Mixed bioturbation

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... 

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). 

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 ... 

Bioturbation The physical mixing of sediments caused by the burrowing and feeding activities of benthic organisms. 

Fauna also influence soil carbon cycling. Bioturbation mixes and aerates soil, physically breaks down litter, creates flow paths for water in soil, and can reduce surface litter stocks and enhance erosion (Bohlen et al., 2004). For example, along a gradient of European earthworm (Lumbricus terrestris) colonization in a deciduous forest of northern Michigan, earthworms are associated with a decrease in litter-layer thickness, apparently mixing some forest floor organic matter into the mineral soil. Thus, fauna can create spatial patterns in SOM stocks. 

In addition to calcite, aragonite and occasionally magnesian calcites may also reach the ocean floor. They may play an important role even in sediments where they do not accumulate, by contributing carbonate ion to pore waters because of their dissolution, thus causing less calcite dissolution to occur than would be the case in their absence. This process is enhanced by the benthic process of bioturbation, whereby organisms cause a physical mixing of sediments down to... 

Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates (Armentano and Woodwell, 1975 Krishnaswami et al., 1980 Church et al., 1981 Kuehl et al., 1982 Olsen et al., 1985 Paez-Osuna and Mandelli, 1985 McKee et al., 1986 Lynch et al., 1989 Bricker-Urso, 1989 Moore, 1992 Smoak et al., 1996 Dellapenna et al., 1998, 2001 Benoit, 2001 Corbett et al., 2003). Lead-210 is considered to be a reliable method for dating sediments deposited over the last 100 to 110 y (Krishnaswami et al., 1971). In the absence of bioturbation/mixing the activity gradient of excess 210Pb in sediments, which is the net result of accumulation and radioactive decay, can provide information on the sedimentation rate of recent sediments. Unfortunately, in many... 

Assuming that the transport (e.g., physical mixing and bioturbation), depositional inputs (e.g., sources/quality and amounts of POM), mass sediment accumulation rate, temperature, and decomposition are in steady state, as well as lateral homogeneity of the deposit, the concentration of POM will not change over time. Although these conditions are almost never met in estuarine systems (Berner, 1980), steady-state conditions will be used here for the general purposes of this discussion. Thus, assuming steady-state conditions, the GDE for POM decay is as follows (Rice and Rhoads, 1989) ... 

Figure 8.25 Depth of mixing and bioturbation rates as they relate to seasonal temperature changes in different regions of Chesapeake Bay (USA). (Modified from Schaffner et al., 2001.)...

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