996 resultados para LAST GLACIAL PERIOD


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La cuenca del Duero constituye un vasto territorio ibérico cuyo paisaje se encuentra actualmente muy alterado por la mano del hombre y es prácticamente imposible localizar alguna manifestación de su cubierta vegetal natural. Aunque la historia de la vegetación en los sectores central y oriental es relativamente bien conocida, en su mitad occidental los registros paleoecológicos estudiados hasta la fecha son prácticamente inexistentes. Esto hace que se desconozca la respuesta de la vegetación a las diferentes oscilaciones climáticas que se han producido desde el Último Máximo Glaciar, cuál fue el impacto de las diferentes culturas sobre el medio, cuándo se produjo una alteración profunda de la vegetación natural y cuál ha sido la historia de los incendios. Este último aspecto, el papel e importancia de los incendios, reviste un especial interés en la península Ibérica dada su situación geográfica y climática dentro de la cuenca Mediterránea, donde el fuego es un factor ecológico de primer nivel. Las distintas técnicas paleoecológicas son las más adecuadas para abordar todas estas preguntas. De este modo, los avatares de la vegetación a través del tiempo se han reconstruido mediante el análisis polínico y de macrofósiles, el impacto humano se ha trazado utilizando indicadores polínicos ligados a actividades antrópicas y esporas de hongos coprófilos, estudiándose los incendios a partir del registro de partículas microscópicas de carbón. La alta resolución temporal y taxonómica alcanzada en estos análisis, así como la amplia superficie abarcada con los yacimientos estudiados, hacen que la información obtenida sea altamente detallada y complete el conocimiento que se tiene sobre la cuenca del Duero. El Tardiglaciar se registra por primera vez en la Meseta Norte Ibérica en la secuencia de Ayoó de Vidriales, donde una vegetación esteparia prácticamente desarbolada domina durante los periodos fríos. Durante el interestadial Bølling/Allerød la expansión forestal (pinos, abedules) fue muy leve y tardía y fue interrumpida bruscamente por el Dryas Reciente. Al final del Dryas Reciente o al inicio del Holoceno se produjo una acusada y rápida expansión de los bosques. Esta dinámica sugiere que no hubo refugios glaciares importantes en esta zona de la Meseta durante el “Mystery Interval”, aparentemente el periodo más frío y seco. Los incendios fueron muy escasos, aumentando de forma muy brusca al inicio del Holoceno por el aumento de biomasa y las condiciones relativamente cálidas y secas. A partir de los registros de Ayoó y El Maíllo se consolida la importancia del gradiente oceanicidad-continentalidad en el Sistema Central y la Meseta Norte, que se manifiesta principalmente en la dominancia más prolongada de los pinares hacia el interior. Además, otra de las principales contribuciones de la presente tesis es proporcionar evidencia sobre la sucesión de distintos tipos de bosques en el noroeste de la Meseta, precisando el marco temporal en el que suceden. Así, se ha constatado que hubo un máximo desarrollo del bosque caducifolio durante el Holoceno medio en Ayoó de Vidriales y una baja incidencia del fuego que sugieren que el clima fue más húmedo que en el Holoceno inicial. El estudio de macrofósiles leñosos ha permitido detectar procesos que con el análisis polínico habrían pasado desapercibidos, como la persistencia hasta el Holoceno final de Pinus sylvestris en la sierra del Teleno y la sustitución de P. sylvestris por P. pinaster en la sierra de Francia durante el Holoceno inicial. También el estudio de los carbones procedentes de los arenales de Tierra de Pinares han proporcionado la prueba definitiva de la naturalidad de los pinares de P. pinaster. El impacto humano se detecta temprano en las secuencias del oeste de la cuenca del Duero, durante el Neolítico, aunque ha sido mucho más acusado desde la Edad del Hierro (ca 2700-2500 años cal BP). Para la detección del impacto humano temprano ha sido clave el análisis de esporas de hongos coprófilos, cuyo análisis fue incorporado en la secuencia de Ayoó de Vidriales. Una de sus principales consecuencias fue el establecimiento de comunidades de matorral (brezales, piornales) sobre amplias extensiones del occidente de la cuenca del Duero, vinculado al recrudecimiento de los regímenes de incendios. A pesar de que los incendios han sido ecológicamente importantes desde el inicio del Holoceno, los cambios introducidos por el hombre en sus regímenes sobrepasaron la resiliencia de los bosques originales, lo cual condujo a su sustitución sostenida por matorrales. ABSTRACT The Duero Basin constitutes a vast Iberian territory which is currently strongly disturbed due to human activities, so it is very difficult to find any remnant of the natural vegetation. Vegetation history for the eastern and western sectors of the Basin is relatively well-known but, in contrast, there is an almost complete lack of palaeoecological record in the western area. Consequently, there exists a profound ignorance about vegetation responses to the climatic oscillations occurred since the Last Glacial Maximum, the environmental impact of the different cultures, when a severe disturbance of the natural vegetation took place and fire history. The last question, the role and importance of fire, has a special interest in the Iberian Peninsula due to its geographic and climatic framework, the Mediterranean Basin, where fire is a major ecological factor. The diverse palaeoecological techmiques are the most suitable tools to address all these questions. Thus, vegetation shifts through time have been reconstructed using pollen and macrofossil analyses, human impact has been tracked by means of anthropogenic pollen indicators and dung fungal spores, while fire history has been studied from the quantification of microscopic charcoal particles. The high taxonomic and time resolution attained along with the extensive surface covered by the studied sites provide detailed information very useful to complete the knowledge on landscape dynamics in the Duero Basin. The Lateglacial is recorded for the first time in the Northern Iberian Plateau in the sequence from Ayoó de Vidriales, showing that almost treeless steppic vegetation dominated during the cold periods. Tree expansion (pines, birches) was late and slight during the Bølling/Allerød interstadial and was sharply interrupted by the Younger Dryas (YD) climatic reversal. By the end of the YD or the onset of the Holocene, a rapid forest expansion occurred. This forest dynamics suggests an absence of important glacial refugia for trees in this area of the Plateau during the Mystery Interval, apparently the coldest and driest period. Fires were fairly rare, increasing abruptly at the beginning of the Holocene due to the relatively warm and dry climate and the accumulation of biomass. The records from Ayoó and El Maíllo reinforce the role of the oceanicity-continentality gradient in the vegetation history of the Iberian Central Range and the Iberian Northern Plateau, reflected mainly in the longer dominance of pine forests towards inland areas. Further, another important contribution of this PhD Thesis is providing evidence on the succession of different forest types in the northestern fringe of the Plateau, specifying the chronological framework. A maximum of deciduous forest development and low fire activity have been detected in Ayoó de Vidriales during the mid-Holocene, suggesting that climate was wetter than in the early Holocene. The study of woody macrofossils has allowed detecting processes which would have remained unnoticed using pollen analysis alone, such as the persistence of Pinus sylvestris until the late Holocene in the Teleno Mountains and the early Holocene replacement of P. sylvestris with P. pinaster in the sierra de Francia range. The study of macroscopic charcoal fragments from palaeosols of the Tierra de Pinares has also provided the definitive proof of naturalness for the P. pinaster stands gorwing over this area at present. Early human impact, during the Neolithic, has been detected in the sequences from the western sector of the Duero Basin, although human disturbance has been more severe from the Iron Age onwards (ca 2700-2500 cal yr BP). The analysis of coprophilous fungi incorporated in the sequence of Ayoó de Vidriales has played a key role in recognizing that early human impact. One of the main consequences of human disturbance was the establishment of shrubland communities (heaths, brooms) over huge areas of the western Duero Basin, linked to severe and/or frequent fires. Despite fires has been ecologically important since the onset of the Holocene, human-induced changes in fire regimes have exceeded the resilience of original forests leading to a sustained replacement with shrublands.

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Widespread species- and genus-level extinctions of mammals in North America and Europe occurred during the last deglaciation [16,000–9,000 yr B.P. (by 14C)], a period of rapid and often abrupt climatic and vegetational change. These extinctions are variously ascribed to environmental change and overkill by human hunters. By contrast, plant extinctions since the Middle Pleistocene are undocumented, suggesting that plant species have been able to respond to environmental changes of the past several glacial/interglacial cycles by migration. We provide evidence from morphological studies of fossil cones and anatomical studies of fossil needles that a now-extinct species of spruce (Picea critchfieldii sp. nov.) was widespread in eastern North America during the Last Glacial Maximum. P. critchfieldii was dominant in vegetation of the Lower Mississippi Valley, and extended at least as far east as western Georgia. P. critchfieldii disappeared during the last deglaciation, and its extinction is not directly attributable to human activities. Similarly widespread plant species may be at risk of extinction in the face of future climate change.

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The magnetic properties of a sediment core from a high altitude lake in the Swiss Alps were compared with palynological and geochemical data to link climatic and mineral magnetic variations. According to pollen data, the sediments extend from the present to the Younger Dryas, i.e., they cover more than 10,000 years of environmental change in the Alps. The major change in magnetic properties corresponds to the climatic warming of the early Holocene. High-coercivity magnetic minerals that characterize the Late Glacial period almost disappeared during the Holocene and the concentration of ferrimagnetic minerals increased sharply. The contribution of superparamagnetic grains also decreased in the Holocene sediments. Similar variations in {SP} content and coercivity, of smaller magnitude, are found in the Holocene and are interpreted to represent minor climatic variations. Comparison with the historical record of the last 1000 years confirms this interpretation. The magnetic mineralogy, the superparamagnetic contents, and the {IRM} intensity in the coarse-grained, Late Glacial sediments are similar to those measured in the catchment bedrock. This indicates a detrital origin. The different properties and the higher concentration of magnetic minerals in the Holocene sediments are due to authigenic phases. Magnetic properties provide a high resolution record of climatic change. They are sensitive even to small variations that are not recorded in the pollen or {LOI} data. Magnetic parameters show fine-scale variation and constitute a valuable supplement to conventional climatic indicators.

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Knowledge about the glacial refugia of the thermophilous European Castanea sativa Mill. (sweet chestnut) is still inadequate. Its original range of distribution has been masked by strong human impact. Moreover, under natural conditions the species was probably admixed with other taxa (such as Quercus, Fraxinus, Fagus, Tilia) and thus possibly represented by low percentages in pollen records. In this paper we try to overcome the difficulties related to the scarcity and irregularity of chestnut pollen records by considering 1471 sites and extending the palynological approach to develop a Castanea refugium probability index (IRP), aimed at detecting possible chestnut refugia where chestnuts survived during the last glaciation. The results are in close agreement with the current literature on the refugia of other thermophilous European trees. The few divergences are most probably due to the large amount of new data integrated in this study, rather than to fundamental disagreements about data and data interpretation. The main chestnut refugia are located in the Transcaucasian region, north-western Anatolia, the hinterland of the Tyrrhenian coast from Liguria to Lazio along the Apennine range, the region around Lago di Monticchio (Monte Vulture) in southern Italy, and the Cantabrian coast on the Iberian peninsula. Despite the high likelihood of Castanea refugia in the Balkan Peninsula and north-eastern Italy (Colli Euganei, Monti Berici, Emilia-Romagna) as suggested by the IRP, additional palaeobotanical investigations are needed to assess whether these regions effectively sheltered chestnut during the last glaciation. Other regions, such as the Isère Département in France, the region across north-west Portugal and Galicia, and the hilly region along the Mediterranean coast of Syria and Lebanon were classified as areas of medium refugium probability. Our results reveal an unexpected spatial richness of potential Castanea refugia. It is likely that other European trees had similar distribution ranges during the last glaciation. It is thus conceivable that shelter zones with favourable microclimates were probably more numerous and more widely dispersed across Europe than so far assumed. In the future, more attention should be paid to pollen traces of sporadic taxa thought to have disappeared from a given area during the last glacial and post-glacial period.

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During four expeditions with RV "Polarstern" at the continental margin of the southern Weddell Sea, profiling and geological sampling were carried out. A detailed bathymetric map was constructed from echo-sounding data. Sub-bottom profiles, classified into nine echotypes, have been mapped and interpreted. Sedimentological analyses were carried out on 32 undisturbed box grab surface samples, as well as on sediment cores from 9 sites. Apart from the description of the sediments and the investigation of sedimentary structures on X-radiographs the following characteristics were determined: grain-size distributions; carbonate and Corg content; component distibutions in different grain-size fractions; stable oxygen and carbon isotopes in planktic and, partly, in benthic foraminifers; and physical properties. The stratigraphy is based On 14C-dating, oxygen isotope Stages and, at one site, On paleomagnetic measurements and 230Th-analyses The sediments represent the period of deposition from the last glacial maximum until recent time. They are composed predominantly of terrigenous components. The formation of the sediments was controlled by glaciological, hydrographical and gravitational processes. Variations in the sea-ice coverage influenced biogenic production. The ice sheet and icebergs were important media for sediment transport; their grounding caused compaction and erosion of glacial marine sediments on the outer continental shelf. The circulation and the physical and chemical properties of the water masses controlled the transport of fine-grained material, biogenic production and its preservation. Gravitational transport processes were the inain mode of sediment movements on the continental slope. The continental ice sheet advanced to the shelf edge and grounded On the sea-floor, presumably later than 31,000 y.B.P. This ice movement was linked with erosion of shelf sediments and a very high sediment supply to the upper continental slope from the adiacent southern shelf. The erosional surface On the shelf is documented in the sub-bottom profiles as a regular, acoustically hard reflector. Dense sea-ice coverage above the lower and middle continental slope resulted in the almost total breakdown of biogenic production. Immediately in front of the ice sheet, above the upper continental slope, a <50 km broad coastal polynya existed at least periodically. Biogenic production was much higher in this polynya than elsewhere. Intense sea-ice formation in the polynya probably led to the development of a high salinity and, consequently, dense water mass, which flowed as a stream near bottom across the continental slope into the deep sea, possibly contributing to bottom water formation. The current velocities of this water mass presumably had seasonal variations. The near-bottom flow of the dense water mass, in combination with the gravity transport processes that arose from the high rates of sediment accumulation, probably led to erosion that progressed laterally from east to West along a SW to NE-trending, 200 to 400 m high morphological step at the continental slope. During the period 14,000 to 13,000 y.B.P., during the postglacial temperature and sea-level rise, intense changes in the environmental conditions occured. Primarily, the ice masses on the outer continental shelf started to float. Intense calving processes resulted in a rapid retreat of the ice edge to the south. A consequence of this retreat was, that the source area of the ice-rafted debris changed from the adjacent southern shelf to the eastern Weddell Sea. As the ice retreated, the gravitational transport processes On the continental slope ceased. Soon after the beginning of the ice retreat, the sea-ice coverage in the whole research area decreased. Simultaneously, the formation of the high salinity dense bottom water ceased, and the sediment composition at the continental slope then became influenced by the water masses of the Weddell Gyre. The formation of very cold Ice Shelf Water (ISW) started beneath the southward retreating Filchner-Ronne Ice Shelf somewhat later than 12,000 y.B.P. The ISW streamed primarily with lower velocities than those of today across the continental slope, and was conducted along the erosional step on the slope into the deep sea. At 7,500 y.B.P., the grounding line of the ice masses had retreated > 400 km to the south. A progressive retreat by additional 200 to 300 km probably led to the development of an Open water column beneath the ice south of Berkner Island at about 4,000 y.B.P. This in turn may have led to an additional ISW, which had formed beneath the Ronne Ice Shelf, to flow towards the Filcher Ice Shelf. As a result, increased flow of ISW took place over the continental margin, possibly enabling the ISW to spill over the erosional step On the upper continental slope towards the West. Since that time, there is no longer any documentation of the ISW in the sedimentary Parameters on the lower continental slope. There, recent sediments reflect the lower water masses of the Weddell Gyre. The sea-ice coverage in early Holocene time was again so dense that biogenic production was significantly restricted.

