Upwelling of Hot Gas
The Deep, Hot Biosphere. Thomas Gold. 235 pp. Copernicus, 1999. $27.
This book's title is Thomas Gold's hypothesis: that most subsurface microbial food chains are based on nonbiological hydrocarbons upwelling from deep in the earth. This hypothesis is not very well defended. Only two of the chapters discuss microbial communities, and these chapters really avoid the topic of subsurface microbial metabolism, the key aspect of the hypothesis. In fact, this theory is at odds with the observation that most known deep microbes are found in fermentative communities that produce hydrocarbons (methane) rather than consume it.
Despite the title, the real focus of this book is the deep-earth gas hypothesis. Gold believes that most recoverable hydrocarbons were derived from space as the earth accreted. These primordial hydrocarbons, the theory goes, reside in the lower crust and upper mantle and slowly charge shallower conventional petroleum accumulations along planes of tectonic weakness. Because chemicals derived exclusively from bacteria are found in oil, Gold calls on subsurface microbial metabolism to contaminate the primordial oil. Most of the book elaborates and presents supporting evidence for this theory to a general audience.
Arguments in favor of the deep-earth gas hypothesis sound quite convincing on the surface to the uninitiated. But those arguments are deceptive. Well documented and theoretically justified criticisms of this hypothesis and explanations for almost all of the proposed inadequacies of the conventional model for petroleum generation have been published in the past decade, yet these criticisms and alternate interpretations are not mentioned or rebutted here. Key issues unexplained in this book are the occurrence of other chemicals in oils (such as steroids), indicative of specific eukaryotic sources, carbon isotopic fractionation patterns observed in normal alkane species inconsistent with an inorganic origin, and the ability of the conventional petroleum-generation model to predict where and how much of a specific oil type will be found in a geological basin.
I do not recommend this book for either its microbiological or geological aspects. The exciting and diverse world of subsurface microbiology is reduced to a pale caricature of itself. Arguments supporting the deep-earth gas hypothesis may have been valid 30 years ago, but we now know too much about the subsurface and about petroleum geochemistry to seriously consider these ideas in the form in which they are presented here. Like all evolving scientific theories, the conventional model of petroleum generation is still subject to modification in its details. But overall, it is alive and well, and the criticisms raised here are not likely to substantially change the model.—Alton Brown, Atlantic Richfield Company, Plano, Texas
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