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  • 1.
    Ahmed, Engy
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Science for Life Laboratory (SciLifeLab).
    Parducci, Laura
    Unneberg, Per
    Ågren, Rasmus
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rattray, Jayne E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Han, Lu
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. Columbia University, USA.
    Pedersen, Mikkel W.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Afrifa Yamoah, Kweku
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Slotte, Tanja
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Stockholm University, Science for Life Laboratory (SciLifeLab).
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Archaeal community changes in Lateglacial lake sediments: Evidence from ancient DNA2018In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 181, p. 19-29Article in journal (Refereed)
    Abstract [en]

    The Lateglacial/early Holocene sediments from the ancient lake at Hasseldala Port, southern Sweden provide an important archive for the environmental and climatic shifts at the end of the last ice age and the transition into the present Interglacial. The existing multi-proxy data set highlights the complex interplay of physical and ecological changes in response to climatic shifts and lake status changes. Yet, it remains unclear how microorganisms, such as Archaea, which do not leave microscopic features in the sedimentary record, were affected by these climatic shifts. Here we present the metagenomic data set of Hasseldala Port with a special focus on the abundance and biodiversity of Archaea. This allows reconstructing for the first time the temporal succession of major Archaea groups between 13.9 and 10.8 ka BP by using ancient environmental DNA metagenomics and fossil archaeal cell membrane lipids. We then evaluate to which extent these findings reflect physical changes of the lake system, due to changes in lake-water summer temperature and seasonal lake-ice cover. We show that variations in archaeal composition and diversity were related to a variety of factors (e.g., changes in lake water temperature, duration of lake ice cover, rapid sediment infilling), which influenced bottom water conditions and the sediment-water interface. Methanogenic Archaea dominated during the Allerod and Younger Dryas pollen zones, when the ancient lake was likely stratified and anoxic for large parts of the year. The increase in archaeal diversity at the Younger Dryas/Holocene transition is explained by sediment infilling and formation of a mire/peatbog.

  • 2. Barcikowska, Monika J.
    et al.
    Weaver, Scott J.
    Feser, Frauke
    Russo, Simone
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stone, Daithi A.
    Wehner, Michael F.
    Zahn, Matthias
    Euro-Atlantic winter storminess and precipitation extremes under 1.5 degrees C vs. 2 degrees C warming scenarios2018In: Earth System Dynamics, ISSN 2190-4979, E-ISSN 2190-4987, Vol. 9, no 2, p. 679-699Article in journal (Refereed)
    Abstract [en]

    Severe winter storms in combination with precipitation extremes pose a serious threat to Europe. Located at the southeastern exit of the North Atlantic's storm track, European coastlines are directly exposed to impacts by high wind speeds, storm floods and coastal erosion. In this study we analyze potential changes in simulated winter storminess and extreme precipitation, which may occur under 1.5 or 2 degrees C warming scenarios. Here we focus on a first simulation suite of the atmospheric model CAM5 performed within the HAPPI project and evaluate how changes of the horizontal model resolution impact the results regarding atmospheric pressure, storm tracks, wind speed and precipitation extremes. The comparison of CAM5 simulations with different resolutions indicates that an increased horizontal resolution to 0.25 degrees not only refines regional-scale information but also improves large-scale atmospheric circulation features over the Euro-Atlantic region. The zonal bias in monthly pressure at mean sea level and wind fields, which is typically found in low-resolution models, is considerably reduced. This allows us to analyze potential changes in regional-to local-scale extreme wind speeds and precipitation in a more realistic way. Our analysis of the future response for the 2 degrees C warming scenario generally confirms previous model simulations suggesting a poleward shift and intensification of the meridional circulation in the Euro-Atlantic region. Additional analysis suggests that this shift occurs mainly after exceeding the 1.5 degrees C global warming level, when the midlatitude jet stream manifests a strengthening northeastward. At the same time, this northeastern shift of the storm tracks allows an intensification and northeastern expansion of the Azores high, leading to a tendency of less precipitation across the Bay of Biscay and North Sea. Regions impacted by the strengthening of the midlatitude jet, such as the northwestern coasts of the British Isles, Scandinavia and the Norwegian Sea, and over the North Atlantic east of Newfoundland, experience an increase in the mean as well as daily and sub-daily precipitation, wind extremes and storminess, suggesting an important influence of increasing storm activity in these regions in response to global warming.

