ABSTRACT: Chironomid-based paleoenvironmental study of a lacustrine sediment corespanning the Holocene for Western Victoria Island in the Canadian Arctic.

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Abstract

A chironomid-based reconstruction spanning the Holocene was constructed from a lacustrine sediment core taken from a shallow lake on Western Victoria Island in the Canadian Arctic Islands. The primary purpose of this study was to investigate long-term climatic variations and their impacts on aquatic ecosystem functioning in the Canadian Arctic. The sampled lake, informally called WB02 is located in the Middle Arctic and is in a prostrate shrub tundra vegetation zone (CAVM, 2003). The 183 cm core was collected from the ice surface of the lake in June 1997. The top 5cm were extruded in the field in 0.5 cm increments to preserve the integrity of the sediment at the water interface.The core was dated by 7 AMS radiocarbon dates. Lead-210 dating was used for the top 14 cm of the core (14 dates). The material sent for radiocarbon dating consisted mostly of terrestrial and aquatic plant material as well as chironomid head capsules. The core has a basal date of 8300 BP. The sediment accumulation in this lake followed a fairly constant rate. However, we were unable to obtain dates between 144 BP and 6090 BP (5946 years or 76 cm) due to the presence of only minute amounts of organic material in the sediment. So far, eight different depths within this interval have been picked for radiocarbon dating but have yielded insufficient amounts of organic material. A total of 10 more depths in this interval have been sampled and remain to be picked to obtain a few more dates in this gap. Sediment parameters such as loss-on ignition and biogenic silica were analyzed on this core. LOI wasmeasured continually throughout the core by 0.5 cm increments. In general, organic weight percent showed an inverse relationship to carbonate weight percent. Throughout the core, carbonate weight percent was relatively high (approx. 15%), and organic weight percent remained relatively low (approx. 5%), indicative of a low productivity system with high buffering capability. The early Holocene (8300-5250 BP) appears to have been the most productive period, as it is marked by high variability and high values of LOI. During the next 5000 years, the organic and carbonate weight percent values remained constant at 5% and 12%, respectively. The significant and rapid increase in organic weight percent over the last 100 years may be indicative of increases in productivity due to warmer climate. Biogenic silica was measured every 2 cm throughout the core, and every 0.5 cm for the top 5 cm of sediment.BSi values were very low throughout the core, ranging from 0-2.5 weight percent BSi. The highest values of BSi occurred in the top most sediment of the core. Preliminary analysis of the core indicated that no diatoms were preserved in the sediment. It is possible to hypothesize that the siliciclastic organisms in this core, such as diatoms, may have undergone dissolution or post-depositional diagenesis. Finally, chironomid assemblages were examined at 5 cm intervals downcore and every 0.5 cm for the top 5 cm. A total of 48 levels were analysed and a total of 33 different taxon were found in the sediment. The early Holocene was marked by the highest abundances of head capsules, with Heterotrissocladius and Paracladius being the dominant taxa. A shift in assemblages occurred during this early Holocene (7500-7000BP) where organisms from the subtribe Tanytarsina increased significantly in abundance. The later Holocene was marked by a decrease in chironomid abundance until recent times, when chironomid abundance increased significantly. During this period, Abiskomyia and taxa from the subfamily Diamesinae became increasingly abundant.

Bibliography of Canadian Geomorphology