Examinando por Autor "Yokoyama, Yusuke"
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Ítem Foraminifera and coccolithophorid assemblage changes in the Panama Basin during the last deglaciation: Response to sea-surface productivity induced by a transient climate change(ELSEVIER SCIENCE BV, 2006-05-03) Martinez, Ignacio; Rincon, Daniel; Yokoyama, Yusuke; Barrows, Timothy; Universidad EAFIT. Departamento de Geología; Ciencias del MarThe responses of community assemblages of planktonic and benthonic foraminifera and coccolithophorids to transient climate change are explored for the uppermost 2 m of cores ODP677B (1.2°N; 83.74°W, 3461 m) and TR163-38 (1.34°S; 81.58°W, 2200 m), for the last ~ 40 ka. Results suggest that the deglaciation interval was a time of increased productivity and a major reorganization of planktonic trophic webs. The succession in dominance between the planktonic foraminifera species Globorotalia inflata, Globigerina bulloides, and Neogloboquadrina pachyderma denote four periods of oceanographic change: (1) advection (24-20 ka), (2) strong upwelling (20-15 ka), (3) weak upwelling (14-8 ka) and (4) oligotrophy (8 ka to present). Strong upwelling for the deglaciation interval is supported by the low Florisphaera profunda/other coccolithophorids ratio and the high percentage abundance of Gephyrocapsa oceanica. Benthonic foraminifera assemblage changes are different in both cores and suggest significant regional variations in surface productivity and/or oxygen content at the seafloor, and a decoupling between surface productivity and export production to the seafloor. This decoupling is evidenced by the inverse relationship between the percentage abundance of infaunal benthonic foraminifera and the percentage abundance of N. pachyderma. The terrigenous input of the Colombian Pacific rivers, particularly the San Juan River, is suggested as a possible mechanism. Finally, the Globorotalia cultrata/Neogloboquadrina dutertrei ratio is used to reconstruct the past influence of the Costa Rica Dome-Panama Bight and cold tongue upwelling systems in the Panama Basin. A northern influence is suggested for the late Holocene (after 5 ka) and the last glacial (before 20 ka), whereas a southern influence is suggested for the 20-5 ka interval. There is a correspondence between our reconstructed northern and southern influences and previously proposed positions of the Intertropical Convergence Zone (ITCZ) after the Last Glacial Maximum (LGM). © 2005 Elsevier B.V. All rights reserved.Ítem La Niña-like conditions in the eastern equatorial Pacific and a stronger Choco jet in the northern Andes during the last glaciation(American Geophysical Union (AGU), 2002-12-30) Martínez, Ignacio; Keigwin, Lloyd; Barrows T., Timothy; Yokoyama, Yusuke; Southon, John; Universidad EAFIT. Departamento de Geología; Martínez, Ignacio (jimartin@eafit.edu.co); Ciencias del MarSix deep sea cores from the eastern equatorial Pacific (EEP) were analyzed for planktonic foraminifera and stable isotopes in order to reconstruct sea surface temperatures (SST) for the last 40 ka. South of the Equatorial Front the abundance of Globorotalia inflata increased, and SST decreased by >5°C (core ODP846B), creating a stronger SST meridional gradient and advection of the Peru Current than present for the ∼16–35 ka interval. A sharper SST meridional gradient forced stronger Choco jet events and a moisture increase in western Colombia, which supplied, through the San Juan River and the south-flowing equatorial and the Peru-Chile countercurrents, abundant hemipelagic quartz over the northern Peru basin (core TR163-31B). The Choco jet, and its associated mesoscale convective cells, provoked an increase in snow precipitation over the Central Cordillera of Colombia and the advance of the Murillo glacier. In synchrony with the intensified Choco jet events, the “dry island” effect over the Eastern Cordillera of Colombia intensified, and the level of Fuquene Lake dropped.Ítem Late Holocene marine terraces of the Cartagena region, southern Caribbean: The product of neotectonism or a former high stand in sea-level?(PERGAMON-ELSEVIER SCIENCE LTD, 2010-03-01) Ignacio Martinez, J.; Yokoyama, Yusuke; Gomez, Andres; Delgado, Adriana; Matsuzaki, Hiroyuki; Rendon, Esteban; Universidad EAFIT. Departamento de Geología; Ciencias del MarThe detailed stratigraphic survey and paleontological study (mollusks, corals, foraminifera and ostracods) of four low-level, ~3 m, marine terrace sections: Punta Canoas, Manzanillo del Mar, Playa de Oro, and Tierra Bomba Island, from the Cartagena region, southern Caribbean, supplemented with 22 radiocarbon dates, reveals that the northern terraces were deposited as parasequences in a clastic depositional system compared to the Tierra Bomba Island succession that was deposited in a carbonate depositional system between ~3600 and ~1700 cal yrs BP. Drier conditions and the southern location of the ITCZ at about 3 ka triggered stronger easterly Trades and more dynamic southwestward sediment drift fed by the Magdalena River mouth, thus promoting the formation of sand spits that ultimately isolated the Cienaga de Tesca coastal lagoon from the Caribbean Sea. Our estimates support the hypothesis that the present position of the terraces is the product of neotectonism rather than a higher 3 ka, sea-level. Upheaval of the terraces varies between ~3.8 mmyr-1 at Punta Canoas and ~2.2 mmyr-1 at Tierra Bomba to ~1.5 mmyr-1 at Manzanillo del Mar and Playa de Oro terraces. Our study corroborates previous contentions on the role of mud diapirism and the dynamics of the Dique Fault as late Holocene upheaval mechanisms. © 2009 Elsevier Ltd. All rights reserved.