To better understand past environments of south-central Alaska, as well as broader atmospheric and oceanic systems that influence the regional climate, members of the “South Alaska Lakes” project from Northern Arizona University and partnering institutions are analyzing lake sediments from both large, glacier-fed lakes and small, non-glacier-fed lakes in the Kenai Peninsula. By using multiple climate proxies in these different types of lakes, we will contribute to a more comprehensive understanding of important climate and environmental phenomena in south-central Alaska and the broader north Pacific region.
If you are new to paleoclimate studies, or to studies about Alaska’s climate, we suggest visiting the “Background for Beginners” section as you explore the site.
A MULTICORE, 2300-YEAR VARVE CHRONOLOGY FROM EKLUTNA LAKE
David Fortin
In collaboration with researchers from the University of Gent in Belgium, we are developing chronologies on five long cores taken along a transect at Eklutna Lake, a varved glacial lake located approximately 45 km northeast of the city of Anchorage. We use different approaches to quantify the uncertainty in our varve chronologies, including repeated counts by independent observers, and a Bayesian model that takes into account priors such as varve quality and marker beds that are traced across the entire basin. Our chronologies, which are validated using radiocarbon, will serve as a chronological framework for the late Holocene reconstruction of glacial activity in the Eklutna Lake watershed as well as for an analysis of the seismites and other turbidites found in the Eklutna Lake cores. As part of the South Alaska Lakes project, a similarly detailed varve chronology will be developed for nearby Skilak Lake, another deep glacial lake.
Typical examples of varve and turbidites facies at Eklutna Lake. A and B: 5 cm long linescan images on the left panels and Ct-Scans topograms on the right of typical varve sequences. The coarse grain unit appears dark on the digital photograph and pale on the topograms and inversely for the clay cap. The vertical white line represent the axis along which the varve thickness is measured and white horizontal lines represent varve boundaries as defined for the varve count. A: Simple varve unit composed of two couplets. B: Varve composed of more than one sub-annual unit of coarse sediments.
DIATOM OXYGEN ISOTOPES RECORDS FROM LAKES IN THE KENAI LOWLANDS
Ellie Broadman
While annually-resolved sediments in deep glacial-fed lakes like Eklutna and Skilak provide detailed accounts of climate conditions as reflected by glacier fluctuations, it is also important to examine indicators that yield information about the source and amount of precipitation over time, as changes in these moisture conditions can have pronounced effects on terrestrial climate. To this end, it is useful to analyze changes in the isotopic composition of oxygen in lake water over time, as the relative abundance of heavy (18O) versus light (16O) oxygen isotopes is sensitive to changes in both precipitation/ evaporation balance (P-E) and changes in the source and trajectory of storm tracks and precipitation. Changes in lake water oxygen isotopes are recorded in the isotopic composition of the diatoms (single-cellular, green algae) living in a given lake– specifically, in the diatoms’ skeletons, or “frustules,” which are hard, silicious, and remain preserved in lake sediments as they rapidly bloom and die. Therefore, it is possible to extract and purify diatom frustules throughout a lake sediment sequence, and to infer the changes in the lake water’s oxygen isotopes that diatoms have recorded over time.
Diatom frustules from Hidden Lake, Kenai National Wildlife Refuge, purified and shown under a light microscope
By analyzing diatom oxygen isotopes in several lakes in the Kenai lowlands, where the biogenic silica (diatom) content in lake sediments is high enough to complete these analyses, the glacier fluctuations inferred from Eklutna and Skilak lakes will be placed in a broader paleohydrological context. Both hydrologically closed (sensitive to P-E) and hydrologically open (sensitive to changes in the source and trajectory of precipitation) lakes will be analyzed in order to get a improve our understanding of multiple aspects of the region’s climate history. These records will inform our understanding of how important atmospheric and oceanic phenomena, such as the Aleutian Low atmospheric pressure cell and the Pacific Decadal Oscillation, have behaved over the course of the Holocene.
Sunken Island Lake, Kenai Peninsula lowlands; a target for diatom oxygen isotope analysis
A RECORD OF GLACIAL ACTIVITY AND SEDIMENT FLUX FROM POTHOLE LAKE
Annie Wong
Pothole Lake formed when the outwash plain of Skilak River, an outlet of Skilak Glacier, rose and began to flow into the valley now occupied by Pothole Lake, where it began to deposit varved glacial sediment. Because Pothole Lake was formed by a dam deposited by Skilak River’s glacial outwash plain, it is in the perfect position to help us answer the following question: do glaciers produce and deposit the most sediment during advance or retreat? The timing and progression of dam building by the Skilak River’s glacial outwash plain is captured by the onset and subsequent termination of sediment deposition into Pothole Lake. Therefore, studying Pothole’s sediment sequence will improve our understanding of the timing and dynamics of sediment deposition associated with glacial activity.
