HR: 16:30h
AN: OS14A-03    [Abstracts]
TI: The Peru Margin as an Authigenic Mineral Factory, Evidence From Surface Sediments and Oceanography
AU: * Dean, W E
EM: dean@usgs.gov
AF: U.S. Geological Survey, Box 25046, MS980, Federal Center, Denver, CO 80225 United States
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: Department of Geosciences, Pennsylvania State University, University Park, PA 16802 United States
AB: Characteristics of sediments deposited within an intense oxygen-minimum zone (OMZ) on the Peru continental margin were mapped by submersible, and studied in samples collected in deck-deployed box cores and submersible push cores on two east-west transects over water depths of 75 to 1000 m at 12 degrees and 13.5 degrees S. On the basis of sampling of the top 1-2 cm of available cores, three main belts of sediments were identified in each transect with increasing depth: 1) organic-carbon (OC)-rich muds; 2) authigenic phosphatic mineral crusts; and 3) glaucony facies. These facies patterns are primarily controlled by redox conditions and strength of bottom currents. OC-rich sediments on the 12-degree transect were mainly located on the outer shelf and upper slope (150-350 m), but they occurred in much shallower water (ca. 100 m) on the 13.5-degree transect. The organic matter is almost entirely marine, resulting from very high primary productivity. The OC concentrations are highest (up to 18%) in sediments where intermediate water masses with low dissolved oxygen concentrations (less than 5 micromoles/kg) impinge on the slope at water depths between 75 and 450 m. The region between 175 and 350 m depth is characterized by bedforms stabilized by bacterial mats, extensive authigenic mineral crusts, and (or) thick organic flocs. Currents as high as 30 cm/sec were measured over that depth interval. Current-resuspension of surficial organic matter, activity of organisms, and transport to and from more oxygenated zones contribute to greater oxidation and poorer preservation of organic matter than occur under oxygen-deficient conditions. Phosphate-rich sediments occurred at depths of about 300 to 550 m on both transects. Nodular crusts cemented by carbonate-fluorapatite (CFA; phosphorite) or dolomite form within the OMZ. The crusts start by cementation of sediment near the sediment-water interface forming stiff but friable phosphatizes claystone "protocrusts". The protocrusts evolve into dense, dark phosphorite crusts, cemented breccias, and pavements. The degree of phosphatization and thickness of the phosphorite crusts depends on rates of sediment supply and strength and frequency of currents that re-expose crusts on the seafloor. Glaucony-rich surficial sediments, relatively undiluted by other components, mainly were found in deeper water on the 13.5 degree transect (750 m to at least 1067 m). These sediments consist almost entirely of sand-size glaucony pellets (aggregates of clay minerals with pelletoid shapes). These widespread glaucony sands possibly formed in situ and were then concentrated and reworked by strong currents that winnowed away the fine-grained matrix. Overall, sedimentation rate must be slow in order for the glaucony minerals to remain in contact with seawater, which is the source of cations during growth. The close association of glaucony and phosphorite indicates that there is a delicate balance between slightly oxidizing and slightly reducing conditions at the base of the OMZ- slightly reducing to mobilize iron and phosphate, and slightly oxidizing to form glaucony.
DE: 4825 Geochemistry
DE: 4802 Anoxic environments
DE: 3022 Marine sediments--processes and transport
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting