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The Late Holocene Atmospheric Methane Budget Reconstructed from Ice Cores

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The Late Holocene Atmospheric Methane Budget Reconstructed from Ice Cores AN ABSTRACT OF THE DISSERTATION OF Logan E. Mitchell for the degree of Doctor of Philosophy in Geology presented on March 4, 2013. Title: The Late Holocene Atmospheric Methane Budget Reconstructed from Ice Cores Abstract approved: ______________________________________________________ Edward J. Brook Ice cores are considered the gold standard for recording past climate and biogeochemical changes. However, gas records derived from ice core analysis have until now been largely limited to centennial and longer timescales because sufficient temporal resolution and analytical precision have been lacking, except during rare times when atmospheric concentrations changed rapidly. In this thesis I used a newly developed methane measurement line to make high-resolution, high-precision measurements of methane during the late Holocene (2800 years BP to present). This new measurement line is capable of an analytical precision of < 3 ppb using ~120 g samples whereas the previous highest resolution measurements attained a precision of ± 4.1 ppb using 500- 1500g samples [MacFarling Meure et al., 2006]. The reduced sample size requirements as well as automation of a significant portion of the analysis process have enabled me to make >1500 discrete ice core methane measurements and construct the highest resolution records of methane available over the late Holocene. Ice core samples came from the recently completed West Antarctic Ice Sheet (WAIS) Divide ice core which has as one of its primary scientific objectives to produce the highest resolution records of greenhouse gases, and from the Greenland Ice Sheet Project (GISP2) ice core which is a proven paleoclimate archive. My thesis has the following three components. I first used a shallow ice core from WAIS Divide (WDC05A) to produce a 1000 year long methane record with a ~9 year temporal resolution. This record confirmed the existence of multidecadal scale variations that were first observed in the Law Dome, Antarctica ice core. I then explored a range of paleoclimate archives for possible mechanistic connections with methane concentrations on multidecadal timescales. In addition, I present a detailed description of the analytical methods used to obtain high- precision measurements of methane including the effects of solubility and a new chronology for the WDC05A ice core. I found that, in general, the correlations with paleoclimate proxies for temperature and precipitation were low over a range of geographic regions. Of these, the highest correlations were found from 1400-1600 C.E. during the onset of the Little Ice Age and with a drought index in the headwater region of the major East Asian rivers. Large population losses in Asia and the Americas are also coincident with methane concentration decreases indicating that anthropogenic activities may have been impacting multidecadal scale methane variability. In the second component I extended the WAIS Divide record back to 2800 years B.P. and also measured methane from GISP2D over this time interval. These records allowed me to examine the methane Inter-Polar Difference (IPD) which is created by greater northern hemispheric sources. The IPD provides an important constraint on changes in the latitudinal distribution of sources. We used this constraint and an 8-box global methane chemical transport model to examine the Early Anthropogenic Hypothesis which posits that humans began influencing climate thousands of years ago by increasing greenhouse gas emissions and preventing the onset of the next ice age. I found that most of the increase in methane sources over this time came from tropical regions with a smaller contribution coming from the extratropical northern hemisphere. Based on previous modeling estimates of natural methane source changes, I found that the increase in the southern hemisphere tropical methane emissions was likely natural and that the northern hemispheric increase in methane emissions was likely due to anthropogenic activities. These results also provide new constraints on the total magnitude of pre-industrial anthropogenic methane emissions, which I found to be between the high and low estimates that have been previously published in the literature. For the final component of my thesis I assembled a coalition of scientists to investigate the effects of layering on the process of air enclosure in ice at WAIS Divide. Air bubbles are trapped in ice 60-100m below the surface of an ice sheet as snow compacts into solid ice in a region that is known as the Lock-In Zone (LIZ). The details of this process are not known and in the absence of direct measurements previous researchers have assumed it to be a smooth process. This project utilized high-resolution 15 methane and air content measurements as well as density of ice, δ N of N2, and bubble number density measurements to show that air entrapment is affected by high frequency (mm scale) layering in the density of ice within the LIZ. I show that previous parameterizations of the bubble closure process in firn models have not accounted for this variability and present a new parameterization which does. This has implications for interpreting rapid changes in trace gases measured in ice cores since variable bubble closure will impact the smoothing of those records. In particular it is essential to understand the details of this process as new high resolution ice core records from Antarctica and Greenland examine the relative timing between greenhouse gases and rapid climate changes. © Copyright by Logan E. Mitchell March 4, 2013 All Rights Reserved The Late Holocene Atmospheric Methane Budget Reconstructed from Ice Cores by Logan E. Mitchell A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented March 4, 2013 Commencement June 2013 Doctor of Philosophy dissertation of Logan E. Mitchell presented March 4, 2013. APPROVED: Major Professor, representing Geology Dean of the College of Earth, Ocean, and Atmospheric Sciences Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Logan E. Mitchell, Author ACKNOWLEDGEMENTS The winding path of my life has taken me to a great many places where I’ve met amazing people and had wonderful experiences. As I stand here at the end of a major milestone along this path it is important to look back and thank those who have helped me along the way. At the top of this list is my adviser, Ed Brook who has been a better mentor than I could have ever asked for. It was at Ed’s suggestion that I applied to graduate school and he has not only patiently guided me along each step of my Ph.D. program, but he also has been a positive role model on many levels from developing professional relationships, work-life balance, writing techniques, experimental objectiveness, intellectual curiosity, etc., to name but a few. Above all else it is his kindness and graciousness to others as well as his ethic of hard work that I will always remember. It has been an honor to be your student. Peter, it was your glacial geology class that introduced me to paleoclimatology many years ago and that class has played a pivotal role in the direction of my life since then. Someday I hope to become as good of a teacher as you are. Thank you especially for all of the encouragement you’ve given me over the years. Alan, thank you for having the humility and confidence to tell me and the other paleo students that no paper is perfect, not even the ones that you wrote. By encouraging us to think of constructive criticism while reading papers you helped us to grow as scientists. Andreas, even though I didn’t take your modeling class, I still learned a bunch from that class through Julia and Thomas. Thank you for you continued presence and insight within the paleoclimate community. Mark, thank you for being willing to join my committee, I hope you have enjoyed learning a little bit about methane! I have been fortunate to have awesome co-conspirators over the past few years. Thomas, you are one of the funniest and smartest people I know. I wish we could have gone to the field together! Julia, I love your voice, and I’m so excited that you have found such a good fit with science writing. I look forward to reading your articles for a long time to come! James, where do I start? Thank you so much for all of the long hours you put into making the methane measurements and for all of your help with math over the years! I’ll never be able to run as fast as you, bike as hard as you, or play Frisbee as well as you, but I can beat you at pool. The best thing though is that I can count you among my best friends. Jon, your enthusiasm is infectious! The methane line is yours now, take care of it! I can’t wait to see what you turn up. Mike, it has been awesome to see your smiling face every morning over the years. Shaun, we’ve come a long way since that intro geology class we took a decade ago. Thanks for being an awesome TA, and for letting me copy your thesis format. Christo, of all the awesome things you’ve done, I’m still most impressed by you making pasta from scratch with wheat flour on Taylor Glacier. Adrian, I’ll always remember your love of salad! Rachael, its been awesome to get to know you, and I hope we can continue to work together in the future! A lot of manual labor went into producing the methane records presented here.
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