Ice-free Arctic winters could explain amplified warming during Pliocene

Year-round ice-free conditions across the surface of the Arctic Ocean could explain why the Earth was substantially warmer during the Pliocene Epoch than it is today, despite similar concentrations of carbon dioxide in the atmosphere, according to new research carried out at the University of Colorado Boulder.

In early May, instruments at the Mauna Loa Observatory in Hawaii marked a new record: The concentration of carbon dioxide climbed to 400 parts per million for the first time in modern history.

The last time researchers believe the carbon dioxide concentration in the atmosphere reached 400 ppm-between 3 and 5 million years ago during the Pliocene-the Earth was about 3.5 to 9 degrees Fahrenheit warmer (2 to 5 degrees Celsius) than it is today. During that time period, trees overtook the tundra, sprouting right to the edges of the Arctic Ocean, and the seas swelled, pushing ocean levels 65 to 80 feet higher.

Scientists’ understanding of the climate during the Pliocene has largely been pieced together from fossil records preserved in sediments deposited beneath lakes and on the ocean floor.

“When we put 400 ppm carbon dioxide into a model, we don’t get as warm a planet as we see when we look at paleorecords from the Pliocene,” said Jim White, director of CU-Boulder’s Institute of Arctic and Alpine Research and co-author of the new study published online in the journal Palaeogeography, Paleoclimatology, Palaeoecology. “That tells us that there may be something missing in the climate models.”

Scientists have proposed several hypotheses in the past to explain the warmer Pliocene climate. One idea, for example, was that the formation of the Isthmus of Panama, the narrow strip of land linking North and South America, could have altered ocean circulations during the Pliocene, forcing warmer waters toward the Arctic. But many of those hypotheses, including the Panama possibility, have not proved viable.

For the new study, led by Ashley Ballantyne, a former CU-Boulder doctoral student who is now an assistant professor of bioclimatology at the University of Montana, the research team decided to see what would happen if they forced the model to assume that the Arctic was free of ice in the winter as well as the summer during the Pliocene. Without these additional parameters, climate models set to emulate atmospheric conditions during the Pliocene show ice-free summers followed by a layer of ice reforming during the sunless winters.

“We tried a simple experiment in which we said, ‘We don’t know why sea ice might be gone all year round, but let’s just make it go away,’ ” said White, who also is a professor of geological sciences. “And what we found was that we got the right kind of temperature change and we got a dampened seasonal cycle, both of which are things we think we see in the Pliocene.”

In the model simulation, year-round ice-free conditions caused warmer conditions in the Arctic because the open water surface allowed for evaporation. Evaporation requires energy, and the water vapor then stored that energy as heat in the atmosphere. The water vapor also created clouds, which trapped heat near the planet’s surface.

“Basically, when you take away the sea ice, the Arctic Ocean responds by creating a blanket of water vapor and clouds that keeps the Arctic warmer,” White said.

White and his colleagues are now trying to understand what types of conditions could bridge the standard model simulations with the simulations in which ice-free conditions in the Arctic are imposed. If they’re successful, computer models would be able to model the transition between a time when ice reformed in the winter to a time when the ocean remained devoid of ice throughout the year.

Such a model also would offer insight into what could happen in our future. Currently, about 70 percent of sea ice disappears during the summertime before reforming in the winter.

“We’re trying to understand what happened in the past but with a very keen eye to the future and the present,” White said. “The piece that we’re looking at in the future is what is going to happen as the Arctic Ocean warms up and becomes more ice-free in the summertime.

“Will we continue to return to an ice-covered Arctic in the wintertime? Or will we start to see some of the feedbacks that now aren’t very well represented in our climate models? If we do, that’s a big game changer.”

Geochemical ‘fingerprints’ leave evidence that megafloods eroded steep gorge

This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. -  U.S. Geological Survey
This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. – U.S. Geological Survey

The Yarlung-Tsangpo River in southern Asia drops rapidly through the Himalaya Mountains on its way to the Bay of Bengal, losing about 7,000 feet of elevation through the precipitously steep Tsangpo Gorge.

