Cambios climáticos a escala orbital y milenaria en el Atlántico norte entre 800.000 y 400.000 años
[EN]From a climatic and oceanographic point of view, the North Atlantic is a very important area because the main sources of deep water formation, which drive thermohaline circulation, are in the Norwegian-Greenland Sea and the Labrador Sea. Hence, changes in North Atlantic circulation have a great...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2010 |
| País: | España |
| Institución: | Universidad de Salamanca (USAL) |
| Repositorio: | GREDOS. Repositorio Institucional de la Universidad de Salamanca |
| OAI Identifier: | oai:gredos.usal.es:10366/83196 |
| Acceso en línea: | http://hdl.handle.net/10366/83196 |
| Access Level: | acceso abierto |
| Palabra clave: | Tesis y disertaciones académicas Universidad de Salamanca (España) Tesis Doctoral Academic dissertations Cambio climático Atlántico norte Climate change North Atlantic Paleoclimatología Paleoclimatology 2502.05 Paleoclimatología |
| Sumario: | [EN]From a climatic and oceanographic point of view, the North Atlantic is a very important area because the main sources of deep water formation, which drive thermohaline circulation, are in the Norwegian-Greenland Sea and the Labrador Sea. Hence, changes in North Atlantic circulation have a great impact on global circulation. In this Thesis a section of the North Atlantic sediment core IODP Site U1314 was studied. This core was recovered in a key area for studying North Atlantic current variations and IRD discharges as well as changes in the strength of the Atlantic Meridional Overturning circulation (AMOC). Several records were obtained from micropaleontological and geochemical analysis on foraminifers: stable oxygen and carbon isotopes from benthic foraminifers (mainly C. wuellerstorfi), stable oxygen and carbon isotopes from two planktic foraminifer species N. pachyderma sin y dex, IRD fluxes, planktic foraminifer assemblages and trace elements from the planktic foraminifer N. pachyderma sin. Additionally, time series analyses were performed on all the records above mentioned and on the records from the Antarctic ice core EDC. The first aim of this Thesis was establishing the chronological framework for the studied interval. We tuned our benthic δ18O with the Antarctic temperatures from EDC ice core. The studied interval encompasses from MIS 19 to 11, which means from 800 to 400 thousand years (ka) approximately. Using the paleoceanographic and paleoclimatic proxies above mentioned, we propose that glacial-interglacial cycles can be divided in five stages: 1) Early interglacial stage. During this stage the Arctic and Polar fronts were located in a position similar to their present day position. The AMOC was very active and the NAC reached the Norwegian-Greenland Sea and the Labrador Sea where it sank to generate deep waters, like it happens nowadays. 2) Late interglacial stage. During this stage the Arctic front started to migrate southeastwards reaching a position near Site U1314. However, the low benthic δ18O values indicated that the ice sheets did not started to grow. Deep water formation in the Labrador Sea was reduced lowering the input of NAC waters in the subpolar gyre and hence the salinity of the gyre. Conversely in the Norwegian-Greenland Sea deep water formation was very strong. 3) First stage of glacial periods. The beginning of glacial periods was established at the inflection of benthic δ18O record, in other word when the ice volume began to grow. During this stage the AMOC was active in the Norwegian-Greenland Sea because the Arctic and Polar fronts were rather north in this area. That favoured the fresh water transport towards high latitudes and the accumulation of snow in the continents. In the Labrador Sea a perennial sea ice cover prevented convection and deep waters were not produced at this area. The Arctic front was located between Site U1314 and ODP Site 980, whereby the NAC flowed through the East Atlantic and still reached the Norwegian-Greenland Sea. It is likely that deep waters progressively cooled and that cooling was transmitted through the AMOC to the Southern Hemisphere, and the whole ocean. 4) Second stage of glacial periods. This stage began with the first ice-rafting event and it is characterised by a progressive ice sheet growth that was interrupted by several millennial-scale events that we denominated ice sheet collapse events (ISCE). As a consequence of the first iceberg discharges the AMOC was disturbed and intermediate water (GNAIW) was generated instead of deep water (NADW). The ISCE present two phases, first iceberg discharges dampened the formation of GNAIW and because of the see-saw effect, the Southern Hemisphere warmed and CO2 was outgased from the ocean to the atmosphere. Subsequently an abrupt warming occurred in the Northern Hemisphere melting part of the ice sheets. The ISCE present a similar sequence of events as the Heinrich events and the subsequent interstadial that where described for the last glacial period. During the progressive ice sheet growth the Arctic front was at approximately E-W and was located at 55 º N. The NAC reached Site 980 and GNAIW was likely generated south of 60º N. Conversely, during the warm phase of the ISCE the Arctic front migrated northwards, the NAC reached northernmost positions and the GNAIW was generated northernmost too. 5) Late glacial periods-termination. At the end of glacial periods the Arctic front began to retreat slowly and then at the Termination the Arctic front migrated northwestwards abruptly as the ice volume was reduced. The NAC entered in the Norwegian-Greenland Sea and the Labrador Sea where it resume the deep water formation, and hence, the AMOC. Planktic foraminifer assemblages showed abrupt shifts between cold and warm assemblages. N. pachyderma dex, G. inflata y G. bulloides were dominant during the warm periods whereas during cold periods the assemblage was almost monospecific and N. pachyderma sin dominated. The abrupt shifts were related to the position of the Arctic front respect to the studied area, which determined the presence of NAC waters or Arctic waters. When the Arctic front was near Site U1314 high percentages of T. quinqueloba were recorded. Paleotemperature reconstructions performed with transfer functions for planktic foraminifers and Mg/Ca paleothermometry showed that Mg/Ca paleothermometry is reliable when the Arctic front was north of Site U1314 whereas the presence of the Arctic waters interfered in the Mg/Catemperature relationship, supporting previous works. Additionally the analysis of temperatures and δ18O records on planktic foraminifer tests of N. pachyderma sin and dex suggests that during interglacial periods the difference between both species is related to seasonality and stratification of the water column. During MIS 15 and 11 seasonality was higher, with cooler winters and warmer summers than during MIS 17 and 13 summer stratification of the water column was higher than during MIS 17 and 13. The differences in seasonality might be related with the strength of summer winds. During interglacial periods the presence of sea ice during most of the year favoured that both species lived during the same season and in the same water mass because the sea ice melting might have produced a low salinity lid that prevented water mixing. The iceberg discharges did not produce significant changes in the seawater δ18O except at the Terminations. Time series analyses performed on the different records of the Thesis and their comparison with the spectral analyses of EDC records, showed a progressive importance of the precession band in the climatic cycles. From the marine-establishment of the East Antarctic Ice Sheet the Southern Hemisphere fluctuated between in and out phase responses respect to the Northern hemisphere until MIS 12. This change produced a northward shift in the ITCZ increasing moisture transport to higher latitudes and contributing to the snow accumulation and the large ice sheet growth of MIS 16. From MIS 16 to 12 high latitude northern hemisphere insolation controlled global climate changes and the CO2 concentration, but the climate was still highly influenced by obliquity. After MIS 12 the ITCZ migrated near the equator, sea surface temperatures in the Southern Hemisphere increased and ice volume changes occurred in phase. |
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