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Abrupt Environmental Change in Ward Hunt Lake, Canada's Northernmost Lake
5 Oct, 2007 12:15 pm
The northernmost lake in North America has not escaped the effects of recent global warming, according to a study recently completed by an international team of researchers. These effects were recorded by dramatic recent shifts in the microfossils found in the lake?s sediments.
We chose to study Ward Hunt Lake because it sits in a region that is among the most ice-dominated in the northern hemisphere. Ward Hunt Lake itself is covered by a 4m thick layer of ice that never disappears. Even in summer, the only open water is a moat around the fringes of the ice cover that can be 10-15m wide. Ward Hunt Island is surrounded by the Ward Hunt Ice Shelf, a 40m thick platform of floating ice that recently fractured into large pieces (Mueller et al. 2003). To the south, there is the mountainous and glacier dominated coast of Ellesmere Island, and to the north is the permanent polar pack ice of the Arctic Ocean. Ice controls everything around Ward Hunt Island. As such, we thought that if there remained places in the Arctic that were buffered against the effects of recent climate change, this might be such a place.
In May 2003, two members of our team squeezed into the claustrophobic space beneath the ice to take a sediment core. Given that the lake is 5.5m deep, and 4m of this is occupied by ice, this job was not for the weak of heart. They recovered an 18 cm sediment core, which we dated using methods that are often used in paleoenvironmental studies – the widely known 14C, as well as 210Pb and 137Cs, two isotopes that give us age information on shorter timescales where 14C is not as effective. These analyses told us that our core represented 8540 years of sediment accumulation.
Our goal was to reconstruct the history of the lake and its surrounding environment by examining fossils contained within this sediment core. Specifically, we looked for diatoms, a group of algae with cell walls made of silica, and for fossil pigments –chemical signatures that allow us to infer changes in the abundance of different algal groups in the lake’s past.
The results were striking. We saw that for the majority of the lake’s history, no diatoms were present in the sedimentary record. Given that diatoms are capable of surviving almost anywhere that there is liquid water, this was remarkable. When they did appear, at a horizon in the sediments which corresponded to roughly 200 years ago, essentially the entire community was composed of one species. A second species was found, but only one single specimen was present. A similar situation of single species dominance was observed from a lake in the Antarctic, but during the peak of a glacial period (Hodgson et al. 2005).
Changes in the pigments were just as pronounced. Major pigments, such as chlorophyll a and b, and other pigments indicative of different algal groups, were present in low concentrations throughout the core. We also identified a number of the products formed by the degradation of these pigments over time. Levels fluctuated slightly during the 8450 years represented by the core, but at about the same level as the appearance of diatoms began an almost 500-fold increase in the concentration of most of the major pigments. Two pigments that are indicators of diatom populations, fucoxanthin and diadinoxanthin, also appeared at this horizon, and subsequently increased. These results show major changes occurring in Ward Hunt Lake within the last 200 years. They represent the effects of longer growing seasons that resulted from warming, and suggest that, until recently, the lake may have been almost completely frozen throughout the year. While conditions in the lake are still severe, recent warming in the region has made the lake’s environment more conducive to increasingly productive and complex communities.
When viewed in concert with the recent fracturing of the Ward Hunt Ice Shelf in 2001, the disintegration of the nearby Ayles Ice Shelf in 2005, the disappearance of shallow ponds on eastern Ellesmere Island, and the frequent new records for high temperatures and low sea ice extent in the Arctic, these changes paint a picture of a landscape in transition, and one in which many ecosystems as we know them may soon cease to exist.
1. Antoniades D, Crawley C, Douglas MSV, Pienitz R, Andersen D, Doran PT, Hawes I, Pollard W, Vincent WF. 28 September 2007. Abrupt environmental change in Canada's northernmost lake inferred from fossil diatom and pigment stratigraphy, Geophysical Research Letters 34, L18708, doi:10.1029/2007GL030947.
2. Hodgson D.A., E. Verleyen, K. Sabbe, A.H. Squier, B.J. Keely, M.J. Leng, K.M. Saunders & W. Vyverman. 2005. Late Quaternary climate-driven environmental change in the Larsemann Hills, East Antarctica, multi-proxy evidence from a lake sediment core, Quaternary Research 64: 83-99.
3. Mueller D.R., W.F. Vincent & M.O. Jeffries. 2003. Break-up of the largest Arctic ice shelf and associated loss of an epishelf lake, Geophysical Research Letters 30 (20): 2031, doi:10.1029/2003GL017931.