Core observation, X-radiography, and scanning electron microanalysis of distinctly laminated, indistinctly laminated, and nonlaminated sediments from ODP Site 893 show that laminated as well as nonlaminated (i.e., massive) hemipelagic sediments can form in the absence of bioturbation. This view contrasts with conventional sedimentological interpretations equating laminated hemipelagic sediments with anoxic water masses (i.e., absence of bioturbation), and nonlaminated sedimentary intervals exclusively with oxygenation episodes that permitted infaunal colonization. We conclude that heterogeneities in the texture and/or composition of sediment supply are necessary for the production of laminated sediments; the absence of hydraulic and biological reworking permits their preservation . Laminae at Site 893 have two prominent components: biosilica and terrigenous detritus. Compositional contrasts between adjacent laminae in X-radiographs reflect short-term (seasonal and subseasonal) heterogeneities in sediment flux. Core intervals with large contrasts in density and/or composition between adjacent laminae are termed as having high bimodality (HB); HB couplets produce distinctly laminated sediments that intergrade vertically with indistinctly laminated and nonlaminated sediments. Adjacent laminae in indistinctly laminated sediments possess minimal contrasts in bulk density and composition; nonlaminated sediments are compositionally and texturally homogeneous. Many biosiliceous laminae responsible for distinctly laminated, HB couplets record mass flocculation and sedimentation of ungrazed diatom frustules formed during discrete bloom events. Ecological interpretation of diatomite laminae suggests that many were effectively self-sedimenting (i.e., the production of gelatinous exudates by phytoplankton facilitated their aggregation and rapid sedimentation). The results indicate that lamination style preserves meaningful ocean climate data concerning episodicity, heterogeneity, and export efficiency of biologically mediated sedimentary flux in upwelling-dominated coastal ecosystems.

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