The past : a compass for future earth

Antarctic sea ice impacts on the ocean-atmosphere heat and gas fluxes, the formation of deep and intermediate waters, the nutrient distribution and primary productivity, the so-called &#8216;biological carbon pump&#8217;, one of the most active in the global ocean. In this study, we explore the link between sea ice dynamic, biological production and nutrient cycling during the late Holocene (the last 2,000 yrs) in the Adélie Basin, East Antarctica, from the well-dated sediments of the Ocean Drilling Program (ODP) Site U1357. This archive, composed from ~32 meters of seasonal to annual laminated diatomaceous sequences, allows reconstructions at an unprecedented time resolution (5-10 yrs). Our study combines records of diatom census counts and diatom-specific biomarkers (a ratio (D/T) of di- and tri-unsaturated Highly Branched Isoprenoid lipids (HBI)) as indicators of sea ice and biological production changes, XRF data as markers for terrigenous inputs and bulk nitrogen isotopes (d15N) and d15N on chlorins as proxies for reconstructing nitrogen cycle. The diatom and HBI records reveal five distinct periods. From 0 to 350 yrs AD, decreasing occurrences of sea ice-related diatom species (e.g. Fragilariopsis curta + F. cylindrus) together with low D/T values and increasing open ocean diatom species (large centrics, Chaetoceros Resting Spores (CRS)) document a progressive decline of sea ice presence during the year (>9 months per year) with spring melting occurring earlier in the year and autumn sea ice formation appearing later. In contrast, between 350 and 750 yrs AD, high production of open ocean diatom species and low low D/T values and sea ice related species indicate a short duration of sea ice cover (~10 months per year) is illustrated by a pronounced increase of sea ice-associated diatom species and high D/T values. Between ~1400 and 1850 yrs AD, seasonal sea ice strongly declines (<~7 months per year) as a result of early spring melting (increasing CRS production) and late autumn waxing (high occurrences of Thalassiosira antarctica). Longer growing seasons promoted a substantial development of phytoplankton communities (especially large centric diatoms) that conducted to lower D/T values. Consistent with diatom and HBI reconstructions, XRF data show higher Fe/Al and Zr/Al ratios values during inferred warmer periods and lower ratio values during inferred cooler and icier periods, thus supporting a strong impact of the sea ice seasonal cycle on glacial runoffs. The link between sea ice conditions, biological production and nutrient cycling is still being explored and we will discuss its relationship by combining all the cited records cited above with the d15N records that we are currently generated. Based on our results, we find that sea ice dynamic and associated diatom production in the Adélie Basin revealed an opposite climatic trend than that identified in the Northern Hemisphere for the last 2000 years. The 'Little Ice Age' (1400-1850 yrs AD) or the 'Dark Ages' (400-750 yrs AD) corresponded to warmer climate conditions in the Adélie Basin, while the 'Roman Warm Period' (0-350 yrs AD) or the 'Medieval Warm Period' (900-1200 yrs AD) were associated to colder conditions. We therefore emphasize that Northern and Southern Hemisphere climate evolved in anti-phase seesaw pattern during the late Holocene

Aromatic acids in a Eurasian Arctic ice core: a 2,600-year proxy record of biomass burning

Wildfires and their emissions have significant impacts on ecosystems, climate, atmospheric chemistry, and carbon cycling. Well-dated proxy records are needed to study the long-term climatic controls on biomass burning and the associated climate feedbacks. There is a particular lack of information about long-term biomass burning variations in Siberia, the largest forested area in the Northern Hemisphere. In this study we report analyses of aromatic acids (vanillic and para-hydroxybenzoic acids) over the past 2600 years in the Eurasian Arctic Akademii Nauk ice core. These compounds are aerosol-borne, semi-volatile organic compounds derived from lignin combustion. The analyses were made using ion chromatography with electrospray mass spectrometric detection. The levels of these aromatic acids ranged from below the detection limit (0.01 to 0.05 ppb; 1 ppb  =  1000 ng L−1) to about 1 ppb, with roughly 30 % of the samples above the detection limit. In the preindustrial late Holocene, highly elevated aromatic acid levels are observed during three distinct periods (650–300 BCE, 340–660 CE, and 1460–1660 CE). The timing of the two most recent periods coincides with the episodic pulsing of ice-rafted debris in the North Atlantic known as Bond events and a weakened Asian monsoon, suggesting a link between fires and large-scale climate variability on millennial timescales. Aromatic acid levels also are elevated during the onset of the industrial period from 1780 to 1860 CE, but with a different ratio of vanillic and para-hydroxybenzoic acid than is observed during the preindustrial period. This study provides the first millennial-scale record of aromatic acids. This study clearly demonstrates that coherent aromatic acid signals are recorded in polar ice cores that can be used as proxies for past trends in biomass burning