For the first time, scientists have direct geochemical evidence that the 150-mile long gorge, possibly the world’s deepest, was the conduit by which megafloods from glacial lakes, perhaps half the volume of Lake Erie, drained suddenly and catastrophically through the Himalayas when their ice dams failed at times during the last 2 million years.

“You would expect that if a three-day long flood occurred, there would be some pretty significant impacts downstream,” said Karl Lang, a University of Washington doctoral candidate in Earth and space sciences.

In this case, the water moved rapidly through bedrock gorge, carving away the base of slopes so steep they already were near the failure threshold. Because the riverbed through the Tsangpo Gorge is essentially bedrock and the slope is so steep and narrow, the deep flood waters could build enormous speed and erosive power.

As the base of the slopes eroded, areas higher on the bedrock hillsides tumbled into the channel, freeing microscopic grains of zircon that were carried out of the gorge by the fast-moving water and deposited downstream.

Uranium-bearing zircon grains carry a sort of geochemical signature for the place where they originated, so grains found downstream can be traced back to the rocks from which they eroded. Lang found that normal annual river flow carries about 40 percent of the grains from the Tsangpo Gorge downstream. But grains from the gorge found in prehistoric megaflood deposits make up as much as 80 percent of the total.

He is the lead author of a paper documenting the work published in the September edition of Geology. Co-authors are Katharine Huntington and David Montgomery, both UW faculty members in Earth and space sciences.

The Yarlung-Tsangpo is the highest major river in the world. It begins on the Tibetan Plateau at about 14,500 feet, or more than 2.5 miles, above sea level. It travels more than 1,700 miles, crossing the plateau and plunging through the Himalayas before reaching India’s Assam Valley, where it becomes the Brahmaputra River. From there it continues its course to the Ganges River delta and the Bay of Benga

At the head of the Tsangpo Gorge, the river makes a sharp bend around Namche Barwa, a 25,500-foot peak that is the eastern anchor of the Himalayas. Evidence indicates that giant lakes were impounded behind glacial dams farther inland from Namche Barwa at various times during the last 2.5 million years ago.

Lang matched zircons in the megaflood deposits far downstream with zircons known to come only from Namche Barwa, and those signature zircons turned up in the flood deposits at a much greater proportion than they would in sediments from normal river flows. Finding the zircons in deposits so far downstream is evidence for the prehistoric megafloods and their role in forming the gorge.

Lang noted that a huge landslide in early 2000 created a giant dam on the Yiggong River, a tributary of the main river just upstream from the Gorge. The dam failed catastrophically in June 2000, triggering a flood that caused numerous fatalities and much property damage downstream.

That provided a vivid, though much smaller, illustration of what likely occurred when large ice dams failed millions of years ago, he said. It also shows the potential danger if humans decide to build dams in that area for hydroelectric generation.

“We are interested in it scientifically, but there is certainly a societal element to it,” Lang said. “This takes us a step beyond speculating what those ancient floods did. There is circumstantial evidence that, yes, they did do a lot of damage.”

The process in the Tsangpo Gorge is similar to what happened with Lake Missoula in Western Montana 12,000 to 15,000 years ago. That lake was more than 10,000 feet lower in elevation than lakes associated with the Tsangpo Gorge, though its water discharge was 10 times greater. Evidence suggests that Lake Missoula’s ice dam failed numerous times, unleashing a torrent equal to half the volume of Lake Michigan across eastern Washington, where it carved the Channeled Scablands before continuing down the Columbia River basin.

“This is a geomorphic process that we know shapes the landscape, and we can look to eastern Washington to see that,” Lang said.

Researchers shed new light on supraglacial lake drainage

Supraglacial lakes – bodies of water that collect on the surface of the Greenland ice sheet – lubricate the bottom of the sheet when they drain, causing it to flow faster. Differences in how the lakes drain can impact glacial movement’s speed and direction, researchers from The City College of New York (CCNY), University of Cambridge and Los Alamos National Laboratory report in “Environmental Research Letters.”

