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
Y. Furukawa et al., 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, J MARINE RE, 58(3), 2000, pp. 493-522
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.