1,721,039 research outputs found
Millenian-scale changes in North Atlantic Deep Water circulation during marine isotope stages 11-12: Linkage to Antartic climate
No full textThe time interval represented by marine isotope stages (MIS) 11 and 12 (ca. 360-470 ka) may contain the most extreme glacial and interglacial climate conditions of the late Pleistocene. Sediments from the Bermuda Rise (western North Atlantic) provide clues to the nature of climate variability during this period. Our geochemical records indicate that millennial-scale climate instability and associated changes in North Atlantic Deep Water (NADW) production occurred during both interglacial MIS 11 and glacial MIS 12. Stage 12 is punctuated by a series of interstadial events that occurred at a 5-6 k.y. periodicity, and the occurrence of ice-rafted debris at various times during stage 12 indicates that icebergs were present at least as far south as 34°N during this glacial period. Within the limits of our correlation, the atmospheric temperature changes recently reported for the Vostok ice core for the stage 11 time period appear to be represented by coeval changes in NADW flow. Specifically, warming in Antarctica is associated with increased production of NADW
Pliocene-pleistocene vegetational and climatic evolution of the south-central mediterranean
No full textThe vegetational history of the south-central Mediterranean clearly reflects the major global climatic changes of the last four million years. A Mediterranean-type climate, characterized by strong seasonality and a dry summer, may have existed in this region during the early Pliocene. A short-term climatic cooling at approximately 3.2 Ma resulted in the temporary establishment of a humidity-demanding flora; the onset of major Northern Hemisphere glaciation at approximately 2.4 Ma initiated an alternation between humid (glacial) and dry (interglacial) conditions. Vegetational differences between this region and the north-west Mediterranean indicate that distinct latitudinal climatic gradients probably existed in the Mediterranean during the Pliocene-early Pleistocene. © 1989
PLIOCENE LOWER PLEISTOCENE CHRONOSTRATIGRAPHY - A REEVALUATION OF MEDITERRANEAN TYPE SECTIONS
No full textThe recently developed calcareous plankton biochronology based on results from Ocean Drilling Program Leg 107 and numerous outcrop sections provides the necessary time control for evaluating the ages and stratigraphic relationships for the various Pliocene-Pleistocene stages and their associated stratotypes. Results indicate that the currently accepted stratotypes do not provide a continuous stratigraphic representation for all of Pliocene-early Pleistocene time. On the basis of these findings, a revised chronostratigraphic scheme for this time period is propose
Laminated sediments from the Vrica section (Calabria, S. Italy): evidence for plio-pleistocene climatic change in the Mediterranean region
No full textSix laminated sedimentary units (laminites) from the Vrica section (Calabria, southern Italy) have been analyzed geochemically and micropaleontologically to assess paleoclimatic conditions in this region during the transition from the Pliocene to the Pleistocene. Two laminites formed during the early Pleistocene and one formed immediately below the Pliocene/Pleistocene boundary; all have a greater organic carbon content relative to the surrounding marls. By contrast, three late Pliocene laminites exhibit no significant increase in organic carbon, which indicates that the conditions associated with the formation of these three units were somewhat different than those associated with the formation of the three younger laminites. Stable isotope analyses of planktonic foraminifera from the laminites indicate that all of these units were formed during periods of decreased salinity concentrations in surface waters. The laminites were formed as a result of basin anoxia brought on by increased surface-water stratification during periods of high runoff. The Vrica planktonic foraminiferal census data indicates that the laminites contain a distinctive fauna which can be partially attributed to increased nutrient concentrations. By contrast, species assemblages within the marls seem to be primarily controlled by surface-water temperatures. Through the use of polytopic vector analysis, the foraminiferal census data can be simplified into five assemblages, each of which reflects temperature, salinity and/or nutrient concentrations. The nutrient-dependent assemblage indicates an intensification in surface water productivity during periods of laminite formation during the later Pliocene and early Pleistocene, while the temperature-dependent assemblages indicate an overall cooling of surface-water temperatures during the same period. The results of the planktonic foraminiferal analyses support the concept of a regional cooling across the Pliocene/Pleistocene boundary in the Mediterranean. In addition, the results of this study indicate that these climatic changes resulted in differences in the nature of the laminites. Cooler climatic conditions during the latest Pliocene and the early Pleistocene favored the formation of laminites slightly richer in organic carbon and nitrogen because of increased organic carbon input or enhanced organic matter preservation. The warmer climatic conditions of the late Pliocene produced laminites relatively poor in organic carbon and nitrogen as a result of either decreased runoff or decreased organic matter preservation. © 1990