“Knowledge of the draining mechanisms allows us to improve our understanding of how surface melting can impact sea-level rise, not only through the direct contribution of meltwater from the surface, but also through the indirect contribution on the mass loss through ice dynamics,” says Dr. Marco Tedesco, the principal investigator and lead author.

Dr. Tedesco is an associate professor in CCNY’s Department of Earth and Atmospheric Sciences at CCNY and is currently serving as temporary program director for the National Science Foundation’s Polar Cyberinfrastructure Program. The research described in the paper was funded before Dr. Tedesco accepted the position at NSF.

NSF supported the research along with NASA’s cryosphere program, the Natural Environment Research Council, the U.S. Department of Energy’s earth systems modeling program, St. Catherine’s College (Cambridge), the Scandinavian Studies Fund and the B.B. Roberts Fund.

Over the past decade, surface melting in Greenland has increased considerably.

Previous research already suggested that the water injected from the rapid draining of the supraglacial lakes controlled sliding of ice over the bed beneath it. However, there was no evidence of the impact of the slow draining mechanism, which the paper identified.

Professor Tedesco and colleagues documented that supraglacial lakes have two different drainage mechanisms that cause them to empty rapidly or slowly. The findings are based on analysis of data collected in 2011 from five GPS stations the team installed around two supraglacial lakes in the Paakistoq region of West Greenland.

The smaller of the two lakes, Lake Half Moon, overflowed its banks and drained from the side to reach a moulin. It took approximately 45 hours to empty. The larger lake, Lake Ponting, drained through a crack in the ice beneath it and was voided in around two hours.

“At first, a crack in the ice beneath the lake may be small, but it deepens as water enters it because the pressure of the water overcomes the compressive action of the ice, which is trying to close the crack,” Professor Tedesco explains. “When the crack reaches the bed beneath the glacier, which could be 1,000 meters or more below the surface, the lake empties rapidly, like a bathtub after its plug is pulled.”

Drainage from both lakes accelerated glacial movement. However, water from Lake Ponting caused the glacier to move faster and further. While the slower drainage from Lake Half Moon caused the glacial pace to increase from baseline values of 90 – 100 meters per year to a maximum of around 420 meters a year, glacial movement in the area affected by Lake Ponting reached maximum velocities of 1,500 – 1,600 meters per year, nearly four times greater.

The drainage of the two lakes impacted the glacier’s trajectory differently, as well. The emptying of Lake Half Moon via the moulin did not change the direction of glacial movement. However, when Lake Ponting drained a slight southerly shift in the glacier’s direction was detected.

“Because the different draining mechanisms affect ice velocity, they could also affect the amount of ice lost through calving of glaciers, which results in icebergs,” Professor Tedesco points out. “Because what happens on a glacier’s surface impacts what is going on below, researchers are trying to look at glaciers as a system instead of independent components,” he adds.

“The surface is like the skin of a tissue and the subglacial and englacial channels that develop because of the surface water act like arteries or veins that redistribute this water internally.”

Irish chronicles reveal links between cold weather and volcanic eruptions

Medieval chronicles have given an international group of researchers a glimpse into the past to assess how historical volcanic eruptions affected the weather in Ireland up to 1500 years ago.

By critically assessing over 40,000 written entries in the Irish Annals and comparing them with measurements taken from ice cores, the researchers successfully linked the climatic aftermath of volcanic eruptions to extreme cold weather events in Ireland over a 1200-year period from 431 to 1649.

Their study, which has been published today, 6 June, in IOP Publishing’s journal Environmental Research Letters, showed that over this timescale up to 48 explosive volcanic eruptions could be identified in the Greenland Ice Sheet Project (GISP2) ice-core, which records the deposition of volcanic sulfate in annual layers of ice.

