Attempts should be made to investigate integrative parameters which can be used to map 

 net fluxes. For example, to what extent does sedimentary chlorophyll concentration serve as a 

 flux measure for metabolizable organic matter? Is the technology for measuring dissolved 

 oxygen concentration sufficiently refined to assess apparent sedimentary oxygen utilization by 

 measuring oxygen deficits in the overlying benthic boundary layer, as a means of mapping 

 benthic metabolism over large areas? 



ii. Sediment accumulation and mixing rates 



Continental margins are regions of the oceans where rates of sediment accumulation and 

 carbon burial are high, reflecting sediment and nutrient inputs from adjacent landmasses. 

 Moreover, modem inputs of nutrients to the coastal ocean may be elevated by human 

 activities, potentially increasing the modem rate of carbon burial above the long-term average. 

 We can directiy address the role of the continental margin in drawing down present 

 atmospheric CO2 by determining modem rates of carbon burial in margin sediments. To also 

 recognize possible global change we must provide an historical context for the modem burial 

 rates by extending determinations of carbon burial rates thousands of years into the past. 



Particle-associated radionuclide tracers are used for determination of rates of sediment 

 accumulation; combined with measurements of carbon concentrations, sediment accumulation 

 rates yield carbon burial rates. Natural excess ^^°Pb and weapons-fallout nuclides (Pu, ^^^Cs) 

 provide information on rates of sediment accumulation and mixing for the past 100 years. ^'^C 

 itself is used to determine accumulation rates over longer time-scales. To assess the impact of 

 carbon burial on atmospheric CO2 it is necessary to determine the burial rates of both organic 

 and inorganic (CaC03) carbon. 



Determination of sediment accumulation rates from sediment tracer profiles rests upon a 

 number of assumptions as to sources of the radionuclide tracers and processes of tracer 

 transport within the sediment column. At a minimum, tracer transport by physical or 

 biological mixing must be separated from tracer transport due to sediment accumulation. 

 Various models, involving multiple tracers of different, but overlapping, time-scales (half-lives 

 or input histories), have been used successfully with tracer profiles of simple shape. Using 

 these conventional models, complex tracer profiles are often simply left uninterpreted. A new 

 procedure applicable to both simple and complex radionuclide tracer profiles has been 

 designed. The procedure combines inverse numerical techniques with predictive models to 

 separate effects of variable ti-acer strength and variable sediment accumulation rate in complex 

 profiles. The numerical algorithm can be used to derive accumulation rates from different 

 tracer profiles in the same core. This method will be used with radionucUde data (at least 

 ^^°Pb, fallout Pu, ^''C) to determine time variation in accumulation rates in sediment cores 

 from shelf-rise environments off Cape Hatteras. Relative to conventional methods of 

 interpretation, improved spatial and temporal resolution in sediment accumulation is 

 anticipated. 



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

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

 reworking by macrobenthos dominates particle transport in most subtidal sediments underlying 

 oxygenated waters. Bioturbation rates should be measured using a variety of tracers including 

 •^^''Th, ^Be, chloropigment profiles, and injected luminophores. This has the dual purpose of 

 potentially evaluating changes in decomposition patterns in relation to bioturbation and 



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