Scientific objectives

Our objectives are to recover sediments that overlie a hydrological flow path previously studied in the basement underlying North Pond sediments (see below), recover basement at one site (NP-1), and establish CORKs to address two major scientific questions:

1. Where do deep-seated microbial communities come from?

Viable, diverse, and distinct microbiological communities occur in deeply buried marine sediments. Where they originate is unknown. There are (at least) two possibilities: (1) microorganisms from overlying bottom seawater are a steady source of inoculum that seeds microorganisms (particle attached and free living) to sediments. Hence, each sediment layer, no matter how deeply buried, can in principle harbor a population derived from this initial deepwater inoculum; or (2) microbial inoculum is provided by active transport (e.g., by vertical advective transport from the basement [passive transport] or by lateral active transport [swimming] from adjacent, older sediments following redox gradients). This hypothesis is consistent with known mechanisms (i.e., swimming by chemotactic response to chemical gradients or advective flow) and would explain how microbial communities persist in such nutrient-limited, deeply buried sedimentary sequences, that is, they have evolved very specifically for this niche. However, this mechanism implies that sediments need to be in physical contact for effective inoculum transfer in order for these specialized niches to be exploited. Hence, an isolated sedimentary sequence may be inactive, dormant, or harbor some evolutionarily distinct populations of microorganisms compared to sedimentary communities that receive the "ancient inoculum." North Pond is the ideal location to test these opposing hypotheses, which have important mechanistic implications concerning dispersal mechanisms in the deep biosphere and evolutionary consequences for microbial life on Earth. We will analyze the microbial communities in both the deep sediments (obtained from cores taken during the expedition) and the basement crustal fluids (obtained with the CORKs postexpedition).

2. What is the nature of microbial communities harbored in young ridge flanks, and what is their role in ocean crust weathering?

In the subseafloor, the extent of direct participation in weathering by extant communities is not clear. Abundant petrographic and geochemical data indicate that oxidative seafloor alteration occurs during the first 10–20 m.y. of crustal age and thereafter slows or ceases. These preliminary lines of evidence suggest that the most reasonable place to search for active subsurface microbial communities should be in young ridge flanks (<10 Ma). Most young ridge flanks lack sediment cover, which presents difficulty for intact recovery of upper ocean crust. One major exception is the Juan de Fuca Ridge flank. However, the high heat flow, rapid chemical reaction rates, and anaerobic conditions of this setting make it a poor site choice, given our principal interest in characterizing more "average," cold ridge flanks in order to reveal the role that microorganisms may play on a global basis in promoting weathering. Chemical reaction kinetics are inhibited at low temperatures, providing a window of opportunity for biological catalysts. The low heat flow ridge flank at North Pond represents an ideal model system for studying biologically mediated oxidative basement alteration. The work will also provide an excellent point of comparison for the studies taking place at the Juan de Fuca Ridge, which represents the warm, sedimented end-member in the global spectrum of ridge flanks.