Geologists prove early Tibetan Plateau was larger than previously thought

This is Syracuse University professor Gregory Hoke. -  Syracuse University
This is Syracuse University professor Gregory Hoke. – Syracuse University

Earth scientists in Syracuse University’s College of Arts and Sciences have determined that the Tibetan Plateau-the world’s largest, highest, and flattest plateau-had a larger initial extent than previously documented.

Their discovery is the subject of an article in the journal Earth and Planetary Science Letters (Elsevier, 2014).

Gregory Hoke, assistant professor of Earth sciences, and Gregory Wissink, a Ph.D. student in his lab, have co-authored the article with Jing Liu-Zeng, director of the Division of Neotectonics and Geomorphology at the Institute for Geology, part of the China Earthquake Administration; Michael Hren, assistant professor of chemistry at the University of Connecticut; and Carmala Garzione, professor and chair of Earth and environmental sciences at the University of Rochester.

“We’ve determined the elevation history of the southeast margin of the Tibetan Plateau,” says Hoke, who specializes in the interplay between the Earth’s tectonic and surface processes. “By the Eocene epoch (approximately 40 million years ago), the southern part of the plateau extended some 600 miles more to the east than previously documented. This discovery upends a popular model for plateau formation.”

Known as the “Roof of the World,” the Tibetan Plateau covers more than 970,000 square miles in Asia and India and reaches heights of over 15,000 feet. The plateau also contains a host of natural resources, including large mineral deposits and tens of thousands of glaciers, and is the headwaters of many major drainage basins.

Hoke says he was attracted to the topography of the plateau’s southeast margin because it presented an opportunity to use information from minerals formed at the Earth’s surface to infer what happened below them in the crust.

“The tectonic and topographic evolution of the southeast margin has been the subject of considerable controversy,” he says. “Our study provides the first quantitative estimate of the past elevation of the eastern portions of the plateau.”

Historically, geologists have thought that lower crustal flow- a process by which hot, ductile rock material flows from high- to low-pressure zones-helped elevate parts of the plateau about 20 million years ago. (This uplift model has also been used to explain watershed reorganization among some of the world’s largest rivers, including the Yangtze in China.)

But years of studying rock and water samples from the plateau have led Hoke to rethink the area’s history. For starters, his data indicates that the plateau has been at or near its present elevation since the Eocene epoch. Moreover, surface uplift in the southernmost part of the plateau-in and around southern China and northern Vietnam-has been historically small.

“Surface uplift, caused by lower crustal flow, doesn’t explain the evolution of regional river networks,” says Hoke, referring to the process by which a river drainage system is diverted, or captured, from its own bed into that of a neighboring bed. “Our study suggests that river capture and drainage reorganization must have been the result of a slip on the major faults bounding the southeast plateau margin.”

Hoke’s discovery not only makes the plateau larger than previously thought, but also suggests that some of the topography is millions of years younger.

“Our data provides the first direct documentation of the magnitude and geographic extent of elevation change on the southeast margin of the Tibetan Plateau, tens of millions years ago,” Hoke adds. “Constraining the age, spatial extent, and magnitude of ancient topography has a profound effect on how we understand the construction of mountain ranges and high plateaus, such as those in Tibet and the Altiplano region in Bolivia.”

Geologists prove early Tibetan Plateau was larger than previously thought

This is Syracuse University professor Gregory Hoke. -  Syracuse University
This is Syracuse University professor Gregory Hoke. – Syracuse University

Earth scientists in Syracuse University’s College of Arts and Sciences have determined that the Tibetan Plateau-the world’s largest, highest, and flattest plateau-had a larger initial extent than previously documented.

Their discovery is the subject of an article in the journal Earth and Planetary Science Letters (Elsevier, 2014).

Gregory Hoke, assistant professor of Earth sciences, and Gregory Wissink, a Ph.D. student in his lab, have co-authored the article with Jing Liu-Zeng, director of the Division of Neotectonics and Geomorphology at the Institute for Geology, part of the China Earthquake Administration; Michael Hren, assistant professor of chemistry at the University of Connecticut; and Carmala Garzione, professor and chair of Earth and environmental sciences at the University of Rochester.