Of these 48 volcanic events, 38 were associated, closely in time, with 37 extreme cold events, which were identified by systematically examining written entries in the Irish Annals and picking out directly observed meteorological phenomena and conditions, such as heavy snowfall and frost, prolonged ice covering lakes and rivers, and contemporary descriptions of abnormally cold weather.

Lead author of the study, Dr Francis Ludlow, from the Harvard University Center for the Environment and Department of History, said: “It’s clear that the scribes of the Irish Annals were diligent reporters of severe cold weather, most probably because of the negative impacts this had on society and the biosphere.

“Our major result is that explosive volcanic eruptions are strongly, and persistently, implicated in the occurrence of cold weather events over this long timescale in Ireland. In their severity, these events are quite rare for the country’s mild maritime climate.”

Through the injection of sulphur dioxide gas into the stratosphere, volcanic eruptions can play a significant role in the regulation of the Earth’s climate. Sulphur dioxide gas is converted into sulphate aerosol particles after eruptions which reflect incoming sunlight and result in an overall temporary cooling of the Earth’s surface.

Whilst the global effects of recent eruptions are quite well-known, such as the Mount Pinatubo eruption almost 22 years ago (15 June 1991), less is known about their effects on climate before the beginning of instrumental weather recording, or their effects on regional scales; the Irish Annals provided an opportunity to explore both of these issues.

The Irish Annals contain over one million written words and around 40,000 distinct written entries, detailing major historical events on an annual basis, and providing both systematic and sustained reporting of meteorological extremes.

The dating and reliability of the Annals can be gauged by comparing reported events to those which are independently known, such as solar and lunar eclipses.

“With a few honourable exceptions, the Irish record of extreme events has only been used anecdotally, rather than systematically surveyed and exploited for the study of the climate history of Ireland and the North Atlantic, and so the richness of the record has been largely unrecognized,” continued Dr Ludlow.

Although the effect of big eruptions on the climate in summer is largely to cause cooling, during the winter, low-latitude eruptions in the tropics have instead been known to warm large parts of the northern hemisphere as they cause a strengthening of the westerly winds that brings, for example, warmer oceanic air to Europe; however, this study identified several instances when low-latitude eruptions appeared to correspond to extreme cold winters in Ireland.

One example is the 1600 eruption in Peru of Huaynaputina, which the researchers found, against expectations, to be associated with extreme cold winter weather in Ireland in the following years.

“The possibility that tropical eruptions may result in severe winter cooling for Ireland highlights the considerable complexity of the volcano-climate system in terms of the regional expression of the response of climate to volcanic disturbances.

“It is on the regional scale that we need to refine our understanding of this relationship as ultimately, it is on this scale that individuals and societies plan for extreme weather,” continued Dr Ludlow.

LiDAR technology reveals faults near Lake Tahoe

Results of a new U.S. Geological Survey study conclude that faults west of Lake Tahoe, Calif., referred to as the Tahoe-Sierra frontal fault zone, pose a substantial increase in the seismic hazard assessment for the Lake Tahoe region of California and Nevada, and could potentially generate earthquakes with magnitudes ranging from 6.3 to 6.9. A close association of landslide deposits and active faults also suggests that there is an earthquake-induced landslide hazard along the steep fault-formed range front west of Lake Tahoe.

Using a new high-resolution imaging technology, known as bare-earth airborne LiDAR (Light Detection And Ranging), combined with field observations and modern geochronology, USGS scientists, and their colleagues from the University of Nevada, Reno; the University of California, Berkeley; and the U.S. Army Corps of Engineers, have confirmed the existence of previously suspected faults.

LiDAR imagery allows scientists to “see” through dense forest cover and recognize earthquake faults that are not detectable with conventional aerial photography.

“This study is yet one more stunning example of how the availability of LiDAR information to precisely and accurately map the shape of the solid Earth surface beneath vegetation is revolutionizing the geosciences,” said USGS Director Marcia McNutt. “From investigations of geologic hazards to calculations of carbon stored in the forest canopy to simply making the most accurate maps possible, LiDAR returns its investment many times over.”