Pothole Lake, looking south at Skilak Glacier in the distance
PALEOCLIMATE INFERRED FROM VARVE THICKNESS AT EKLUTNA AND SKILAK LAKES
Joseph Thomas
Eklutna and Skilak lakes provide valuable records of climatic and glaciological shifts over the past 2,000 years. Both lakes contain varved or annually deposited sediments, a rare type of paleoclimatic proxy. Varve thickness at these lakes is influenced by glacial activity, and fluctuations in varve thickness can therefore be used to interpret how the sizes of glaciers feeding the lakes have varied in the past. Both Eklutna and Skilak contain over 2,000 years of varved sediments, and are the longest known varve records in Alaska.
Varve thickness at Eklutna Lake, with estimated error shown by gray shading. Y-axis shows varve thickness, and x-axis shows time (Year AD).
This project will examine changes in varve thickness over the past 2,000 years in both lakes. Changes in inferred glacial activity as reflected by varve thickness will be compared to climate variables of interest, including regional precipitation and air temperature, which may in turn be related to atmospheric circulation patterns and known multi-decadal climatic trends, such as modern warming.
CLIMATE AND ENVIRONMENTAL CHANGE RECONSTRUCTED FROM MARL DEPOSITS AT KELLY LAKE
Emmy Wrobleski
Kelly Lake, a kettle lake in the Kenai lowlands, contains a rich record of freshwater carbonate deposits, known as a “marl,” from ~14,500 to ~7,500 years ago. We have measured changes in stable isotopes of carbon (changes in the relative amount of 13C versus 12C) and oxygen (changes in the relative amount of 18O versus 16O), which record environmental changes both at the lake and in the broader geographic region.
The interpretation of the oxygen isotope data depends in part on the prevailing hydrologic conditions at Kelly Lake when the marl was deposited. There is geomorphic evidence of an input stream into the Kelly Lake basin, but its historical activity is unknown. Using carbon/nitrogen ratios (C:N), as well as carbon and nitrogen isotopes (15N versus 14N) from organic matter, we are attempting to reconstruct changes in the dominant source (terrestrial versus aquatic) of organic input to the lake. This information will help us understand stream activity at Kelly Lake, which will in turn help us interpret the oxygen isotope record.
Part of the layered marl deposit in the Kelly Lake sediment core.
People
PRINCIPAL INVESTIGATOR
GRADUATE STUDENTS
Darrell Kaufman Northern Arizona University
Darrell is a Regents Professor in the School of Earth and Sustainability at NAU and is the PI of the project. He has been studying the Quaternary geology of Alaska for more than 30 years, and has been coring lakes there for more than two decades. He has a special interest in geochronology and in facilitating large collaborative data synthesis projects.
Ellie is a PhD student in School of Earth and Sustainability at NAU. Her doctoral research involves analyzing changes in oxygen isotopes in diatoms found in lake sediments to reconstruct Holocene changes in hydrological conditions in both the Kenai lowlands and the northeastern Brooks Range. Prior to beginning her graduate work at NAU, Ellie worked at the US Geological Survey in Menlo Park, CA after receiving her B.A. in Geography from UC Berkeley.
Joseph Thomas Northern Arizona University
Joseph is a PhD student in the Earth Science and Environmental Sustainability program. His research focus is the Holocene paleoclimate of Alaska. He is analyzing varve thickness in glacial lakes sediments from southern Alaska, and studying long-term permafrost carbon dynamics in interior Alaska. Joseph holds a B.S. in Geology from Western Colorado University, and a M.S. in Geology from Idaho State University, where he studied northern Rocky Mountain hydroclimate using stable isotopes in lake sediments.
Emmy Wrobleski Northern Arizona University
Emmy is a M.S. student in Environmental Science and Policy, PaleoEnvironments emphasis. She is reconstructing lake level and environmental changes in the Kenai lowlands, Alaska by analyzing stable isotopes, macrofossils, and organic matter content in marl deposits. She received her B.A. in Geology from Mount Holyoke College in 2019.
Annie Wong Northern Arizona University
Annie completed her M.S. at NAU in Environmental Science & Policy in May 2019. Her M.S. research focused on determining the timing of glacial sediment deposition across Skilak River’s glacial outwash plain on the Kenai Peninsula in southern Alaska. Prior to her graduate work at NAU, Annie received her B.A. in Biology and Geology from Mount Holyoke College.
COLLABORATORS
Abby Boak Mount Holyoke College
Abby is a student at Mount Holyoke College and will be graduating in 2020 with a B.A. in Geology. She completed an undergraduate research project on marl deposits from a lake in the Kenai lowlands. Her main interests include volcanology and Arctic geomorphology, but she is passionate about all areas of geology.
