LABORATORY SIMULATIONS OF ABIOTIC HYDROCARBON FORMATION IN EARTH'S DEEP SUBSURFACE

Over the past several decades, methane and other light hydrocarbons with an apparently abiotic origin have been identified in an increasing number of geologic fluids on the Earth, with settings that include seafloor hydrothermal systems, fracture networks in crystalline rocks within continental and oceanic crust, volcanic fumaroles and hot springs, and fluids discharged from serpentinites. Understanding the origin of these compounds has significant implications for the global carbon cycle, the distribution of life in the deep subsurface, hydrocarbon resources, and the origin of life. This presentation will provide an overview and synthesis of laboratory experimental simulations of the abiotic formation of organic compounds in geologic systems. In broad terms, there are two potential sources for abiotic hydrocarbons found within rocks and fluids at the Earth’s surface: (1) fluids from deep within the mantle that have migrated towards the surface, and (2) inorganic carbon in fluids that has been reduced to hydrocarbons during circulation through the crust. Experiments performed at conditions relevant to the deep upper mantle indicate that inorganic carbon is rapidly reduced, resulting in equilibrium distributions of carbon species dominated by methane, along with small amounts of ethane and other hydrocarbons. Transport of these compounds to the surface, however, would require them to survive metastably through the less reducing conditions of the shallow upper mantle. Experimental studies have also demonstrated several different potential pathways for the reduction of inorganic carbon to hydrocarbons at conditions found in the Earth’s crust, most of which require involvement of minerals as a source of reducing power and as reaction catalysts. However, the ubiquitous presence of biologically derived organic matter in shallow geologic environments complicates efforts to identify hydrocarbons with an unambiguous abiotic origin, and an ongoing challenge for experimental studies is to develop robust criteria to differentiate between biotic and abiotic hydrocarbons in geologic fluids.