Examinando por Autor "Martinez, JI"

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    Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?
    (AMER GEOPHYSICAL UNION, 2005-01-28) Lyle, M; Mitchell, N; Pisias, N; Mix, A; Martinez, JI; Paytan, A; Universidad EAFIT. Departamento de Geología; Ciencias del Mar
    The paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing (bioturbation and horizontal sediment transport). Diagenesis and bioturbation are relatively well-studied, but the effects of physical sedimentation have been largely ignored. Modeling U series isotopes (e.g., 230Th) can potentially quantify horizontal sediment movement, but model horizontal sediment focusing often equals or exceeds the vertical particle rain. We find no evidence of this level of sediment focusing in the equatorial Pacific from geophysical data or sediment core comparisons. The overestimate of sediment focusing by 230Th is probably caused by poor model assumptions: that sediment does not fractionate (does not sort according to size during transport) and that 230Th cannot leak from slowly accumulating sediments. Both assumptions are weak. U series methods do hold promise to quantify sediment focusing if properly calibrated. With calibration the trade-offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed. Copyright 2005 by the American Geophysical Union.
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    Late Quaternary vegetation and climate change in the Panama Basin: Palynological evidence from marine cores ODP 677B and TR 163-38
    (ELSEVIER SCIENCE BV, 2006-05-03) Gonzalez, C; Urrego, LE; Martinez, JI; Universidad EAFIT. Departamento de Geología; Ciencias del Mar
    The Late Quaternary paleoenvironmental history from Pacific slopes of the western Andes is reconstructed by pollen analysis of 32 samples from two marine sediment cores from the Panama Basin, eastern equatorial Pacific: core ODP 677B (83°44.2200' W, 1°12.1440' N, 3473 m water depth) is 185 cm long and spans the last 39,410 years, core TR 163-38 (81.583° W, 1.337° N, 2200 m water depth) is 103 cm long and covers the last 17,380 years. Six ecological groups were established: mangrove, brackish and fresh water swamps, terra firma lowland forests, broad range taxa, Andean forests, and open vegetation. A good correspondence was found between the changes of these ecological groups in the two cores. The records evidence the continuous presence of all vegetation types during the last 39,410 years and specially the uninterrupted occurrence of tropical rain forest. They record a development from: (1) a cold and humid phase (39,410-28,120 yr cal BP) with moderately high sea levels, (2) the coldest and driest phase in the record (28,120-14,500 yr cal BP) accompanied by the lowest sea levels, (3) a transitional phase when sea level rose and humid conditions dominated, (4) a stage (11,300-5600 yr cal BP) of the highest sea levels and moisture conditions including a drier period ~7000 yr BP, to (5) a final period (5600 yr cal BP-Present) when sea level reached its present height, humidity persisted, and indicators of disturbance expanded. Peaks in pollen and spore concentration, associated with high river discharge periods, indicate periods of higher precipitation around 33,500, 28,000 and 12,000-9000 yr cal BP. Although main vegetation responses seem to reflect rainfall and moisture variations, a good correspondence was found between d18O values and percentages of Andean and lowland pollen, suggesting that vegetation also responded to tempearture changes. © 2005 Elsevier B.V. All rights reserved.
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    The paleoecology of Late Cretaceous upwelling events from the upper Magdalena basin, Colombia
    (SEPM-SOC SEDIMENTARY GEOLOGY, 2003-10-01) Martinez, JI; Universidad EAFIT. Departamento de Geología; Ciencias del Mar
    Ten planktonic foraminiferal zones are identified and six stratigraphic sequences are interpreted in the Turonian-Maastrichtian interval of the Michu-1 well in the Upper Magdalena Basin. Anoxic bottom-water conditions prevailed during the Turonian and Coniacian followed by dysoxic conditions in the Santonian and Campanian. Oxic conditions were established during the Maastrichtian age. Upwelling occurred for most of the Late Cretaceous. Coccolithophorids, planktonic foraminifera, radialaria, ammonites, clupeoid fish, and mosasaurs dominated the food-web structure of the Turonian-Coniacian interval. Following a major turnover during the Coniacian-Santonian boundary interval, dinoflagellates were the primary producers that sustained radiolarians, a sparse population of planktonic foraminifera, clupeoid fish, and ammonites. Dinaflagellate blooms (peridinoids) were frequent during the Campanian interval and sustained a sparse population of planktonic foraminifera and abundant clupeoid fish whose feces (phosphatic pellets) were grazed extensively by a specialized population of buliminids dominated by the genus Siphogenerinoides.
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    Sea-surface temperatures of the southwest Pacific Ocean during the Last Glacial Maximum
    (AMER GEOPHYSICAL UNION, 2000-02-01) Barrows, TT; Juggins, S; De Deckker, P; Thiede, J; Martinez, JI; Universidad EAFIT. Departamento de Geología; Ciencias del Mar
    The southwest Pacific Ocean covers a broad range of surface-water conditions ranging from warm, salty water in the subtropical East Australian Current to fresher, cold water in the Circumpolar Current. Using a new database of planktonic foraminifera assemblages (AUSMAT-F2), we demonstrate that the modern analog technique can be used to accurately reconstruct the magnitude of sea-surfacetemperature (SST) in this region. We apply this technique to data from 29 deep-sea cores along a meridional transect of the southwest Pacific Ocean to estimate the magnitude of SST cooling during the Last Glacial Maximum. We find minimal cooling in the tropics (0°-2°C), moderate cooling in the subtropical midlatitudes (2°-6°C), and maximum cooling to the southeast of New Zealand (6°-10°C). The magnitude of cooling at the sea surface from the tropics to the temperate latitudes is found to generally be less than cooling at the surface of adjacent land masses. Copyright 2000 by the American Geophysical Union.