Page 10. MAINE COASTAL NEWS August 2018


Waterfront News Following the Fresh Water


Fingerprint of ancient abrupt climate change found in Arctic


JULY 9, 2018 – A research team led by Woods Hole Oceanographic Institution (WHOI) found the fi ngerprint of a massive fl ood of fresh water in the western Arctic, thought to be the cause of an ancient cold snap that began around 13,000 years ago. "This abrupt climate change—known


as the Younger Dryas—ended more than 1,000 years of warming," explains Lloyd Keigwin, an oceanographer at WHOI and lead author of the paper published online July 9, 2018, in the journal Nature Geosci- ence.


The cause of the cooling event, which is


named after a fl ower (Dryas octopetala) that fl ourished in the cold conditions in Europe throughout the time, has remained a mystery and a source of debate for decades. Many researchers believed the source


was a huge infl ux of freshwater from melting ice sheets and glaciers that gushed into the North Atlantic, disrupting the deep-water circulation system—Atlantic Meridional Overturning Circulation (AMOC)— that transports warmer waters and releases heat to the atmosphere. However, geologic evidence tracing its exact path had been lacking. In 2013, a team of researchers from


WHOI, Scripps Institution of Oceanography at the University of California San Diego, and Oregon State University, set sail to the eastern Beaufort Sea in search of evidence for the fl ood near where the Mackenzie River enters the Arctic Ocean, forming the border between Canada’s Yukon and Northwest territories. From aboard the U.S. Coast Guard Cutter Healy, the team gathered sediment cores from along the continental slope east of the Mackenzie River. After analyzing the shells of fossil plankton found in the sediment cores, they found the long sought-after geochemical signal from the fl ood.


“The signature of oxygen isotopes


recorded in foraminifera shells preserved in the sediment allowed us to fi ngerprint the source of the glacial lake discharge down the MacKenzie River 13,000 years ago,” said co-principal investigator Neal Driscoll, a professor of geology and geophysics at Scripps Oceanography. “Radiocarbon dating on the shells provided the age con- straints. Circulation models for the Arctic Ocean reveal that low-salinity surface water is effi ciently transported to the North Atlan- tic. How exciting it is when the pieces of a more than 100-year puzzle come together." Next steps in future research, Keig-


win says, will be for scientists to answer remaining questions about the quantity of fresh water delivered to the North Atlantic preceding the Younger Dryas event and over how long of a period of time. "Events like this are really important,


and we have to understand them better," adds Keigwin. "In the long run, I think the fi ndings from this paper will stimulate more research on how much fresh water is really necessary to cause a change in the system and weakening of the AMOC. It certainly calls further attention to the warming we're seeing in the Arctic today, and the acceler- ated melting of Greenland ice." Earlier this year, a paper by research-


ers at the University College London and WHOI found evidence that the AMOC hasn’t been running at peak strength since the mid-1800s and is currently at its weak- est point in the past 1,600 years. Continued weakening could disrupt weather patterns from the U.S. and Europe to the African Sahel.


Additional co-authors of the paper


News From Woods Hole Oceanographic Institute “Many processes can aff ect sea level,


published in Nature Geoscience are: Ning Zhao and Liviu Giosan of WHOI; Shannon Klotsko of Scripps Oceanography; and Brendan Reilly of Oregon State University. This work was supported by a grant


from the National Science Foundation, Of- fi ce of Polar Programs. Study Finds Link Between River Outflow and Coastal Sea Level


Increased fresh river outfl ow enters the coastal environment, where it mixes with ambient salty ocean water. Under the in- fl uence of the Earth's rotation, this plume of fresher water "turns to the right" (in the northern hemisphere), fl owing downstream as an alongshore current "trapped" to the coast. These fresher waters "piled up" along the shore result in an increase in sea level at the coast. Illustration by Natalie Renier, Woods Hole Oceanographic Insti- tution


