comparing the agreement and resolution provided by different tracers in the examination of 

 mixing processes. 



Sedimentary environments where sediment mixing caused by both biological and physical 

 processes is occurring will also be analyzed using numerical models. The bioturbation 

 signature wdll be separated from the radioisotope data profiles using two isotope profiles for a 

 single core. 



The rate and style of particle transport within deposits have a major influence on 

 decomposition pathways, recycling patterns, and storage of reaction products. Particle 

 reworking by macrobenthos dominates particle transport in most subtidal sediment underlying 

 oxygenated waters. These activities depend on the types and abundances of animals. 

 Biological community structure, including species composition, abundances, sizes and 

 functional groupings can vary significantly both spatially and seasonally in shelf environments 

 and may be correlated with seabed topography. Erosion of "ridge" areas by storms removes 

 organic matter and disturbs the benthic community. These regions are dominated by fossorid 

 and interstitial feeding macrofauna and tubiculous polychaetes which are typical early 

 colonizers. Depressions or swales that may be located a few hundred meters away are less 

 frequently disturbed and are the foci for organic matter deposition. Dense populations of 

 deposit feeding tubiculous amphipods and burrowing polychaetes, typical of higher-order 

 successional stage communities, dominate swales. The characterization of benthic communities 

 in our study will be used largely for interpreting transport processes in radionuclide 

 distribution, pore water profiles, and solid phase diagenetic reactions. The relative importance 

 of biological and physical processes is essential for understanding the dynamics of the shelf 

 and slope environments. Macrobenthos wiU also be measured as one component of biomass 

 cycling representing in situ synthesis of new carbon from planktonic inputs. 



While the measurements described above will provide much information concerning the 

 deposition, resuspension, transformation, dissolution and burial of particles on the shelf and 

 adjacent areas, morphological features of the sea floor may significantiy influence the carbon 

 dynamics of a particular location. For example, ridge and swale structures on the shelf may 

 concentrate or focus the deposition of fresh organic materials into the topographically low 

 areas. This wovild, in turn, intensify metabolic and diagenetic processes in these regions 

 relative to the surrounding higher locations. Depending on the horizontal scale of such 

 features, significant variability of benthic processes may result. Similarly, on the slope, 

 canyons may serve as major conduits for the off-shelf transport of particulate materials and 

 away from canyons, slope morphology suggests that mass wasting is an important down-slope 

 transport process. 



To understand and interpret the results of mechanistic and process studies described in 

 the previous sections, it will be necessary to assess sediment morphology and type 

 distributions on the shelf and slope. Broad surveys using side-scan and 3.5 KHz sonar should 

 be used to identify the scales of horizontal variability. Sampling at specific, known locations 

 using ROV's or submersibles will be required to relate sediment types and processes to the 

 larger-scale morphologic features. Attempts should be made to resolve sediment properties 

 such as grain size and water content, as these properties may serve as correlative measures for 

 organic matter content or biotic communities. 



20 