Motion on the faults has offset linear moraines (the boulders, cobbles, gravel, and sand deposited by an advancing glacier) providing a record of tectonic deformation since the moraines were deposited. The authors developed new three-dimensional techniques to measure the amount of tectonic displacement of moraine crests caused by repeated earthquakes. Dating of the moraines from the last two glaciations in the Tahoe basin, around 21 thousand and 70 thousand years ago, allowed the study authors to calculate the rates of tectonic displacement.

“Although the Tahoe-Sierra frontal fault zone has long been recognized as forming the tectonic boundary between the Sierra Nevada to the west, and the Basin and Range Province to the east, its level of activity and hence seismic hazard was not fully recognized because dense vegetation obscured the surface expressions of the faults,” said USGS scientist and lead author, James Howle. “Using the new LiDAR technology has improved and clarified previous field mapping, has provided visualization of the surface expressions of the faults, and has allowed for accurate measurement of the amount of motion that has occurred on the faults. The results of the study demonstrate that the Tahoe-Sierra frontal fault zone is an important seismic source for the region.”

New sources found for accumulated dust on Chinese Loess Plateau

Geologists have long thought the loess-or fine silt-that accumulated on the Chinese Loess Plateau was carried on winds from desert regions to the northwest over the past 2.6 million years. New research indicates the loess may actually have come from due west, which would change conventional thinking about wind patterns during that period.

A team of geologists from the U.S. and China-led by the University of Rochester-compared the composition of uranium and lead in zircon crystals excavated from the Chinese Loess Plateau and potential source sites. The scientists found that the ages of the crystals from the Chinese Loess Plateau matched with samples from the northern Tibetan Plateau and the Qaidam Basin, both of which are due west.

The results have been published in a recent issue of the journal Geology.

“The data suggest a dramatic shift in atmospheric winds,” said lead author Alex Pullen.

By testing for the ages of the embedded zircon crystals, the researchers determined that the loess came from the west during recent glacial periods, which suggests that the atmospheric jet streams shifted equatorward during those periods. That would mean there have been alternating northwesterly and westerly sources for the loess during warm interglacial and cold glacial periods, respectively. The geological team says additional studies of ancient soil (paleosol) layers of the Chinese Loess Plateau are needed to test that theory.

“The research should help us better understand how the earth behaves as a system,” said Pullen. “With that knowledge, we’ll be able to improve our climate models.”

Evidence emerges of ancient lake in California’s Eel River

A catastrophic landslide 22,500 years ago dammed the upper reaches of northern California’s Eel River, forming a 30-mile-long lake which has since disappeared. It left a living legacy found today in the genes of the region’s steelhead trout.

Using remote-sensing technology known as airborne Light Detection and Ranging (LiDAR) and hand-held global-positioning-systems (GPS) units, scientists recently found evidence for a late Pleistocene, landslide-dammed lake along the river.

Today the Eel river is 200 miles long, carved into the ground from high in the California Coast Ranges to the river’s mouth in the Pacific Ocean in Humboldt County.

The evidence for the ancient landslide, which, scientists say, blocked the river with a 400-foot-wall of loose rock and debris, is detailed this week in a paper appearing on-line in the journal Proceedings of the National Academy of Sciences.


The research provides a rare glimpse into the geological history of this rapidly evolving mountainous region.

“This study reminds us that there are still significant surprises to be unearthed about past landscape dynamics and their broad impacts,” said Paul Cutler, program director in the National Science Foundation’s Division of Earth Sciences, which funded the research. “For example, it provides valuable information for assessing modern landslide hazard potential in this region.”

It also helps to explain emerging evidence from other studies that show a dramatic decrease in the amount of sediment deposited from the river in the ocean just offshore at about the same time period, says lead author of the paper Benjamin Mackey of the California Institute of Technology.

“Perhaps of most interest, the presence of this landslide dam also provides an explanation for the results of previous research on the genetics of steelhead trout in the Eel River,” Mackey said.

