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Scientific objectives

Our objectives for Expedition 336 were to recover sediment and basement cores and to install new CORKs to address two major scientific questions:

Where do deep-seated microbial communities come from?

Viable, diverse, and distinct microbiological communities occur in deeply buried marine sediments, but the origin of these communities is currently unknown. One possibility is that microorganisms from overlying bottom seawater are a steady source of inoculum that seeds microorganisms (particle attached and free living) to sediments, allowing each sediment layer, no matter how deeply buried, to harbor (in principle) a population that derives from this initial deepwater inoculum. Another possibility is that 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). The latter 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 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 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 deep sediments (obtained from cores taken during the expedition) and basement crustal fluids (obtained with the CORKs postexpedition).

What is the nature of the microbial communities harbored in young ridge flanks, and what is their role in the 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 is in young ridge flanks (<10 Ma). Most young ridge flanks lack a sediment cover, which presents difficulty for recovering intact 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 preclude using Juan de Fuca for our principal interest, which is to characterize more average, cold ridge flanks to reveal the role of microorganisms in promoting weathering on a global basis. Chemical reaction kinetics are inhibited at low temperatures, providing a window of opportunity for biological catalysts (Knab and Edwards, in press). 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.