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Eustatic Signals in Deep-Marine Sedimentary Sequences Recovered at ODP Site 978, Alboran Basin, Western Mediterranean Sea

¹ Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, California 91330-8266, U.S.A.; kathie.marsaglia{at}csun.edu
² Department of Geography, Faculty of Science, Tokyo Metropolitan University, Hachiohji, Tokyo 192-03, Japan
³ Department of Geosciences, The University of Tulsa, 600 South College Avenue, Tulsa, Oklahoma 74104-3189, U.S.A.
⁴ Department of Environmental Sciences, University of Technology, Sydney, P.O. Box 123 Broadway, New South Wales 2007, Australia
⁵ Department of Geological Sciences, The University of Michigan, 3514 C.C. Little Building, 425 East University Avenue, Ann Arbor, Michigan 48109-1063, U.S.A.
⁶ Geophysics Department, 397 Panama Mall, Stanford University, Stanford, California 94305-2215, U.S.A.
⁷ Ocean Drilling Program, 1000 Discovery Drive, College Station, Texas 77845, U.S.A

A continuous section of Pliocene marine sediments was recovered at Ocean Drilling Program Site 978, located in the Alboran Sea between Spain and Morocco. Three Pliocene subunits have been defined at Site 978: the lowermost (Subunit IC, 129.2 m thick) is characterized by alternating beds of lighter, more calcareous, and darker, less calcareous, claystone with bioturbated upper and lower contacts (Type 1 cycles); the middle (Subunit IB, 67.1 m thick) is composed of relatively homogeneous nannofossil claystone; and the uppermost (Subunit IA, 211.6 m thick) contains abrupt-based darker, terrigenous layers interpreted as turbidites that are interstratified with lighter nannofossil claystone (Type 2 cycles). The rhythmically bedded light and dark layers in Subunit IC correlate with those in the Rosello Composite Section of Sicily, a global reference standard for the Pliocene time scale. These sedimentary cycles are products of variations in precession and resulting continental runoff. Missing cycles occur during eustatic highstands. The shift to more homogeneous sedimentation in Subunit IB is represented in similar-aged sequences throughout the Mediterranean which display evidence of submarine mass wasting. Mediterranean-wide slope degradation was likely a response to rapid sea-level change at approximately 3 Ma. This change in sedimentation style was accompanied by an upsection increase in sediment accumulation rates associated with turbidite influx in Subunit IA. Turbidite frequency throughout the Pliocene section can be linked to eustatic changes in sea level, with turbidite maxima corresponding with mid-sequence downlap surfaces and their associated condensed sections.