In that study, scientists found a striking relationship between two types of ocean-going steelhead in the river–a genetic similarity not seen among summer-run and winter-run steelhead in other nearby waterways.

An interbreeding of the two fish, in a process known as genetic introgression, may have occurred among the fish brought together while the river was dammed, Mackey said.

“The dam likely would have been impassable to the fish migrating upstream, meaning both ecotypes would have been forced to spawn and inadvertently breed downstream of the dam. This period of gene flow between the two types of steelhead can explain the genetic similarity observed today.”

Once the dam burst, the fish would have reoccupied their preferred spawning grounds and resumed different genetic trajectories.

“The damming of the river was a dramatic, punctuated event that greatly altered the landscape,” said co-author Joshua Roering, a geologist at the University of Oregon.

“Although current physical evidence for the landslide dam and ancient lake is subtle, its effects are recorded in the Pacific Ocean and persist in the genetic make-up of today’s Eel River steelhead,” said Roering. “It’s rare for scientists to be able to connect the dots between such diverse phenomena.”

The lake formed by the landslide, the researchers theorize, covered about 18 square miles.

After the dam was breached, the flow of water would have generated one of North America’s largest landslide-dam outburst floods.

Landslide activity and erosion have erased much of the evidence for the now-gone lake. Without the acquisition of LiDAR mapping, the lake’s existence may have never been discovered, the scientists said.

The area affected by the landslide-caused dam accounts for about 58 percent of the modern Eel River watershed. Based on today’s general erosion rates, the geologists believe that the lake could have filled in with sediment within about 600 years.

“The presence of a dam of this size was unexpected in the Eel River, given the abundance of easily eroded sandstone and mudstone, which are generally not considered strong enough to form long-lived dams,” Mackey said.

He and colleagues were drawn to the Eel River–among the most-studied erosion systems in the world–to study large, slow-moving landslides.

“While analyzing the elevation of terraces along the river, we discovered they clustered at a common elevation rather than decreased in elevation downstream paralleling the river profile, as would be expected for river terraces,” said Mackey.

“That was the first sign of something unusual, and it clued us into the possibility of an ancient lake.”

Heavy metal — in and around the lakes

Heavy metal pollution of lakes has a seriously detrimental impact on people and ecosystems that rely on such bodies of water. According to a study published in the current issue of Interdisciplinary Environmental Review, researchers have focused on the physicochemical properties and toxicology of water from and around Thane City of Maharashtra.

Environmental chemist Pravin Singare of Bhavan’s College, in Mumbai, and colleagues highlight the fact that fresh water bodies all over the world are becoming increasingly polluted day by day and that this represents a growing problem in the developing world and beyond. They suggest that regular monitoring is crucial for the well-being and health of the surrounding population and as such, the team has carried out a systematic study to estimate the physico-chemical parameters and level of toxic heavy metal content in the Jail Talav and Kalwa Lakes of Thane City, as perhaps being indicative of similar problems with other bodies of water.

The team’s measurements suggest that the presence of heavy metals such as iron, copper, nickel and zinc, which are essential for life at trace levels are well above permissible concentrations making them a significant threat to ecosystems and a problem for those who rely on the lakes for drinking water or crop irrigation. In addition mercury, arsenic and cadmium were all present at much higher than acceptable concentrations.

South Asia is home to more a fifth of the world’s population, the researchers say, and is facing a serious water crisis. “This region, which is in the grip of flood and drought cycles, needs a long-term strategy for management of its water resources,” the team says. Unfortunately, strategies adopted so far have all failed in India, the team asserts, this is obvious given the poor quality of the water revealed by their measurements of Jail Talav and Kalwa Lakes assuming these are typical of the region as a whole.

Food chain contamination by heavy metals has become an important issue partly because of the potential accumulation in biosystems, through contaminated water, the team adds. “A better understanding of heavy metal sources, their accumulation in water and the effect of their presence in water on plant systems are particularly impertinent in ongoing risk assessments,” the researchers say.