An overview of the post-Messinian paleoenvironmental history of the Western Mediterranean
No full textA review of the Pliocene-Pleistocene paleoenvironmental history of the western Mediterranean is presented based on micropaleontological and geochemical studies of Ocean Drilling Program (ODP) leg 107 material and marine outcrops in southern Italy. The early Pliocene period in the Mediterranean was marked by relatively high-amplitude climatic oscillations. In marginal areas, these climatic oscillations resulted in the rhythmic deposition of limestone-marl couplets which have an average duration of approximately 19 000 years. The global cooling at ~2.4 Ma and the associated expansion of northern hemisphere glaciation resulted in a shift to more arid conditions in the Mediterranean region. The δ18O record for site 653 is marked by a sharp cooling across the Pliocene/Pleistocene boundary
LIMESTONE-MARL COUPLETS - ORIGIN OF THE EARLY PLIOCENE TRUBI MARLS IN CALABRIA, SOUTHERN ITALY
No full textIndicates that the rhythmic sedimentation pattern is related to changes in surface water productivity. Relative to the marls, the limestone units are typically marked by higher abundances of the planktonic foraminifera G. bulloides, higher numbers of benthic foraminifera, higher δ13C values and lower δ18C values. These characteristics are interpreted as indicating that cool, highly productive conditions existed during the deposition of the limestone units.
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
Late neogene laminated and opal-rich facies from the Mediterranean region: Geochemical evidence for mechanisms of formation
No full textThe mechanisms for the formation of laminated and opal-rich sediments from southern Italy (Bianco and Vrica) and southern Sicily (Gela) are investigated using a geochemical approach. The Vrica laminates are relatively rich in organic carbon while the Gela laminites are not. Carbon/nitrogen ratios, carbon isotopic composition and lipid content of the organic matter at Vrica indicate that the laminates represent periods of increased input of terrestrial organic matter and enhanced preservation. Based on oxygen isotopic studies of planktonic foraminifera, a basin stratification model is presented to explain the formation of laminites at both Vrica and Gela. Biogenic silica and carbonate content, and the carbon isotopic composition of benthic foraminifera suggest that the Bianco diatomites represent periods of increased productivity. The isotopic composition of planktonic foraminifera from Bianco indicates that the increased productivity was a result of basin eutrophication brought on by continental runoff. The low organic carbon content of the Bianco diatomites is attributed to continuous bottom water ventilation. The results of laminites studies indicate that they are not always rich in organic matter, as generally assumed. Moreover the formation of organic-rich laminites can be the result of enhanced preservation, not increased productivity. Results from Bianco indicate that preservation plays a primary role in controlling the organic carbon content of sediments. © 1988
Chronology of the pleistocene oxygen isotope record: 0–1.88 m.y. B.P
No full textDetailed oxygen isotope records from various ocean basins and representing the last 1.88 m.y. are correlated using nannofossil biostratigraphy and paleomagnetic stratigraphy. These correlations establish the global nature of oxygen isotope stages 23 through 63 in the early Pleistocene to latest Pliocene. A composite isotope record for the last 1.88 m.y. reveals that the mid-Pleistocene change in climate regime was a complicated response that lasted from approximately 0.9 to 0.6 Ma and not a simple shift from one climate mode to another. The proposed chronology for the extended isotope stages provides a chronostratigraphic framework for detailed studies of paleoceanographic processes in the early Pleistocene and paves the way for application of oxygen isotope stratigraphy to early Pleistocene deep-sea and continental margin drilled sections. © 198
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