Methane cold seeps: Paleo proxies, modern measurements, and instrument development

Marine cold seeps are one of the main conduits by which methane (CH4) from anoxic marine sediments, the largest CH4 reservoir on Earth’s surface, is transferred from the sediments into the ocean. Most of this CH4 is oxidized by microbes in the sediments, making the anaerobic oxidation of CH4 (AOM) a filter that prevents a significant fraction of dissolved CH4 from reaching the seawater. Despite its importance, our current constraints on AOM rates at marine cold seeps are very limited. Modern rate measurements are challenging and sparse, which hinders our ability to draw robust global inferences about AOM rates in the modern ocean and geological past. In this dissertation, I have developed a geochemical proxy to reconstruct AOM rates from methane-derived authigenic carbonates (MDACs) at cold seeps, applied this proxy to globally-distributed seeps, quantified CH4 emissions from seeps along the central Chilean Margin, and developed novel prototype instruments for improving my proxy measurements. Using an isotope-enabled reactive-transport model, I show that the clumped isotopic composition of MDACs (the frequency of 13C–18O bonding within the carbonate lattice) quantitatively records AOM rates (Chapter 2) and derived global estimates of AOM rates at both modern seeps and over the past as recorded in MDACs. I then apply this novel proxy to three specific marine seep settings (Chapter 3): the North American Margin, Vestnesa Ridge (Norwegian Margin), and Jiulong Methane Reef (South China Sea). Comparing clumped isotope measurements of North American Margin MDACs with AOM rates produced by their associated microbes highlights the power of this proxy to constrain long-term geologic AOM rates, and the applications of this proxy to dated MDACs from Vestnesa Ridge and Jiulong Methane Reef reveal important environmental controls on CH4 seepage at these sites, such as hydrate stability and organic carbon supply. I further determine the transport and fate of CH4 that passes the AOM filter and enters the water column by measuring dissolved CH4 concentration and acoustically detecting gas flares along the central Chilean Margin (Chapter 4). Finally, I describe two prototype isotope ratio laser spectrometers that I developed to enable future high-throughput benchtop clumped isotope measurements on very small amounts of carbonate and thus expand the application of carbonate clumped isotope proxies (Chapters 5 and 6). Taken as a whole, this dissertation presents a cross-disciplinary exploration of CH4 seeps in marine settings and advances our understanding of dynamics and controls on CH4 seepage in the modern ocean and geological past.

Grantor dc:publisher

Massachusetts Institute of Technology and Woods Hole Oceanographic Institution

Year dc:date.issued

2026

Author dc:creator

• Wieman, Scott T.

Advisors dc:contributor.advisor

• Guo, Weifu
• Michel, Anna P. M.

• Methane seep
• Clumped isotopes
• Anaerobic oidation of methane

DOI

10.1575/1912/73004

OAI identifier oai:identifier

oai:darchive.mblwhoilibrary.org:1912/73004