Late Glacial and Holocene Paleoenvironmental Change Recorded in Lake Sediments, Brock Plateau (Melville Hills), Northwest Territories, Canada
Sediments from South Lake, Brock Plateau (Melville Hills), Northwest Territories, provide one of the longest postglacial records from the mainland western Canadian Arctic, outside of eastern Beringia. Sedimentation commenced at least 13,900 cal yr B.P., possibly as early as 15,000 cal yr B.P. in response to early deglaciation of the site. Pollen is present throughout the entire record, with an initial Artemisia-Salix assemblage indicative of very cold conditions, consistent with a locally severe Younger Dryas Stade (~12,700-11,500 cal yr B.P.). At ~11,000 cal yr B.P., abiotic proxies signal a transition to warmer conditions, which is confirmed by a pollen shift to an assemblage dominated by Betula and Cyperaceae. Although South Lake was biologically productive during the early Holocene (~11,000-7,000 cal yr B.P.), diatoms and other siliceous organisms are notably absent from the record, suggesting severe silica limitation. Rises in Alnus crispa and Picea mariana pollen at ~7500 cal yr B.P. suggest cooling and/or an increase in effective moisture. Following this vegetation change, wetter conditions likely led to the establishment of a pioneering diatom community at ~6500 cal yr B.P. as a result of increased hydrological inputs and thus, the silica supply. Decreased organic sedimentation after 2000 cal yr B.P. suggests cooler conditions. Additionally, changing niveo-eolian deposition of sand on lake ice varied with a ~3000-year periodicity through the entire record. The long South Lake pollen record lends support to the hypothesis that the Brock Plateau was one of the earliest deglaciated regions, if not an actual nunatak during the late Wisconsinan.
Late Quaternary paleoceanography and paleo-sea ice conditions in the Mackenzie Trough and Canyon, Beaufort Sea
The Mackenzie Trough provides a high resolution signal for paleoceanography as a result of high sedimentation rates at the mouth of the Mackenzie River. Three cores were collected along a transect covering a depth range of 58-671 m and the time period of the last 11,500 calBP. Prior to the last ~10,000 calBP the distal core is characterized by laminated sediment and a foraminiferal fauna of Arctic Bottom Water calcareous species and abundant planktic foraminifera suggesting little freshwater runoff and/or perennial sea ice cover. This occurs at a similar time as laminated sediments from the west of this site, which have been suggested to be part of the Lake Agassiz flood outburst/and or cold period. If this outburst occurred, the very positive oxygen isotope values from our core (PC3; >+3.0ppm) indicate that it did not flow through the Mackenzie Trough. After 9000calBP, the faunas in the three cores differ because of timing and different water depths. However it is possible to see a progression of cold saline water prior to 10,000calBP, with a freshening of surface water after 10kcalBP where tintinnids (brackish water ciliates) occur with incursions of deep water Arctic calcareous species to ~3000ybp. A sequence of mixed faunas appears sea ice returns, at least periodically, in the last 3000 calBP but (in core PC2 only) a return to more sea ice is recorded by both foraminifera and dinocysts in the last few hundred years.
Pre-Industrial Climate Change in the Mackenzie Trough Area, Beaufort Sea, Canada
Two short sediment cores were collected along a nearshore-offshore transect in the Mackenzie Trough (Canadian Beaufort Sea) in order to reconstruct the evolution of sea surface parameters (temperature, salinity, sea ice cover) during the industrial and pre-industrial periods, and assess the impact of anthropogenic activities. Dinoflagellate cysts (dinocysts) and freshwater palynomorphs were used as proxy indicators of sea surface conditions and freshwater input, respectively. The cores were dated using measurements and range from approximately 1550 A.D. to the modern period, providing a multi-annual to decadal-scale resolution for our reconstructions. Sedimentation rates are maximum at the offshore site (2.3 mm/yr) and lower at the nearshore site (0.7 mm/yr). This is due to the energy at the river mouth, which induces a bypass of the proximal trench and shelf areas, allowing sediment deposition further offshore. In both cores we observe the passage from a heterotrophic to an autotrophic regime over the last 450 years, which we associate with increased nutrient input and/or increased open water conditions throughout the study area. The Little Ice Age period (LIA; 1550 to 1850 A.D.) is marked by low, but increasing, dinocyst fluxes (enhanced productivity) and low sea surface temperature in surface waters. Freshwater palynomorhs reach their maximum abundance toward the end of the LIA, and start decreasing toward modern times. This is accompanied by a reduction of salinity suggesting decreased freshwater inflow from the Mackenzie River. The industrial period (1850 A.D. and onward) is marked by a decrease of dinocyst fluxes by a factor of 2.5. These data suggest that the observed decrease in surface productivity in the Beaufort Sea may be related to the apparent reduction in the rate of summer sea surface temperature increase that occurred at the onset of the industrial period (increase rate of 1.2°C/century during the LIA and increase rate of 0.33°C/century during the industrial period). However, there is no clear link between anthropogenic activities and the warming trend observed with the reconstructed sea surface parameters.
A multi-isotopic study (U-series, 14C, 13C, 18O) on growth of Arctic fissure calcretes (endostromatolites) from Northern Canada.
