Graduate Thesis Or Dissertation

Augmenting and interpreting ice core greenhouse gas records

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  • The three studies that comprise this dissertation seek to answer significant questions in paleoclimatology through unconventional applications of ice core greenhouse gas data. These studies involve different gases and span the interval of time between the Last Glacial Maximum and the Industrial Revolution, but are united by their nontraditional use of greenhouse gases and their attempt to realize the potential for greenhouse gases to reveal important information about Earth’s climate. Ever since their discovery, the abrupt climate changes of the last glacial period known as Dansgaard-Oeschger (D-O) events have proved challenging to explain. The dominant hypothesis involves periodic freshwater discharges into the North Atlantic, which may regulate the strength of the Meridional Overturning Circulation (MOC) and its role in transporting heat to high latitudes. These events were not restricted to the North Atlantic, and can also be recognized in paleoclimate archives around the world. However, numerous uncertainties surrounding the mechanism behind D-O events remain, including how they are communicated to low latitudes and whether other hypotheses can be definitively ruled out. To constrain the mechanism behind abrupt climate changes, we investigate the phasing of climate changes in high- and low-latitude regions at the Bølling Transition, the penultimate abrupt warming event of the last glacial period. We use methane and the ¹⁵N/¹⁴N ratio of N₂ from the North Greenland Eemian (NEEM) ice core, which serve as proxies for tropical climate and Greenland temperature, respectively. We find that these gases change synchronously in the ice core record, and use a firn air model together with a Monte Carlo approach to constrain the phase lag to within several decades. Our results indicate that the mechanism behind the Bølling Transition was capable of rapidly transmitting the climate signal across the planet in a matter of years, and must therefore involve components of the climate system that are suitably reactive. The glacial-interglacial change in atmospheric methane concentrations revealed in ice core records has spurred a decade of debate about its cause. The most likely explanations involve dramatic changes in methane emissions, which originate from both high- and low-latitude wetlands. One method of investigating the changing latitudinal distribution of methane sources is to quantify the difference in methane concentrations between Greenland and Antarctica, which changes in proportion to the fraction of methane produced at high northern latitudes. Previous attempts to determine the methane interpolar difference (IPD) abound, but many have been hampered by complications in synchronizing bipolar ice core records and analytical uncertainties. We present the first continuous estimate of the methane IPD across the termination using high-resolution methane data from the NEEM and West Antarctic Ice Sheet (WAIS) Divide ice cores. Our results reveal the dominant role of tropical sources in driving abrupt changes in atmospheric methane concentrations, and show that boreal methane sources were surprisingly insensitive to dramatic climate changes. We hypothesize that changes in Northern Hemisphere snow and ice cover exerted strong control over tropical methane emissions, while gradually increasing solar insolation and land area allowed boreal sources to grow during the termination. We also investigate the IPD across the major climate transitions of the termination, and during four centennial-scale methane variations, and find opposing trends in boreal and tropical source strengths during these transient events. We propose that temporary decoupling of the locus of interhemispheric mixing, the position of the Intertropical Convergence Zone, and tropical precipitation may explain these results. Atmospheric concentrations of nitrous oxide (N₂O) have risen by ~20% from preindustrial to modern times, but the cause of this increase is not fully understood. The change has been previously attributed to various agricultural activities which perturb microbial processes in soils, but exactly how remains an outstanding question with important implications for future mitigation of N₂O emissions. We present the first measurements of the isotopomers of tropospheric N₂O over the interval from 1450 to 1920 CE. Our results confirm that the preindustrial atmosphere was enriched in all isotopes relative to the modern atmosphere. Furthermore, we estimate that the net anthropogenic source of nitrous oxide must be depleted in all heavy isotopes and have a strong site preference, consistent with a strong role for agricultural emissions and characteristic of N₂O derived from nitrification. We also find a large oscillation in the site preference of the ¹⁵N in N₂O during the Little Ice Age between 1500 and 1700 CE. We hypothesize that this excursion may be due to changing climate conditions that led to an increase in the amount of N₂O produced by nitrification vs. denitrification.
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