Field Notes: The Polar Field Services Newsletter

Lake Cores Offer Clues About Climate Change

Darrell Kaufman mans the motor en route to a remote Alaskan Lake. Image courtesy PolarTREC

Darrell Kaufman (Northern Arizona University), one of our favorite veteran Alaska scientists, has sampled lake sediments around the state since 1996 – sometimes more than once a year, and the 2012 field season was no exception. In two collaborative studies Kaufman and colleagues scouted out some new study lakes in the Brooks Range and Kenai Peninsula to help piece together thousands of years of Alaska’s climate history.

Specifically, Kaufman and his colleagues aim to better understand how Alaskan lakes (and climate) have changed since the Pleistocene glacier receded about 14,000 years ago.

Lakes in the Brooks Range

Last summer, Kaufman headed to the north-central Brooks Range with graduate student Brandon Boldt (NAU) and collaborator Al Warner (Mount Holyoke College) in an NSF-funded study of two glacier-fed lakes –  Karupa and Shainin Lakes — and two non-glacial lakes — Cascade Lake and Shainin Pond — all in and around Gates of the Arctic National Park & Preserve.

Sediments from the glacial lakes contain a record of glacier fluctuations in the headwaters of the lakes, whereas the non-glacial lakes contain sediment rich in biological materials that reflect changes in the ecology of the lake and the watershed. In a land of many lakes Kaufman says “the trick is to narrow the choices from a zillion to a handful.”

Acoustic Profiling

Using a geophysical technique called acoustic profiling, the team mapped each lake with a sound-emitting transducer mounted to a surfboard and towed behind a rubber boat. The transducer emits a range of frequencies that penetrate below the lake floor. The energy reflected back to the sensor provides an image of the sub-bottom sedimentary layers and structures.

“This is a tried-and-true technique. We want an image of the lake sub-bottom stratigraphy so that we can choose locations to core when we return in April. We can get a longer core, which means more represented time, where sediment deposits are thicker. We also want to avoid areas where the mud is disturbed by methane bubbling up or by landslides below lake level,” Kaufman explains.

Muddy Cores

Following each acoustic profile survey, the team took several surface cores to get a first look at some of the mud. They used a light-weight sampler fitted with a plastic tube to extract 1-3 feet of sediment, including the intact lake floor.

Next Kaufman and Boldt flew to Homer where they met up with NAU graduate students Taylor LaBreque  and Jon Griffith. The team of four chartered a floatplane to Emerald Lake across Kachemak Bay on the Kenai Peninsula. Emerald Lake is presently separated from Grenwink Glacier, which spills out from Kenai’s large ice fields. Several hundred years ago, during the Little Ice Age, the Grenwink Glacier advanced and overtopped the drainage divide, contributing meltwater and sediment to Emerald Lake. Grenwink Glacier has recently retreated, but its history is preserved in the lake.

Coring Operation Explained

There, the team embarked on a full coring operation, using a  Zodiac as a tender to the coring platform, which gets anchored to the shoreline via boulders lassoed with parachute cord. The core tube consists of two-meter-long sections of clear tubing, or multi-meter-long sections of PVC pipe. Coring from a “cataraft,” two pontoons with a plywood platform in between, the scientists lowered the coring contraption into the water using ropes and cables, while 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 into the lake bottom as far as possible. Then they pulled up the tube hand over hand, using cleats to tie off rope, and a rubber-stopper-like piston to provide suction at the top of the tube to prevent the mud from slipping out the bottom.

The team then placed rubber caps on the tube bottoms. After gently pressing down on a piece of floral foam on top, they drained the water from the tube. Before transporting the cores from the field, the team split some of them lengthwise for inspection.

“Our coring operation has really evolved over the years. We are much less likely to lose the mud from the core tube or get our cables tangled up now. We are always innovating and we just make things work creatively,” Kaufman says. “We now have special tube cutters. We inject an absorbent powder that makes the watery mud at the top of the core gel so it’s stable for shipping. We’ve really got it down to an art!”

Analysis

Cores are shipped to the Limnological Research Center (University of Minnesota) where they are split in half lengthwise, photographed, and one-half is archived in a repository for future studies, while the working half is forwarded to Kaufman’s lab at Northern Arizona University. First analyses include radiocarbon studies for sediment dating.

Colleagues will study pollen, chlorophyll, and microfossils to reconstruct vegetation history and lake productivity. Chemical and isotopic methods are used to infer precipitation rates and sources.

Kaufman’s goal is to synthesize data from lakes across the Arctic for a more comprehensive look at Holocene climate history. A big job, he admits, and climate is complicated.

“Just like the changes that are taking place now, when global changes occurred in the past, the effects were not felt uniformly from place to place. We need to study lots of sites to capture and to understand the variability,” says Kaufman. “A lot of people consider the Holocene — the present interglacial period — as a time of a relatively uniform climate that allowed humans to flourish, but out lake sediments show substantial changes in climate. Alaska’s glaciers are sensitive to variations in ocean and atmospheric patterns. The changes are not entirely random, but can often be tied to solar activities, sea ice and volcanic activity.”  –Marcy Davis

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