This dissertation presents results from three studies that address major scientific questions in glacial geology and paleoclimatology for the late Pleistocene and Holocene using relatively new geochemical and statistical techniques. Each of the studies attempts to answer a longstanding question in the respective field using geochemical or statistical methods that have not been applied to the problem thus far. A longstanding question in glaciology is the nature and mechanism of the so- called "Heinrich events" of the last ~60 ka. These massive iceberg discharge events into the North Atlantic from the partial breakup of the Laurentide Ice Sheet are identified from distinct ice rafted debris and detrital carbonate layers in marine sediment cores. The mechanism associated with the initiation of these events is commonly thought to be related to internal ice sheet instabilities. However, Heinrich events consistently occur following a long cooling trend that culminates in an extreme cold event, thus suggesting a possible triggering mechanism by climate. Recent modeling work has proposed an oceanic mechanism associated with ocean warming, but no physical evidence has been made available to date. To test this ocean-warming hypothesis, we measured temperature sensitive trace metals and stable isotopes in benthic foraminifera from a sediment core collected in the western North Atlantic that spans the last six Heinrich events and compared our results to climate model simulations using CCSM3. Our results show subsurface warming occurred prior to or coeval with nearly all of the Heinrich events of the last ~60 ka, thus implicating subsurface ocean warming as the main trigger of these rapid breakups of the Laurentide Ice Sheet. In the field of glacial geology a longstanding question has been the timing of alpine glacial advances during the Holocene. A number of studies have interpreted several Holocene glacial advances in western North America, but age control is based largely on relative dating techniques, which have been shown to be in error by up to 10,000 yrs in some cases. Based on 124 ¹⁰Be surface exposure ages from twenty cirque moraines in ten mountain ranges across western North America, glacier were retreating from moraine positions during the latest Pleistocene or earliest Holocene and not throughout the Holocene epoch as previously assumed, thus requiring a refined interpretation of Holocene glacial activity in western North America and the associated climate forcing. In the field of paleoclimatology a question regarding how global temperature varied over the entirety of the Holocene epoch has remained to be answered for some time. While many temperature reconstructions exist for the last 2000 years, a full Holocene temperature stack does not exist, despite its potential utility of putting modern climate change into a full interglacial perspective. Based on a global composite of 73 proxy based temperature record, a Holocene temperature stack was constructed and used to demonstrate that a general cooling of ~1°C has occurred from the early to mid Holocene and that centennial and millennial scale variability is modest. We account for both temperature calibration and chronologic uncertainties using a Monte Carlo based approach. Our results are consistent with prior reconstructions of the last 2000 years and now allow for a full Holocene temperature perspective for evaluation with present and future climate change. / Graduation date: 2011 / Access restricted to the OSU Community, at author's request, from May 5, 2011 - May 5, 2012

NE Brazil is a semi-arid region influenced by the southward displacement of the Intertropical Convergence Zone (ITCZ), which is the main atmospheric system that modulates the seasonal increasing in precipitation over this region. It is well established that these wet periods correlate temporally with the Heinrich Events. However, the local oceanographic response to the fresh water pulses in North Atlantic and how it influences sediment transport along the margin off NE Brazil, is still a matter of discussion. This thesis presents a rock magnetic and element data along four marine cores collected in the south equatorial Atlantic Ocean, covering the last 85 ka BP. Moreover, it was performed an end-member analysis based on the IRM acquisition curves bringing better understanding about sediment sources. Magnetic characterization permitted us to investigate a possible relationship between river supply/longitudinal transport and precipitation enhancement caused by cold events over the North Atlantic. The results suggest a differential deposit of magnetic minerals along the coast, with highly coercive phases (hematite and goethite) further west, and gradually softer phases, like (titano)magnetite to the east. This difference is due mainly to differential oxidation over land, but secondarily we suggest a transport from W-E of coercive material, caused by weakness of NBC. / O NE do Brasil é uma região semiárida influenciada pela migração para sul da Zona de Convergência Intertropical (ITCZ), que é o principal sistema atmosférico que controla o aumento sazonal da precipitação nessa região. É bem estabelecido que esses períodos úmidos se correlacionam temporalmente com os Eventos Heinrich. Entretanto, a resposta oceanográfica local a esse input de água doce no Atlântico Norte e como isso influencia o transporte de sedimentos ao longo da margem adjacente ao NE do Brasil é ainda discutido. Este trabalho apresenta dados magnéticos e geoquímicos para quatro testemunhos coletados na parte sul do Oceano Atlântico equatorial, para os últimos 85 ka BP. Além disso, foi feito um estudo de end-members baseado nas curvas de aquisição de IRM, a partir do qual elucidamos sobre possíveis fontes de sedimento. O estudo magnético permitiu investigar uma possível relação entre input de rios/transporte longitudinal e aumento da precipitação causado pelos eventos frios do Atlântico Norte. Os resultados sugerem uma deposição diferencial de minerais magnéticos a longo da costa, com fases altamente coercivas (hematita e goethita) na parte oeste, e fases gradualmente menos coercivas, como titano(magnetita) para leste. Essa diferença é devida, principalmente, à oxidação diferencial no continente, mas secundariamente nós sugerimos um transporte W-E de material coercivo, causado pelo enfraquecimento da Corrente Norte do Brasil (NBC).

