Isotopic fractionation

This is the second of my posts on Thomas Gold’s book Deep Hot Biosphere, in which Gold espouses the radical idea that global petroleum and gas reserves are formed from geochemical reactions deep in the Earth’s crust. In contrast, the vast majority of experts think he's wrong. Fascinated by this book and the controversy around it, I decided to look into the scientific data on petroleum’s origins and arrive at my own conclusions. Like I said before, I’m not a geologist; I just did the best I could with the limited time I had.

One of the strongest pieces of evidence in favor of the biogenic theory is the study of stable isotope fractionation. All elements have isotopes. Hydrogen vs. deuterium, carbon-12 vs. carbon-13, oxygen-16 vs. oxygen-17 or oxygen-18, and so on. The only difference between an atom of carbon-12 and an atom of carbon-13 is that carbon-13 is about 8% heavier. Since carbon-13 is more-or-less chemically identical to carbon-12, we find it in all the same places that we find the usual carbon isotope. It’s in atmospheric carbon dioxide, in the carbon in coal, oil, natural gas, and in our the carbon that comprises our bodies. Carbon-13 atoms hang around all over the place and do the exact same things as carbon-12 atoms do...

Except. Being just a tad heavier that its more populous cousin makes carbon-13 undergo some chemical reactions and processes at very slightly different rates. For example, its heavier, so carbon-13 dioxide evaporates from sea water into the atmosphere at slightly slower rate than its more common cousin carbon-12 dioxide. So, the ratio of carbon-13 to carbon-12 in atmospheric carbon dioxide is ever so slightly lower than in pools of carbon.

Over the decades, geologists have gotten good at tracing the minute differences in the carbon-13 to carbon-12 ratio, and similar rations for other isotopes. One thing they found, that proponents of both theories seem to agree on, is that oil and gas have are depleted in carbon-13.

The biogenic camp (e.g., these guys) explains the depletion of carbon-13 through the chemical reactions of photosynthesis: the atmospheric carbon dioxide has to diffuse through leaves. In oceans, it has to dissolve in water before aquatic plants can fix it. These processes favors lighter carbon. It has to react chemically with other metabolites in the plants biochemistry. These reactions, too, happen faster to lighter carbons. So the organics sediments that eventually formed oil are depleted in light carbon. Viola. Oil and natural gas will naturally be depleted in heavy carbon, carbon-13, as a result.

Gold’s theory holds that oil and gas come from methane trapped in the planet since its formation and slowly diffusing outward from the Earth’s mantle. But, on its way out, it gets trapped in the very tiny pores of a variety of rocks. It has to diffuse through. Diffusion is faster for light carbon. Eventually, in the deep crust, chemical reactions transform the methane (now depleted in heavy carbon) to oil and gas. Viola.

But, both camps can’t be right. Luckily, it seems recently geologists have gotten more sophisticated with their isotopic analyses. They’ve been analyzing the isotopic fractionation of not only the carbon in oil and gas, but also the hydrogen. And they’ve been correlating these results.

Also, geologists have turned to helium-3 for answers. Helium-4, the more common isotope of helium, is formed from the decay of various radioactive minerals in the crust. Helium-3, in contrast, is formed only in stars. The helium-3 on Earth, then , is left over from when the planet formed from stardust four billion years ago. It’s still leaking slowly out of the mantle. So if sample of natural gas has a ratio of helium-3 to helium-4 that is relatively high, it’s a sign that at least some of the gas may have originated in the mantle.

With these new techniques, scientists have found abiogenic hydrocarbons in the Earth’s crust! But, the isotopic signature of this abiogenic gases differed strongly from what the Gold’s abiogenic theory would have predicted, and way different than the observed signature for commercially exploited gas reserves. The upshot: abiogenic hydrocarbons apparently account for only a minute portion of the subterranean hydrocarbons on Earth. Gold's theory, at least as he spelled it out in his book, doesn't seem to overcome the more recent isotopic measurements.