Geochemistry of Cold Hydrocarbon Seeps: An Overview

Abstract

A fundamental geochemical process operating at methane seeps is the anaerobic oxidation of methane (AOM) by which methane is oxidized and sulfate is reduced. This process takes place in the sulfate-methane transition zone (SMTZ), generally located below the sediment-water interface. Methane has a low δ13C signature, and this is transferred to the dissolved inorganic carbon (DIC) reservoir during AOM. The increase in alkalinity from AOM can cause various carbonate minerals to precipitate with low δ13C. Indeed, very low δ13C values of ancient calcium carbonates are taken as prima facie evidence that the carbonates formed in a cold hydrocarbon seep environment. Isotope systems applied to ancient hydrocarbon seeps include those of carbon, oxygen, strontium, neodymium, and sulfur. These provide information on carbon source, carbonate formation temperature, the involvement of deep-sourced fluids, and fluid pathways in transferring methane to the SMTZ. Variations of rare earth elements (REEs) provide clues to the environmental conditions under which seep carbonates formed, with implications for the precipitation depth and flow regime. Other trace elements (Fe, Mn, Sr, Mg) in seep carbonates reflect mineralogical differences, and redox-sensitive trace elements (Mo, U, Cd, Sb, As) provide constraints on fluid flux and the dynamics of redox conditions.