1,721,333 research outputs found
Calcareous nannofossils at the Paleocene/Eocene Transition in the Equatorial Pacific Ocean (ODP leg 199)
No full textIODP Proposal 626: "Cenozoic Equatorial Age Transect – Following the Palaeo-equator"
No full textAs the largest ocean, the Pacific is intricately linked to major changes in the global climate system that took place during the Cenozoic. Throughout the Cenozoic the Pacific plate has had a northward component. Thus, the Pacific is unique, in that the thick sediment bulge of biogenic rich deposits from the currently narrowly focused zone of equatorial upwelling is slowly moving away from the equator. Hence, older sections are not deeply buried and can be recovered by drilling. Previous ODP Legs 138 and 199 were designed as transects across the paleo-equator in order to study the changing patterns of sediment deposition across equatorial regions, while this proposal aims to recover an orthogonal “age-transect” along the paleo-equator. Both previous legs were remarkably successful in giving us new insights into the workings of the climate and carbon system, productivity changes across the zone of divergence, time dependent calcium carbonate dissolution, bio- and magnetostratigraphy, the location of the ITCZ, and evolutionary patterns for times of climatic change and upheaval. Together with older DSDP drilling in the eastern equatorial Pacific, both Legs also helped to delineate the position of the paleo-equator and variations in sediment thickness from approximately 150°W to 110°W. As we have gained more information about the past movement of plates, and where in time “critical” climate events are located, we now propose to drill an age-transect (“flow-line”) along the position of the paleo-equator in the Pacific, targeting selected time-slices of interest where calcareous sediments have been preserved best. Leg 199 enhanced our understanding of extreme changes of the calcium carbonate compensation depth across major geological boundaries during the last 55 million years. A very shallow CCD during most of the Paleogene makes it difficult to obtain well preserved sediments, but we believe our siting strategy will allow us to drill the most promising sites and to obtain a unique sedimentary biogenic carbonate archive for time periods just after the Paleocene- Eocene boundary event, the Eocene cooling, the Eocene/Oligocene transition, the “one cold pole” Oligocene, the Oligocene-Miocene transition, and the Miocene, contributing to the objectives of the IODP Extreme Climates Initiative, and providing material that the previous legs were not able to recover
Evolutionary trends of tropical calcareous nannofossils in the Late Neogene
No full textExamination of Middle-Late Miocene sediments recovered during ODP Leg 154 in western equatorial Atlantic has led to identification of evolutionary transitions in some groups of late Neogene calcareous nannofossils. Through analyses of high resolution samples (10-cm sample interval equivalent to average interval of 6 kyr) we were able to document the origin of the genera Catinaster, Amaurolithus, and Ceratolithus, and the nannofossil species Discoaster berggrenii and D. quinqueramus. The presence of intermediate morphotypes between end-members representing distinct species sheds new light on phylogenetic relationships and/or confirms relationships suggested in previous studies. The mode and timing of the evolutionary transitions described are discussed. Successive branching from Triquetrorhabdulus rugosus is demonstrated for Amaurolithus primus, 'Amaurolithus amplificus', and Ceratolithus acutus. A new genus is therefore established, Nicklithus, type species Nicklithus amplificus n. comb. A new species is described, Ceratolithus larrymayeri n. sp. The genera Triquetrorhabdulus, Amaurolithus, Nicklithus and Ceratolithus all belong to the family Ceratolithaceae
Late Pleistocene and Holocene calcareous nannofossil biochronology and paleoceonography across the Arctic Basin
No full textBiochronology and paleoceanography of late Pleistocene and Holocene calcareous nannofossil abundances across the Arctic Basin
No full textLate Pleistocene and Holocene stratigraphic distributions of calcareous nannofossils have been investigated in seven short cores raised from four major ridges in the Arctic Basin and in one core from the Norwegian Sea. Observed assemblages in the Arctic Ocean cores mostly represent the influence of Atlantic shallow waters. On the Gakkel and Lomonosov Ridges, a distinct cross-over in abundance occurs between Emiliania huxleyi and Gephyrocapsa spp. in the upper part of Marine Isotope Stage (MIS) 3. The lowest abundances of indigenous calcareous nannofossils occur in the southern Mendeleev Ridge core and in the two Alpha Ridge cores. These areas show a minor production of calcareous nannoplankton, probably because of the progressively diminishing influence of shallow Atlantic waters with the increasing distance from the Fram Strait gateway, the key conduit of shallow water inflow to the Arctic Ocean. It remains unknown whether or not the observed abundance patterns have been modified by glacial-interglacial scale changes in preservation. Obvious reworking becomes intensified around MIS boundaries
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Plio-Pleistocene nannofossil biostratigraphy and calibration to oxygen isotope stratigraphies from Deep Sea Drilling Project Site 607 and Ocean Drilling Program Site 677
No full textEvolutionary events within early Pleistocene Gephyrocapsa assemblages provide at least four biostratigraphically useful events occurring in the Matuyama Chron after the Olduvai subchron. Four additional, late Pliocene-early Pleistocene events are also investigated. The successful astronomical calibration of oxygen isotope stratigraphies from Deep Sea Drilling Project site 607 and Ocean Drilling Program site 677 in the Matuyama Chron permits calibration of the biostratigraphic events to these uniquely resolved isotope chronologie
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