Graduate Thesis Or Dissertation
 

Interpreting Climate of the Past from High-Resolution Ice Core Records of Methane

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  • Methane is a product of biogeochemical processes which respond to changes in climate. The history of atmospheric methane is recorded by ice cores providing insight into past changes in these biogeochemical processes. This dissertation is comprised of three studies which focus on centennial- and millenial-scale variability of methane from ice core records; first these variations are used as a tool to reconstruct the age of ice in an ice core, then, we assess the fidelity which ice cores record atmospheric methane, and finally we investigate how abrupt climate events affected different methane sources. Whether the recession of the West Antarctic Ice Sheet (WAIS) contributed to sea level rise during the deglaciation is an open question (Denton et al., 1989; Clark et al., 2002; Lambeck et al., 2014; Halberstadt et al., 2016). To better constrain the history of WAIS, the Roosevelt Island Climate Evolution (RICE) project drilledan ice core on Roosevelt Island in the East Ross Sea, West Antarctica. In Chapter 2, we establish an age scale for the RICE ice core and discuss initial observations from its climate records. The age scale is developed by matching gas records to the well-dated WAIS Divide ice core, initially by an automated matching routine developed here and then by visual matching for the bottom section of the core, and by modeling the ice age-gas age offset. A continuous age-depth relationship without a temporal hiatus and climate records comparable to other Antarctic ice cores implies that the ice dome persisted as an independent ice feature from WAIS. Additionally, the RICE methane record is the highest resolution record for the Holocene (0-11.7 ka) and clearly captures centennial-scale variability throughout this period; a mode of variability previously used as evidence of human influence on climate in the late Holocene (Ferretti et al., 2005; Houweling et al., 2008; Mitchell et al., 2013). Methane records have been measured from a number of ice cores, with recent records greatly improving precision and resolution. In Chapter 3, we review methane records from Greenland and observe discrepancies which coincide on depth with high calcium concentrations (a proxy for dust content) and depletions in δD-CH4. This is the first evidence of enrichments of methane in glacial ice absent of refrozen melt layers or basal sediment. We quantify the release or production of non-atmospheric methane (referred to as excess methane) with a multiple melt-refreeze analysis procedure on samples from Greenland and Antarctica. Excess methane was only observed in Greenlandic samples, was unaffected by treatment with a microbial inhibitor, and was highly correlated with the abun-dance of calcium (R=0.88). We propose that excess methane is released from dust particles during the sample analysis and was adsorbed onto the particle either at the dust source or from methane production in situ. This study concludes with a discussion of the impact excess methane has had on interpreting methane records, the evidence it provides for potential life within the ice sheets, and by proposing future work to clarify what the mechanism is and how to more accurately interpret the paleo-atmosphere. The mechanism which links the temperature of Greenland and atmospheric methane concentration during millennial-scale events, Dansgaard-Oeschger events (DO-events), has been debated for over 25 years (Chappellaz et al., 1990, 1993). Recently, methane emissions have been proposed to also respond to Heinrich Events (H-events) (Rhodes et al., 2015). In Chapter 4, we present new records of the isotopic composition of methane from the WAIS Divide replicate ice core and use this data to reconstruct the evolution of different types of methane sources between 37.8 and 40.5 ka, a time period which includes DO-8 and DO-9 (Rasmussen et al., 2014) and H-4 (Hemming, 2004; Rhodes et al., 2015). Our results indicate that the balance of sources is stable between stadial and interstadial periods with northern hemisphere biogenic sources dominant in terms of absolute emissions. Thermogenic emissions were estimated to be ∼30 Tg C/yr, significantly higher than another recent estimate (Petrenko et al., 2017). A negative isotopic excursion at the onset of DO-8 (38.5 ka) can be explained by the faster growth of wetlands and biogenic emissions compared to forest ecosystems and pyrogenic emissions. A positive isotopic excursion at 39.5 ka, believed to be related to H-4 (Rhodes et al.,2015), is possibly related to the southward shift of tropical rainbelts and a rapid dessication of northern hemisphere forest ecosystems
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  • Intellectual Property (patent, etc.)
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  • 2017-12-08 to 2018-07-08
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