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The AAPG/Datapages Combined Publications Database
Cores and bottom photographs from the continental margin and the Peru-Chile trench off Callao, Peru, show large amounts of organic material in the sediments, a direct reflection of the high productivity in the surface waters of the Peru (Humboldt) Current. Organic material ranges from 6.0 to 17.7% by weight and is roughly seven times the amount found in samples seaward of the trench. Values of C/N show a wide range but average 14.2 for sediments of the trench-continental margin and 13.3 for sediments seaward of the trench. Sediment at water depth of 1,000 m has greater CaCO3 content and mean-size diameter, and better sorting than the sediments of the shelf and slope. These changes in chemical and textural parameters are the result of maximum abundance of Forami ifera at this depth. Bottom photographs show the presence of cobble and other coarse clastic material at considerable depths off Callao, and of slumped sediment, scour, "streams" of megarippled sediment, and patches of thin-crested oscillation (?) ripple-marks in the trench axis (5,200 m) off Valparaiso, Chile.
Sediments south of the Nasca ridge are generally brown calcareous clay, buff clay with fecal pellets, manganese micronodules, and interlayered pyroclastics. Dredge hauls and bottom photographs indicate several regions of abundant manganese nodules.
The distribution of CaCO3 is a function of productivity, water masses and currents, bathymetry, and distance from land. Variations of carbonate with time indicate a trend similar to Arrhenius' model of lower CaCO3 production for Holocene and interglacial sediments in the equatorial Pacific. On the assumption that two cores, approximately 500 km apart, have a complete record of Quaternary sediments, a sedimentation rate for the total Pleistocene is calculated at 1.3-1.4 mm/1,000 years. Along the Nasca ridge, as a result of higher carbonate production, Holocene sediments have accumulated at the rate of 1.4-2.7 mm/1,000 years.
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