Diking, event plumes, and the subsurface biosphere at Mid-Ocean Ridges

Diking events along the Mid-Ocean Ridge (MOR) drive long, narrow intrusions of lava into the upper ocean crust in response to far-field tectonic plate stresses. Intrusion of dikes is the primary accretion mechanism of the upper ocean crust, providing a pathway for eruption of lavas onto the ocean floor. This extrusive layer, which has bulk porosity greater than 25% in the youngest ocean crust, provides a potential extensive habitat for a robust and diverse microbial subsurface biosphere. The extrusive layer is hundreds of meters to a kilometer or greater in thickness in much of the ocean crust generated by fast and intermediate-rate spreading rates. Its thickness is more variable at slower spreading rates, and in some places it may be missing entirely. The porosity and permeability decrease by a factor of ten in the sheeted dikes, which lie beneath the extrusive layer. As the ocean crust ages and cools, its porosity decreases and the vigor of hydrothermal circulation decreases. In addition to these time varying factors, other crustal accretion variables such as the spacing of first, second, and lower order tectonic boundaries, time-variable magma supply, and faulting create a spatially variable habitat for the oceanic subsurface biosphere. Dike intrusion at the Mid-Ocean Ridge is commonly accompanied by graben formation at the seafloor and, in some cases by eruption(s) of lava. The geometry of the dike and the location of the eruption(s) (if any) depend on many factors, including the stress state of the crust and topographic gradient. The concomitant increase in the release of volatiles and the creation of fractures often create ideal conditions for significant microbial blooms. The commonly observed "snow-blower" vents are the most obvious manifestation of this process. The diking process is also intimately tied to the generation of event plumes, probably by driving existing subsurface fluid reservoirs up into the ocean. Expulsions of subsurface microbes in event plumes that accompany dike injections and the pulse of heat and volatiles that follow, them provide windows of opportunity to sample the microbial populations that could be similar to those that normally reside at greater depth in the ocean crust. In situ sampling of the microbial communities in the youngest upper oceanic crust remains challenging, and further success depends on further. advances in drilling technology.