RECONSTRUCTION OF HOLOCENE ENVIRONMENTAL CHANGES IN NORTHERN BRITISH COLUMBIA USING FOSSIL MIDGES by Erin Mattea Fleming B.Sc., Okanagan University College, 2005 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in The College of Graduate Studies (Environmental Science) UNIVERSITY OF BRITISH COLUMBIA (Okanagan) August 2008 ©Erin Mattea Fleming, 2008 ABSTRACT Lake sediments contain the remains of midge communities that may be used as biological proxies for inferring past environmental changes. Freshwater midges, including Chironomidae and Chaoboridae, from two alpine tarns (Pyramid Lake and Bullwinkle Lake) in the Cassiar Mountains of northern British Columbia were used to estimate Holocene palaeotemperature changes, and more specifically, to test for the presence of the Milankovitch thermal maximum, an early Holocene warm interval coinciding with peak Holocene summer solar insolation. Mean July air temperatures were reconstructed using midge-inference models developed via weighted averaging- partial least squares (WA-PLS) regression. Cold-tolerant midge taxa dominate the stratigraphies from both Pyramid and Bullwinkle Lakes; however, warm-adapted species are more common in Bullwinkle Lake. Early Holocene warming is apparent at both lakes, however it is unclear whether this is indicative of the Milankovitch thermal maximum. A decrease in temperature occurs from 8,700-7,900 cal. yr BP at Pyramid Lake, around the same time that the 8,200 cal. yr BP cooling event occurred in the northern hemisphere. During the middle Holocene, records from Pyramid Lake indicate an overall decrease in temperature, with a short period of warmer temperatures that peak at 5,100 cal. yr BP. Temperatures fluctuate little during this time at Bullwinkle Lake. A short warming phase is apparent at both lakes during the late Holocene. July temperatures are highest at 2,000 cal. yr BP (10.5°C) in Pyramid Lake and at 1,200 cal. yr BP (13°C) in Bullwinkle Lake. Thereafter, temperatures return to what they were before the warming occurred, and at Bullwinkle Lake, vary little throughout the remainder of the Holocene. ii TABLE OF CONTENTS ABSTRACT.......................................................................................................... ii TABLE OF CONTENTS...................................................................................... iii LIST OF TABLES................................................................................................ v LIST OF FIGURES............................................................................................... vi ACKNOWLEDGEMENTS.................................................................................. ix DEDICATION...................................................................................................... x Chapter I INTRODUCTION.............................................................................. 1 Chapter II LITERATURE REVIEW.................................................................. 3 2.1 Chironomids as Indicators of Climate Change................................... 3 2.2 The Cordilleran Ice Sheet in Northern British Columbia................... 5 2.3 Holocene Climatic Events.................................................................. 9 2.3.1 Milankovitch Thermal Maximum....................................... 9 2.3.2 Preboreal Oscillation........................................................... 12 2.3.3 8,200 cal. yr BP Cooling Event........................................... 14 2.3.4 Little Ice Age....................................................................... 15 2.4 Palaeoclimate of Northern British Columbia..................................... 17 2.4.1 The Late Glacial and Early Holocene................................ 21 2.4.2 The Middle Holocene.......................................................... 22 2.4.3 The Late Holocene.............................................................. 22 Chapter III STUDY AREAS.............................................................................. 24 3.1 Pyramid Lake...................................................................................... 24 3.2 Bullwinkle Lake................................................................................. 27 Chapter IV METHODS...................................................................................... 29 4.1 Core Collection and Lithology........................................................... 29 4.2 Chronology......................................................................................... 30 4.3 Midge Analysis................................................................................... 30 4.4 Data Analysis...................................................................................... 31 Chapter V RESULTS........................................................................................ 33 5.1 Pyramid Lake...................................................................................... 33 5.1.1 Sediment Lithology............................................................. 33 iii 5.1.2 Chronology.......................................................................... 35 5.1.3 Midge Stratigraphy.............................................................. 37 5.1.3.1 Zone PYR-1.......................................................... 41 5.1.3.2 Zone PYR-2.......................................................... 41 5.1.3.3 Zone PYR-3a........................................................ 42 5.1.3.4 Zone PYR-3b........................................................ 42 5.1.4 Temperature Reconstructions.............................................. 43 5.1.4.1 Zone PYR-1.......................................................... 45 5.1.4.2 Zone PYR-2.......................................................... 45 5.1.4.3 Zone PYR-3a........................................................ 45 5.1.4.4 Zone PYR-3b........................................................ 45 5.2 Bullwinkle Lake................................................................................. 46 5.2.1 Sediment Lithology............................................................. 46 5.2.2 Chronology.......................................................................... 50 5.2.3 Midge Stratigraphy.............................................................. 53 5.2.3.1 Zone BULL-1....................................................... 53 5.2.3.2 Zone BULL-2....................................................... 56 5.2.4 Temperature Reconstructions.............................................. 57 5.2.4.1 Zone BULL-1....................................................... 57 5.2.4.2 Zone BULL-2....................................................... 57 Chapter VI DISCUSSION.................................................................................. 59 6.1 Pyramid Lake..................................................................................... 59 6.1.1 Lithology............................................................................. 59 6.1.2 Chronology.......................................................................... 60 6.1.3 Climate................................................................................ 60 6.1.3.1 The Early Holocene.............................................. 60 6.1.3.2 The Middle Holocene........................................... 62 6.1.3.3 The Late Holocene............................................... 62 6.2 Bullwinkle Lake................................................................................. 63 6.2.1 Lithology............................................................................. 63 6.2.2 Chronology.......................................................................... 64 6.2.3 Climate................................................................................ 66 6.2.3.1 The Early-Middle Holocene................................. 67 6.2.3.2 The Middle-Late Holocene................................... 68 6.3 Regional Synthesis............................................................................. 69 6.3.1 The Early Holocene............................................................. 70 6.3.2 The Middle Holocene.......................................................... 74 6.3.3 The Late Holocene.............................................................. 76 Chapter VII CONCLUSIONS............................................................................ 77 REFERENCES….................................................................................................. 80 iv LIST OF TABLES Table 2.1 A comparison of palaeoclimatic reconstructions performed in northern British Columbia and surrounding areas. The study sites are organized on a west to east gradient, with Windmill Lake, Alaska located the farthest west and BC2 Lake, British Columbia located the farthest east................................................. 19 Table 5.1 AMS radiocarbon dates for Pyramid Lake (Mazzucchi et al., 2003).............................................................................................. 36 Table 5.2 Zonation of the Pyramid Lake chironomid stratigraphy............... 40 Table 5.3 AMS radiocarbon dates for Bullwinkle Lake................................ 51 Table 5.4 Zonation of the Bullwinkle Lake chironomid stratigraphy........... 55 v LIST OF FIGURES Figure 2.1 Map showing palaeoenvironmental study sites in northern British Columbia and surrounding areas. Windmill, Birch and Jan Lake were