots came back up to the surface, to be collected in Togo, Mali and northeast Brazil, by Brazilian co-researcher Carlos Ganade de Araujo, from the University of Sao Paolo.

Dr Ganade de Araujo recognised the samples were unique and brought the rocks to ANU where, using world-leading equipment, the research team accurately identified that the rocks were of similar age, and had been formed at similar, great depths.

The research team involved specialists from a range of different areas of Earth Science sharing their knowledge, said Professor Rubatto.

“With everyone cooperating to study tiny crystals, we have managed to discover a huge mountain range,” she said.




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Scientists from Australian National University reveal how they found a mountain range that fed an explosion of life 600 million years ago. The range stretched 2,500 km across Gondwana from modern west Africa to Northeast Brazil. Tiny mineral crystals formed in the roots of these huge mountains were the key to reconstructing their age and size. – ANU Media