ABSTRACT: Using carbon and nitrogen isotopic ratios as paleoclimate proxies in a Baffin Island lake.

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Abstract

Carbon and nitrogen stable isotopic ratios of lake sediment organic matter can provide valuable information about past productivity and nutrient cycling of lake basins (Meyers and Teranes 2001). In Arctic lakes, which are highly sensitive to changes in climate, the isotopic composition of bulk sediment can thus be used as a climate proxy. In this study, a sediment core from a shallow (3.5m), oligotrophic lake located on the highlands of the Qivitu Peninsula (67° 57’ 33.20” N, 65° 02’ 35.53” W) of eastern central Baffin Island has been analyzed. There is interesting structure in the isotopes of both elements during the time period suspected to be the Little Ice Age, which is associated with a strong negative excursion in percent organic matter in the sediment. An age model however, is yet to be developed for this core, so any statements about the age of sediment are purely speculative. A rapid d15N increase of two per mil during this time period represents a large shift in the nitrogen cycle of the lake. The relatively light d15N seen throughout the majority of the core could be due to nitrogen fixation or, as suggested by Briner et al. in 2006 in a study of a similar Baffin Island lake, phosphorus limitation. In a phosphorus limited environment, a relatively large DIN pool could exist so that primary producers are able to easily fractionate against 15N. Because of the number of factors influencing the d15N of the sediment, a multi proxy approach is necessary to determine what is driving the signal (Talbot 2001). d13C does not track percent organic matter during this period suggesting that something in addition to primary productivity of the lake is being recorded. Additional proxies, such as the concentration of fossil algal pigment, will be measured to help characterize lake primary productivity throughout the period, as well as help deconvolve the d15N signal. It is possible that this enrichment of d13C contemporaneous with low percent organic matter could be due to perennial ice cover preventing the surface waters from equilibrating with the atmosphere (making it easier for a small amount of primary productivity to cause enrichment of the lake surface water). Bulk sediment inorganic geochemical analysis will be used to determine if the bottom waters were anoxic during this period, as would be expected during perennial ice cover. Increased denitrification of the sediment due to anoxia could be an additional explanation for the increase in d15N seen. Samples will be collected in the summer of 2010 with the goal of characterizing the current biogeochemistry of the lake basin, giving context to what is seen in the sediment core.

Bibliography of Canadian Geomorphology