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On the continental margin of the southeastern Weddell Sea, Antarctica, several channel-ridge systems can be traced on the eastern side of the Crary Fan. Swath mapping of the bathymetry reveals three southwest-northeast trending ridges up to 300 m high with channels on their southeastern side. The structures occur on a terrace of the continental slope in water depths of 2000 - 3300 m. We carried out sedimentological studies on cores from three sites. Two of the studied cores are from ridges, one is from the northwestern part of the terrace. The stratigraphy of the recovered sediments is based on accelerator mass spectrometer 14C determinations, stable oxygen and carbon isotopes analyses and paleomagnetic measurements. The sediments represent a period from the last glacial maximum (LGM) to recent time. They are composed predominantly of terrigenous components. We distinguish four different sedimentary facies and assign them to processes controlling sedimentation. Microlaminated muds and cross-stratified coarse-silty sediments originated from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with high contents of ice-rafted debris characterize slumps. The inferred contour currents shaping the continental slope during the LGM were canalized within the channels and supplied microlaminated mud to the western sedimentary ridges due to deflection to the left induced by the Coriolis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents were directed to the northeast and hence opposite to the Weddell Gyre. Cross-stratified coarse-silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Average sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth phases of the Crary Fan during glacial intervals. A substantial environmental change at 19.5 - 20 ka is documented in the sediments by a gradual change from lamination to bioturbation. During the recent interglacial, bioturbated sediments were deposited in all parts of the terrace. Because of a reduction of the contour current velocities (4-7 cm/s), the water masses of the Weddell Gyre, supplying fine-grained sediments from northeast, gain a greater influence on sedimentation on the continental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by greater amounts of ice-rafted debris. The interglacial sedimentation rates decrease to a few cm/ka and indicate that the Crary Fan became relatively sediment-starved during interglacial intervals.

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Millennial-scale variability in the behavior of North Pacific Intermediate Water during the last glacial and deglacial period, and its association with Dansgaard-Oeschger (D-O) cycles and Heinrich events, are examined based on benthic foraminiferal oxygen and carbon isotopes (d18Obf and d13Cbf) and %CaCO3 using a sediment core recovered from the northeastern slope of the Bering Sea. A suite of positive d18Obf excursions at intermediate depths of the Bering Sea, which seem at least in part associated with increases in the d18Obf gradients between the Bering and Okhotsk Seas, suggest the Bering Sea as a proximate source of intermediate water during several severe stadial episodes in the last glacial and deglacial period. Absence of such d18Obf gradients during periods of high surface productivity in the Bering and Okhotsk Seas, which we correlate to D-O interstadials, suggests a reduction in intermediate water production in the Bering Sea and subsequent introduction of nutrient-rich deep waters from the North Pacific into intermediate depths of the Bering Sea. We argue that a reorganization of atmospheric circulation in the high-latitude North Pacific during severe cold episodes in the last glacial and deglacial period created favorable conditions for brine rejection in the northeastern Bering Sea. The resulting salinity increase in the cold surface waters could have initiated intermediate (and deep) water formation that spread out to the North Pacific.