Stable Isotopes in Tree Rings

This Open Access volume highlights how tree ring stable isotopes have been used to address a range of environmental issues from paleoclimatology to forest management, and anthropogenic impacts on forest growth. It will further evaluate weaknesses and strengths of isotope applications in tree rings. In contrast to older tree ring studies, which predominantly applied a pure statistical approach this book will focus on physiological mechanisms that influence isotopic signals and reflect environmental impacts. Focusing on connections between physiological responses and drivers of isotope variation will also clarify why environmental impacts are not linearly reflected in isotope ratios and tree ring widths. This volume will be of interest to any researcher and educator who uses tree rings (and other organic matter proxies) to reconstruct paleoclimate as well as to understand contemporary functional processes and anthropogenic influences on native ecosystems. The use of stable isotopes in biogeochemical studies has expanded greatly in recent years, making this volume a valuable resource to a growing and vibrant community of researchers

Late Quaternary and Holocene paleoclimate and paleoenvironmental reconstruction - a multi-proxy approach on Swiss speleothems

Detailed knowledge of the past climate and environmental variability is vital in order to predict future scenarios with more confidence. High resolution and highly-resolved natural archives are therefore in growing demand (PAGES report, 2009). This thesis is part of a coordinated effort, STALCLIM project, where a series of state-of-theart methods in speleothems proxies were used in order to develop the full potential of speleothemsas paleoclimate and paleoenvironmental archive. The thesis investigates climate variations as recorded by speleothems from Schafsloch and Milandre caves, spanning time intervals of high scientific interest: 0 to 14 550 years BP and 130 000 to 230 000 years BP. The locations of the caves, Jura Mountains and the Alpstein Mountains (Swiss Alps) are regions highly sensitive to climate-induced changes in temperature, atmospheric pathways and/or environment evolution. In order to establish new climate records and resolve the paleoclimate questions, continuous geochemical analyses were applied, with high resolution stable isotopic and trace elements analysis and 230Th age measurements, and these were combined with thin section analysis of the calcite petrology. Where possible, we conducted continuous and temporal measurements in the caves where the samples were collected, to provide a better understanding of the cave environment. Chapters 2 and 3 highlight the sensitivity of high altitude speleothem proxies to small changes in temperature and water availability, both induced by internal and external climate drivers. The MF3 stalagmite, from Schafsloch Cave, provides a unique opportunity to investigate a regional expression of millennial-scale climate variability in Central Europe during MIS 6 and Late MIS 7. The sample records in high detail the structure of the penultimate deglaciation in the Alps, and brings new evidence regarding the nature of this important climatic transition. Chapters 4 and 5 show the high resolution composite δ18O record and elemental composition of speleothems from Milandre Cave, covering the Lateglacial- Holocene interval. A detailed comparison with Greenland ice cores and Gerzensee lake sediments (Central Switzerland) indicates similarities in the long timescale changes of atmospheric circulation patterns in the whole North Atlantic region after the last deglaciation. In contrast, during the Holocene, the signature pattern in the isotopic calcite δ18O data differs to that seen in Central Europe. This provides important insights into the regional teleconnections and local environmental reaction to changes in the temperature, seasonality of precipitation and atmospheric processes. The use of a robust δ18O/temperature transfer function has enabled a high resolution temperature reconstruction of the autumn to spring temperatures at the Milandre site for the last 14 550 y BP. In summary, this thesis provides new results on environmental and climatic variability in Switzerland during specific intervals over the last 230 000 years. It suports large teleconnections over the northern hemisphere and emphasizes the role of insolation and obliquity as important climate triggers in the region. These new high-resolution and precisely dated records of δ18O and cold season temperature reconstruction for the Jura Mountains over the last 14 550 y BP, are proposed as being representative for Central Europe