Researchers test cutting-edge technology for underwater mapping at Tahoe basin

A borrowed boat, a small mountain lake and the inaugural run of a half-a-million dollar state-of-the-art multi-beam sonar system made history this month with the successful high-definition mapping by University of Nevada, Reno, and Scripps Institution of Oceanography researchers of the bottom of Fallen Leaf Lake, a tributary lake just upstream from Lake Tahoe. -  Photo by Mike Wolterbeek, University of Nevada, Reno
A borrowed boat, a small mountain lake and the inaugural run of a half-a-million dollar state-of-the-art multi-beam sonar system made history this month with the successful high-definition mapping by University of Nevada, Reno, and Scripps Institution of Oceanography researchers of the bottom of Fallen Leaf Lake, a tributary lake just upstream from Lake Tahoe. – Photo by Mike Wolterbeek, University of Nevada, Reno

A borrowed boat, a small mountain lake and the inaugural run of a half-a-million dollar state-of-the-art multi-beam sonar system made history this month with the successful high-definition mapping of the bottom of Fallen Leaf Lake, a tributary lake just upstream from Lake Tahoe.

“The clarity of the images we produced is unmatched in detail,” said University of Nevada, Reno Seismology Lab Director Graham Kent and co-lead investigator of the project. “We can clearly see 1,000-year-old trees standing upright under 100 feet of water and remnants of earthquake activity along the West Tahoe Fault line. This is a valuable tool for a number of scientific pursuits.”

What the scientists can see:

  • Fault mapping such as the West Tahoe Fault which runs through Fallen Leaf Lake – it’s a magnitude 7.3 capable normal fault that’s approaching the end its characteristic earthquake cycle (almost overdue)
  • The effects of drought, including the Medieval Warm Period (approximately 950-1250 AD); features include: old shorelines at 80- to140-feet underwater; and standing, rooted trees at 110-foot level below the lake’s current surface
  • Substrate identification that has potential uses for biohabitat mapping of various aquatic species, both native and invasive

“The centerpiece of the system comes from rocket technology, with an inertial guidance/gyro system, which allows image stability even as the boat rocks back and forth in the waves,” said Kent, also a professor in the University’s College of Science. “It’s also positioned with a phased GPS array and sound velocity corrections to align or properly register lake-floor pixels. It’s a half-million dollar acoustic system, but mo
st of the cost is in the guidance system.”

While there are many commercial applications of this type of mapping, this system is owned by only a handful of academic institutions worldwide. The technology allows for several centimeter depth resolution (with less than one-meter spatial resolution), giving definition similar to airborne lidar.

“This system helps document the best estimate of how severe the Medieval Warm Period drought was, with perhaps 40 percent less precipitation than we get today, for more than two centuries,” Kent said. “It’s disturbing to think it could happen again. This is possibly the best estimate of medieval drought anywhere in the Sierra.”

Kent and his colleagues from Scripps Institution of Oceanography in San Diego, Calif., geophysicists Jeff Babcock and Neal Driscoll, have been studying the glacially carved lake bottom in conjunction with seismic studies at Lake Tahoe for nearly a decade, and they are excited to use the new tool they have developed to continue and enhance those studies.

“Not only are we using this cutting-edge system to map the geologic substrate but we can use this, for example, to quickly find potential habitat for invasive species at Lake Tahoe such as the Asian Clam,” Kent said.

These expeditions also provide an ideal environment to train the next generation of research scientists; two graduate students, one from the University and one from SIO, participated in the mapping. The team has also been joined by Emeritus Professor John Kleppe from the University’s College of Engineering, who was one of the first to document the submerged trees beneath Fallen Leaf, and his involvement has been significant in both the science and educational aspects of this project.

The Fallen Leaf Lake area was the largest unmapped region of the Tahoe Basin. The lake is three miles long and one mile wide, and the surface is about 150 feet higher in elevation than Lake Tahoe. The lake is about 415 feet deep at its deepest point.

