Abstract

The nature of the energy yielding mechanisms in the lowenergy organic-poor sedimentary environment underlying the South Pacific Gyre (SPG) is not fully constrained. We used the approach of Wang et al. (2008) to quantify rates of organic-fuelled metabolic activities at most IODP Expedition 329 Sites (U1365 through U1370). At Site U1366 and U1370 net rates of oxygen-reducing organic oxidation averaged 1.77E^-2 and 1.64E^-3 fmol O₂ cell^-1 yr^-1, respectively, representing a tremendously low cellular metabolism. At Site U1370, we observe net oxygen reduction throughout the entire sediment column. At Site U1366, statistically significant net oxygen reduction is not detected at depths greater than 11 meters below seafloor. Despite these low rates of organic oxidation, most cell counts are above the minimum detection limit throughout the entire sequence at both sites. Hydrogen from natural radioactive splitting of water has been hypothesized to be a significant electron donor in organic-poor sediment of the SPG. Becauses water radiolysis produces H₂ and – O₂ simultaneously, oxidation of this H₂ does not contribute to net O₂ reduction in the sediment. Our calculation of radiolytic H₂ production, based on radioactive element content and sediment physical properties, indicate that on average 5.63E^-1 and 9.79E^-2 fmol H₂ yr^-1 cell^-1 is available throughout the sequence at Sites U1366 and U1370, respectively. Despite these relatively high production rates, dissolved H₂ abundances are below detection at both sites. These results suggest that H₂ from in situ water radiolysis fuels the predominant energy-yielding pathway for microbes in SPG sediment.