Abstract

To discover both the extent and limits of microbial life in the subsurface oceanic crustal biosphere, we must first understand fluid circulation patterns and the evolution of rock/fluid composition as circulation occurs within permeable crust. The initial goal of this project was to assess the viability of the geophysical model (Stein and Fisher, 2001) proposed for subseafloor fluid circulation in Middle Valley, a sedimented rift at the northern end of the Juan de Fuca Ridge. Using existing geochemical, mineralogical, and physical properties datasets, we determined that the overall concept of the model is correct: secondary circulation through the sediment section inside the Area of Active Venting (AAV) is indeed occurring, albeit slowly, and is not chemically influencing hydrothermal fluids discharging from vents within the AAV. We also agree that there is a boundary layer within the sediment section that separates deep, hydrothermal conditions from shallow, recharging conditions. An important side note to this finding is that the slow vertical recharge through sediment may be driven by large scale lateral flow in the subsurface that is dissolving and re-precipitating anhydrite within the boundary layer at ~90ºC. This temperature is within the current proposed limit for life and thus this layer may be a refuge for microbes in this otherwise high temperature (~270ºC) system. Widespread subsurface lateral flow in a cooler/mixing hydrothermal environment has significant implications for the deep biosphere: chemical availability from hydrothermal fluids and minerals (sulfates and/or sulfides), but in a cooler environment, and the potential for subsurface microbial transport.