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Evaluation of Near-Surface Gases in Marine Sediments


Gases contained within near-surface marine sediments can be derived from multiple sources: shallow microbial activity; thermal cracking of organic matter and inorganic materials; or magmatic-mantle degassing. Each origin will display a distinctive hydrocarbon and non-hydrocarbon composition as well as compound specific isotope signature and thus the interpretation of origin should be relatively straight forward. Unfortunately, this is not always the case due to in-situ microbial alteration, non equilibrium phase partitioning, mixing, and fractionation related to gas extraction method. Sediment gases can reside in the interstitial spaces, bound to mineral or organic surfaces, and/or entrapped in carbonate inclusions. The interstitial sediment gases are contained within the sediment pore space, either dissolved in the pore waters (solute), or as free (vapor) gas. The bound gases are believed to be attached to organic and/or mineral surfaces, entrapped in structured water, or entrapped in authigenic carbonate inclusions. There are multiple protocols to collect, prepare, extract, and analyze near-surface gases from marine sediments resulting in different gas compositions and compound specific isotopes. The purpose of this paper is to provide a review of gas types found within shallow marine sediments and review issues related to gas sampling and extraction. In addition, the paper will discuss how to recognize mixing, alteration, and fractionation issues to best interpret the seabed geochemical results and determine gas origin relative to regional charge assessment.