ORGANIC COMPOUNDS AS INDICATORS OF FLUID FLOW IN THE OCEANIC CRUST

Hydrothermal vent fluids contain a range of organic compounds derived from the interaction of hot waters with sea-floor sediments and oceanic crust. These compounds are entrained in hydrothermal fluids during recharge, and undergo further reactions as geochemical conditions change along subsurface flow paths. Thus, the observed concentrations and isotopic compositions of organic compounds in fluids vented at the seafloor reflect subsurface hydrologic processes.

Hydrothermal vent fluids from two regions in Middle Valley—the Dead Dog and ODP Mound fields, and the Main Endeavour Field (MEF), both sediment-influenced hydrothermal vent systems, were collected in July, 2000. The aqueous organic compounds in fluids from all three sites have been derived from a combination of bacterial processes (methanogenesis in low-temperature surface sediments) and thermogenic processes in higher-temperature portions of the sediment column. Sedimentary organic matter degrades via a stepwise oxidation reaction pathway during heating, which releases carbon dioxide and hydrocarbons to circulating fluids. As these fluids flow through the subsurface, dissolved organic compounds undergo further alteration reactions. Because the relative stabilities of different redox couples varies with changes in geochemical conditions, the integration of a suite of organic and inorganic geochemical proxies provides a powerful tool to resolve differences in hydrologic processes in the subsurface. For example, at Middle Valley, differences in dissolved Cl, alkane, alkene and CO₂ concentrations indicate that the Dead Dog and ODP Mound fields are characterized by quite different subsurface temperature regimes. Additionally, at ODP Mound and MEF, fluids of seawater chlorinity experience different degrees of mixing with a vapor-rich phase, relative to fluids venting at the Dead Dog field. The amount of mixing can be quantified through the use of both organic and inorganic proxies, providing further insights into the subsurface hydrologic regime than can be deduced through a consideration of the inorganic geochemistry alone.