A high-resolution multi-proxy reconstruction of the Arabian Sea Summer Monsoon (ASSM) intensity over the past 90,000 years has been determined using two marine sediment cores: one from the Somali margin and one from the Indian margin. This reconstruction indicates that changes in monsoon- induced upwelling, primary productivity and denitrification have varied in synchrony with Dansgaard-Oeschger (D-O) cycles. Increased monsoon intensity correlates with warm climate events (interstadials) and decreased monsoon intensity, which coincides with stadials and Heinrich Events, is confirmed by elevated dust concentrations in the marine cores. A comparison of the Somali and Indian margin cores with previously reported studies from the Northern and Western Basin allows the identification of discrete sediment signals from the Indus River, the Arabian Peninsula and from local riverine runoff. Sedimentary deposition on the Indian margin during interglacials is dominated by local terrestrial runoff, whereas during glacial periods increased dust input from the Arabian Peninsula is evident. Both signals are related to changes in the intensity of the ASSM. Monsoon intensity has decreased during the Holocene as the Intertropical Convergence Zone (ITCZ) has moved to a more southerly position. The ASSM-ITCZ relationship (increased ASSM intensity and a northern ITCZ, decreased ASSM intensity and a southern ITCZ) has remained consistent over the last glacial cycle suggesting that global millennial scale climatic variability is in part driven by modulations in tropical hydrological cycle. This ASSM reconstruction provides evidence that rearrangements in the tropical convection system affected atmospheric dust concentrations as well as the concentration and location of atmospheric water vapour. In addition to modulating terrestrial and marine emissions of greenhouse gases, variation in the tropical hydrological cycle provides a mechanism of amplifying and perpetuating millennial-scale climatic changes.

NE Brazil is a semi-arid region influenced by the southward displacement of the Intertropical Convergence Zone (ITCZ), which is the main atmospheric system that modulates the seasonal increasing in precipitation over this region. It is well established that these wet periods correlate temporally with the Heinrich Events. However, the local oceanographic response to the fresh water pulses in North Atlantic and how it influences sediment transport along the margin off NE Brazil, is still a matter of discussion. This thesis presents a rock magnetic and element data along four marine cores collected in the south equatorial Atlantic Ocean, covering the last 85 ka BP. Moreover, it was performed an end-member analysis based on the IRM acquisition curves bringing better understanding about sediment sources. Magnetic characterization permitted us to investigate a possible relationship between river supply/longitudinal transport and precipitation enhancement caused by cold events over the North Atlantic. The results suggest a differential deposit of magnetic minerals along the coast, with highly coercive phases (hematite and goethite) further west, and gradually softer phases, like (titano)magnetite to the east. This difference is due mainly to differential oxidation over land, but secondarily we suggest a transport from W-E of coercive material, caused by weakness of NBC. / O NE do Brasil é uma região semiárida influenciada pela migração para sul da Zona de Convergência Intertropical (ITCZ), que é o principal sistema atmosférico que controla o aumento sazonal da precipitação nessa região. É bem estabelecido que esses períodos úmidos se correlacionam temporalmente com os Eventos Heinrich. Entretanto, a resposta oceanográfica local a esse input de água doce no Atlântico Norte e como isso influencia o transporte de sedimentos ao longo da margem adjacente ao NE do Brasil é ainda discutido. Este trabalho apresenta dados magnéticos e geoquímicos para quatro testemunhos coletados na parte sul do Oceano Atlântico equatorial, para os últimos 85 ka BP. Além disso, foi feito um estudo de end-members baseado nas curvas de aquisição de IRM, a partir do qual elucidamos sobre possíveis fontes de sedimento. O estudo magnético permitiu investigar uma possível relação entre input de rios/transporte longitudinal e aumento da precipitação causado pelos eventos frios do Atlântico Norte. Os resultados sugerem uma deposição diferencial de minerais magnéticos a longo da costa, com fases altamente coercivas (hematita e goethita) na parte oeste, e fases gradualmente menos coercivas, como titano(magnetita) para leste. Essa diferença é devida, principalmente, à oxidação diferencial no continente, mas secundariamente nós sugerimos um transporte W-E de material coercivo, causado pelo enfraquecimento da Corrente Norte do Brasil (NBC).