Sea levels in coastal areas can be af- fected by a number of factors: tides, winds, waves, and even barometric pressure all play a role in the ebb and fl ow of the ocean. For the fi rst time, however, a new study led by the Woods Hole Oceanographic Institution (WHOI) has shown that river outfl ow could play a role in sea level change as well. The study, published July 9 in the jour-


nal Proceedings of the National Academy of Sciences, examined decades’ worth of river level and tidal data from gauges installed throughout the eastern United States. The researchers then combined that data with information on water density, salinity, and the Earth’s rotation, creating a mathematical model that describes the link between river discharge and sea level on an annual basis. “The equation we derived lets us predict


how much sea level will rise based on river fl ow, and then compare that prediction to actual measurements and observations,” says Chris Piecuch, a physical oceanogra- pher at WHOI and lead author on the paper. “Based on our model and the observations, we’re fi nding that variations in the amount of water that comes out of a river annually can raise or lower coastal mean sea level by several centimeters.” Notably, the study found the majority of


that sea level change occurs only one side of a river’s mouth. Since freshwater is naturally less dense than saltwater, river outfl ow fl oats along the ocean’s surface, where the Earth’s rotation forces it to turn sharply along the coast. In the northern hemisphere, that water follows the right hand side of the river; in the southern hemisphere, the left hand side. In both cases, the freshwater forms a current that pushes water up against the shoreline, raising localized sea levels in the process. At the moment, says Piecuch, the model


is still just a proof-of-concept, but could already help calculate the eff ects of sea level rise on certain coastal regions. That data is missing from current satellite mea- surements, since the resolution of existing sensors isn’t fi ne enough to get accurate readings of ocean height within a few miles of the coast. “When you think of societal impact,


you want to know what’s happening at the coast,” Piecuch says. “In low lying areas like Bangladesh, we don’t yet know how sea level and river outfl ow combine. But if a major storm comes through, even a small rise in the background mean sea level could have a huge impact on fl ooding.” Right now, the WHOI model has only


been used to calculate average sea level on an annual basis—but Piecuch and his colleagues are working to change that. Ulti- mately, they hope to consider more detailed and granular data, so they can understand how individual events, like a hurricane or massive rainfall, might aff ect ocean levels.


making predictions of regional sea level change a challenging endeavor,” says Larry Peterson, a program director in the National Science Foundation (NSF)’s Division of Ocean Sciences, which funded the research. “These scientists show that discharge from rivers can play a signifi cant but overlooked role in the interpretation of sea level from downstream tide gauges. The work has important implications for climate models, remote sensing, and the projection of coastal fl ood risks.” Also collaborating on the study were


Steven J. Lentz of WHOI, Klaus Bittermann and Andrew C. Kemp of Tufts University, Rui M. Ponte and Chistopher M. Little of Atmospheric and Environmental Research, Inc., and Simon E. Engelhart of the Univer- sity of Rhode Island. The research was supported by grants


from NASA, NSF, and the Investment in Science Fund at Woods Hole Oceanographic Institution.


Geologic History of Ayeyawady River Delta Mapped for the First Time


The Ayeyawady River delta in Myan-


mar is home to millions of people, and is a hub of agricultural activity. Unlike other large rivers across the world, however, the Ayeyawady has been relatively untouched by large infrastructure and dam projects for the past 50 years, and its geologic evolution has never previously been studied. A team from the Woods Hole Oceano-


graphic Institution (WHOI) has collaborated with Myanmar scientists to present the fi rst extensive view of the delta’s history, reveal- ing how its fl oodplains and shorelines on the Andaman Sea have grown and shrunk over the past 10,000 years. The study, published June 12, 2018, in the journal Earth Surface Dynamics, shows that shifts in monsoon cli- mate have changed the shape of delta in the past, and suggests that human impact could drastically alter the region in the future, says lead author Liviu Giosan, a geologist at WHOI. "In geological terms, the Ayeyawady, as


any delta, is kind of an ephemeral feature. If the river can dump enough sediment onto the area through seasonal fl oods, the surround- ing land will stay above sea level. But if those sediments can’t keep up with sea level rise, the delta will eventually be submerged and overtaken by the sea” Giosan says. According to the new study, the delta


has changed repeatedly over the past several thousand years, showing that the region is particularly vulnerable to changes in out- fl ow, ocean currents, or storm frequency. To map the delta’s geologic history,