David Fortin File Hills Qu’Appelle Tribal Council
After obtaining his PhD in Physical Geography at Queen’s University (Kingston, Ontario), David worked at the Institut National de la Recherche Scientifique in Québec City and then as a Postdoc and Research Associate at the School of Earth and Sustainability at Northern Arizona University. From 2017-2020, David worked at Wicehtowak Limnos Consulting Serves Ltd., a Regina-based environmental consulting firm owned by George Gordon First Nation.
Britta Jensen University of Alberta
Britta is an Assistant Professor in the Department of Earth and Atmospheric Sciences at the University of Alberta in Edmonton. Her research focus has been in Quaternary geology, with a particular interest in the loess deposits and tephrochronology of Alaska and the Yukon. More recent research directions include using lake and marine records to improve late Pleistocene and Holocene eruption records in Alaska to aid distal tephra correlations and test ideas about potential links between climatic events and eruption frequency.
Ed Berg US Fish and Wildlife Service
Ed Berg, PhD, is a retired USFWS Ecologist whose research has focused on large-scale disturbance agents, such as fire and spruce bark beetles, and on the ecological effects of climate change on the Kenai Peninsula. He teaches geology at the Kenai Peninsula College in Homer, where he lives with his wife Sara and is presently writing a book on the landscape history of the western Kenai Peninsula.
Al Werner Mount Holyoke College
Al is a Professor of Geology at Mount Holyoke College and is a collaborator on the project. He has worked in Alaska and in the high Arctic for the last 40 years on a variety of glacial history and lake coring projects. He Co-directed the Svalbard-REU program from 2004 to 2014.
Nicholas McKay Northern Arizona University
Nick is an Associate Professor at the School of Earth and Sustainability at NAU. He uses process models to improve paleoclimate reconstructions and uncertainty quantification. He has studied sedimentation processes and paleoclimate records from several Alaskan lakes. He will assist with establishing the geochronology for lake sediment cores for the South Alaska Lakes project.
In April 2019, Annie Wong successfully defended her thesis for her Master’s degree in Environmental Science and Policy: “Timing and rate of glacially driven outwash plain aggradation, Pothole Lake, south-central Alaska.” Congratulations, Annie!
In January 2019, David Fortin and others published a paper on the Eklutna Lake varve chronology in Quaternary Science Reviews: “New approach to assessing age uncertainties – The 2300-year varve chronology from Eklutna Lake, Alaska (USA)”.
For a few days in the June 2018 field season, the crew was fortunate to have the company of Emily Stone, Naturalist and Education Director at the Cable Natural History Museum in Cable, Wisconsin. Emily is also a talented writer; we are featured in one edition of her weekly column, “Natural Connections”, as well as in a blog post about sediment coring methods. Please enjoy Emily’s articles below! Thank you, Emily, for making us look good!
In June 2018, a team of researchers and students returned to the Kenai Peninsula for a second field season. The team included: PI Darrell Kaufman (NAU); graduate students Ellie Broadman and Annie Wong (NAU); collaborators Ed Berg (USFWS, emeritus), Scott Anderson (NAU), and Al Werner (Mount Holyoke); and undergraduates Emmy Wrobleski and Abby Boak (Mount Holyoke). The team recovered 64.4 meters of sediment from Petersen and Kelly Lakes on the Kenai lowlands, and Pothole Lake near the Skilak River floodplain, as well as water samples from 20+ sites in the region. Broadman, Wong, Wrobleski, and Boak then undertook the first round of processing and analysis of the lake sediment samples at LacCore (the National Lacustrine Core Facility) in Minneapolis. We are grateful for support from the Kenai National Wildlife Refuge and CH2MHill Polar Services, and look forward to a year of good science following the field season. Stay tuned for more updates and results!
For two weeks in June, a team of two PIs (Darrell Kaufman, David Fortin) and two graduate students (Ellie Broadman, Annie Wong) from Northern Arizona University, as well as collaborator Ed Berg of the US Fish and Wildlife Service (emeritus) completed the first “South Alaska Lakes” field season! The team recovered lake sediment cores from four lakes (Sunken Island, Paradox, Fish, and Petersen) and collected water samples for isotope analyses from twenty-five lakes throughout the Kenai lowlands. We are grateful for help and support from the US Fish and Wildlife Service – Kenai Refuge, and look forward to an exciting year of analyzing samples, and to returning to the field next summer.
Contact
If you have questions about our research, we’d be happy to hear from you!
Please contact Ellie Broadman <ebb42@nau.edu> or Darrell Kaufman <darrell.kaufman@nau.edu>
