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

Abstract

Biologically enhanced pore water irrigation affects the course of early diagenesis in shallow marine sediments, as illustrated here for the carbonate sediments from North Key Harbor, Dry Tortugas National Park, Florida. Whereas macrofaunal activity at the study site extends approximately 15 cm below the water-sediment interface, measured O 2 microprofiles only show O 2 penetration to depths of a few mm. This apparent discrepancy can be explained by considering the 3-D O 2 distribution in the burrowed sediments. Calculations based on an idealized tube model for burrow irrigation show that measureable O 2 concentrations are limited to the immediate vicinity of burrows. Given the observed burrow density (705 ± 15 m -2 ), a randomly positioned 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 sediment via the burrows. Other characteristic features observed in the upper 15-20 cm of the sediments, in particular, the absence of SO 4 2- depletion and the presence of subsurface maxima in the profiles of NH 4 + and TCO 2 , are also the result of pore water irrigation. These features are reproduced by the multicomponent reactive transport model STEADYSED1. Results of the model simulations indicate that bacterial SO 4 2- reduction is the dominant pathway of organic carbon degradation, but that consumption of SO 4 2- in the sediments is compensated by its enhanced transport by irrigation. Thus, depth profiles of SO 4 2- may be poor indicators of the importance of SO 4 2- reduction in irrigated sediments.