In situ microscale variation in distribution and consumption of O-2: A case study from a deep ocean margin sediment (Sagami Bay, Japan)

A transecting microprofiler documented a pronounced small-scale variation in the benthic O-2 concentration at 1450-m water depth (Sagami Bay, Japan). Data obtained during a single deployment revealed that within a sediment area of 190 cm(2) the O-2 penetration depth varied from 2.6 mm to 17.8 mm (average; 6.6 +/- 2.5 mm) and the diffusive O-2 uptake, calculated from the vertical concentration gradient within the diffusive boundary layer, ranged from 0.6 mmol m(-2) d(-1) to 3.9 mmol m(-2) d(-1) (average; 1.8 +/- 0.7 mmol m(-2) d(-1), n = 129). However, correction for microtopography and horizontal diffusion increased the average diffusive O-2 uptake by a factor of 1.26 +/- 0.06. Detailed 2D calculations on the volume-specific O-2 consumption exhibited high variability. The oxic zone was characterized by a mosaic of sediment parcels with markedly different activity levels. Millimeter- to centimeter-sized "hot spots" with O-2 consumption rates up to 10 pmol cm(-3) s(-1) were separated by parcels of low or insignificant O-2 consumption. The variation in aerobic activity must reflect an inhomogeneous distribution of electron donors and suggests that the turnover of material within the oxic zone to a large extent was confined to hot spots. The in situ benthic O-2 uptakes, measured during four cruises, reflected a seasonal signal overlying the observed small-scale variability. The annual benthic mineralization balanced similar to 50% of the estimated pelagic production. However, the central bay is characterized by a significant downslope sediment transport, and mass balance estimates indicate 90% retention of the total organic material reaching the bottom of the central bay.