Carbon mineralization and oxygen dynamics in sediments with deep oxygen penetration, Lake Superior

To understand carbon and oxygen dynamics in sediments with deep oxygen penetration, we investigated eight locations (160-318-m depth) throughout Lake Superior. Despite the 2-4 weight percent organic carbon content, oxygen penetrated into the sediment by 3.5 to. >12 cm at all locations. Such deep penetration is explained by low sedimentation rates (0.01-0.04 cm yr^-1), high solubility of oxygen in freshwater, and a shallow (~ 2 cm) bioturbation zone. In response mainly to oxygen variations in the bottom waters, the sediment oxygen penetration varied seasonally by as much as several centimeters, suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged. The oxygen uptake rates (4.4-7.7 mmol m^-2 d^-1, average 6.1 mmol m^-2 d^-1) and carbon mineralization efficiency (~ 90% of deposited carbon) were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates. The reactivity of organic carbon was found to decrease with age similarly to the power-law documented in marine environments. The burial flux of carbon into the deep sediment (0.7 mmol m^-2 d^-1) was 2.5% of the previously estimated primary production. Maximum volume-specific carbon degradation rates were 0.3-1.5 μmol cm^-3 d^-1; bioturbation coefficient near the sediment surface was 3-8 cm² yr^-1. These results indicate that carbon cycling in large freshwater systems conforms to many of the same trends as in marine systems.