Growth rates, δ18O and δ13C of five stalagmites from the west coasts of North and South Islands, New Zealand, provide records of millennial-scale climate variability over the last ~75 kyr. Thirty-five uranium-series ages were used to provide the chronology. δ18O of stalagmite calcite was influenced by changes in moisture source region, temperature and both δ18O and δ13C primarily display a negative relationship with rainfall. To assist interpretation of climatic signals δ18O profiles were adjusted for the ice-volume effect. Changes in these proxies reflect changes in the strength of the circumpolar westerly circulation and the frequency of southwesterly flow across New Zealand. MIS 4 was a period of wet and cool climate lasting from 67.7 to 61.3 kyr B.P., expressed in the stalagmites by an interval of strongly negative isotope ratios and increased growth rate. This contrasts with less negative δ18O and δ13C, and slow growth, interpreted as dry and cold climate, during much of MIS 2. This difference between MIS 2 and MIS 4 provides an explanation for why glacial moraines in the Southern Alps of MIS 4 age lie beyond those deposited during the last glacial maximum (MIS 2). Heinrich events, with the exception of H0 (the Younger Dryas), are interpreted from high-resolution South Island stalagmite HW05-3, from Hollywood Cave, West Coast, as times of wetter and cooler climate. Minima in δ18O and δ13C (wet periods) occurred at 67.7-61.0, 56-55, 50.5-47.5, 40-39, 30.5-29, 25.5-24.3 and 16.1-15. kyr B.P. matching Heinrich events H6-H1 (including H5a) respectively. This demonstrates a robust teleconnection between events in the North Atlantic and New Zealand climate. Minima in δ18O also occurred at similar times in less well-dated North Island stalagmite RK05-3 from Ruakuri Cave, Waitomo. Speleothems from low-latitudes have revealed that Heinrich events forced southerly displacement of the Intertropical Convergence Zone. This caused steepening of the temperature gradient across mid-southern latitudes, increased westerly circulation and resulted in wet conditions on the west coast of both islands. Immediately following H1 in the HW05-3 stable isotope profiles is another excursion to more negative isotopic values, suggesting wet and cold climate, lasting from 14.6 to 13.0 kyr B.P. Such a climate on the West Coast at this time has been previously suggested from glacier advance (e.g. Waiho Loop moraine) and decreased abundance of tall trees on the landscape. This event occurred too early to be a response to H0, but is synchronous with a return to cool climate in Antarctica. Thus West Coast climate appears to have been sensitive to changes in Antarctica as well as the North Atlantic. Isotopic minima (wet and cool climate) in South Island stalagmite GT05-5, which formed during the Holocene, first occurred 4.6 kyr B.P. This began a series of four oscillations in isotope ratios, the last terminating when the stalagmite was collected (2006). Onset of these oscillations is associated with initiation of ice advance in the Southern Alps, and beginning of the Neoglacial. The last oscillation displays enriched isotope ratios lasting from 1.2 to 0.8 kyr B.P. succeeded by depleted ratios lasting until 0.15 kyr B.P., mirroring the Medieval Climate Optimum and Little Ice Age, respectively, of European palaeoclimate records.

La dernière période glaciaire a été ponctuée d’évènements climatiques abrupts connus sous le nom d’évènements d’Heinrich et évènements de Dansgaard-Oeschger. Cette variabilité millénaire a fait l’objet de nombreuses études, mais plusieurs incertitudes demeurent. Ce travail de doctorat vise à étendre et compléter les connaissances existantes sur cette variabilité climatique rapide en ciblant l’étude des variations hydrographiques telles qu’enregistrées au sein de deux archives sédimentaires prélevées au niveau des îles Féroé. Nos principaux résultats, basés sur une approche intégrée multiproxies, mettent en évidence un schéma atypique en Mer de Norvège, où les épisodes froids (stadiaires, évènements d’Heinrich inclus) sont marqués par des températures océaniques de surface relativement élevées (notamment en été) et un couvert de glace de mer réduit à quelques mois par an, et inversement pour les périodes chaudes (interstadiaires) qui enregistrent des conditions océaniques de surface froides et une expansion du couvert de glace de mer. Le caractère atypique des stadiaires paraît lié à une advection accrue d’eaux chaudes atlantiques dans les Mers Nordiques, couplée à un réchauffement de subsurface généralisé au bassin subpolaire Nord-Atlantique et ses mers bordières. Ces deux processus semblent jouer un rôle majeur dans la déstabilisation des ice-shelves et glaciers boréaux, et in fine dans leur effondrement final et les débâcles d’icebergs qui en résultent. Nos travaux nous permettent également de dégager les principaux mécanismes à l’origine des changements de circulation océanique en Atlantique Nord et des variations de température atmosphérique associées. Sur la base de l’ensemble de nos résultats et de ceux émanant de précédentes études, nous proposons ainsi un nouveau scénario de fonctionnement couplé océan-glace-atmosphère permettant d’expliquer les évènements climatiques abrupts de la dernière période glaciaire. / The last glacial period was punctuated by abrupt climatic events known as Dansgaard-Oeschger and Heinrich events. Many studies have focused on this millennial climatic variability, but several uncertainties remain. The present work aims at improving our knowledge on this topic through the study of the hydrographical changes recorded in two marine archives retrieved off Faeroes. Our main results, based on a multiproxy approach, reveal a paradoxical scheme in the Norwegian Sea where cold episodes (stadials, including Heinrich events) are characterized by relatively warm sea-surface temperatures (especially during summer) and a reduced sea-ice cover, and warm periods (interstadials) are marked by a reverse pattern with cold sea surface conditions and extended sea ice cover. The atypical stadial features seem to be related to enhanced advection of warm Atlantic waters in the Nordic Seas, combined to a subpolar North-Atlantic and adjacent seas basin-wide subsurface warming. These two processes seem to play a key role in the destabilization of boreal ice-shelves and ice-sheets, and in fine to their final collapse and subsequent iceberg discharges. Our work also allows us to identify the main mechanisms responsible for Atlantic Meridional Overturning Circulation changes and associated atmospheric temperature variations. On the basis of our results and of those coming from previous studies, we thus propose a new hydrographical scenario which could explain the abrupt climate events of the last glacial period