The chronology of Arctic terrestrial environments is poorly documented due to the scarcity of representative deposits allowing radiometric age determinations. Endostromatolites are secondary carbonate precipitations filling fractures in karstic carbonate terrains found in various sites in Yukon, North-West Territories. Expectations about their use to document environmental change were high. The specimens studied here originate from the Bear Cave Mountains (Yukon). They fill presently unsaturated fissures along cliffs, above karstic formations, within a few meters from the bedrock surface. They occur on southern oriented surfaces, and systematically develop on the outer side surface of the fractures. These concretions are likely of biogenic origin, and are thought to form during periods with relatively warm conditions in the Arctic (i.e., maximum insolation intervals). We report here on 238U, 234U, 230Th, 226Ra, 14C, 13C and 18O analyses of a few thick specimens (2 cm). A first sample yielded inconsistent 14C- and 230Th-ages (ranging from bottom to top of the concretion10-4 ka and 260-55 ka, respectively). Higher resolution measurements in a second sample better illustrate trends from the layer immediately attached to the host rock (here Paleozoic limestone) and the outer, columnar surface of the concretion: i) 230Th/238U activity ratio decrease from near secular equilibrium values to appr. 0.2; ii) 238U- concentrations increase from ~ 2 to ~ 6 ppm, and iii) 226Ra/230Th activity ratios increase from near secular equilibrium values to 1.2. This pattern suggest a pseudo-Rayleigh fractionation process with redistribution of U-series isotopes from the host-rock into the calcrete growth layers, more or less in function of their relative solubility. Some addition of more soluble elements relating to water fluxes cannot be ruled out. Stable carbon isotopes suggest a similar process with a progressive enrichment in 13C (up to +8.5 ‰ vs. VPDB) attributed to kinetic fractionation with freezing of water inducing outgazing of an isotopically light CO2 with precipitation of a 13C-enriched calcite. The trend for a progressive enrichment in 14C suggests partial exchanges with the atmospheric CO2 circulating in bedrock fissures. Thus, if 14C and U-series methods cannot be used to set the age of such calcrete, they provide information on their accretion process. In view of the excess 226Ra observed throughout most of the concretion, the overall age of the study specimen cannot exceed a few thousand years (i.e., mid- to late-Holocene), but one cannot estimate any precise duration for the growth phase within this interval. Both their stable isotope properties and complex age structure made them unsuitable for precise paleoclimate reconstructions but allow inferences about the presence of seasonal/occasional active layer above the permafrost during their growth stages.
Foraminiferal and Isotopic Evidence for Sea-Ice Extent and Paleoceanography of the Amundsen Gulf, Canadian Arctic
Paleoclimatic reconstructions based on the identities and isotopic signatures of marine microfossils assist in estimating natural variability of ocean-climate dynamics. Sea-ice extent and its response to warming affects those dynamics, yet remains poorly known in many Arctic regions, including the Western Canadian Arctic Archipelago. Reconstructions of Late Holocene sea ice cover were created from foraminiferal and stable isotopic proxies from sediment cores collected by the Canadian Arctic Shelf Exchange Study (CASES) and ArcticNet. The five boxcores examined were collected in 2004 along a transect through the Central Amundsen Gulf. The top 30 cm of sediment represents an average of 1200 years cal BP, assuming constant sedimentation rates. The benthic calcareous and agglutinated foraminiferal assemblages suggest a decline in perennial sea-ice cover over the last millennium, and that the Amundsen Gulf may have experienced seasonally open waters within the last 300 years cal BP. The oxygen and carbon stable isotopes of benthic and planktonic species both demonstrate the spatial and temporal extent of variations in sea-ice cover and suggest changes in benthic conditions. In addition to facilitating stratigraphic correlation among boxcores through the Amundsen Gulf, the stable isotopes of foraminifera will assist in correlating the paleoceanographic conditions with those from other areas of the Arctic.
Quaternary evolution of the central canadian Arctic
Pleistocene ice streams of Lancaster Strait and Boothia Bay are important because of their effects on global climate, through meltwater output and icebergs calving in the Baffin Bay. Important concepts of ice streams genesis were developed from numerous studies of glacial geology of this area. Study of relative chronology and intensity of multiples ice flow is possible though field observations (microforms), detailed bathymetric data, satellite images and aerial photographs. Glacial transport in till (clasts, heavy minerals and geochemistry) yields useful information concerning the importance of successive ice flows. As the area is located north of the Keewatin ice divide, an early, northward movement is interpreted to reflect unconstrained ice movement emanating from the ice divide. Later, the development of the Boothia/Lancaster ice streams pulled in the ice flow lines gradually a zone of convergence located in the ice stream main axis. The mineral prospectivity of these regions is estimated from glacio-sedimentary methods, notably diamonds and metals. Absolute geochronological methods (14C AMS, IRSL and cosmogenic elements) help better constrain the temporal evolution of glacial and post-glacial events, and their correlation with global climatostratigraphy. Local reservoir effect is calculated using 14C AMS dating on coupled marine/terrestrial materials. Deglacial and last interglacial climate and chronology will be better understood from 14C AMS and IRSL dating of Paisley River section sediments.