BIOLOGICAL FORCING OF HEMIPELAGIC SEDIMENTARY LAMINAE - EVIDENCE FROMODP SITE-893, SANTA-BARBARA BASIN, CALIFORNIA

Core observation, X radiography, and scanning electron microanalysis o f distinctly laminated, indistinctly laminated, and nonlaminated sedim ents from ODP Site 893 show that laminated as well as nonlaminated (i. e., massive) hemipelagic sediments can form in the absence of bioturba tion, This view contrasts with conventional sedimentological interpret ations equating laminated hemipelagic sediments with anoxic water mass es (i.e., absence of bioturbation), and nonlaminated sedimentary inter vals exclusively with oxygenation episodes that permitted infaunal col onization, We conclude that heterogeneities in the texture and/or comp osition of sediment supply are necessary for the production of laminat ed sediments; the absence of hydraulic and biological reworking permit s their preservation. Laminae at Site 893 have two prominent component s: biosilica and terrigenous detritus, Compositional contrasts between adjacent laminae in X-radiographs reflect short-term (seasonal and su bseasonal) heterogeneities in sediment Aux, Core intervals with large contrasts in density and/or composition between adjacent laminae are t ermed as having high bimodality (HE); HE couplets produce distinctly l aminated sediments that intergrade vertically with indistinctly lamina ted and nonlaminated sediments, Adjacent laminae in indistinctly lamin ated sediments possess minimal contrasts in bulk density and compositi on; nonlaminated sediments are compositionally and texturally homogene ous, Many biosiliceous laminae responsible for distinctly laminated, H E couplets record mass flocculation and sedimentation of ungrazed diat om frustules formed during discrete bloom events, Ecological interpret ation of diatomite laminae suggests that many were effectively self-se dimenting (i.e., the production of gelatinous exudates by phytoplankto n facilitated their aggregation and rapid sedimentation), The results indicate that lamination style preserves meaningful ocean climate data concerning episodicity, heterogeneity, and export efficiency of biolo gically mediated sedimentary flux in upwelling dominated coastal ecosy stems.