ABSTRACT: Identifying coherent links between subannual sedimentary structures and daily weather observations.

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Abstract

Surface sediment cores obtained from Lake R (unofficial name), an ice-dammed lake in the central part of the Colin Archer Peninsula, northwest Devon Island, are shown to be varved using 137Cs profiles. These varves, however, correlate poorly with annual melting degree-day (MDD) records from Resolute, Nunavut. Examination of the internal structures of the varves shows a variety of subannual sediment deposition units, and these are well correlated with daily MDD records (R2 = 0.747, p < 0.001 , n = 26). Unusually coarse units, and units which interrupt clay cap deposition, coincide with large, late season rainfall events. The varve record covers the time period from 1959 to 2003, with several years represented by thin, simple varve couplets. For the majority of years, however, the varves are composed of a complex arrangement of coarser and finer structures, indicative of differing modes and intensities of sediment delivery to the lake. The thickness of these units is measured using high-resolution scans of thin sections, and range from 0.24 to 3.27 mm. By identifying and cataloguing the physical characteristics of the subannual units (texture, colour, grain size), broad interpretations of the delivery mechanism are made and compared to known external forces (temperature and precipitation)In order to develop the links between the MDD and varve records, a binary-type approach was used to divide the temperature record into blocks of time corresponding to either positive or negative values. Blocks of positive temperature, defined by their duration (days) and intensity (average temperature), represent periods of potential snow and ice melt, which are seen to be important sources of hydrologic energy in Arctic catchments. Such positive blocks, here called ‘melt events’, drive sediment delivery and ultimately deposition at the study site. Blocks with a total MDD (duration x average temperature) below 1, and negative blocks, represent lulls in the hydrologic system, when sediment transport is reduced and deposition is restricted tosuspension settling of the finest particle fraction, thus forming the end of a depositional period. Observations from 9 of the thickest varves provide the following rules for the attribution of a given subannual unit to one or more melt events: 1) melt events less than 1 MDD do not create subannual units; 2) melt events separated by 4 or more days form separate subannual units; 3) melt events separated by less than 4 days are: a. grouped together if the first event is larger than subsequent events, or b. separated if an event is larger thanthe preceeding event; 4) unusually coarse units are derived from large (> 7.5 mm d-1) rainfall events.

Bibliography of Canadian Geomorphology