A Sedimentary Record of Holocene Neoglaciation from the Uinta Mountains, Utah

A SEDIMENTARY RECORD OF HOLOCENE NEOGLACIATION FROM THE UINTA MOUNTAINS, UTAH By Catherine Klem Submitted in partial fulfillment of the requirements for the degree of Bachelor of Arts Department of Geology Middlebury College Middlebury, Vermont May 2010 Klem, Catherine M. 2010. A Sedimentary Record of Holocene Neoglaciation from the Uinta Mountains, Utah. Unpublished Senior Thesis: Middlebury College, Middlebury Vermont. 77 pgs. ABSTRACT Considerable paleoclimate research is focused on climatic variability during the Holocene, including the enigmatic period of renewed glaciations known as the Neoglaciation. Alpine lake sedimentary records are particularly useful in Neoglacial studies because they can provide uninterrupted, high-resolution records of environmental changes. This study focused on analysis of biogenic silica, phosphorus, and carbon:nitrogen from an alpine lake sediment record collected in northeastern Utah, adding to an extensive dataset that suggests several periods of Neoglacial activity in the Uinta Mountains. The sediment core, which spans the past 5300 years, was taken from EJOD Lake, a small lake with a maximum depth of 4.3 m, and a surface area of 2.7 ha, located above modern treeline at 3323 m. The lake is situated in a large glacial cirque ~500 m downslope from a complex of Neoglacial end moraines. Dry meltwater channels run from these moraines directly into EJOD Lake, thus establishing a clear connection between glacial activity and the lake sediment record. Biogenic silica, loss-on-ignition, and clastic flux time series all exhibit an overall decreasing trend over the past 5000 years, suggesting a gradual shift of the lake environment towards a colder, less productive climate. Superimposed on this downward trend are smaller-scale fluctuations characterized by intervals of notably low values in biogenic silica and LOI, and high values in C:N. These intervals suggest episodes of Neoglacial advance at 3600-3400, 2500-2300, and 2000-1600 BP. Subsequent increases in mineral P and clastic flux and decreases in median grain size at 3000-2900, 2300-2000, and 1400-1200 BP mark times when glacial retreat washed fine-grained rock flour out of the moraines into the lake. Less pronounced variability at 800-550 and 400-200 BP show similar trends which may indicate two episodes of Neoglacial advance associated with the Little Ice Age. The dating of these episodes shows some synchronicity with periods of Neoglacial advance in western North America, but also indicates that such advances were extremely regionally variable. A dramatic shift in proxies indicates a decline in soil development between AD 1750 and AD 2005 likely due to the onset of grazing ca. AD 1850 in the area. i ACKNOWLEDGEMENTS First and foremost, I would like to thank Jeff Munroe, my thesis and academic advisor. You patiently answered all of my questions, introduced me to lab work, and somehow always made me believe that this was possible. You taught me everything from how to use a repeat pipette to how to efficiently navigate excel, and I really appreciate that. Also, thank you to the entire geology department, both the professors and my fellow seniors. I can confidently say to all of my peers that I studied in the coolest department and whether my future studies delve deeper into geology or lead me to other endeavors, I will always greatly cherish my Middlebury education. And to my friends and family who have supported me throughout this project and who listened to my stories of long hours in bihall and entertained me while in the lab. Thank you for the moments when you offered a little perspective on life and for the moments when you supported my inner-geek. Finally, thank you to the Middlebury College Senior Work Fund for supporting my research and to the Middlebury College travel fund for making it possible for me to travel to Maryland and see what a real geology conference is all about. ii TABLE OF CONTENTS Abstract..............................................................................................................................i Acknowledgements…………………………………………………………………….….ii Table of Contents ………………………………………..……………………...………..iii List of Figures and Tables …………………………………………..……...……..……..iv Chapter I: Introduction………………………………...............................................……..1 The Importance of Paleoclimate studies…………………………………………..1 Holocene Neoglaciation……………………………..…………………………….2 The Study of Paleoclimate……………………………………..………………….7 Climate Proxies……………………………………..……………………………10 Chapter II: Study Site…………………………………………………………………….13 Uinta Mountains………………………...………………….……………….……13 EJOD Lake……………………………………………....……………………….15 Previous studies of Glacial History in the Rocky Mountains…………………....20 Chapter III: Methods…………………………………………………………….