GLOBAL PATTERNS OF NET RESPIRATION IN SUBSEAFLOOR SEDIMENT

Microbial life is abundant in subseafloor sediment. Rates of respiration in subseafloor sediment appear to be very slow, but are generally poorly constrained. This thesis aims to address this problem by quantifying respiration rates in subseafloor sediment at sites across the globe and determining geographic patterns of depth-integrated respiration rates and average per-cell respiration rates. First, we compiled pore water profiles and descriptive data from scientific coring and drilling sediment cores deposited in publicly accessible databases. Using these profiles, we quantified the global distribution of net respiration rates in subseafloor sediment (\u3e1.5 meters below seafloor) using (i) a reaction-transport algorithm, (ii) sediment physical properties, and dissolved oxygen, sulfate, and inorganic carbon concentration data from interstitial waters of sediment cores, and (iii) correlations of net respiration rates to sedimentation rate and sea-surface chlorophyll-a concentration. Our results indicate depth-integrated net global respiration in subseafloor sediment to be 2.6 x 1012 ± 0.25 x 1012 mol e-/yr. Comparison to the global rate of organic-matter burial in marine sediment indicates that most organic matter in subseafloor sediment (\u3e1.5 mbsf) goes unconsumed. Respiration rates per unit area vary between abyssal regions, where aerobic respiration dominates and rates are lowest, and continental margin regions, where anaerobic respiration dominates and rates are generally higher. Regional areal net respiration can vary by as much as six orders of magnitude. Mean per-cell respiration rates are much slower in subseafloor sediment than in the ocean, although both environments have equivalent numbers of prokaryotic cells. Per-cell respiration rates in subseafloor sediment are generally higher in regions dominated by aerobic respiration than in regions of anaerobic respiration. When combined with a recent estimate of radiolytic chemical production in subseafloor sediment, the potential total respiration rate in the sediment increases to 1.3 x 1013 mol e-/yr, but is 10,000 times lower than the electron-equivalent rate of carbon fixation in the ocean. This work contributes to the growing collection of studies showing cells in subseafloor sediment live at rates much slower than those seen in the ocean and surface world