The role of biologically-enhanced pore water transport in early diagenesis: An example from carbonate sediments in the vicinity of North Key Harbor, Dry Tortugas National Park, Florida

Biologically enhanced pore water irrigation affects the course of early dia genesis in shallow marine sediments, as illustrated here for the carbonate sediments from North Key Harbor, Dry Tortugas National Park, Florida. Where as macrofaunal activity at the study site extends approximately 15 cm below the water-sediment interface, measured O-2 microprofiles only show O-2 pen etration to depths of a few mm. This apparent discrepancy can be explained by considering the 3-D O-2 distribution in the burrowed sediments. Calculat ions based on an idealized tube model for burrow irrigation show that measu reable O-2 concentrations are limited to the immediate vicinity of burrows. Given the observed burrow density (705 +/- 15 m(-2)), a randomly positione d O-2 microprofile has a high probability (>90%) to fall outside the reach of radial O-2 diffusion from burrows. Hence, the shallow penetration depths recorded at the site do not exclude a much deeper supply of O-2 in the sed iment via the burrows. Other characteristic features observed in the upper 15-20 cm of the sediments, in particular, the absence of SO42- depletion an d the presence of subsurface maxims in the profiles of NH4+ and TCO2, are a lso the result of pore water irrigation. These features are reproduced by t he multicomponent reactive transport model STEADYSED1. Results of the model simulations indicate that bacterial SO42- reduction is the dominant pathwa y of organic carbon degradation, but that consumption of SO42- in the sedim ents is compensated by its enhanced transport by irrigation. Thus, depth pr ofiles of SO42- may be poor indicators of the importance of SO42- reduction in irrigated sediments.