Analysis of chemical signals from complex oceanic gas hydrate ecosystems with infrared spectroscopy
Dobbs, Gary T.
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Substantial amounts of methane are sequestered in naturally occurring ice-like formations known as gas hydrates. In particular, oceanic gas hydrates are globally distributed in complex heterogeneous ecosystems that typically occur at depths exceeding 300 m. Gas hydrates have received attention for their potential as an alternative energy resource, as marine geohazards, and their role in cycling of greenhouse gases. In addition, chemosynthetic communities often play a vital role in the cycling and sequestration of carbon emanating from cold hydrocarbon seeps surrounding hydrate sites. Research efforts are presently striving to better understand the significance and complexity of these ecosystems through the establishment of seafloor observatories capable of long-term monitoring with integrated sensor networks. In this thesis, infrared (IR) spectroscopy has been implemented for the investigation of molecular-specific signatures to monitor gas hydrate growth dynamics and evaluate carbonate minerals, which are intimately connected with complex chemosynthetic processes occurring in these harsh environments. The first fundamental principles and data evaluation strategies for monitoring and quantifying gas hydrate growth dynamics utilizing mid-infrared (MIR) fiber-optic evanescent field spectroscopy have been established by exploiting the state-responsive IR absorption behavior of water. This has been achieved by peak area evaluation of the O-H stretch, H-O-H bend, and libration modes and assessing peak shifts in the 3rd libration overtone and libration bands during the formation and dissociation of simple clathrate hydrates of methane, ethane, and propane formed from aqueous solution. Hydrate growth and monitoring was facilitated with a customized pressure cell enabling operation up to ~5.9 MPa with spectroscopic, temperature, pressure, and video monitoring capabilities. Furthermore, the initial feasibility for extending the developed IR spectroscopic hydrate monitoring strategies into oceanic gas hydrate ecosystems has been demonstrated through the evaluation of potential spectroscopic interferences from sediment matrices in samples collected from two hydrate sites in the Gulf of Mexico (GoM). With exception of the libration band, the primary IR absorption features of water are readily accessed within hydrated sediment samples. Additional consideration for potential long-term hydrate monitoring applications revealed that the collection of approx. 2 IR spectra per day should enable direct insight into the temporal dynamics of hydrates...