Arctic Lakes Help Explain Ancient Ancient Climate Patterns

Greenland’s unglaciated landscape is one of starkly exposed bedrock undulating in gentle ridges and swales. Small lakes form in the topographic lows where rainwater and glacial ice melt collect. University of Buffalo’s Jason Briner is part of a National Science Foundation-funded collaboration of 10 institutions looking to use these small lakes to reconstruct the last 2000 – 8000 years of arctic climate change.

Last summer, Jason Briner spent a month on Canada's Ellesmere Island last summer, hiking, camping, and boating on Ayr Lake. Oh yeah, he and his team also got many lake core sediments.

Darrell Kaufman, Northern Arizona U, is the lead PI on this massive effort.

Lakes Provide Important Climate Change Evidence

The collaborators will double the amount of existing high-resolution climate records reconstructed from arctic lake sediments. In particular, Briner and colleagues will focus on collecting cores from Alaska and Greenland likely to record the transition between the Holocene Thermal Maximum, a warm period between 5,000 and 9,000 years ago, and the Little Ice Age, a cooler period between the 16th and 19th centuries.

“We have good climate information from ice cores, ocean sediment cores, and tree rings, but not from lake sediments. So, for this project we pulled together people who are good at studying lake cores. Ultimately, we want to extend back to 8000 years and collect cores from around the Arctic,” says Briner.

Middle Holocene as Proxy for Today

“The middle Holocene period climate was warmer than today so it is our closest analog to modern climate change. By studying how and when the ice sheet retreated during a warming climate and constraining the timing and degree of margin fluctuation during the subsequent Little Ice Age, we will place constraints on the sensitivity of the GIS to temperature change. Understanding this sensitivity is urgent given contemporary issues.”

Briner and UB graduate students Nicolas Young, Sam Kelley and undergraduate student Stefan Truex worked out of three field camps near Ilulissat in 2011. The team collected cores from lakes a few hundred meters across where the Greenland Ice Sheet (GIS) spilled its melt along with pulverized glacial flour, very fine-grained glacial sediments. Briner had many lakes to choose from. Ultimately he and his team sampled from lakes closest to camp, portaging their sediment cores in a small, motorized Zodiac boat.

How to Get a Lake Sediment Core

Once on a lake, the team used GPS and a fish-finder device to get a sense of lake bottom topography and water depth. After creating a rough map of the lake bottom, they chose a coring location with a water depth that would accommodate their coring apparatus: two meter-long sections of clear tubing, or several-meter-long sections of PVC pipe. Coring from a “cataraft,” two pontoons with a plywood platform in between, Briner’s team successfully cored five lakes.

Using ropes and cables, the scientists lowered the coring contraption into the water whilst keeping track of the length deployed by measuring the amount of rope out. Once the tubing was on the bottom, they hammered the top of the PVC pipe until they couldn’t any longer. Then they pulled up the tube, making certain to hold it vertical to contain the muck.

On the boat, the core bottoms were capped, and a piece of floral foam was put on top. By gently pressing down, the team drained the water from the tube; back at camp the core sat upright over several days to let the sediments compact. Before transporting the cores from the field, the team split some of them lengthwise for inspection.

What the Sediment Cores Tell Us

Briner, who runs the UB Paleoclimate Lab, uses the relative thickness of varves, layers of sediment that accumulate in response to seasonal weather changes, to get a sense of how warm or cool a particular year was. During warm summers, the GIS melts more readily and, consequently, transports more sediment downstream to the lakes. Consequently. the varve layer will be relatively thick for that year. Conversely, in mild summers with low discharge, the sediment transport will also be low and the varve layer relatively thin.

“Eventually, we can say ‘when it’s this thick, it was this temperature’ and we can mathematically extrapolate that assumption back in time to get a detailed climate picture from these very fine sediment laminations that correspond to annual sedimentation,” explains Briner.

Reconstructing the Climate

Dating lake sediments is also important in climate reconstruction. Briner correlates the upper two or so centimeters of sediment to historical climate records and then counts sediment layers to get the number of years back to the end of the core. He and his researchers also date organic matter using radiocarbon techniques, which helps constrain the onset of glaciation. Biological markers like pollen grains or insect assemblages, particularly chironomids, two-winged flies that spend much of their lives in arctic lakes, highlight climate details. Ultimately, these data will contribute to climate models that will help predict GIS behavior in a changing climate.

“One of my real passions is finding out where the glaciers have been and how they’ve advanced and retreated. We need more detailed climate records not only to establish the timing of retreat, but because the pattern and rate of future ice sheet retreat are unknown,” says Briner. “We know that the GIS marginal extent fluctuates, but we need to know how long and when the ice sheet was at its maximum as well as how fast the margin retreated and associated volumetric changes given the temperature during that time. If we know these things we will be in a better position to predict the response of the cryosphere to warming–rates of global climatic change, glacier melt, and sea-level rise through the next century.”—Marcy Davis