bably reflect the chemistry of sediments under relatively calm condi- 

 tions. Mixing which occurs as a result of tidal or storm driven wave 

 action might alter the depth to which oxygen can penetrate sediments. 

 This should vary with the nature of the sediment so that muds should be 

 less affected than sandy sediments on high-energy beaches. Over sea- 

 sonal time intervals, it is likely that the aerobic/anoxic boundaries 

 predicted by Eh profiles are preserved in the muddy sediments sampled. 

 However, it is likely at the oiled beach AMC-4 that erosion and deposi- 

 tion created considerable instability in the depth of oxygen penetra- 

 tion and could even have caused vertical redistribution of sediments 

 (Gundlach and Hayes, 1978). It is also possible that oxygen could be 

 introduced to depths below the lower boundary of its diffusion by sedi- 

 ment infauna which can burrow into anaerobic sediments. 



The vertical distribution of the dominant anaerobic process, sul- 

 fate reduction, indicated maximum activity in the surface 0-3 cm inter- 

 val at all stations. The obligately anaerobic sulfate-reducing and 

 methane-producing bacteria were also present in maximum number in the 

 0-3 cm depth interval (Winfrey and Ward, submitted). These observa- 

 tions suggest that at least portions of the 0-3 cm interval at all 

 sites were sufficiently anoxic to allow survival and activity of obli- 

 gately anaerobic microorganisms. 



A survey of the various sediments sampled confirmed the presence 

 of AMOCO CADIZ pollutants in oiled sites. Although control sites were 

 not polluted by the AMOCO CADIZ spill, each contained some hydrocarbons 

 of anthropogenic origin. The extent of oiling was greatest at the sedi- 

 ment surface where anaerobic processes were greatest. Oiling decreased 

 with depth, but there was clear evidence of AMOCO CADIZ hydrocarbons in 

 sediments likely to be free from exposure to oxygen. Sediments below 

 the aerobic/anoxic boundary and above the deepest level of penetration 

 of AMOCO CADIZ pollutants provided an environment suitable for the en- 

 richment of anaeroDic hydrocarbon-degrading microorganisms, and appro- 

 priate for comparison to aerobic surface sediments to study the differ- 

 ences in weathering in situ due to different exposures to oxygen. 



Because of the rapid biodegradation of aliphatic components and 

 relative enrichment of aromatic components of the spilled oil (Atlas, 

 et al, 1981; Ward, et al, 1980), ratios of naphthalenes, phenanthrenes 

 and dibenzothiophenes to the more persistent C -DBT were used as an 

 index of weathering. This index should be independent of absolute 

 amounts of oil within sediment samples which could vary due to patchy 

 distribution of oil. Changes in sediment aromatic hydrocarbons oc- 

 curred in all sediments and at all sediment depths where comparisons 

 were made for one year or longer. The greatest and most rapid changes 

 were noted in surface sediments of the beach station AMC-4 where most 

 compounds had decreased by one year after the spill and extensive 

 losses had occurred by about 20 months after the spill. This seems 

 consistent with the expected mixing and oxygenation of this high energy 

 beach sediment. In muddy sediments , slower changes in relative amounts 

 of aromatic compounds were noted. Extensive losses were observed main- 

 ly among the naphthalenes and DBT's. This may have been related to the 

 low energy nature of these sediments and/or the corresponding lack of 

 oxygenation indicated by reducing conditions. Decreases in these com- 



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