Thesis advisor: Jeremy D. Shakun / Arctic permafrost contains a substantial stock of carbon that could be released to the atmosphere as CH4 and CO2 upon thawing, making it a potentially powerful amplifier of future warming. The sensitivity of permafrost to climate change is uncertain, however, and occurs on time scales longer than those captured by the instrumental record. Speleothems – cave precipitates deposited from flowing or dripping water – in currently frozen regions record past episodes of thaw, which can be used to assess the response of permafrost to long-term warmth. Here, we present 90 uranium-thorium ages on speleothems from across the North American Arctic, sub-Arctic and northern alpine regions to reconstruct a 600-kyr permafrost history. Widespread speleothem growth supports an episode of extensive permafrost thaw during the Marine Isotope Stage 11 interglacial about 400 ka, when global temperature was only slightly warmer than pre-industrial conditions. Additional growth is evident during MIS 13, curiously, a smaller magnitude interglacial. Ice-core records of atmospheric greenhouse gases do not show elevated concentrations at these times, perhaps suggesting that the permafrost carbon pool was smaller than today or released gradually enough to be buffered by other reservoirs. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

L'étude des interglaciaires passés qui sont des périodes chaudes avec un volume de glace réduit comme l’interglaciaire actuel, l'Holocène, est cruciale pour comprendre le climat futur. Ce travail apporte de nouvelles informations sur le climat des interglaciaires clés, les stades isotopiques marins (MIS) 11 et 31, considérés comme des analogues au réchauffement global projeté. Une analyse pollinique des sédiments du Site IODP U1385 (marge sud-ouest ibérique) a été effectuée à haute résolution, ce qui permet de comparer directement les variations de la végétation (atmosphère) avec celles de la température des eaux de surface océaniques. Nos données montrent qu’à l’échelle orbitale, la forêt du sud-ouest de l’Europe pendant le MIS 11 est principalement influencée par la précession alors que pendant le MIS 31, malgré des valeurs de précession extrêmes, le forçage dominant est l’obliquité, favorisant une végétation moins méditerranéenne et un régime climatique tempéré. De plus, la variabilité millénaire apparaît comme une caractéristique persistante mais les épisodes de refroidissement varient en intensité et durée en fonction des conditions limites qui favorisent un forçage prédominant des hautes ou basses latitudes. Enfin, nous examinons l'expression régionale de l'Holocène et de ses analogues orbitaux, les MIS 11c et 19c dans le sud-ouest de l’Europe. Ceci révèle que l'optimum Holocène se distingue par un plus fort développement forestier et donc que les MIS 11c et 19cne sont pas des analogues à l’Holocène pour notre zone d’étude. Grâce à une comparaison modèle-données, nous montrons aussi que la forêt interglaciaire dans cette région est principalement contrôlée par la précession en influençant les précipitations hivernales, facteur critique pour le développement de la forêt méditerranéenne, tandis que le CO2 joue un rôle négligeable. / The study of past interglacials, periods of reduced ice volume like our present interglacial, the Holocene, is crucial for understanding the future climate. This work provides new insights into the intensity and climate variability of key interglacials, namely Marine Isotopic Stages (MIS) 11 and 31, considered as analogues for the projected global warming. A highresolution pollen analysis at IODP Site U1385 off SW Iberia was performed, which enables adirect comparison between atmospheric-driven vegetation changes and sea surface temperature variability. At orbital time scale, this thesis shows that the dominant orbital forcing on the SW European forest was different between the interglacials of the 100-ky (MIS 11) and 41-ky (MIS31) worlds. While during MIS 11 its weak precessional forcing predominates, during MIS 31 itsextreme precession forcing is dwarfed by the prevailing influence of obliquity leading to a temperate climate regime as shown by a less Mediterranean character of the vegetation. This work also shows that millennial-scale variability was a pervasive feature and suggests that the different intensity and duration of the cooling events in SW Iberia was related to different atmospheric and oceanic configurations modulated by high or low-latitude forcing depending on the baseline climate states. Finally, this study examines the dominant forcing underlying the regional expression of the Holocene and its orbital analogues, MIS 11c and 19c, over SW Iberia using a data-model comparison approach. This comparison reveals that the Holocene optimum stands out for its higher forest development and therefore these interglacials cannot be considered as analogues for the Holocene vegetation and climate changes in Iberia. Additionally,it shows that the SW Iberian forest dynamics during these interglacials were primarily controlled by precession through its influence on winter precipitation, which is critical for the Mediterranean forest development whereas CO2 played a negligible role.