C. Transport in Canyons 



During the present interglacial period particulate organic carbon produced on the shelf 

 can be intercepted by canyons as currents sweep material along the bottom. This carbon can 

 be shunted quickly to deeper waters because daily internal tides cause flushing of the canyons 

 which resuspends sediment in the upper canyon and moves it down and away from the canyon 

 along isopycnal surfaces. Some of this material resettles down-canyon and some is moved out 

 of the canyon and along the slope in the direction of prevailing currents. Little transport 

 occurs along the deep canyon axis (>1000 m) except by the extremely rare turbidity cvurents. 



While no large canyons incise the shelf in the OMP region, there is a large canyon 

 (Norfolk Canyon) just north of the study area that could spew out sediments and organic 

 carbon which could then accumulate along the slope in the study area. Through the use of 

 moored current meters and optical sensors hydrodynamics and resuspension in the canyon axis 

 can be studies. Optical instruments can be used to record resuspension of fine-grained 

 sediments, but aggregates are not quantitatively sampled, and a large amount of transport 

 within and away from canyons occurs in that form. To quantify the concentration gradients of 

 both aggregates and suspended particles, a video imaging system for aggregates coupled with a 

 CTD and optical sensors for suspended particles will be employed. It is unknown whether the 

 small canyons and ravines on the slope in the OMP area cause focusing of intemal tides and 

 resuspension of bottom sediments as occurs in canyons. It is important to determine how far 

 away firom the slope the high concentrations of aggregates extend, because this determines 

 how far downslope they may settle. This off-slope transport should also be related to the 

 regions where ^^"^Th deficiencies have been measured down to 800 m, as the aggregates could 

 be important in scavenging Th. Particular attention must be paid to dynamics of the bottom 

 Ekman layer transport, bottom upwelling and downwelling as discussed elsewhere as these 

 processes are important in moving carbon down the slope. 



D. Alongshelf Transport 



At Cape Hatteras, shelf water leaves the continental margin for the ocean interior. 

 Associated wdth this advective transport is a carbon flux. A large proportion of this carbon is 

 ultimately derived firom the ocean interior upstream of Cape Hatteras, including but not limited 

 to, the initial formation of shelf water masses. In order to determine the net entry of carbon 

 into the oceanic carbon sink within the continental margin, it will be necessary to fuUy 

 characterize the difference between advective inputs and outputs of carbon within the Cape 

 Hatteras experimental region. The major advective transport is alongshelf. The magnitude of 

 seasonal variability for aU the relevant properties must be determined so that the seasonal 

 changes in along- and cross-shelf gradients may be differentiated from the changes produced 

 by processes occurring within the study area. 



On a broader scale, the alongshelf transport of carbon can be measured for the entire 

 Middle Atiantic Bight region. This is important for setting boundary conditions for the 

 Hatteras studies as well as examining the potential for carbon export from the shelf as a 

 whole. These alongshelf cruises should lead to a seasonal resolution of shelf dynamics 

 matched to ongoing process work at Hatteras. The cruises will be broadly biogeochemical in 

 nature, examining standing stocks and turnover of POM and DOM, as well as CO2, oxygen 

 and nutrient dynamics. In addition, the survey cruises will focus on biomarker characteristics 



