scavenging and/or exchange processes. It will therefore be important to define the "endpoints" 

 outside the immediate study area where those boundary exchanges with time scales similar to 

 the decay times of ^'^Th, i.e., 35 days, will have subsided. 



2. Benthic boundary layer - sediment measurements 



Benthic environments provide both a temporary processing site and a final storage site for 

 carbon sedimented from the water column. The coarse-grained nature of the shelf in the 

 Hatteras region suggests that this area may not be important as a final storage site, but 

 previous observations have shown rich benthic communities in the swales of sand ripple fields. 

 These observations lead to the hypothesis that microenvironments may be important in 

 processing deposited organic matter for some period of time, and then lose the carbon in a 

 resuspension event associated with high-energy storms characteristic of this region. 



The Hatteras region can be divided into a number of benthic provinces. These include: 



• Shallow shelf of < 15-25 m depth, characterized by the presence of benthic primary 



production in some seasons and frequent exposure to storm waves, topographically 

 consisting of ridges and swales. Benthic primary production has been measured to 

 depths exceeding 30 m in Onslow Bay. The maximum depth could be lower north of 

 Cape Hatteras where there is probably more fine-grained, terrigenous material in the 

 water column. 



• Deeper shelf environments of 25-200 m depth, are characterized by the presense of 



heterotrophic organisms and topographically by ridges and swales. Slope depocenter is 

 characterized by intense accumulations of organic matter at about 900 m, and perhaps 

 areally more extensive regions of organic carbon accumulation at depths greater than 

 typically seen in continental rises and slopes. 



• Estuarine environments which have the potential to be a significant sink for shelf organic 



matter. 



• Canyons that likely act as both an important conduit and perhaps also a significant 



processing zone for shelf-derived organic matter. 



The first two of these environments would be studied as sites of deposition and benthic 

 processing of organic matter. The slope depocenter is of interest primarily as a burial sink for 

 shelf-derived organic matter and, through its metabolic activity, as a detector of the timing and 

 extent of export from the shelf. The estuarine areas would be studied primarily for their 

 potential role as burial sites for shelf-derived organic matter, leaving process-oriented studies 

 for the many existing estuarine research groups. While export through canyons may be a major 

 mechanism by which shelf-derived organic matter is transported to the deep sea, process 

 studies within canyons are given a lower priority than the above studies. Processing of 

 organic matter in canyons may be very different than at shelf locations. 



The ridge and swale topography of the shelf provinces likely sets up a heterogeneity 

 problem that will affect sampling. Fine-grained muds, and possibly organic matter, wUl 

 preferentially accumulate in the swales; benthic responses may be similarly concentrated. It 

 will therefore be necessary to assess the heterogeneity within the ridges and swales, and 

 sample in either specific locations within this topography or in larger numbers to overcome the 

 heterogeneity. Permanent benthic stations, revisited by submersible or ROV, would facilitate 

 reproducible process studies. The temporal sampling strategy is predicated on assessing 



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