Giosan’s team consulted 19th century British charts—which even today are the only available depth soundings off the Ayeyawady—to analyze long-submerged landscapes. They also turned to satellite im- agery and NASA’s digital elevation data to spot past channels but also shorelines, which appear as slight ridges in the otherwise fl at landscape. On the ground, the researchers dug


trenches and drilled cores into those former shorelines and channels, revealing layers of sand and silt that they could then date in the lab. The resulting data showed that the ebb and fl ow of the delta was linked to extreme climate events: during long periods of heavy monsoons, more sediment traveled downriver, maintaining the delta’s coastline. In dryer intervals, that fl ow dwindled, and waves and tides from the Andaman Sea chipped away at the landscape. Most surprisingly, Giosan notes, plate


tectonics has played a major role in the del- ta’s shape. Instead of the uniform fan shape of most deltas, the modern shoreline of the


Ayeyawady advanced far more quickly to the western side of the river than to the east, thanks to a major system of fault lines that run through the area. “Myanmar sits directly where the Indi-


an plate slides underneath the Asian plate. The fault system extends into the Andaman Sea, maintaining deeper waters and stronger tidal currents on the eastern side of river mouth that move sediments off shore. In the past the western side grew faster than the eastern coast, but this situation is about to change as dams are being built upriver, ” Giosan notes. The team’s study will provide a baseline


for future scientifi c work in the region, as well as data that will help inform human de- velopment in the future. Its fi ndings suggest that major dam projects in the future could alter the outfl ow of the Ayeyawady, reducing the sediments deposited downstream. That scenario could upset an already delicate balance, rendering homes and farmland on the delta potentially uninhabitable as the land erodes and seawater encroaches. "Those effects can’t be understood


without showing how the delta evolved, how it formed, what direction it went at certain times, or why it switched from one region to another. You need that basis to address specifi c questions,” Giosan adds. Although he’s passionate on a profes-


sional level about the work he’s doing in Myanmar, Giosan is also quick to note that it appeals to him personally. Like Myanmar, his homeland of Romania was ruled for decades by a closed regime under dictator Nicolae CeauÈescu. "I know what it’s like to live in a closed


country,” he says. “It’s important to me that my colleagues in Myanmar get all the help they can, whether it’s in education, or sci- entifi c areas." Professor Thet Naing of Pathein Uni-


versity led the work in Myanmar together with Myo Min Tun of the University of Mandalay and Swe Thwin of Mawlamyine University. Other national and international partners included Peter D. Clift of Louisiana State University, Florin Filip of the Institute for Fluvial and Marine Studies in Bucharest, Romania, Stefan Constantinescu of Bucha- rest University, Nitesh Khonde of WHOI and Birbal Sahni Institute of Paleosciences in India, Jerzy Blusztajn of WHOI, Jan-Pi- eter Buylaert of the Technical University of Denmark, and Thomas Stevens of Uppsala University in Sweden. Hearing Tests on Wild Whales


Belugas have more sensitive hearing than previously thought


Scientists published the fi rst hearing


tests on a wild population of healthy marine mammals. The tests on beluga whales in Bristol Bay, AK, revealed that the whales have sensitive hearing abilities and the number of animals that experienced exten- sive hearing losses was far less than what scientists had anticipated. The latter fi ndings contrasted with ex-


pectations from previous studies of humans and bottlenose dolphins, which showed more hearing loss as they aged, says Aran Mooney, a biologist at Woods Hole Oceano- graphic Institution (WHOI) and lead author of two new studies on beluga whales. “But unlike the wild beluga population, the dol- phins that were studied lived in a very noisy environment, as most humans do.” At a time when noise in the ocean is


increasing from human activities, such as oil and gas exploration and ship traffi c, un- derstanding the natural hearing abilities of whales and other endangered marine mam- mals is crucial to assessing potential noise


Continued on Page 23.


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