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Dissertation/Thesis Abstract

Assessing Methane Dynamics in and Around Seafloor Gas Seep Environments using Stable Isotopes
by Leonte, Mihai, Ph.D., University of Rochester, 2019, 210; 13884953
Abstract (Summary)

This work is motivated by the fact that methane is currently the second largest contributor to atmospheric radiative forcing behind carbon dioxide. Although a small source to the atmosphere, the oceans are the second largest global reservoir of methane, whose dynamics in response to a warming ocean are not well understood. This makes future climate projections difficult especially since rising temperatures can influence both sources and sinks of methane in seawater. Methane that is found in the water column, whether sourced from seafloor sediments, gas seeps, or produced in situ, is actively oxidized by bacteria. Understanding the capacity of oxidizing bacteria to remove methane from the water column is crucial in predicting how this biological sink may respond to future methane release. In chapters 2 & 3, we investigated several locations along the mid-U.S. Atlantic Margin, where hundreds of methane gas seeps have been identified. Without relying solely on the more traditional ex situ incubations of seawater samples, our measurements of natural distributions of in situ CH4 concentration and δ13C-CH4 show that CH4 emitted from seafloor gas seeps is readily oxidized and quantitatively removed from the water column before migrating vertically to the surface. Furthermore, supersaturation of surface waters is driven by aerobic CH4 production in the subsurface, above 200 m depth, rather than the more dramatic seafloor gas seeps. Chapter 4 examines the dissolution of methane from bubbles above an active gas seep in the Gulf of Mexico. While gas bubbles appear to quickly transport CH4 vertically, our goal was to experimentally determine the extent of CH4 dissolution during ascension. Previous studies have shown that CH4 isotopes are fractionated as dissolution occurs and we used δ13C-CH4 measurements from water and gas bubble samples to determine the fraction of CH4 dissolved since bubble emission at the seafloor. Our results show that on average, over 90 % of CH4 is dissolved at an altitude of 400 m above the seafloor. This dissertation explores the use of δ13C-CH4 measurements to fingerprint different CH4 sources, assess CH4 dissolution to the water column, and quantify the extent and rate of CH4 oxidation. Our results show that CH4 emitted from gas seeps quickly dissolves above the seafloor after which it is readily oxidized. The gas seeps investigated here did not contribute CH4 to the atmosphere directly and thus CH4 supersaturation in surface waters is driven by in-situ CH4 production.

Indexing (document details)
Advisor: Kessler, John D.
Commitee: Garzione, Carmala N., Kelley, Douglas H., Tenhaeff, Wyatt
School: University of Rochester
Department: School of Arts and Sciences
School Location: United States -- New York
Source: DAI-B 81/2(E), Dissertation Abstracts International
Subjects: Chemical Oceanography
Keywords: Dissolution, Fractionation, Gas seep, Isotope, Methane, Oxidation
Publication Number: 13884953
ISBN: 9781085751254
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