SEDIMENT MAGNETIC ANALYSES REVEAL PALEOCLIMATIC HISTORY OF HANGING LAKE, YUKON

Two sediment cores from near the deepest part of Hanging Lake, a small lake in unglaciated Northern Yukon Territory (lat. 68°23'N, long. 138°23'W), record climatic variations for the Holocene and late Pleistocene. Holocene sediment consists of organic rich massive to poorly laminated silt, which is underlaid by sand and gravel of Pleistocene age. Sediment magnetic analyses reveal pseudo-single-domain magnetite as the primary magnetic component during the middle and late Holocene. The magnetic properties of older samples, however, are strongly influenced by greigite an metastable iron-sulfide. Sediment containing greigite is characterized by low values of ARM/IRM, SIRM/c ratios above 40 kA/m and hysteresis parameters characteristic of single-domain grains. The magnetic data are confirmed by XRD analyses performed on magnetic extracts. Smearslide analyses show that early Holocene and Pleistocene sediment also contains framboidal pyrite, indicating high S:Fe ratios during the early history of the lake. We interpret the changes in S:Fe ratios as expressed in the magnetic and smearslide analyses as changes in residence time and probably ice cover with higher S:Fe ratios corresponding to colder climates. During the mid and late Holocene mixing of the water column during extended ice free periods leads to a well oxygenated hypolimnion and the deposition of magnetite rather than iron-sulfide minerals.