“We’ve determined the elevation history of the southeast margin of the Tibetan Plateau,” says Hoke, who specializes in the interplay between the Earth’s tectonic and surface processes. “By the Eocene epoch (approximately 40 million years ago), the southern part of the plateau extended some 600 miles more to the east than previously documented. This discovery upends a popular model for plateau formation.”

Known as the “Roof of the World,” the Tibetan Plateau covers more than 970,000 square miles in Asia and India and reaches heights of over 15,000 feet. The plateau also contains a host of natural resources, including large mineral deposits and tens of thousands of glaciers, and is the headwaters of many major drainage basins.

Hoke says he was attracted to the topography of the plateau’s southeast margin because it presented an opportunity to use information from minerals formed at the Earth’s surface to infer what happened below them in the crust.

“The tectonic and topographic evolution of the southeast margin has been the subject of considerable controversy,” he says. “Our study provides the first quantitative estimate of the past elevation of the eastern portions of the plateau.”

Historically, geologists have thought that lower crustal flow- a process by which hot, ductile rock material flows from high- to low-pressure zones-helped elevate parts of the plateau about 20 million years ago. (This uplift model has also been used to explain watershed reorganization among some of the world’s largest rivers, including the Yangtze in China.)

But years of studying rock and water samples from the plateau have led Hoke to rethink the area’s history. For starters, his data indicates that the plateau has been at or near its present elevation since the Eocene epoch. Moreover, surface uplift in the southernmost part of the plateau-in and around southern China and northern Vietnam-has been historically small.

“Surface uplift, caused by lower crustal flow, doesn’t explain the evolution of regional river networks,” says Hoke, referring to the process by which a river drainage system is diverted, or captured, from its own bed into that of a neighboring bed. “Our study suggests that river capture and drainage reorganization must have been the result of a slip on the major faults bounding the southeast plateau margin.”

Hoke’s discovery not only makes the plateau larger than previously thought, but also suggests that some of the topography is millions of years younger.

“Our data provides the first direct documentation of the magnitude and geographic extent of elevation change on the southeast margin of the Tibetan Plateau, tens of millions years ago,” Hoke adds. “Constraining the age, spatial extent, and magnitude of ancient topography has a profound effect on how we understand the construction of mountain ranges and high plateaus, such as those in Tibet and the Altiplano region in Bolivia.”

From greenhouse to icehouse — reconstructing the environment of the Voring Plateau

<IMG SRC="/Images/335517525.jpg" WIDTH="350" HEIGHT="325" BORDER="0" ALT="This is a scanning electron micrograph of one of the characteristic brackish water species of the genus Wezteliella. – NOCS”>
This is a scanning electron micrograph of one of the characteristic brackish water species of the genus Wezteliella. – NOCS

The analysis of microfossils found in ocean sediment cores is illuminating the environmental conditions that prevailed at high latitudes during a critical period of Earth history.

Around 55 million years ago at the beginning of the Eocene epoch, the Earth’s poles are believed to have been free of ice. But by the early Oligocene around 25 million years later, ice sheets covered Antarctica and continental ice had developed on Greenland.

“This change from greenhouse to icehouse conditions resulted from decreasing greenhouse gas concentrations and changes in Earth’s orbit,” said Dr Ian Harding of the University of Southampton’s School of Ocean and Earth Science (SOES) at the National Oceanography Centre, Southampton (NOCS): “However, the opening or closing of various marine gateways and shifts in ocean currents may also have influenced regional climate in polar high-latitudes.”

The separation of Eurasia and Greenland due to shifting tectonic plates led to the partial or complete submergence of former land barriers such as the Vøring Plateau of the Norwegian continental margin. For the first time, waters could exchange between the Norwegian-Greenland Sea, the Arctic Ocean and the North Atlantic.

Dr Harding and his former PhD student Dr James Eldrett have reconstructed the environmental conditions over the Vøring Plateau over this time period by carefully analyzing the fossilized remains of organic debris and cysts of tiny aquatic organisms called dinoflagellates from sediment cores.

