Graduate Thesis Or Dissertation
 

Late quaternary oceanic circulation along the Pacific Coast of South America

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  • The distribution patterns of quartz and opal contents and the distribution patterns of radiolarian assemblages, in the sediments of the southeastern Pacific, are largely controlled by both the surface winds and the mixed-layer oceanic circulation. The distribution pattern of quartz is related to eolian and ice-rafted transport, as well as to runoff. In the subtropical region, quartz distribution extends offshore of Peru as a tongue in the same direction and position as the prevailing trade winds. Along the South American and Antarctic coast, high quartz concentrations are found in patches near shore or abundances commonly decrease rapidly seaward. Opal distribution follows the mixed-layer circulation around the subtropical anticyclonic gyre and shows high values under areas of oceanic divergence, characterized by high primary productivity. Along the continental margins, opal abundances are diluted by terrigenous debris; even off the Peruvian coast, where strong upwelling occurs and consequently high primary productivity prevails The distribution patterns of six radiolarian assemblages, defined through Q-mode factor analysis, reflect the geographical distribution of water masses associated with both surface and subsurface circulation. These are: 1) the surface Antarctic water mass, 2) the surface Subantarctic-Temperate water mass, 3) the Chile Current, 4) the Peru Current, 5) the surface Subtropical water mass and 6) the Equatorial Undercurrent. The Equatorial Undercurrent surfaces either because of strong trade winds or because of considerable relaxed trade winds. The above mentioned distribution patterns have shifted geographically during the Late Quaternary because of global climatic changes (glacial and interglacial stages). Fluctuations in the strength of the winds have been concurrent with fluctuations in the amount of ice stored on the continents, represented by the δ18O stratigraphic record, however, the direction and position of the winds have varied in a rather complex manner. Consequently, the oceanic circulation and associated water masses have presented a different behavior within each of the last five δ18O stages. Approximately 127,000 years ago, at the δ18O stage 5/6 boundary, the oceanographic conditions along the Pacific coast of South America were like the ones observed presently. During δ18O stage 5, weak trade winds, probably less parallel to the Peruvian coast than at the present, caused reduction in coastal upwelling and allowed advection of subantarctic-temperate waters, via the Chi1 Current, along the Peruvian coast. During δ18O stage 4, the trade winds were strong and more parallel to the equator, even to the east of the Galapagos Islands This gave rise to surfacing of the Equatorial Undercurrent through divergence processes Because of weak trade winds, the Galapagos front was positioned nearer to the South American coast during the lower part of stage 3. During the upper δ18O stage 3 and stage 2, the Pacific coast of South America experienced an intensified circulation, a strong development of upwelling, and as a result "cool" surface water temperatures. Approximately 18,000 years ago, at the climax of the last glaciation (δ18O stage 2), sea surface temperatures in the subtropical southeastern Pacific, were as much as 4°C cooler than present during the austral winter. It appears that the trend to the present oceanographic conditions began first in high latitudes and later in the tropical regions. Similar features in oceanographic changes were also observed during the onset of δ18O stage 5. A significant waning of the Antarctic ice occurred approximately 400,000 years ago. This was preceded by a considerable waxing of the Antarctic ice close to the Brunhes-Matuyama paleo-magnetic boundary (approximately 700,000 years ago). During the last 700,000 years the Polar front zone has shifted from its present position toward the north.
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