Ítem Mangrove dynamics in the southwestern Caribbean since the 'Little Ice Age': A history of human and natural disturbances(SAGE PUBLICATIONS LTD, 2010-09-01) Gonzalez, Catalina; Estela Urrego, Ligia; Ignacio Martinez, Jose; Polania, Jaime; Yokoyama, Yusuke; Universidad EAFIT. Departamento de Geología; Ciencias del MarRelatively little is known about the long-term response of Caribbean mangroves to human and natural disturbances during the 'Little Ice Age' (LIA). We present new palynological information on the dynamics of the Bahia Honda mangrove from the eastern coast of San Andres Island in the southwestern Caribbean for the late Holocene. Major changes in the Bahia Honda pollen record show the combined effects of natural events (strong storms and sea-level rise), and human disturbances. These changes are supported by 14C dates, sedimentological and palynological information. A storm (most probably a hurricane) was recorded around AD 1600, caused sediment reworking and the subsequent loss of about 2000 years of the vegetation record. The devastation of tree vegetation by this event allowed the expansion of heliophytic vegetation (e.g. grasses and vines). Mangroves and coastal vegetation started to recover at AD 1700, reaching their maximum extent within a few decades, when microforaminifera shells became abundant at the coring site, thus suggesting a relative sea-level rise because of the geomorphic reconfiguration of the coastal plain after the storm. Furthermore, the pollen evidence indicates more humid regional climates during the late LIA (AD 1700-1850). Mangrove and coastal vegetation declined sharply as a consequence of the establishment of coconut plantations around AD 1850. The recovery of the mangroves after AD 1960 is a result of the combined effect of relative sea-level rise and drastic changes in the local economy from coconut plantations to commerce. © 2010 The Author(s).Ítem Palynofacies analysis of the late Holocene San Nicolás terrace of the Cauca paleolake and paleohydrology of northern South America(ELSEVIER SCIENCE BV, 2011-01-01) Garcia, Yuri C.; Ignacio Martinez, J.; Velez, Maria I.; Yokoyama, Yusuke; Battarbee, Richard W.; Suter, Fiore D.; Universidad EAFIT. Departamento de Geología; Ciencias del MarThe detailed palynofacies analysis of the late Holocene laminated succession of the San Nicolás terrace in the middle Cauca Valley, northern Colombia, is interpreted as the product of sedimentation in a lacustrine to fluvial dominated setting. Radiocarbon analyses reveal that the succession was deposited between ~. 3500 and a few hundreds of years with sedimentary rates in excess of ~. 600. cm/ka. The milimetric laminae and the high frequency fluctuations in the content of the palynodebris of terrestrial origin, i.e., the high altered ligno-cellulosic debris/amorphous organic matter ratio, together with diatoms typical of fluvial conditions, such as Aulacoseira granulata do suggest an intermittent and continuous hydrological and biological connectivity with the Cauca River, akin to a varzea lacustrine environment. Drier to wetter conditions appear to occur, moving upward in the succession. This might reflect regional precipitation conditions all along the Cauca Valley and the southern migration of the intertropical convergence zone. © 2010 Elsevier B.V.Ítem The San Nicolás succession of the Cauca paleolake: A late Holocene laminated ria lake record from the Neotropics(SPRINGER, 2013-02-01) Ignacio Martinez, J.; Mayr, Christoph; Yokoyama, Yusuke; Velez, Maria I.; Battarbee, Richard W.; Universidad EAFIT. Departamento de Geología; Ciencias del MarThe stratigraphic, geochemical, and organic matter study of the late Holocene San Nicolás succession of the Cauca paleolake (Santa Fé-Sopetrán pull-apart basin) in the middle Cauca Valley, northern Colombia, suggests that it was deposited in a ria lake environment, at sedimentary accumulation rates in excess of 600 cm ky-1 between ~3500 and ~500 yr BP. Laminated deposition occurred, first under igapo (black-water), and then under varzea (white-water) conditions. The transition occurred ca. 3000 yr BP, a time of major change in El Niño/Southern Oscillation (ENSO) behavior in the Cariaco Basin, thus reflecting the southern migration of the intertropical convergence zone and intensified rain upstream the Cauca Valley. A second, but less conspicuous change occurred ca. 2000 yr BP, which apparently corresponds to the intensified and/or more frequent ENSO activity in the Galapagos Islands. Our contribution describes, for the first time, a ria lake sedimentary succession from the northern Andes and demonstrates the high potential of these hitherto undervalued deposits for the reconstruction of the paleohydrological history of the northern Andes. © 2012 Springer Science+Business Media Dordrecht.