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The Neogene sedimentary basins of the Colombian Pacific coast were formed in forearc basins associated with the uplift of the Andes. In this area, some marine sequences remain unexplored and are marked by a lack of biostratigraphic information. Additionally, available high-resolution biostratigraphic and biochronologic data from this region are limited almost exclusively to open sea research conducted by the Integrated Ocean Drilling Programs (IODP). The Ladrilleros-Juanchaco section is a continuous and well-exposed marine terrigenous sedimentary sequence with abundant Miocene microfossils. Biostratigraphic and biochronologic studies with calcareous nannofossils, planktonic foraminifera and diatoms carried out there revealed the presence of seventeen astronomically calibrated and standard calcareous microfossil biohorizons as well as two tropical diatom biozones. Such information enabled the establishment of a chronologic framework between the Base of Praeorbulina glomerosa and Catinaster coalitus Base, corresponding to 16.27 and 10.79 Ma, respectively (Burdigalian–Tortonian). The proposed biozonation favoured the observation of a high correlation between low-latitude planktonic foraminifera and calcareous nannofossil bioevents whereas diatom biozones and calcareous nannofossil biohorizons are marked by variations since the lowest occurrence of diatoms around 12.2 Ma. Furthermore, by means of interpolating age between the Sphenolithus heteromorphus Top and Fohsella praefohsi Base bioevents, the fact that the Reticulofenestra pseudoumbilicus (>7 µm) Base common took place around 13.6 Ma is interpreted and correlated with other sites, therefore revealing that it can be considered a good bioevent for the base of the Serravallian in these tropical areas. BIOESTRATIGRAFIA DE MICROFOSILES CALCAREOS Y SILICEOS DEL MIOCENO EN BAJAS LATITUDES DEL NW DE SUR AMERICA: SECCION DE LADRILLEROS-JUANCHACO, PACIFICO COLOMBIANO. El relleno sedimentario del Neogeno de las cuencas de antearco del Pacifico Colombiano esta estrechamente relacionado con la dinamica tectonica de los Andes y los cambios paleoceanograficos que ocurrieron durante el Cenozoico. Las unidades marinas en el area permanecen inexploradas y carecen de estudios bioestratigraficos detallados. Los trabajos de este tipo en la zona estan restringidos a los realizados en las campanas del Integrated Ocean Drilling Programs (IODP) en el Pacifico ecuatorial oriental. La seccion de Ladrilleros-Juanchaco es una secuencia continua y bien expuesta con abundantes microfosiles del Mioceno que pueden llenar este vacio en la informacion. El analisis bioestratigrafico y biocronologico de nanofosiles calcareos, foraminiferos planctonicos y diatomeas ha permitido generar un modelo de edad de media-alta resolucion, identificando 17 bioeventos bioestratigraficos estandares calibrados astronomicamente en estudios previos. Esto permitio establecer un modelo de edad y establecer el tiempo de sedimentacion entre los bioeventos Praeorbulina glomerosa Base y Base de Catinaster coalitus, correspondientes a 16.27 y 10.79 Ma (Burdigaliense–Tortoniense, respectivamente). Por otro lado, la biozonacion propuesta facilito observar una alta correlacion entre los biohorizontes de nanofosiles calcareos y foraminiferos planctonicos, mientras que el nanoplancton y las biozonas de diatomeas tropicales demuestran variaciones desde su aparicion a los ~12.2 Ma. Ademas, por medio de la interpolacion entre los bioeventos Sphenolithus heteromorphus Top y Fohsella praefohsi Base se logro estimar la edad del Base comun de Reticulofenestra pseudoumbilicus (>7 µm) a ~13.6 Ma. Esta edad habia sido propuesta anteriormente, sin embargo no se le habia dado importancia a este bioevento.

Two gravity cores, Gebra-1 and Gebra-2 from the central and eastern basins of Bransfield Strait, West Antarctica, consist mainly of hemipelagic, laminated muds with black layers rich in sand-sized volcanic ash. Micropalaeontological (diatoms and radiolarians) and geochemical (organic and inorganic) analyses, together with radiometric dating (U/Th, 14C and 210Pb) have been performed on both cores. AMS analyses on Total Organic Carbon yielded a 14C-age older than expected, 2810 yr BP for the core top of Gebra-1 and 2596 yr BP for Gebra-2. The downcore pattern of ages indicates a sedimentation rate of 130 cm kyr1 for Gebra-1 and 160 cm kyr1 for Gebra-2 210Pb anomalies suggest the core top of Gebra-1 is present-day sediment. The diatom and radiolarian assemblages are related to the sequence of neoglacial events over the last three millennia. The recent significant reduction in Chaetoceros resting spores is interpreted as a reduction in palaeoproductivity. The progressive increase in sea-ice taxa for the last three millennia may indicate a cooling trend. Greater sea-ice coverage during the coldest neoglacial events in the Bransfield Basin, as well as in the Weddell Sea and Bellingshausen Sea, is documented by increases in sea-ice taxa and reductions in Thalassiosira antarctica/ T. scotia resting spores, Fragilariopsis kerguelensis, the Lithomelissa group and the “circumpolar” group of radiolarians. For these periods, we postulate a restricted communication between the Weddell Sea, Bellingshausen Sea and Bransfield basin. The millenial-scale changes are overprinted by a high frequency cyclicity at about 200–300 yrs, which might be related to the 200–yrs solar cycle.

20 pages, 8 figures, 1 table.-- Printed version published Mar 30, 2004., The response of coccolithophore assemblages to short-term millennial and centennial climatic changes over the last 50 kyr is studied in core M39029-7 from the Gulf of Cadiz (Atlantic Ocean) and core MD95-2043 from the Alboran Sea (Western Mediterranean Sea). The nannoplankton data are compared with oxygen isotopes, U37k′-SST and other biogeochemical and sedimentological proxies to obtain a new perspective of the paleoceanography and paleoecology of the area during these short-term climatic changes. Coccolithophores indicate that Heinrich events (HE) were characterised by the presence of cold, low salinity and turbid water masses on both sides of the Strait of Gibraltar. High abundances of the Lower Photic Zone (LPZ) species Florisphaera profunda in the Gulf of Cadiz synchronous with HE are interpreted as being an evidence of upper water-column stratification on the Atlantic side. These episodes coincided with input of terrigenous material, especially during H2. Surface waters during Dansgaard-Oeschger (DO) Stadials were also cold and turbid, while most of DO Interstadials were characterised by warm-water flora. The increase in small placoliths (small Gephyrocapsa and Emiliania huxleyi (4 μm), which is considered here as a cold-water indicator. The sharp decrease in large specimens of E. huxleyi related to the warming trend in the last deglaciation can be used as a biostratigraphic reference level that marks—at least approximately—the deglaciation and the beginning of the Holocene. Peaks of F. profunda are observed during Termination 1 (T1) in the Alboran Sea. They are isochronous with the “Organic-Rich Layer 1” recognised in the Western Mediterranean. In this interval, the water column of the region was stratified as a consequence of the global increase in the sea level following the deglaciation. Quantitative analyses of the coccolithophore assemblage allow us to interpret that coccolithophore production has been higher in the Alboran Sea than in the Gulf of Cadiz during the Holocene., This study was supported by an FPU grant from the Ministry of Education, Culture and Sport of Spain to E. Colmenero-Hidalgo and by Spanish project CLI 1998-1002-CO2-O2 and FEDER project 1FD97-1178. Sedimentological and stable-isotope work on core M39029-7 was funded by the Deutsche Forschungsgemeinschaft (Grants Scha355/15 and Za157/15).