Bioturbation

The intensity of bioturbation is expressed as biodiffusivity (Dg) and the effect of bioturbation versus the molecular diffusion is described as the ratio between Dg and Dj. A vast amount of literature is available on the role of macrobenthos on the exchange of solutes across sediment-water interface in freshwater and marine sediments. It should be noted that in wetlands, rooting of vegetation in soils creates additional complexity on exchange of solutes across soil-floodwater interface. The macrobenthos can influence the vertical distribution of sediments and POM, and the 

FIGURE 14.7 Schematic representation of major burrowing fauna in soils and sediments. (Redrawn from Aller, 1982.) 

Biogeochemistry of Wetlands Science and Applications Undisturbed Bioturbated 

FIGURE 14.8 Schematic showing solute concentration gradients influenced by bioturbation. 

Stratification of electron acceptor distribution in sediments and its maintenance in four major ways (Aller, 1982 Kristensen, 2000)  

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Bioturbation, due to the burrowing action of organisms, may connect sand layers otherwise separated by clay laminae, thus enhancing vertical permeability. On the other hand, bioturbation may homogenise a layered reservoir resulting in an unproducible sandy shale. 

Geochemically significant mixing and transport can sometimes be accomplished by biological processes. An interesting example is redistribution of sediment material caused by the movements of worms and other organisms (bioturbation). 

Mixing of sediment and water at the interface (bioturbation and water turbulence). 

Green MA. Aller RC, Cochran JK, Lee C, Aller JY (2002). Bioturbation in shelf/slope sediments off Cape Hatteras, North Carolina The use of " Th, Chl-a, and Br to evaluate rates of particle and solute transport. Deep-Sea Res 1149(20) 4627-4644... 

Henderson GM, Lindsay FN, Slowey NC (1999) Variation in bioturbation with water depth on marine slopes a study on the Little Bahamas Bank. Mar Geol 160 105-118 Henderson GM, Anderson RF (2003) The U-series toolbox for paleoceanography. Rev Mineral Geochem 52 493-531... 

Pope RH, DeMaster DJ, Smith CR, Seltman Jr H (1996) Rapid bioturbation in equatorial Pacific sediments evidence from excess measurements. Deep-Sea Res 1143 1339-1364. 

Where Ao is the activity at the sediment surface, w is the sedimentation rate (cm yr ), D is the mixing rate (cm yr ), is the decay constant for the nuclide of interest (yr ) and z is the depth in the sediment (cm). In some near-shore environments both sedimentation and bioturbation must be considered. But in most open marine environments the sedimentation rate is sufficiently slow that it can be ignored and the equation simplifies to ... 

Boudreau BP (1994) Is burial velocity a master parameter for bioturbation Geochim Cosmochim Acta 58(4) 1243-1249... 

Bioturbation Disruption of sediment layers by activity of living organisms... 

Diffusion is characterized by a mass transfer coefficient U8 of 104 m/h, which can be regarded as a molecular diffusivity of 2 x 10 6 m2/h divided by a path length of 0.02 m. In practice, bioturbation may contribute substantially to this exchange process, and in shallow water current-induced turbulence may also increase the rate of transport. Diffusion in association with organic colloids is not included. The D value is thus given as Us AwZ2. 

Sediments are important compartments for many organic contaminants in the aquatic environment, in particular for hydrophobic POPs such as PAHs and PCBs. Sediments have been recognised as important sinks for these compounds but with the reduction in levels of them in water, the question arises of whether the older highly contaminated sediments will function in the future as secondary sources of the compounds or whether burial by recent, cleaner sediment will prevent exchange with the water phase. This will depend on the strength of turbulence/bioturbation and on anthropogenic influences such as dredging. 

The simplest approach considers a perfectly mixed bioturbated layer of thickness L and homogeneous concentration C. If v is the sedimentation rate, the mass balance condition for element i reads... 

Fora bioturbation of constant thickness L, the conservation equation now becomes... 

Michel et al. (1990) have measured the iridium content in Ocean Drilling Project hole 690 sediments from the Weddel Sea across the Cretaceous-Tertiary (K-T) boundary (Table 8.1). Depth z is relative to an arbitrary level in the core. Compare the reduced iridium flux at each depth down the core assuming the bioturbated layer is either 4 or 8 cm thick. 

Figure 8.4 Smearing of a burst in the input of an element i at the surface of the sediment by bioturbation over a layer of constant thickness L (from Ruddiman and Glover, 1972). With time, combined sedimentation at rate v and bioturbation, smears the concentration peak up the sedimentary column.

Reichardt W (1987) Burial of Antarctic macroalgal debris in bioturbated deep-sea sediments. Deep-Sea Res 34 1761-1770... 

Other techniques are used for shorter time scales, including the measurement of the 226Ra Ba ratio in barite extracted from sediments (Paytan et al., 1996). This technique has a time scale of about five thousand years. Alternately, assessments of rapid sedimentation and bioturbation on time scales of days to centuries require shorter half-life nuclides such as 210Pb, 228Th, 234Th, and 222Rn. 

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