New: ARCSS 8k project
Laminated sediments in Cape Hurd Lake, a coastal isolation basin on southwestern Devon Island, Nunavut, Canada
Michael Retelle, Beverly Johnson
Cape Hurd Lake (informal name), is an isolation basin situated at sea level on the south coast of Devon Island along Lancaster Sound (74°32’58”N; 89°37’29”W). The basin is separated from the sea by a narrow bedrock sill that is covered with raised shingle beach deposits (figure 1). The floor of the basin is relatively flat with a maximum depth of 55 m. Saline (34‰) and oxygenated surface water overlies anoxic hypersaline bottom water (40‰) below a well-defined chemocline at 30 m depth. Anoxia in the deep central basin favors the preservation of finely laminated sediments. The basin has two major fluvial inlets draining 39 km2. The north inlet is fed by glacial meltwater from a plateau ice cap in the northern reaches of the watershed and an eastern nival sub-catchment. The western inlet is fed primarily from a nival catchment. In the south the basin is connected to Lancaster Sound by a tidal inlet that opens when a sea-ice ridge on its seaward end melts in the summer.
Varves, biogenic silica (BSi), total organic content (%TOC), C/N, and the δ13C and δ15N composition of decalcified sediment. Laminated sediments include fine mm to sub-mm annual clastic couplets and thicker subannual layers interpreted as discharge events. Biogenic silica and δ13C are used as proxies for primary production. The %TOC represents the amount of organic matter produced and preserved in the sediments. Both BSi and %TOC are influenced by the amount of minerogenic material deposited. The δ15N reflects the presence of marine verses terrestrial primary producers and the C/N values reflects the terrestrial verses aquatic inputs. However, caution should be used when interpreting C/N due to the preferential degradation of N relative to C in most settings. Both BSi and δ13C increase with temperature and nutrient availability.
Laminated sediments in Cape Hurd Lake, comprised primarily of sub-millimeter couplets extend back to ca. 8000 BP; however, the couplets are occasionally interrupted by massive zones (Saenger, 2002). The uppermost continuous varve sequence extends from present day back to ca. 400 yr BP (Putnam, 2004). Two periods of higher sedimentation, i.e. thicker couplets, (1640 to 1780 and 1850 to 2000 AD) bracket a period with the lowest rates between 1790 and 1850 AD. In addition, thick subannual layers within the varves, interpreted as intense spring and summer rain events ( Lamoureux 2000), increase in frequency and thickness in the late 19th and 20th centuries.
The δ13C and biogenic silica records from the short core indicate that Cape Hurd Lake has experienced the highest rates of primary productivity and warmest temperatures in the most recent part of the record (i.e., upper 15 cm of the core). Colder temperatures with low rates of primary productivity occurred prior to this period (i.e., between 15 and 25 cm). Between 25 and 40 cm, the biogenic silica and δ13C records are decoupled, with low BSi and increasing δ13C values suggesting that primary productivity is low and limited while temperatures are rising. Alternately, the carbon isotope composition of the organic matter may be driven by in-lake cycling of isotopically variable carbon sources.
Hafsten, U., 1983, Biostratigraphical evidence for late Weichselian and Holocene sea-level changes in southern Norway, Chapter 7 in D.E. Smith and A.G. Dawson, eds., Shorelines and Isostasy, Institute of British Geographers Special Publication number 16, p. 161-181.
Lamoureux, S., 2000, Five centuries of interannual sediment yield and rainfall-induced erosion in the Canadian High Arctic recorded in lacustrine varves, Water Resources Research, vol. 36, no.1, p. 309-318.
Putnam, A., 2004, Recent sedimentation in a transect of high arctic isolation basins, Southern Queen Elizabeth Archipelago, Nunavut, Canada, unpublished B.S. Thesis, Bates College.
Saenger, C.P., 2002, Holocene basin evolution and paleoclimate reconstruction from laminated