Next time you consider whacking the tiny, bloodthirsty mosquito persistently singing in your ear, consider for a moment that he and his kind may hold the key to past climate change. Mosquitoes are relatives to the diverse “non-biting midge” family, chironomids, small, two-winged flies that spend much of their life cycle in and around water. Yarrow Axford (Northwestern University) is looking to the mosquito’s relatives, preserved in lake mud, to help quantify western Alaska’s climate during the Holocene epoch, over the last 11,000 years.
Small lakes form in the topographic lows carved by Alaska’s ancient glaciers. The lakes collect sediments during warm interglacial intervals like the Holocene. With funding from the National Science Foundation, Axford and Darrell Kaufman (Northern Arizona University) are collecting and studying sediment cores from these lakes in a transect across southern Alaska between Adak, one of the Aleutian Islands midway between Alaska and Russia, and Valdez.
Back in the lab, Axford hunts for chironomids preserved in the lake sediments. During their larval stage these tiny flies molt four times. Chironomids shed the outer part of their entire bodies, but the head capsule, which looks like a teeny tiny helmet with teeth, is the part that allows Axford to identify the midges. The head capsules fall to the lake floor where they are preserved in the sediment.
“Chironomids are very abundant in fresh water lakes across the Arctic and subarctic regions,” explains Axford. “So, this equates to lots and lots of larvae remains raining down on the lake floor. Their abundance makes them very useful”
Under a microscope, Axford identifies midge species by examining minute features including each larva’s teeth: teeth patterns and shape; the number of teeth and whether that number is odd or even; and tooth positioning—all of these factors provide clues to the species. One benefit of using chironomids in paleoclimate studies is their sheer abundance. In only a gram of lake mud, Axford can find a statistically representative sample of chironomids—tens to hundreds of helmet-heads representing many different species. Since chironomids’ habitats are partly temperature-dependent, the different types of chironomids present in lake sediments can help constrain past temperatures and climate variability.
A geologist by training, Axford first studied chironomids as part of her Ph.D. research at the University of Colorado, Boulder. With training from paleoclimatologist Donna Francis (University of Massachussets) Axford looked to midge assemblages to help reconstruct ecosystems and climate changes on Baffin Island, in the Canadian Arctic, and Iceland. Axford has found midge remains in 200,000-year-old lake sediments from Baffin Island and says that chironomids would probably be found in even older sediments
Axford finds deep satisfaction from spending a day looking under the microscope. She admits she enjoys “the tedious, time-consuming work of identifying chironomids. The process is a bit old-fashioned when compared to black-box-ish geochemical methods, but concrete.”
Meanwhile, Alaska’s Adak Island is a unique research location for a couple of reasons. First, there were no mosquitoes or bears complicating field work on Adak, which made it different than most other Alaska field locales (the tradeoff: wondering whether the field party would come across any “unexploded ordnance,” as signs posted at Adak’s decommissioned military base warn).
“Adak is also likely to be biologically interesting. As an isolated north-Pacific Island which was completely ice-covered during the last glacial maximum, the chironomids were likely wiped out and then returned following glacial retreat and lake development,” says Axford. “I look forward to finding out what flies returned and where they were from. Which species showed up first, and were they from Asia or North America?”
Finally, as the westernmost location in Axford and Kaufman’s Alaska transect, this site will help them meet their goal of reconstructing southern Alaska’s Holocene climate, specifically in the context of the Aleutian Low, an area of low pressure that is centered on the Aleutian Islands in winter. The Aleutian Low is a major component of Northern Hemisphere circulation. Through detailed studies of lake sediments across many different sites, Kaufman and his students Anne Kraweic and David Vaillencourt hope to track the strength and position of a changing Aleutian Low as it migrated over time. This, says Axford, will help scientists identify important regional differences in Arctic climate. -- Marcy Davis