Paleo-oceanography of the subtropical southeastern Pacific during late Quaternary : a study of radiolaria, opal and quartz contents of deep-sea sediments Public Deposited

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  • Micropaleontological data have been analyzed through mathematical and statistical procedures, in order to: (1) establish the distribution pattern of radiolarian assemblages in the surface sediments, (2) establish the relationship of these assemblages with oceanographic variables, (3) determine the faunal composition down-core with respect to the faunal surface pattern, and (4) estimate water temperatures and 1OO M PO₄ within the geological past. This, together with the analysis of opal and quartz content of the sediments, has permitted the inference of the paleo-oceanography of the subtropical southeastern Pacific during the last 75,000 years. Factor analysis shows that five major radiolarian assemblages (factors), accounting for 93.6% of the variance, reflect the mixed layer circulation and its associated water masses. According to their position in the area, these factors are called: (1) the Subtropical factor, (2) the Equatorial factor, (3) the Peru (current) factor, (4) the Chile (current) factor, and (5) the Backwater factor. The subtropical factor is correlated with "warm" water temperatures, the Equatorial factor with opal production, the Peru factor with coastal upwelling, the Chile factor with "cool" water temperatures, and the Backwater factor with mixing of water masses. The distribution pattern of quartz reflects the position of the southeast trade winds which largely control the oceanographic conditions of the subtropical southeastern Pacific. The opal distribution resembles the patterns of surface primary productivity. Opal content is "high" below the Equatorial Undercurrent. However, it is "low" along the South American coast probably due to dilution by terrigenous input. Cores RC11-230 (8°48'S-110°48'W), V19-29 (3°35'S-83°56'W), and Y71-6-12 (16°26'S-77°23'W) were used in the stratigraphic study. Their stratigraphic control (correlation) is made through their δ ¹⁸O records. The changes of quartz, opal and radiolarian assemblages contents down-core are considered to be the result of shifts or changes in intensity of the atmospheric and oceanic circulation. Oceanographic changes have occurred in the subtropical southeastern Pacific during the last 75,000 years, but they are not manifested at each location in the same form. These changes have not been synchronous with changes of global ice volume; rather, during pronounced climatic changes, a sequence which is initiated by changes in the wind circulation is observed. The oxygen isotope stage 4 is characterized by mixing of water masses, "deep" thermocline and low concentration of nutrients. The lower interval of isotope stage 3 is characterized by weakened wind-driven circulation and "high" water temperatures. The middle of isotope stage 3 is a time in which wind-driven conditions become dominant in the region. Oxygen isotope stage 2 is "cool" and has strong wind-driven conditions. The shift to the present oceanographic conditions in the subtropical southeastern Pacific occurred 11,000 years ago.
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