“Because different dinoflagellate species are adapted to different surface water conditions, their fossilized remains help us reconstruct past environments,” said Dr Harding.

The evidence from the sediments cores suggests the development of shallow marine environments across parts of the Vøring Plateau during the early Eocene. However, the presence of fossilized species that lived in fresh or brackish water indicates that northerly parts of the plateau as well as the crest of the Vøring Escarpment were still above water.

In the late Eocene sediments (around 44 million years old) only marine plankton species were found, indicating that the entire Vøring Plateau had by then subsided and become submerged. This demonstrates that marine connections were established between the various Nordic sea basins much earlier than had previously been thought. These surface water connections may have promoted the increased surface water productivity evidenced by the abundance of planktonic fossils preserved in the sediment cores of this age.

“Increased productivity would have drawn carbon dioxide down from the atmosphere,” said Dr Harding: “Because carbon dioxide is a greenhouse gas, this may have contributed to declining global temperatures and led to the early development of continental ice on Greenland in the latest Eocene.

When palm trees gave way to spruce trees

New research reveals the demise of an ancient forest. These are dawn redwood stumps on Axel Heiberg Island, Nunavut. -  David Greenwood
New research reveals the demise of an ancient forest. These are dawn redwood stumps on Axel Heiberg Island, Nunavut. – David Greenwood

For climatologists, part of the challenge in predicting the future is figuring out exactly what happened during previous periods of global climate change.

One long-standing climate puzzle relates to a sequence of events 33.5 million years ago in the Late Eocene and Early Oligocene. Profound changes were underway. Globally, carbon dioxide levels were falling and the hothouse warmth of the dinosaur age and Eocene Period was waning. In Antarctica, ice sheets had formed and covered much of the southern polar continent.

But what exactly was happening on land, in northern latitudes? When and how did Northern glaciation begin, and what does this knowledge add to the understanding of the relationship between carbon dioxide levels and today’s climate?

An international team that included Dr. David Greenwood, an NSERC-funded researcher at Brandon University, now provides some of the very first detailed answers, and they come from an unusual source.

“Fossils of land plants are excellent indicators of past climates,” said Dr. Greenwood. “But the fossil plant localities from the Canadian Arctic and Greenland don’t appear to record this major climate change, and pose problems for precisely dating their age, so we needed to look elsewhere.”

The “where” was in marine sediments entombed when the North Atlantic Ocean was beginning to open, and lying now at the bottom of today’s Norwegian-Greenland Sea. Sediment cores taken from there contained a record of ancient spores and pollen blown from the continent to the west.

“These marine sediment cores give us a very precise chronology of the changes in the dominant land plants,” said Dr. Greenwood “and since many of these species have modern relatives, we can assume that the temperatures and environments they lived in were very similar.”

To arrive at a holistic picture of the climate of the transition, the researchers merged the plant data with physical information about the state of the atmosphere and ocean taken from chemical and isotopic information in the same sediments, and compared this to computer modelling of climate in the period.

“We can see that summer temperatures on land remained relatively warm throughout the Eocene/Oligocene transition, but that the period was marked by increasing seasonality,” said Dr. Greenwood.

“Mean temperatures during the coldest month dropped by five degrees Celsius, to just above freezing,” he said.

“This was probably not enough to create much in the way of continental ice on East Greenland,” he said, “but it did wipe out palms and other subtropical trees such as swamp cypress. They were replaced by temperate climate trees such as spruces and hemlock.”

The researcher said that, nonetheless, the middle period of the transition remained fairly warm. “Hickory and walnut were still present, but these became rare in the final stages,” he said.

Although the march to a cooler world was gradual in northern latitudes, it was inevitable according to Dr. Greenwood.

“Changes in the earth’s position in its orbit were leading a much greater seasonal range in radiation for polar regions and, overall, heat was becoming more concentrated in the tropics, largely due to a global drop in carbon dioxide levels in the atmosphere” he said.