The National Methane Hydrates R&D Program
The DOE/JIP Gulf of Mexico Hydrate Research Cruise
Special Report - Geochemistry Program On-Board Uncle John
From: Miriam Kastner, University of California at San Diego
On-Board Geochemistry Analyses
The objectives of the geochemistry program are to document the pore-water chemical and isotopic compositions at two pairs of drill sites at Atwater Valley (AT) and Keathley Canyon (KC), respectively, and at two mounds at AT. The analytical emphasis, especially at sea is on key components that provide information on:
- The depth and spatial distribution and concentration of gas hydrates and the relation to lithology and other physical properties, of which the latter that will be acquired by other scientists involved in the project;
- The nature and rates of the key microbial reactions responsible for gas generation in situ by microbial processes versus upward advection of methane (and other gases) from deeper sources, and
- The subsurface hydrology that controls the pore water chemistry, thus the gas hydrate distribution and concentration, and potential dissociation.
| Miriam Kastner, of the University of California at San Diego, prepares core samples for analyses on board the Uncle John
The key components are pH, salinity, Cl, alkalinity, sulfate sulfide, Ca, and Mg concentrations, and δ13 DIC. The salinity and Cl concentration provide information on dilution or concentration of the pore water by in situ dissociation or formation of gas hydrate, respectively, or by mixing with a fresh water or a saline fluid, respectively. Resolving between these possibilities is possible by additional shore based chemical and isotopic analyses. Alkalinity is produced both by microbial sulfate reduction at a molar ratio of 1:2, thus the sulfate-sulfide analyses, and by anaerobic methane oxidation at a molar ratio of 1:1. DIC δ13 C values, provide a clear distinction between these two reactions. In addition, the relative importance of alkalinity production by the two processes can be determined by the relation to sulfate reduction, after correcting the alkalinity for Ca and Mg concentrations; moderate to high alkalinities induce authigenic carbonates precipitation, that reduces seawater Ca and Mg concentratons.
Hydraulic press extracts pore water from sample for further geochemical analyses