Award: OCE-1131671

In the past two decades a microbial ecosystem buried deep in the sediments beneath the ocean floor has been discovered, yet we still know extremely little about this hidden biosphere. Most sub-seafloor investigations and ocean drilling expeditions have focused on high productivity regions near continental margins, where organic matter export to the seafloor is high, sustaining an anaerobic subsurface microbial biome. IODP Expeditions 329 (South Pacific Gyre – Microbiology) and 336 (Mid-Atlantic-Ridge/North Pond – Microbiology) were both dedicated to studying subsurface life underlying oligotrophic open ocean regions. Wiebke Ziebis was a shipboard participating scientist of Expedition 329 and a shore-based scientist for Expedition 336. Detailed oxygen measurements during expeditions the site survey cruise to North Pond (Ziebis et al., 2012) and on the drilling expeditions 329 (D’Hondt et al., 2011, D’Hondt et al. subm.) and 336 (Orcutt et al., 2013) revealed that, in contrast to the better-studied ocean margin regions, where oxygen only penetrates a few mm or cm, the seafloor underlying oligotrophic ocean gyres is characterized by extremely deep oxygen penetration (several meters to tens of meters). In the case of the South Pacific Gyre, oxygen penetrates the entire sediment column (> 80 m) and reaches the ocean crust. It was also recently discovered at the North Pond site, which is located on the ridge flank of the Mid-Atlantic-Ridge, that oxygen is diffusing upward from the basaltic basement creating another deep zone just above the crust which is oxic (containing molecular oxygen). The presence of oxygen in deep sediment layers greatly influences the microbiology and biogeochemistry beneath the ocean floor. The discovery that the seafloor beneath ocean gyres is mainly or entirely oxic is an extremely important finding since oligotrophic open ocean regions constitute 48 % of the enti...