  • 3.
    Schenk, Frederik
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. KTH Royal Institute of Technology, Sweden.
    Väliranta, Minna
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Cambridge, UK; Columbia University, USA.
    Tarasov, Lev
    Heikkilä, Maija
    Björck, Svante
    Stockholm University, Faculty of Science, Department of Geological Sciences. Lund University, Sweden.
    Brandefelt, Jenny
    Johansson, Arne V.
    Näslund, Jens-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Swedish Nuclear Fuel and Waste Management Company (SKB), Sweden.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Warm summers during the Younger Dryas cold reversal2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 1634Article in journal (Refereed)
    Abstract [en]

    The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. The sudden cooling is typically linked to an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in response to meltwater discharges from ice sheets. However, inconsistencies regarding the YD-response of European summer temperatures have cast doubt whether the concept provides a sufficient explanation. Here we present results from a high-resolution global climate simulation together with a new July temperature compilation based on plant indicator species and show that European summers remain warm during the YD. Our climate simulation provides robust physical evidence that atmospheric blocking of cold westerly winds over Fennoscandia is a key mechanism counteracting the cooling impact of an AMOC-slowdown during summer. Despite the persistence of short warm summers, the YD is dominated by a shift to a continental climate with extreme winter to spring cooling and short growing seasons.

  • 4.
    Yamoah, Kweku A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chabangborn, Akkaneewut
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chulalongkorn University, Thailand.
    Chawchai, Sakonvan
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chulalongkorn University, Thailand.
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    A 2000-year leaf wax-based hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a centennial timescale2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 148, p. 44-53Article in journal (Refereed)
    Abstract [en]

    Limited understanding of the complex dynamics of the tropical monsoon exists, partly due to inadequate paleo (hydro)-climate proxy data from monsoonal regions. This study presents a 2000-year long record of hydrogen isotope values of leaf wax (delta D-wax) from a sedimentary sequence recovered from Lake Pa Kho, Northern Thailand. Evaluation of present day rainfall patterns and water isotope data indicates that delta D-wax reflects the amount of rainfall and is also influenced by El Nino-Southern Oscillation (ENSO) dynamics. Over the last 2000 years, wettest conditions occurred between ca. 700 AD and ca. 1000 AD, whereas the driest intervals lasted from ca. 50 BCE to ca. 700 AD and from ca. 1300 AD to ca. 1500 AD. Further investigations to establish the spatiotemporal variability of ENSO within the wider tropical Asian-Pacific realm over centennial timescales revealed a low-frequency-tripole pattern between mainland SE Asia (MSEA), the tropical West Pacific, and the central-eastern Pacific, with a wetter than normal MSEA during El Nino-like climate conditions. This pattern stands in contrast to the annual event where El Nino cause drier conditions in MSEA. We hypothesize that on centennial timescales the land-sea contrast, which drives monsoon intensity in MSEA, is modulated by the latitudinal shift of the Walker circulation and associated ENSO dynamics.

  • 5.
    Yamoah, Kweku Afrifa
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Higham, Charles F. W.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chabangborn, Akkaneewut
    Chawchai, Sakonvan
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Societal response to monsoonal fluctuations in NE Thailand during the demise of Angkor Civilisation2017In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 27, no 10, p. 1455-1464Article in journal (Refereed)
    Abstract [en]

    This paper investigates the possible social responses to changes in the strength of the southwest monsoon in northeastern Thailand during the currency of the Angkor civilisation. These assessments are based on hydrogen and carbon isotope records of leaf waxes (delta D-wax and delta 13C(wax)) from a 2000-year-long wetland sequence of Pa Kho in northeastern Thailand, a region that formed the northern boundary of the Angkor Kingdom. Our data indicate anthropogenic flooding of the Pa Kho wetland through the control of water through dam construction from c. AD 1300 in response to the fluctuating strength of monsoon rains. delta D-wax, a proxy for regional hydroclimate variability, corroborates pre-existing evidence that increased summer monsoon rains, which supported the expansion of the agrarian economy, aided the rise of the Angkorian Empire whereas extreme drought contributed to its demise. Interestingly, our delta D-wax record shows already a gradual decreasing monsoon intensity from c. AD 1000 onwards, although Angkor's prosperity reached its peak at c. AD 1200. We suggest that the complex hydrological system established under royal patronage at Angkor provided a resilient buffer against short-term monsoon fluctuations. The long-term decline in monsoon rains over a similar to 300-year period, combined with ongoing urbanisation, may have stretched the hydrological systems to their limit. We suggest that this was a major factor that contributed to the demise of Angkor in the mid-15th century.

  • 6.
    Yamoah, Kweku Kyei Afrifa
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chabangborn, Akkaneewut
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chawchai, Sakonvan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Smittenberg, Rienk
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    A 2000-year leaf wax-based hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a sub-millenial timescaleIn: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457XArticle in journal (Refereed)
1 - 6 of 6
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