Sea level influenced tropical climate during the last ice age

The exposed Sunda Shelf during glacial times greatly affected the atmospheric circulation. The shelf is shown on the left for present-day as the light-blue submerged areas between Java, Sumatra, Borneo, and Thailand, and on the right for the last ice age as the green exposed area. -  Pedro DiNezio
The exposed Sunda Shelf during glacial times greatly affected the atmospheric circulation. The shelf is shown on the left for present-day as the light-blue submerged areas between Java, Sumatra, Borneo, and Thailand, and on the right for the last ice age as the green exposed area. – Pedro DiNezio

Scientists look at past climates to learn about climate change and the ability to simulate it with computer models. One region that has received a great deal of attention is the Indo-Pacific warm pool, the vast pool of warm water stretching along the equator from Africa to the western Pacific Ocean.

In a new study, Pedro DiNezio of the International Pacific Research Center, University of Hawaii at Manoa, and Jessica Tierney of Woods Hole Oceanographic Institution investigated preserved geological clues (called “proxies”) of rainfall patterns during the last ice age when the planet was dramatically colder than today. They compared these patterns with computer model simulations in order to find a physical explanation for the patterns inferred from the proxies.

Their study, which appears in the May 19, online edition of Nature Geoscience, not only reveals unique patterns of rainfall change over the Indo-Pacific warm pool, but also shows that they were caused by the effect of lowered sea level on the configuration of the Indonesian archipelago.

“For our research,” explains lead-author Pedro DiNezio at the International Pacific Research Center, “we compared the climate of the ice age with our recent warmer climate. We analyzed about 100 proxy records of rainfall and salinity stretching from the tropical western Pacific to the western Indian Ocean and eastern Africa. Rainfall and salinity signals recorded in geological sediments can tell us much about past changes in atmospheric circulation over land and the ocean respectively.”

“Our comparisons show that, as many scientists expected, much of the Indo-Pacific warm pool was drier during this glacial period compared with today. But, counter to some theories, several regions, such as the western Pacific and the western Indian Ocean, especially eastern Africa, were wetter,” adds co-author Jessica Tierney from Woods Hole Oceanographic Institute.

In the second step, the scientists matched these rainfall and salinity patterns with simulations from 12 state-of-the-art climate models that are used to also predict future climate change. For this matching they applied a method of categorical data comparison called the ‘Cohen’s kappa’ statistic. Though widely used in the medical field, this method has not yet been used to match geological climate signals with climate model simulations.

“We were taken aback that only one model out of the 12 showed statistical agreement with the proxy-inferred patterns of the rainfall changes. This model, though, agrees well with both the rainfall and salinity indicators – two entirely independent sets of proxy data covering distinct areas of the tropics,” says DiNezio.

The model reveals that the dry climate during the glacial period was driven by reduced convection over a region of the warm pool called the Sunda Shelf. Today the shelf is submerged beneath the Gulf of Thailand, but was above sea level during the glacial period, when sea level was about 120 m lower.

“The exposure of the Sunda Shelf greatly weakened convection over the warm pool, with far-reaching impacts on the large-scale circulation and on rainfall patterns from Africa to the western Pacific and northern Australia,” explains DiNezio.

The main weakness of the other models, according to the authors, is their limited ability to simulate convection, the vertical air motions that lift humid air into the atmosphere. Differences in the way each model simulates convection may explain why the results for the glacial period are so different.

“Our research resolves a decades-old question of what the response of tropical climate was to glaciation,” concludes DiNezio. “The study, moreover, presents a fine benchmark for assessing the ability of climate models to simulate the response of tropical convection to altered land masses and global temperatures.

Scientist finds topography of Eastern Seaboard muddles ancient sea level changes

The distortion of the ancient shoreline and flooding surface of the U.S. Atlantic Coastal Plain are the direct result of fluctuations in topography in the region and could have implications on understanding long-term climate change, according to a new study.

Sedimentary rocks from Virginia through Florida show marine flooding during the mid-Pliocene Epoch, which correlates to approximately 4 million years ago. Several wave-cut scarps, (rock exposures) which originally would have been horizontal, are now draped over a warped surface with up to 60 meters variation.

Nathan Simmons of Lawrence Livermore National Laboratory and colleagues from the University of Chicago, Université du Québec à Montréal, Syracuse University, Harvard University and the University of Texas at Austin modeled the active topography using mantle convection simulations that predict the amplitude and broad spatial distribution of this distortion. The results imply that dynamic topography and, to a lesser extent, glacial adjustment, account for the current architecture of the coastal plain and nearby shelf.

The results appear in the May 16 edition of Science Express, and will appear at a later date in Science Magazine,

“Our simulations of dynamic topography of the Eastern Seaboard have implications for inferences of global long-term sea-level change,” Simmons said.

The eastern coast of the United States is considered an archetypal Atlantic-type or passive-type continental margin.

“The highlight is that mantle flow is a major component in distorting the Earth’s surface over geologic time, even in so-called ‘passive’ continental margins,” Simmons said. “Reconstructing long-term global sea-level change based on stratigraphic relations must account for this effect. In other words, did the water level change or did the ground move? This could have implications on understanding very long-term climate change.”

The mantle is not a passive player in determining long-term sea level changes. Mantle flow influences surface topography, through perturbations of the dynamic topography, in a manner that varies both spatially and temporally. As a result, it is it difficult to invert for the global long-term sea level signal and, in turn, the size of the Antarctic Ice Sheet, using east coast shoreline data.