The team has mapped lakes and shallow-seas such as Pyramid Lake, Lake Tahoe, Salton Sea and the Great Salt Lake to name a few of their latest endeavors.

Geologists show unprecedented warming in Lake Tanganyika

Jessica Tierney is a geologist at Brown University. -  Brown University
Jessica Tierney is a geologist at Brown University. – Brown University

Lake Tanganyika, the second oldest and the second-deepest lake in the world, could be in for some rough waters.

Geologists led by Brown University have determined the east African rift lake has experienced unprecedented warming during the last century, and its surface waters are the warmest on record. That finding is important, the scientists write in the journal Nature Geoscience, because the warm surface waters likely will affect fish stocks upon which millions of people in the region depend.

The team took core samples from the lakebed that laid out a 1,500-year history of the lake’s surface temperature. The data showed the lake’s surface temperature, 26 degrees Celsius (78.8°F), last measured in 2003, is the warmest the lake has been for a millennium and a half. The team also documented that Lake Tanganyika experienced its biggest temperature change in the 20th century, which has affected its unique ecosystem that relies upon the natural conveyance of nutrients from the depths to jumpstart the food chain upon which the fish survive.

“Our data show a consistent relationship between lake surface temperature and productivity (such as fish stocks),” said Jessica Tierney, a Brown graduate student who this spring earned her Ph.D. and is the paper’s lead author. “As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the [fishing] industry.”

The research grew out of two coring expeditions sponsored by the Nyanza Project in 2001 and 2004. Cores were taken by Andrew Cohen, professor of geological sciences at the University of Arizona and director of the Nyanza project, and James Russell, professor of geological sciences at Brown, who is also Tierney’s adviser.

Lake Tanganyika is bordered by Burundi, the Democratic Republic of Congo, Tanzania, and Zambia – four of the poorest countries in the world, according to the United Nations Human Development Index. An estimated 10 million people live near the lake, and they depend upon it for drinking water and for food. Fishing is a crucial component for the region’s diet and livelihood: Up to 200,000 tons of sardines and four other fish species are harvested annually from Lake Tanganyika, a haul that makes up a significant portion of local residents’ diets, according to a 2001 report by the Lake Tanganyika Biodiversity Project.

Lake Tanganyika, one of the richest freshwater ecosystems in the world, is divided into two general levels. Most of the animal species live in the upper 100 meters, including the valuable sardines. Below that, the lake holds less and less oxygen, and at certain depths, it is anoxic, meaning it has no oxygen at all. What this all means is the lake is highly stratified and depends on wind to churn the waters and send nutrients from the depths toward the surface as food for algae, which supports the entire food web of the lake. But as Lake Tanganyika warms, the mixing of waters is lessened, the scientists find, meaning less nutrients are funneled from the depths toward the surface. Worse, more warming at the surface magnifies the difference in density between the two levels; even more wind is needed to churn the waters enough to ferry the nutrients toward the fish-dwelling upper layer.

The researchers’ data show that during the last 1,500 years, intervals of prolonged warming and cooling are linked with low and high algal productivity, respectively, indicating a clear link between past temperature changes and biological productivity in the lake.

“The people throughout southcentral Africa depend on the fish from Lake Tanganyika as a crucial source of protein,” Cohen noted. “This resource is likely threatened by the lake’s unprecedented warming since the late 19th century and the associated loss of lake productivity.”

Climate change models show a general warming in the region, which, if accurate, would cause even greater warming of the Lake Tanganyika’s surface waters and more stratification in the lake as a whole. “So, as you move forward, you can imagine that density gradient increasing,” Russell said.

Some researchers have posited that the declining fish stocks in Lake Tanganyika can be attributed mainly to overfishing, and Tierney and Russell say that may be a reason. But they note that the warming in the lake, and the lessened mixing of critical nutrients is exacerbating the stocks’ decline, if not causing it in the first place. “It’s almost impossible for it not to,” Russell said.