ABSTRACT: Using the ITRAXTM Core Scanner for high resolution grain size and varve thickness estimates in case of lake sediment cores from the Canadian High Arctic.

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Abstract

Recent environmental changes in the Arctic highlight the need for a better understanding of long-term climatic variability of this sensitive environment. At present, there are few examples of long paleolimnological records from the High Arctic due the logistical difficulties surrounding collection of this kind of records. Furthermore, it is difficult to accurately date lacustrine paleoclimate records due to the lack of terrestrial carbon and the remobilization of old carbon stored in the watersheds limiting the use of 14C dating (Wetzel, 2001). Two similar adjacent lakes were studied at Cape Bounty, Melville Island, Nunavut, Canada. The sedimentary records of West Lake (74°53’N, 109°35’W) and East Lake (74°53’N, 109°31’W) suggest the presence of varves, composed of clay layers produced under the ice-covered lake during winter and silt layers produced by snow melt during the spring (De Geer, 1912). Sand layers are also frequently observed and represent summer rain events. Depositional processes, water depth, morphology and catchment area are similar for thesetwo lakes. The sedimentary profiles obtained at Cape Bounty potentially represent a 4 000 year records of terrestrial sediment flux in the Western part of the Canadian High Arctic. Here, we tested the possibility of using the ITRAXTM Core Scanner analyses to characterize deposits from the different seasons and hence to count varves and measure their thickness. The ITRAXTM Core Scanner provides digital X-radiography (RX) and X-ray fluorescence (XRF) measurements from sediment cores at high resolution (equal or greater than 100 µm). High-resolution ITRAX data were compared with direct methods to validate the use of the ITRAXTM Core Scanner for paleoclimate reconstructions. The validation methods used in this study were image analysis of thin-sections, EDS (Energy Dispersive Spectroscopy) analysis on SEM images, grain-size on discrete samples by Coulter LS laser particlesize analyzer, 137Cs and 210Pb dating results. At centimetric scale, particle size analysis on discrete samples shows a link between grain-size and relative abundance of chemical elements in Cape Bounty sediment (fig.2). Grain-size is positively correlated with increasing Zirconium, and negatively with Potassium and Iron. The same elements chemical profiles observed ondigital images of thin-sections (0.02 mm resolution) confirm a correlation with grain-size at the varve scale. Finally, EDS analyses also confirms the presence of these relative chemical element abundances in different seasonal deposits:– clay layer: increase in K and weight% Fe; – silt layer: increase in weight% Ti;– sand layer: increase in weight% Zr. We notice the independent distribution of the Silicon regarding the grain-size, probably caused by complex mixed origin of primary production and detritic deposition. We concluded for this site of study: an increase in Zirconium abundance indicates rain events, an increase in Potassium and Iron abundance indicates winter period, and an increase in Titanium abundance indicates spring deposition. The varve-counting with XRF-data is compared with the classical method using thin-sections. Variations inelement abundance with depth are accordant with varve limits. A statistical method is in progress to count varves automatically from XRF-profiles. Varve thickness estimation will be also tested, and will provide information on seasonal hydroclimatic conditions. RX and XRF data are used to estimate grain-size variability related to winter or summer deposition. A new and rapid method to count varves was tested using the XRF-profiles, and has provided a means of estimating varve thickness, which will be employed in future work. We are attempting to verify seasonal sediment deposition cycles linked with the Arctic Oscillation, which influences snow and summer precipitations at this site. The comparison of paleoclimatic reconstructions from West and East Lakes will provide information regarding local and regional climatic controls. The proxies used in this study are chemical elements associated with seasonal deposits, which provide a temporal record of the intensity of winter, spring and rain events. Regional climate influences would be apparent if two lakes exhibit the same paleoclimatic records. In this case, we compare our results with others records in High Arctic. The preliminary results suggest an increase of high frequency and magnitude of rain events for the two last centuries at Cape Bounty, shown by cyclic increases in relative Zirconium abundance from East Lake’s XRF results.

Bibliography of Canadian Geomorphology