The substantial increase in methane concentrations in tropical wetlands can be attributed to the last Glacial Period, when icebergs calving off North America introduced massive influxes of fresh water into the North Atlantic, new research shows.
Calving of glaciers in the North American ice sheet, also known as Heinrich events, released large icebergs into the North Atlantic Ocean. The authors of this new study, Rachael H. Rhodes and her colleagues from Oregon State University, found that Heinrich events in the Hudson Strait may have enhanced rainfall in the Southern Hemisphere, which in turn led to increased methane production in tropical wetlands when the wetlands flooded.
“Essentially what happened was that the cold water influx altered the rainfall patterns at the middle of the globe. The band of tropical rainfall, which includes the monsoons, shifts to the north and south through the year,” Rhodes explained during an interview with Ed Brook, a professor at Oregon State University.
“Our data suggest that when the icebergs entered the North Atlantic causing exceptional cooling, the rainfall belt was condensed into the Southern Hemisphere, causing tropical wetland expansion and abrupt spikes in atmospheric methane,” she added.
According to the study, each individual Heinrich event could have long-term impacts on tropical climate and hydrology, specifically over 740 to 1520 years. Four specific Heinrich events were linked to methane signals. Each of these events deposited “relatively thick and spatially extensive sediment, which was rich in detrital carbonate.”
With a newly developed continuous measurement technique, Rhodes and her colleagues produced an accurate record of atmospheric methane concentrations for West Antarctic Ice Sheet Divide ice core in high resolution. More importantly, they detected methane emission anomalies in Southern Hemisphere.
“Using this new method, we were able to develop a nearly 60,000-year, ultra-high-resolution record of methane much more efficiently and inexpensively than in past ice core studies, while simultaneously measuring a broad range of other chemical parameters on the same small sample of ice,” said Joe McConnell from Desert Research Institute in Reno, Nevada, who contributed to perfecting the measurement technique.
The findings could have implications for better understanding greenhouse gas emissions and the impact of past glacial calving on climate change.
Talk of earthquakes likely calls to mind giant fissures opening up along the earth’s crust, the trembling of rock, buildings crumbling to their knees and, depending on your age and cast of mind, the love of Superman for Lois Lane. But it does not likely conjure up images of giant tongues of sliding ice or the splash of calving icebergs. And yet it should.
Most earthquakes are generated by the friction produced by two bodies of rock rapidly sliding past each other on a fault in the Earth’s crust, but a different breed of earthquakes was discovered in 2003: glacier earthquakes.
These newly documented earthquakes are occurring in glaciated areas of Alaska, Antarctica and Greenland and are caused by the dumping of giant icebergs–equal in size to, say, 400,000 Olympic swimming pools–into the sea. They produce seismic signals equivalent to those found in magnitude 5 earthquakes, which can be felt thousands of kilometers away. And there are many more of them today than there were just a couple of decades ago: six to eight times more than in the early 1990s have been recorded at outlet glaciers along the coast of Greenland.
This sudden surge in glacier earthquakes is expected to set off a series of events that will result in faster sea level rise over the coming century than had previously been estimated, according to research conducted there by Dr. Meredith Nettles, Associate Professor of Earth and Environmental Sciences at Columbia University, and some of her colleagues, as a part of Project SERMI. In 2013, the Intergovernmental Panel on Climate Change (IPCC) revised estimates for the next century dramatically upward (from 11-17 inches by 2100 to 10-39 inches) when taking Dr. Nettles and her colleagues’ earthquake research into account for the first time. This upward revision reflects the fact that the earthquakes change the internal dynamics of the glaciers, causing them to flow more rapidly, and to shed more ice into the ocean.
Nettles gave a talk on glacier earthquakes last November at the American Museum of Natural History. In the summer of 2006, she and 11 other scientists from six institutions in the U.S., Denmark and Spain traveled to a small town in East Greenland to take seismic, GPS and time-lapse photography measurements of the Helheim Glacier. They wanted to examine the location, dynamics and frequency of glacier earthquakes and to develop a method for using seismic data to map changes in the ice. They also wanted to learn how these earthquakes shape the behavior of outlet glaciers, which cluster around coastlines and deposit ice and meltwater into the oceans.
After setting up camp in town, the scientists flew a helicopter out to the glacier, drilled holes 6 feet deep in the ice, and drove 9-foot poles into those holes to anchor their GPS, time-lapse and seismic equipment. From the data they collected, they learned that short-term acceleration of glacier ice flows—up to 25% increases in velocity—coincided with the earthquakes. They also found that the increase in glacier earthquakes corresponded to net retreat of the ice front in Greenland. In particular, the section of the Greenland coast with earthquake-producing glaciers expanded northward. And whereas in the 1990s, a few glaciers were causing earthquakes; by 2005, those glaciers were associated with more frequent earthquakes, and other glaciers began to have seismic activity as well.
Future research should focus on ice-ocean interactions that promote or reduce glacier calving, said Nettles. And scientists still need to better understand the specific mechanisms of loss of ice at the calving front and the effects of loss of ice on flow speeds. Nettles’ current research examines the impact of tides on glacier calving. Preliminary analysis of the data suggests that glacier earthquakes are more likely to occur at low tide.
Nettles and her colleagues collected most of their seismic data and GPS observations of the glacial earthquakes through facilities run jointly by IRIS (Incorporated Research Institutions for Seismology) and the USGS (U.S. Geological Survey). Thanks to grants from the USGS and the National Science Foundation, that data is open sourced and available to the public.