This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (δ18Olakew) collected between 2001 and 2006 show that δ18O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (δ18Op). Changes in local δ18Op depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water δ18O is determined by a combination of the original δ18Olakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various δ18Olakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (δ18Odiatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing δ18Op trend was likely forced by a shift from strong zonal westerly airflow (relatively high δ18Op) in the early Holocene to a more meridional flow pattern (relatively low δ18Op). The large δ18Olakew depletion recorded in the δ18O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: In press. Paper 2: Submitted. Paper 5: In progress.

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A travers l’exemple du froid en Hollande au XVIIe siècle, nous souhaitons montrer que la géographie culturelle a sa place dans les études géoclimatologiques. Cette thèse tisse alors des ponts entre plusieurs champs de la géographie (climatologie, géographie historique, géographie culturelle…) et de l’histoire (histoire du climat, histoire de l’art…). Le XVIIe siècle s’inscrit dans une période climatique marquée par un rafraîchissement global des températures : le petit âge glaciaire (1300 à 1850 environ). En analysant des peintures hollandaises hivernales et des sources écrites avec le regard du géoclimatologue, plusieurs questions se posent. Pourquoi à une certaine période (le Siècle d’or allant de 1600 à 1672 environ) et dans un territoire bien précis (les Pays-Bas), des peintres ont-ils donné ses lettres de noblesse au froid ? Quels éléments météorologiques sont représentés ou non-représentés ? Que disent les sources écrites sur le climat de l’époque où les tableaux ont été peints ? Quelle imagerie du froid est-elle créée et que sous-tend cette imagerie ? Les résultats de recherche montrent que les artistes ont été parfois directement inspirés par certains hivers rudes. Le premier paysage d’hiver hollandais d’Avercamp est daté de 1608, hiver particulièrement rigoureux. Ensuite, la mode des paysages d’hiver perdure et les peintres ne représentent qu’un seul type de temps d’hiver alors même que les sources écrites témoignent de la variabilité du climat à plusieurs échelles de temps. Une imagerie de l’hiver a été créée. Elle participe de la construction identitaire de cette jeune nation que sont les Pays-Bas au XVIIe siècle. / This Ph.D dissertation aims at showing that cultural geography is relevant for geoclimatological studies. At the crossroad of many subfields in geography (climatology, historical geography, cultural geography…) and in history (history of climate, history of art…), it focuses on cold weather and its representations in the Netherlands during the XVIIth century. This period takes place during the Little Ice Age (c. 1300-1850), a limited climatic period, characterized by a cooling of the temperatures in Europe. The analysis of Dutch paintings and written sources with a geoclimatologist’s view point raises many questions. Why did the painters give their letters of nobility to the cold in this period (the Golden Age, c. 1600-1672) and in this bounded territory (the Netherlands) ? What meteorological elements are being represented? Which ones are missing? What pieces of information do the written sources give on the climate at the time the paintings were made? What imagery of the cold was thus created and what do the images stressed? The results of that research show that the artists were at times directly inspired by some rigorous winters. The first winter landscape by Avercamp is painted in 1608, a severe winter. Then, the vogue for winter landscapes continues. However, the painters represent just one type of meteorological condition in winter, whereas the narrative sources reveal the variability of the climate in different time scales. Nevertheless, an imagery of winter was created. It is said to be part of the identity construction process of the Netherlands, an upcoming nation in the XVIIth century.

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.

Climatic change has been the subject of investigation and spirited debate for more than three centuries. One important element of this debate has been the search for and definition of unique, impermanent climatic regimes measurable by historic time. The Medieval Warm Period and the Little Ice Age are the two most commonly referenced and discussed of such regimes. This thesis examines the theories and debates that preceded and surrounded the formal definition of the Little Ice Age as an historic period of approximately 1550-1850 AD. This thesis begins by describing early attempts to measure and record climatic conditions during the late seventeenth and early eighteenth centuries while also demonstrating that climatic change and climatic influence were matters of concern for both the scientific and philosophical elite and the public. By the first decade of the nineteenth century, however, discussion of climatic change had begun to center on comparisons of the medieval past and the cooler present. Climatic change itself often intruded on debates about past climates during the early nineteenth century. By 1900, however, both scholars and laymen had begun to recognize that some form of climatic change had occurred in the sixteenth century. Early twentieth century scholars such as Otto Pettersson, Charles Rabot, and Ellsworth Huntington helped define the boundaries and significance of historical climatic regimes. When Francois Matthes wrote of a "little ice-age" in 1939, he was not creating a wholly new idea; he was instead engaging in a centuries-old debate over the climatic conditions of the last millennium.

