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As part of an ongoing study of anoxic California continental borderland basins, the relationships between dissolved oxygen content, sediment fabric, biogenic structure distribution and size trends, and species richness in the San Pedro and Santa Barbara basins were examined. Results of these analyses are essentially identical to those of a previous study of the adjacent Santa Monica basin. The relationships observed in these basins will be useful as criteria for the reconstruction of ancient anoxic basins, the strata of which have high potential as hydrocarbon source beds.
Six sediment fabric types, subjectively classified on the basis of degree of preservation of primary sedimentary structures versus degree of destruction by biogenic activity, were observed in box core x-radiographs. These are distributed concentrically around the centers of the basins with a progressive trend toward increased preservation of primary structures with depth and decreased oxygen content. Intermediate sediment fabric types are formed by the short-term fluctuations of the position of the dysaerobic-anaerobic boundary in the water column.
Box core x-radiograph analysis of biogenic structures indicates that most burrow types occur over a wide range of depth and dissolved oxygen content. However, burrow diameter analysis indicates that burrow size decreases with a decrease in dissolved oxygen content. Bottom photograph analysis of surface biogenic structures indicates that aerobic and upper dysaerobic environments are characterized by abundant tracks and trails, lower dysaerobic environments are dominated by burrow openings, and anaerobic sea floor lacks any visible biogenic structures. Size trends of burrow openings indicate a decrease in size with decreased oxygen. However, organism-sediment interactions and environmental energy also exert a control on burrow-opening size.
Bottom photograph analysis of visible organisms shows that aerobic and upper dysaerobic environments are dominated by urchins, while lower dysaerobic environments are characterized by holothurians, polychaetes, small arthropods, and the small gastropod, Mitrella permodesta. Anaerobic surface environments are essentially devoid of visible macrobenthic organisms. Although these trends reflect changes in dissolved oxygen content, evidence suggests that grain size also exhibits control on organism distribution.
Species richness data from the upper portion of box cores indicates that diversity of macrobenthic organisms generally decreases with decreased oxygen content. The greatest decrease in diversity is coincident with the shelf break rather than the aerobic-dysaerobic boundary (1.0 mL/L dissolved oxygen). A loss of macrobenthic organisms that significantly affect sediment fabric occurs at the dysaerobic-anaerobic boundary.
Although observations made in these modern basins support parts of previously developed biofacies models designed for use in reconstruction of ancient anoxic basins, several aspects of these models now appear to be invalid. In particular, there appears to be no definitive change in any of the studied parameters at the aerobic-dysaerobic boundary. The use of sediment fabric, biogenic structure trends, and fossil evidence may aid in the reconstruction of ancient basins providing that controlling factors other than oxygen are thoroughly considered.
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