production, community respiration, nutrient and DOC concentrations, microzooplankton 

 abundance and grazing rates, and other parameters tiiat describe the links between bacteria and 

 other trophic levels. 



E. Geochemical Tracers of Organic Matter Transformations 



A multiple natural isotopic tracer approach using ^^C, ^^C, ^'*C, ^^""Th, ^^^Th, and ^^^ 

 on each of the different organic carbon classes will be employed to provide concomitant 

 information on turnover rates, sources and potential sinks of organic carbon. Information on 

 specific biomarkers (lignin, carbohydrates) will be needed as these compounds will help 

 investigators determine the sources and transformations of carbon. Pigment and lipid 

 biomarker analyses provide semi-quantitative estimates of classes of phytoplankton, bacteria, 

 zooplankton and terrestrial organic matter. Stable isotope (^^C and ^^N) analysis will be 

 useful in distinguishing source terms along the shelf and in tracking their decomposition and 

 dispersal. 



The naturally occurring radioactive Th isotopes, ^"^Th (tl/2 = 24.1 y) and ^^^Th (tl/2 = 

 1.91 years) and ^^°Th (tl/2 = 75,000 years) have been identified as excellent tracers for 

 quantifying rates and mechanisms of transformation between the various size classes. The 

 distribution of all three Th isotopes in the oceans is constrained by a balance between supply 

 from soluble parent nuclides, radioactive decay and removal ("scavenging") from the water 

 column onto suspended and sinking particulate matter. By using measurements of ^^"^/^^^U, 

 ^^^Th/^^^Ra and ^^^Th/^^'^U disequilibria in dissolved, colloidal and particulate fractions and 

 existing box models, it is possible to calculate transformation rates between the various size 

 classes. 



Bomb ^^C in surface waters imprints newly-produced biomass with a unique "future-age" 

 radiocarbon signature. Sediments which accumulate only or predominantly modern biodetritus 

 should yield future ^''C ages (b^^C > 0) in surface layers. In fact, while examples of this sort 

 have been found in extreme nearshore settings, b^^C > is rare in surficial sediments from 

 open-ocean settings. Finite ^^C ages {d^^C < 0) in surface sediments result from rapid loss of 

 the bomb ^^C signal by two mechanisms: (i) remineralization of fresh biodetritus in the water 

 column and surface sediments; and (ii) mixing with older organic matter in the sediment 

 column. 



Despite the dearth of clear examples of bomb ^'^C in shelf and slope sediments, surveys 

 of ^'*C in surficial sediments remain important to the question of storage of modern carbon on 

 the continental margin. In the first place, geographic coverage of such measurements is very 

 limited globally, especially on margins not distinguished by exceptional productivity or rapid 

 sediment accumulation. Limited data on the Atlantic continental margin of the United States 

 show sediment accumulation and transport to be laterally heterogeneous. 



For the Mid-Atlantic Bight at least, this heterogeneity extends to ^'^C in surface 

 sediments. We need more observations of ^"^C in surface sediments along sediment dispersal 

 pathways before we can conclude that the bomb signal is absent from the sediments. Indeed, 

 at present we need more such observations before we can intelligently plan a sampling 

 program to search for modem biodetritus (5^'*C > 0) in a designated field area. 



Measurements of ^^C in surface sediments of the continental margin help to constrain 

 maximum storage of modern biodetritus, whether or not unambiguous modern ^"^C (S^'^C > 0) 



35 



