8.5.1 Characterization of Sediment Organic 

 Matter 



RICHARD S. SCALAN' , E. WILLIAM BEHRENS*. MICHAEL E. CAUGHEY*, RICHARD K. ANDERSON*, 

 and PATRICK L. PARKER' 



'Universin- of Texas. Marine Science Institute. Port Aransas. Texas, USA 

 ''University of Texas, Institute for Geophysics. Austin. Te.xas. USA 

 * Aquatic Chemistry Section. Illinois State Water Sun'ey. Champaign. Illinois. USA 

 "Coastal Science Laboratories. Inc.. Austin. Texas, USA 



Introduction 



Carbon isotopic compositions are effective indicators of 

 the source of sedimentary organic matter (Craig, 1953). For 

 example, Sackett and Thompson (1963). Hunt (1970), and 

 Hedges and Parker ( 1 976) used stable carbon isotopes as a 

 tracer of the incursion of terrestrial organic matter into the 

 estuaries bordering the Gulf of Mexico and the Atlantic Ocean. 

 A depletion in carbon of the heavier isotope ("C) is generally 

 associated with terrestrially derived organic matter, while an 

 enrichment suggests incorporation of marine derived material 

 (Fry & Sherr, 1984). 



Nitrogen isotope variations are less associated with the 

 environment of their origin (Sweeney & Kaplan, 1980) but 

 rather reflect the trophic level from which the organic matter 

 has been derived (Miyake & Wada, 1967; Wada & Hattori, 

 1976;Mackof?fl/., 1982; Minigawa& Wada, 1986). Primary 

 producers of organic nitrogen are isotopically like their source, 

 atmospheric nitrogen, while consumers of organic matter 

 become enriched in the heavier isotope ( '^N) with each step up 

 the trophic ladder. The isotopic composition of nitrogen 

 incorporated into marine sediments may indicate not only the 

 trophic shift but also the degree of cycling and recycling of the 

 organic matter in the water column and in the sediments. 

 Remineralization of organic nitrogen may not be accompanied 

 by significant isotopic fractionation; however, many reactions 

 of nitrogenous compounds in natural and cultured systems may 

 lead to considerable nitrogen isotope fractionation (Cifuentes 

 elal.. 1988;Hochf/a/., 1989). 



Carbon and nitrogen elemental content, and more 

 specifically the C/N ratio, are also useful for indicating the 

 sources of organic matter. Terrestrial plants with high carbon 

 to nitrogen ratios are contrasted with marine organisms rich in 

 organic nitrogen. The relative contributions of these two 

 sources can be estimated by their C/N ratios (Fagancli et al.. 

 1988). Of course, the picture is complicated in nature because 

 there are more than two end members in most ecologic systems. 



This report summarizes the results and conclusions of a 

 four year effort to study the isotopic and elemental distribution 

 of organic carbon and nitrogen of sediments within the Chukchi- 

 Bering-Anadyr system. Included in the repertoire of samples 

 are those recovered in the Third Joint US-USSR Bering & 

 Chukchi Seas Expedition in August 1988. 



The present distribution of carbon and nitrogen throughout 

 the arctic marine ecosystem can result from several well known 

 but, perhaps, inadequately studied processes; 7. primary 

 production through fixing atmospheric carbon dioxide and 

 nitrogen by phytoplankton; 2. transport of nutrients into the 

 system from upwelling of deep Pacific waters; i. transportation 

 of primary production of terrestrial origin by rivers and streams; 

 and 4. recycling of the.se elements by remineralization of 

 organic matter both in the sediments and in the water column. 

 In this report we are primarily concerned with the sources and 

 sinks of these elements within the sediments. 



Methods 



Sediment samples were collected on eight different cruises 

 from 487 stations at approximately 280 separate locations. 

 Table 1 lists the cruise identifications, dates, and number of 

 samples, plus replicates of sediments taken for each cruise. 

 Sample station locations are shown on the map of Fig. 1. 

 Because of strong bottom currents, some samples (particularly 

 those recovered from the area of Anadyr Strait west of 

 St. Lawrence Island) were mainly cobbles and boulders with 

 very little fine-grained sediments at the surface. Samples from 

 this locale are largely muds that were found adhering to cobbles 

 or the van Veen grab. 



Samples were frozen at the lime of collection and 

 maintained frozen until prepared for analysis in the laboratory. 

 Samples were thawed, acidified, diluted, and picked free of any 

 visible macro-organic materials such as worms, amphipods, 

 hydroids, bivalves, etc. The residues were filtered, washed, 

 air-dried at 60°C, and very gently disaggregated by passing 

 through a screen of opening size equal to 150 |i meter. Any 

 material not passing this mesh was discarded. 



TTie nitrogen isotope measurements were made on a Nuclide 

 model RMS-6 mass spectrometer. For carbon isotopes a VG 

 Micromass model 602-E instrument was used. The data are 

 expressed by the conventional del notation: 



"("C/'-C), 



[("C/'-O 1 



("C/'=C)„„ J 



10' «/,. 



("C/'=C)„„ 



as the parts per thousand difference in the isotope ratio of the 

 sample and an arbitrary standard, the carbon dioxide generated 

 from the PeedeeBelemnite (Craig, 1957). Asimilarexpression, 



333 