Simmons said the new results provide another powerful piece of evidence that mantle flow is intimately involved in shaping the Earth’s surface and must be considered when attempting to unravel numerous long-term Earth processes such as sea-level variations over millions of years.

Cold conspirators: Ice crystals implicated in Arctic pollution

Field of frost flowers
Field of frost flowers

Frost flowers. Diamond dust. Hoarfrost. These poetically named ice crystal forms are part of the stark beauty of the Arctic. But they also play a role in its pollution, a new study by scientists at the University of Michigan, the Cold Regions Research & Engineering Laboratory and the University of Alaska has found.

After collecting and analyzing hundreds of samples from the Alaskan Arctic, the researchers determined that ice crystals that form from vapor clouds billowing up from cracks in sea ice help concentrate mercury from the atmosphere, and that certain types of crystals are more efficient than others. Their results appear in the cover article for the March 1 issue of Environmental Science & Technology.

“Previous measurements had shown that in polar springtime, the normally steady levels of mercury in the atmosphere drop to near zero, and scientists studying this atmospheric phenomenon had analyzed a few snow samples and found very high levels of mercury,” said Joel Blum, the John D. MacArthur Professor of Geological Sciences at U-M. “We wanted to understand what’s controlling this mercury deposition, where it’s occurring and whether mercury concentrations are related to the type and formation of snow and ice crystals.”

Mercury is a naturally occurring element, but some 150 tons of it enter the environment each year from human-generated sources in the United States, such as incinerators, chlorine-producing plants and coal-fired power plants. Precipitation is a major pathway through which mercury and other pollutants travel from the atmosphere to land and water, said lead author Thomas Douglas of the Cold Regions Research & Engineering Laboratory in Fort Wainwright, Alaska.

“Alaska receives air masses originating in Asia, and with China adding a new coal-fired power plant almost every week, it’s not surprising that we find significant amounts of mercury there,” Douglas said. “The concentrations we measured in some snow are far greater than would be found right next to a waste incinerator or power plant in an industrialized location.”

Once mercury from the atmosphere is deposited onto land or into water, micro-organisms convert some of it to methylmercury, a highly toxic form that builds up in fish and the animals that eat them. In wildlife, exposure to methylmercury can interfere with reproduction, growth, development and behavior and may even cause death. Effects on humans include damage to the central nervous system, heart and immune system. The developing brains of young and unborn children are especially vulnerable.

Douglas, Blum and co-workers discovered that certain types of ice crystals-frost flowers and rime ice-contained the highest concentrations of mercury. Because both types of crystal grow directly by water vapor accretion, the scientists reasoned that breaks in the sea ice, where water vapor rises in great clouds, contribute to Arctic mercury deposition.

“The vapor that rises through these openings in the ice brings with it bromine from the sea water. That gets into the atmosphere, where sunlight plus the bromine cause a catalytic reaction which converts mercury gas into a reactive form. If any ice crystals are present, the mercury sticks to them and comes out of the atmosphere,” Blum said.

Close-up of frost flowers
Close-up of frost flowers

The greater the surface area of the crystals, the more mercury they grab, which explains why frost flowers and rime ice, both delicate formations with high surface areas, end up with so much mercury. The mercury-tainted crystals aren’t, however, confined to the edges of breaks in the ice, the researchers determined. Bromine can travel great distances, resulting in mercury deposition in snow throughout the Arctic coastal region.

Collecting the samples was an undertaking that required a spirit of adventure as well as scientific savvy.

“It’s kind of a scary place to work,” Blum said. “It’s freezing cold, and you’re out on the sea ice as it’s breaking and shifting. You can very easily get stuck on the wrong side of the ice and get stranded. Our Inupiat guides would listen and watch, and when they told us things were shifting, we’d get out of there quickly.”

In one experiment the research team used Teflon containers filled with liquid nitrogen, attached to kites or long poles, to collect newly condensed frost over the open water. They also flew a remote-controlled airplane through the vapor cloud and collected ice from its wings.

Even getting out to the ice to do the work was a challenge. After flying to Barrow, Alaska, the northernmost settlement on the North American mainland, the team took off on snowmobiles, led by their Inupiat guides. That may sound like a lark, but traveling over sea ice was not exactly smooth sailing, Blum said. Though the ice freezes flat, it breaks up, smashes back together and refreezes, forming high ridges through which the team had to chip their way with ice-axes to make pathways for their snowmobiles.

But the results are worth the effort and the risks, Douglas said.

“Research like this will help to further the understanding of mercury deposition to a region that is generally considered pristine,” he said. “In the next phase of our work, we are expanding our knowledge by tracking the mercury during and following snow melt and studying its accumulation on the tundra.”

In addition to Blum and Douglas, the paper’s authors are Matthew Sturm of the Cold Regions Research & Engineering Laboratory in Fort Wainwright, Alaska; William R. Simpson and Laura Alvarez-Aviles of the University of Alaska, Fairbanks; Gerald Keeler, director of the U-M Air Quality Laboratory; Donald Perovich of the Cold Regions Research & Engineering Laboratory in Hanover, N.H.; U-M post-doctoral fellow Abir Biswas and U-M graduate student Kelsey Johnson.

The researchers received funding from the National Science Foundation’s Office of Polar Programs Arctic Science Section.