Satellite imagery and volume-area scaling are used to asses the glacier area and ice volume of Iceland from the Little Ice Age maximum to present day, obtaining a final result in sea level rise between 1890 - 2015. The Little Ice Age was a time of regional cooling, with glaciers reaching their maximum extent (~1890 for Iceland) with warming and glacier retreat after this period ended. Ice volume estimates are important to know due to their relevance in potential sea level rise calculations. Understanding both of these estimations for Iceland connects the impact a changing climate has on regional and global scales. Different scaling parameters used in the volume-area scaling approach to determine ice volume and ultimately sea level equivalents highlight the range of estimates acquired and point out the need in choosing appropriate values based on glacier region. A comparison to using mass balance measurements for volume estimates is also noted, showing differences in ice volume loss over past and present time periods. The Icelandic glacier area for present day is an updated value from previous studies at 10,803 ± 83 km2 and a first ever reported Icelandic Little Ice Age maximum glacier area of 12,201 ± 91 km2. For potential sea level rise, it is found the most reliable estimate from the volume- area scaling assessment is 2.67 mm from the Little Ice Age maximum to present day, with a yearly contribution since 1890 of 0.02 mm. / Satellitbilder och volym-area skalningsmetoden användes för att uppskatta glaciärarea och isvolym på Island från Lilla istiden till nutid, för att få fram hur stor höjningen av havsnivån varit under denna tidsperiod (1890 – 2015). Den lilla istiden var en tid av regional kylning då glaciärer nådde sin maximala utsträckning (~1890 för Island) följt av en snabb reträtt efter att denna period slutade. Uppskattningen av isvolym är viktigt att veta på grund av dess relevans i potentiella beräkningar av höjningen av havsnivån. Att förstå båda dessa uppskattningar för Island är kopplat till den påverkan ett förändrat klimat har på regional och global nivå. De olika skalparametrar som använts i volym-area skalningsmetoden för att bestämma volymen av is, och dess motsvarigheter i havsnivå, gav en rad av olika uppskattningar. Detta pekar på behovet att välja ett lämpligt parametervärde baserat på glaciärregionen. En jämförelse med att använda mätningar av massbalans för volymuppskattningar gjordes också, vilket visar skillnader i isvolymförlust över tidigare och nuvarande tidsperioder. Dagens värde på den isländska glaciärarean är uppdaterat från tidigare studier på 10,803 ± 83 km2 och den första rapporterade maximala isländska glaciärarean från Lilla istiden på 12,201 ± 91 km2. För potentiell höjning av havsnivån, har man funnit att den mest tillförlitlig uppskattning från volym-area skalningsmetoden är 2,67 mm från Lilla istidens maximum till nutid, med ett årligt bidrag sedan 1890 av 0,02 mm. (Översättning Cecilia Bayard.)

The complex and non-linear fluvial river dynamics are characterized by repeated periods of fluvial erosion and re-deposition in different parts of the floodplain. Understanding the fluvial architecture (i.e. the three-dimensional arrangement and genetic interconnectedness of different sediment types) is therefore fundamental to obtain well-based information about controlling factors. However, investigating the fluvial architecture in buried floodplain deposits without natural exposures is challenging. We studied the fluvial architecture of the middle Weiße Elster floodplain in Central Germany, an extraordinary long-standing archive of Holocene flooding and landscape changes in sensitive loess-covered Central European landscapes. We applied a novel systematic approach by coupling two-dimensional transects of electrical resistivity tomography (ERT) measurements and closely spaced core drillings with spatially resolved measurements of electromagnetic induction (EMI) of larger floodplain areas at three study sites. This allowed for (i) time and cost-efficient core drillings based on preceding ERT measurements and (ii) spatially scaling up the main elements of the fluvial architecture, such as the distribution of thick silt-clay overbank deposits and paleochannel patterns from the floodplain transects to larger surrounding areas. We found that fine-grained sand and silt-clay overbank deposits overlying basal gravels were deposited during several periods of intensive flooding. Those were separated from each other by periods of reduced flooding, allowing soil formation. However, the overbank deposits were severely laterally eroded before and during each sedimentation period. This was probably linked with pronounced meandering or even braiding of the river. Our preliminary chronological classification suggests that first fine-grained sedimentation must have occurred during the Early to Middle Holocene, and the last phase of lateral erosion and sedimentation during the Little Ice Age. Our study demonstrates the high archive potential of the buried fluvial sediments of the middle Weiße Elster floodplain and provides a promising time and cost-effective approach for future studies of buried floodplain sediments.

July temperature (TJUL) and total annual precipitation (ANNP) are reconstructed to better understand the spatial and temporal patterns of change in North America over the last 2,000 years using pollen databases. Using a customized application in R, the reconstructions use a composite averaging of multiple site reconstructions that show a distinct warmer Medieval Warm Period (MWP) compared to the colder Little Ice Age (LIA). Results show that, both multi-centennial scale periods are re- constructed as cooler than the last 50 years. Regional time series from several forested ecoregions show positive anomalies up to 0.6ºC during the MWP and anomalies up to -0.3ºC during the LIA. In order to test whether the TJUL reconstructions are biased to the modern calibration climate data, we show a distinct difference between the reconstructions when using station versus reanalysis-based modern TJUL fields. Reconstructions using station-based modern calibration data sets better reflect the centennial to multi-centennial scale climate variability as compared to the reanalysis-based modern calibration data sets that reveal a warm-bias. We justify the choice of the Whitmore et al. (2005) modern data set for large-scale pollen-based paleoclimate reconstructions. Finally we use Local Indicators of Spatial Association (LISA) to spatially filter the ANNP reconstructions in order to distinguish regional hydroclimate patterns from local site-specific conditions. Results show that a La Nina, positive North Atlantic Oscillation (+NAO) and positive Atlantic Multidecadal Oscillation (+AMO) state-like dominated both the MWP and Roman Warm Period (RWP), although the MWP was generally drier. In contrast, the Dark Ages Cold (DAC) period was likely dominated by El Nino, negative NAO and negative AMO state-like circulation. Minimum solar and high volcanic activity is likely to have contributed to more complex hydroclimate regional patterns during the LIA. The results presented in this dissertation can be used as benchmark data sets for future climate data-model comparisons in order to improve our understanding of natural climate variability during the past 2,000 years in the context of modern human-induced climate change.

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