Methane under-saturated fluids in deep-sea sediments: implications for hydrate stability

Dr. Laura Lapham, of Florida State University and the 2008 DOE National Methane Hydrates Program Research Fellow, will speak on Monday, March 2, 2009 at 2:00 p.m. EST at the National Energy Technology Laboratory headquarters. Her talk, "Methane under-saturated fluids in deep-sea sediments: implications for hydrate stability", will discuss her most recent study and its impact on our understanding of hydrate stability.

Oceanic and permafrost gas hydrate deposits contain Earth's largest reservoir of CH₄ trapped within ice-like crystals. Instability of these deposits has been hypothesized to trigger global climate change. It is therefore imperative to determine the effects of thermodynamic factors that control hydrate stability: pressure, temperature, salinity and the degree of gas saturation. Dissolved methane concentrations, however, are rarely quantified on the sea floor.

Using novel instrumentation to obtain un-decompressed samples, in situ CH₄ concentrations in sediments adjacent to seafloor hydrates will be presented based on recent work in the Gulf of Mexico and the Cascadia Margin, offshore Vancouver Island. Because these hydrates are thought to be stable and thermodynamics suggests that stable hydrates are surrounded by saturated fluids, we expected to find saturated methane. However, we found that the fluids were undersaturated with respect to methane which suggests that the hydrates are out of equilibrium and should be dissolving. Based on these novel measurements, a methane flux from hydrates to the surrounding sediments and a hydrate dissolution rate were estimated.

These rates were surprisingly lower than predicted by assuming thermodynamic equilibrium between the hydrates and surrounding fluids and suggest that the hydrates are actually more stable than we predicted. We hypothesize that this meta-stability is due to entrained oils or microbial slimes. These results are significant because they provide additional constraints on the factors that control hydrate stability, which should be considered in future stability models.

Dr. Lapham has spent the past five years conducting researching on carbon-dioxide/methane cycling and organic chemistry through synthesis of novel compounds. Most recently her work has focused on developing a better understanding of the biogeochemical and physical controls on methane and sulfate in cold seep environments. Dr. Lapham works in FSU's Oceanography lab with Dr. Jeff Chanton and at the University of North Carolina with Dr. Chris Martens. For more information about Dr. Lapham's work please see the Methane Hydrates Graduate Fellowship Student Bios webpage.