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Abrupt climate changes from 18 to 15 thousand years before present (kyr BP) associated with Heinrich Event 1 (HE1) had a strong impact on vegetation patterns not only at high latitudes of the Northern Hemisphere, but also in the tropical regions around the Atlantic Ocean. To gain a better understanding of the linkage between high and low latitudes, we used the University of Victoria (UVic) Earth System-Climate Model (ESCM) with dynamical vegetation and land surface components to simulate four scenarios of climate-vegetation interaction: the pre-industrial era, the Last Glacial Maximum (LGM), and a Heinrich-like event with two different climate backgrounds (interglacial and glacial). We calculated mega-biomes from the plant-functional types (PFTs) generated by the model to allow for a direct comparison between model results and palynological vegetation reconstructions. Our calculated mega-biomes for the pre-industrial period and the LGM corresponded well with biome reconstructions of the modern and LGM time slices, respectively, except that our pre-industrial simulation predicted the dominance of grassland in southern Europe and our LGM simulation resulted in more forest cover in tropical and sub-tropical South America. The HE1-like simulation with a glacial climate background produced sea-surface temperature patterns and enhanced inter-hemispheric thermal gradients in accordance with the "bipolar seesaw" hypothesis. We found that the cooling of the Northern Hemisphere caused a southward shift of those PFTs that are indicative of an increased desertification and a retreat of broadleaf forests in West Africa and northern South America. The mega-biomes from our HE1 simulation agreed well with paleovegetation data from tropical Africa and northern South America. Thus, according to our model-data comparison, the reconstructed vegetation changes for the tropical regions around the Atlantic Ocean were physically consistent with the remote effects of a Heinrich event under a glacial climate background.

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Benthic and planktonic 14C ages are presented for the last glacial termination from marine sediment core VM21-30 from 617 m in the eastern equatorial Pacific. The benthic-planktonic 14C age differences in the core increased to more than 6000 years between Heinrich 1 time and the end of the Younger Dryas period. Several replicated 14C ages on different benthic and planktonic species from the same samples within the deglacial section of the core indicate a minimal amount of bioturbation. Scanning electron microscopy reveals no evidence of calcite alteration or contamination. The oxygen isotope stratigraphy of planktonic and benthic foraminifera does not indicate anomalously old (glacial age) values, and there is no evidence of a large negative stable carbon isotope excursion in benthic foraminifera that would indicate input of old carbon from dissociated methane. It appears, therefore, that the benthic 14C excursion in this core is not an artifact of diagenesis, bioturbation, or a pulse of methane. A benthic D14C stratigraphy reconstructed from the 14C ages from the deglacial section of VM21-30 appears to match that of Baja margin core MV99-MC19/GC31/PC08 (705 m), but the magnitude of the low-14C excursion is much larger in the VM21-30 record. This would seem to imply that the VM21-30 core was closer to the source of 14C-depleted waters during the deglaciation, but the source of this CO2 remains elusive.

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Reduced nitrate supply to the subarctic North Pacific (SNP) surface during the last ice age has been inferred from coupled changes in diatom-bound d15N (DB-d15N), bulk sedimentary d15N, and biogenic fluxes. However, the reliability of bulk sedimentary and DB-d15N has been questioned, and a previously reported d15N minimum during Heinrich Stadial 1 (HS1) has proven difficult to explain. In a core from the western SNP, we report the foraminifera-bound d15N (FB-d15N) in Neogloboquadrina pachyderma and Globigerina bulloides, comparing them with DB-d15N in the same core over the past 25 kyr. The d15N of all recorders is higher during the Last Glacial Maximum (LGM) than in the Holocene, indicating more complete nitrate consumption. N. pachyderma FB-d15N is similar to DB-d15N in the Holocene but 2.2 per mil higher during the LGM. This difference suggests a greater sensitivity of FB-d15N to changes in summertime nitrate drawdown and d15N rise, consistent with a lag of the foraminifera relative to diatoms in reaching their summertime production peak in this highly seasonal environment. Unlike DB-d15N, FB-d15N does not decrease from the LGM into HS1, which supports a previous suggestion that the HS1 DB-d15N minimum is due to contamination by sponge spicules. FB-d15N drops in the latter half of the Bølling/Allerød warm period and rises briefly in the Younger Dryas cold period, followed by a decline into the mid-Holocene. The FB-d15N records suggest that the coupling among cold climate, reduced nitrate supply, and more complete nitrate consumption that characterized the LGM also applied to the deglacial cold events.