1
Multi-Proxy Reconstructions of Environmental Change for a Small Glacial Lake in Northern Manitoba, Canada Pooja Shakya 1 , Erin York 2 , Mark Kruger 3 Faculty Advisors: Christoph Geiss 1 , Phil Camill 2 , Charles Umbanhowar 3 , Mark Edlund 4 1 Trinity College, Hartford CT, 2 Bowdoin College, Brunswick, ME, 3 St.Olaf College, Northfield, MN, Minnesota 4 Science Museum, St.Paul, MN Abstract: We analyzed magnetic properties of lake sediments from Lake 396 in Northern Manitoba, Canada in an attempt to reconstruct paleoclimate patterns. The analysis has been conducted employing magnetic susceptibility, anhysteric remanent magnetization (ARM), isothermal remanent magnetization (IRM), macroscopic charcoal analysis and loss on ignition (LOI). Magnetic susceptibility gives a rapid estimate of the abundance of magnetic minerals while ARM reflects the presence of small (0.1 – 1 μm) single domain grains. IRM reflects the presence of all remanence- carrying grains. LOI measure the organic and inorganic sediment fractions. These measurements have been applied to the samples from Lake 396. The values for ARM normalized by IRM, indicate the presence of biogenic magnetite which suggests a possible occurrence of erosion in the site. The peak in the charcoal area from around 40-60 cm corresponds to a greater percentage of inorganic matter in the LOI data. This observation could be explained by an increase of inorganic runoff into the lake after a period of fires. Introduction: In July 2008 together with students from St. Olaf and Bowdoin College, we took part in an NSF-funded, three week expedition to Northern Manitoba to study regional landscape change over the past 8000 years. Our study includes the analysis of lake sediments, peat lands and soils from within the watershed of several small glacial lakes (See Fig. 1). For this project, we report preliminary results from a short sediment core from Lake 396 and interpret magnetic and charcoal analyses as well as sediment composition data in terms of past environmental change. Methods: We cored Lake 396 (informal name) using a modified Livingston piston corer (Wright, 1967), (Fig.2) Sediment composition was analyzed through Loss on Ignition (LOI) analysis (Dean, 1974) To estimate the severity and frequency of past forest fires we counted macroscopic charcoal throughout the core (Camill et al., 2003). Soil erosion from within the watershed and biological activity from within the lake were estimated through magnetic measurements. •Magnetic susceptibility (χ) estimates the total abundance of all magnetic minerals. •Anhysteric Remanent Magnetization (ARM) and Isothermal Remanent Magnetization (IRM) are a proxy for the abundance of small to large, remanence carrying magnetic minerals. •The remanence ratio (ARM/IRM) is used to estimate the relative abundance of small References: Camill, P., C.E. Umbanhowar, R. Teed, C.E. Geiss, L. Dvorak, J. Kenning, J. Limmer, K. Walkup, and J. Aldinger, Late-glacial and Holocene climatic effects on fire and vegetation dynamics at the prairie-forest ecotone in south-central Minnesota, Journal of Ecology, 91, 822-836, 2003. Dean, W.E., Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods, Journal of Sedimentary Petrology, 44, 242- 248, 1974. Wright, H.E., Jr., A square-rod piston sampler for lake sediments, Journal of Sedimentary Petrology, 37, 975-976, 1967. 80 70 60 50 40 30 20 10 0 C o re D ep th (cm ) 0.1 0.05 A R M / IR M -0.5 -0.2 5 0 0.2 5 0.5 / IR M (m/A) 0 0 .0 0 02 IR M (Am 2 /kg ) 0 8E -6 1.6E -5 A R M (Am 2 /kg ) 0 4E -8 8E -8 (m 3 /kg ) 50 60 70 80 90 100 % in o rg a n ic m a terial 0 2 4 6 8 to ta l ch a rco a la re a (mm 2 ) L a k e 3 9 6 stu d e n t co re - m a g n e tic p rop e rties a) b) c) d) e) f) g) Results: The lake cores have 3 distinct sections: • The very bottom (80-75 cm): displays high values of concentration dependent magnetic parameters (χ, ARM, IRM). LOI analyses show that it consists almost entirely of inorganic sediment. Low ARM/IRM ratios suggest a high concentration of coarse grained (> 1 μm) magnetic particles. This material is likely slope wash or till. • The bottom (75-45 cm or 40 cm): Lower values of χ, ARM and IRM suggest a decrease in the concentration of magnetic minerals because either inorganic magnetic mineral supply decreases or sediments become anoxic and the iron oxide minerals dissolve. Low values of ARM/IRM still suggest a relatively coarse-grained magnetic component. • The top (< 45 cm): Very low values of χ, ARM and IRM suggest very low concentrations of magnetic minerals. ARM/IRM increase to 10 – 15%, which is due to the presence of fine Single Domain magnetic particles. These particles may be of biologic origin (formed by magnetotactic bacteria). • Loss-on-Ignition analyses (Fig. 7f) show varying amounts of inorganic matter throughout the core, but the variations in LOI are much smaller than variations in concentration-dependent magnetic parameters. This is likely due to the dissolution of iron-oxide minerals under reducing sediment conditions, which develop as organic matter accumulates in the sediment. However, it is possible that the inorganic component shown in Fig. 7f is mostly composed of biogenic silica and not related to inorganic material derived through slopewash or soil erosion. • Charcoal analyses show a peak in macroscopic charcoal between 40- 60 cm, which corresponds to a greater percentage of inorganic matter in the LOI data. This observation could be explained by an increase in soil erosion into the lake after a period of fires. Lake 396 Fig 1: Map showing the location of Lake 396 and other small glacial lakes analyzed in Northern Manitoba, Canada Fig 2: Sectioning the sediments from Lake 396 using a modified Livingston piston corer Fig 3: Extracting the long core for lake sediments from Lake 396 Fig 4: Aerial photo of Lake 396 and surrounding small glacial lakes Fig 5: Collecting water analysis data on Lake 396 Fig 6: Aerial photo depicting areas of the landscape effected by fire Fig 7: Aerial photo of Lake 396 and surrounding small glacial lakes