No description available.

Tandis que la variabilité climatique à long terme du Quaternaire terminal (oscillations glaciaire/interglaciaire) est relativement bien appréhendée aujourd’hui, l'origine et la modalité des variations climatiques haute-fréquence séculaires à millénaires, depuis la très haute fréquence des cycles de Dansgaard-Oeschger (D/O) jusqu'à la plus grande périodicité des événements d'Heinrich, restent toujours matière à débat. Parmi les éléments encore équivoques se trouvent notamment les modalités de transferts de chaleur méridiens et latitudinaux. A ce titre, la connexion hydrologique entre l’Atlantique et la Méditerranée apparaît comme un exemple d’étude particulièrement approprié vu qu’elle illustre le couplage de phénomènes jouant selon un double gradient Est-Ouest et Nord-Sud, reliant ainsi processus subtropicaux et nord atlantiques. Nous avons ainsi ciblé nos analyses sur des séquences prélevées autour du détroit de Gibraltar, avec pour objectif d’étendre les connaissances spatiales et temporelles de la variabilité climatique haute fréquence des derniers 50 000 ans dans le secteur de l’Atlantique Est subtropical et de la Méditerranée occidentale. La méthodologie de cette thèse est ainsi basée sur une comparaison multi-proxies qui inclut des analyses micropaléontologiques (dinokystes et foraminifères planctoniques) et géochimiques (isotopes stables et alkénones). Nous avons ainsi pu tester la cohérence des changements hydrologiques de surface inter- et intra-bassins et tenter de caractériser la migration des fronts hydrologiques associés. Grâce aux sites des marges marocaine et portugaise, notamment, nous avons pu vérifier l’impact des cycles de D/O sur la variabilité de l’intensité des cellules d’upwelling côtières dans ce secteur et sur la dynamique de la veine d’eau méditerranéenne profonde (MOW), couplant signaux des masses d’eau superficielles (température, salinité et productivité) aux paléo-intensités de la MOW. / While the long-term climatic variability of the Quaternary is relatively well understood today, the causes and processes at the origin of the rapid and brutal climatic variability that characterized the last glacial period (Dansgaard-Oeschger cycles), including meridional and latitudinal heat transfer mechanisms, are still subject to debate. As such, studying the hydrological connection between the Atlantic and the Mediterranean Sea appears particularly appropriate since it illustrates the coupling acting on a double East-West and North-South gradient. It furthermore links subtropical processes and North Atlantic ones. We focused our analysis on sedimentary sequences retrieved around the Strait of Gibraltar, in order to extend the spatial and temporal knowledges about the impact of the high frequency climatic variability of the last 50 000 years in the sector of the eastern subtropical Atlantic and of the western Mediterranean Sea. The methodology of this thesis is based on a multi-proxy compilation coupling micropaleontological (dinocysts and planktonic foraminifera) and geochemical (stable isotopes and alkenones) approaches. We have tested the consistency of the sea-surface paleohydrological changes at inter-and intra-basins scales and tried to characterize the associated migration of the hydrological fronts. Sites of the Portuguese and Moroccan margins, in particular, allowed us to document the impact of the D/O cycles on the intensity of coastal upwelling cells as well as on the dynamics of the MOW, by coupling sea surface signals (temperature, salinity and productivity) to paleointensity of the MOW.