Among the sources of evidence of past climate conditions that lie buried in the layered sediments of lake bottoms are the remains of midge larvae. Today, non-biting midges (of the insect order Diptera, family Chironomidae) are abundant almost everywhere. You may have encountered tiny adult midges lying dead beneath the nightlight in the bathroom in the morning. They lay eggs in water and their larvae go through a series of molts before they metamorphose into pupae and ultimately into flying adults. The larvae (1/16th to 1/8th inch or so in length) are cumulatively so abundant that they typically form the largest single source of animal biomass in lake-bottom sediments. As such, they contribute an important food chain link between the detrital organic matter and algae that they eat and larger benthic (i.e., bottom dwelling) invertebrates or bottom-feeding fishes.
Because of their ecological importance, this group of insects has been well studied with regard to their identity (>5000 species worldwide so far), their distribution and the conditions required for their well-being. As arthropods, their construction includes a non-living exoskeleton covering that remains more or less intact long after the decomposable organic parts of the animal are lost over time. The head capsule of these animals are particularly thick and resilient and can be found identifiably preserved in the sediments into which they settled even thousands of years ago. Features of their heads – especially the appearance of their teeth, their dentition – are used by biologists to identify them at least to the genus level.
To study the chironomids present in the distant past, 3 cubic centimeter subsamples of sediment are removed from various levels within a sediment core. Each sample is immersed in hot 5% potassium hydroxide solution for 20 minutes, which disaggregates clumped sediment particles. The resulting mixture is passed through sieves with mesh sizes of 118 μm (micrometers) and 53 μm and rinsed with distilled water. Particles larger than 118 μm and 53 μm respectively, including the head capsules of chironomids, are retained in the sieves, while the bulk of the material, including most of the disaggregated sediment particles, pass through the 53 μm mesh. The material caught by each sieve is back-washed into a Petri dish and examined using 40X power of a dissecting microscope (see Figure 1).
Figure 1The composition of the chironomid community that settles into surface bottom sediments today would be very different from that found towards the deepest segments of our core sample. The climatic conditions that favor today's community are presumably quite different (e.g., much warmer) than conditions here would have been 10,000 years ago. While many of the chironomid types found in the deepest parts of our core are no longer found locally, they do occur alive and well in today's cold climate areas such as Baffin Island or Labrador or in high elevation alpine tarns. Biologists studying these high latitude or elevation types have been able to determine the environmental conditions that these living representatives require. Distribution of chironomids is particularly highly correlated with mid-summer, surface-water temperatures of the lakes where they are found. We assume that the same critters living here years ago had similar requirements to their currently living representatives that over time followed the cold water conditions they prefer polewardly as the glaciers retreated and the climate here warmed. Species that have narrow preferences for cold water conditions are especially useful as indicator species, i.e., their presence in the past suggests that this narrow range of cold conditions existed then. Such species are referred to as stenothermal (steno – (Greek) narrow + thermal – (Greek) temperature) and they can be used as "proxy" clues to reveal temperature conditions wherever or whenever they are found.
Predictive models have been constructed based on documented requirements of each of these stenothermal species. Each species' optimal mid-summer, surface-water temperature requirement contributes in proportion to that species' abundance within a chironomid community from the past (i.e., from a particular sediment level) to produce a best-guess inferred mid-summer, surface-water temperature present at the time when that layer of sediment settled out.
Figure 2
Figure 3We extracted a first-round of 15 sediment samples at 20 cm intervals from the deepest part of our core. Preliminary results demonstrate that we need to add intervening samples, resulting in 10 cm intervals, to improve the resolution of our model-generated temperature curve. We intend to gather these additional samples (and more) when we return to the core sampling on October 25th. It will be some time before the more refined results will be available. For now, we can say that in comparison to the mid-summer, surface-water temperature of 24.5 C (76 F) at Big Pea Porridge Pond in 2008 (see Scientific Posting # 20), comparable model-generated temperatures from the earliest portions of our core are, appropriately, in the 12-18 C (53.5-64.5 F) range.
Lee Pollock