……….22 Field Work……………………………………………………………………….22 Previous Work on EJOD Lake Core………………………………….………….23 Sample Analysis………………………………………………………….………23 Chapter IV: Results………………………………………………………………………27 Age Control…………………………………………………………………..…..27 Detrending………………………………………………………………………..28 Biogenic Silica……………………………........................................……..…….28 Phosphorus……………………………........................................….……………31 C:N……………………………................................…………………………….34 Flux………………………………………………………………………..……..36 Overall Time Series………………………………………………………...……39 Chapter V: Discussion……………………………………………………………...……43 Relationships between Proxies…………………………………………………..43 Interpretation of EJOD Lake History………………………………….……..…..51 Links to Regional and Global Climate………………………….………..………59 Chapter VI: Conclusions………………………………………………….......………….69 Sources Cited…………………………………………………………………………….71 iii LIST OF FIGURES AND TABLES Chapter I: Figure 1.1 Intervals of Holocene glacial activity in the Western U.S.…………………....5 Chapter II: Figure 2.1 Digital Elevation Model of the Uinta Mountains, UT………………………..16 Figure 2.2 Bathymetric map of EJOD Lake…………………………………………..…16 Figure 2.3 Topographic map of the West Fork of the Black’s Fork……………………..17 Figure 2.4 Photo of the landscape surrounding EJOD Lake…………………….…….…17 Figure 2.5 Photo of Meltwater Channels………………………………………………...18 Figure 2.6 Image of the Surrounding Topography……………………………………....19 Chapter III: Figure 3.1 Photo of lake coring operation……..……………………….................……..22 Figure 3.2 Method for differentiating biologically and minerally derived silica………..24 Chapter IV: Table 4.1 AMS ages used for depth-age model…………………………………….……27 Figure 4.1 Depth-age model……………………………………………………………..27 Figure 4.2 Biogenic Silica time series………………………………………...…………30 Figure 4.3 Total P time series……………………………..……………………………..32 Figure 4.4 Mineral P time series…………………………………………………………33 Figure 4.5 C:N time series……………………………………………………………….35 Figure 4.6 Flux times series……………………………………………………………...37 Table 4.2 Summary statistics………………………………………………………….....41 Figure 4.7 Summary plot………………………………………………………………...42 Chapter V: Figure 5.1 Summary plot with additional proxies…………………………………...…..43 Figure 5.2 Comparison of color data with % mineral P and clastic flux……………...…51 Figure 5.3 Episodes of Neoglacial advance in EJOD Lake record………………………53 Figure 5.4 Comparison of EJOD Lake proxies……………………………….………….54 Figure 5.5 Summer insolation data………………………………………………………61 Figure 5.6 Comparison plot of Neoglacial advance in the Western U.S……………..….64 iv I. INTRODUCTION The Importance of Paleoclimate studies The study of climate change is an important field that is now more relevant than ever. Research has shown that between AD 1860 and present day the concentration of CO2 gas in the Earths atmosphere has increased from 280 to 387 ppmv contributing to an increase of 0.55°C in the global mean annual surface temperature (Lean et al., 1995; NOAA, 2010). Such changes have been largely credited to anthropogenic influences, and could have widespread implications for the future, affecting both plant and animal habitat. However, predicting and understanding human-driven climate changes is greatly complicated by the presence of natural cycles and variability (Lean et al. 1995). Accordingly, a good understanding of past climate cycles and variations is necessary to provide a basis with which to evaluate current climate observations (Kaplan et al., 2002). One of the best ways to better understand these natural variations, and consequently the interface between natural and anthropogenic factors, is by considering past cycles and changes through the study of paleoclimate (Laird et al., 1996; IPCC report, 2007). Paleoclimate reconstructions can shed light on the scale and rapidity of climatic and environmental changes we face today as well as help investigate the forces that drive climate shifts and the earths reactions to such changes. Recently, studies of Holocene Neoglaciation (a period of locally renewed glaciation after the last Pleistocene Ice Age) have become important for understanding paleoclimate drivers and the global synchronicity of periods of warming and cooling (Schaefer et al., 2009). Holocene Neoglaciation studies are especially important as they are on a time scale that humans can 1 relate to and even play a part in, and thus show an interesting interface between natural and anthropogenic climate shifts. The primary goal of this thesis is to carry out a multi-proxy study of Holocene Neoglaciation from a high alpine lake sediment record (EJOD Lake) in the Uinta Mountains of northeastern Utah (Fig. 7). Newly analyzed proxies will be used in conjunction with existing data to reconstruct climate variability

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