I urval från Stockholms universitets publikationsdatabas

• Floral evidence for high summer temperatures in southern Scandinavia during 15-11 cal ka BP

  2020. Frederik Schenk (et al.). Quaternary Science Reviews 233

  Artikel

  The global climate transition from the Lateglacial to the Early Holocene is dominated by a rapid warming trend driven by an increase in orbital summer insolation over high northern latitudes and related feedbacks. The warming trend was interrupted by several abrupt shifts between colder (stadial) and warmer (interstadial) climate states following instabilities of the Atlantic Meridional Overturning Circulation (AMOC) in response to rapidly melting ice sheets. The sequence of abrupt shifts between extreme climate states had profound impacts on ecosystems which make it challenging to reliably quantify state variables like July temperatures within a non-analogue climate envelope. For Europe, there is increasing albeit inconclusive evidence for higher stadial summer temperatures than initially thought. Here we present a comprehensive floral compilation of plant macrofossils from lake sediment cores of 15 sites from S-Scandinavia covering the period similar to 15 to 11 ka BP. We find evidence for a continued presence of plant species indicating high July temperatures throughout the last deglaciation. The presence of hemiboreal plants in close vicinity to the southern margin of the Fennoscandian Ice Sheet implies a strong thermal summer forcing for the rapid ice sheet melt. Consistent with some recent studies, we do not find evidence for a general stadial summer cooling, which indicates that other reasons than summer temperatures caused drastic setbacks in proxy signals possibly driven by extreme winter cooling and/or shorter warm seasons.

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• Warm summers during the Younger Dryas cold reversal

  2018. Frederik Schenk (et al.). Nature Communications 9

  Artikel

  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.

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• A 725-year integrated offshore terrestrial varve chronology for southeastern Sweden suggests rapid ice retreat ~ 15 ka BP

  2021. Rachael S. Avery (et al.). Boreas 50 (2), 477-496

  Artikel

  The Swedish Varve Chronology is an unparalleled tool for linking the deglacial history of Sweden with associated palaeo-environmental change at an annual time scale, and it forms part of Sweden's cultural heritage. A full deglacial chronology connected to the present day does not yet exist; a notable gap is in southeasternmost Sweden, where few varved records are successfully connected to reconstruct ice-margin retreat. Deglaciation in southern Sweden covers both the climate transition to the Bølling warm period (similar to 14.7 ka BP) and the ice-margin transition from a subaqueous to terrestrial terminus. To facilitate investigations into the links between ice-margin dynamics and abrupt climate change, we revisited the varve chronologies of southern Sweden. We digitized unpublished records, reanalysed existing varve thickness records, and obtained and analysed new varve series both on land and offshore. This combined approach has enabled us to refine and extend the existing south coast chronology east and 78 km northwards. Our new Skåne-Småland chronology records 725 years of deglaciation, in addition to a younger floating chronology in parts. This chronology suggests that the glacial-lake terminating Fennoscandian Ice Sheet in southern Sweden initially retreated northwards at similar to 110-160 m a(-1) slowing to 60-70 m a(-1) near the palaeo-shoreline. Between today's mainland and the (now) island of Öland the retreat rates increase three- to fivefold. Ice-margin retreat was initially oriented towards the north (as along the south coast), but later pivoted towards the northwest, signifying a landward retreat of terrestrial 'Swedish' ice that became divorced from the Baltic Sea ice-sheet catchment. Our new 725-year-long varve thickness series reveals repeated multidecadal scale episodes of increased sedimentation. These likely signify phases of enhanced ice-sheet melting that repeat and persist throughout the deglaciation of Skåne-Småland.

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• Ecological turnover in neotropical freshwater and terrestrial communities during episodes of abrupt climate change

  2021. Liseth Pérez (et al.). Quaternary Research 101, 26-36

  Artikel

  The last 85,000 years were characterized by high climate and environmental variability on the Yucatan Peninsula. Heinrich stadials are examples of abrupt climate transitions that involved shifts in regional temperatures and moisture availability. Thus, they serve as natural experiments to evaluate the contrasting responses of aquatic and terrestrial ecosystems. We used ostracodes and pollen preserved in a 75.9-m-long sediment core (PI-6, similar to 85 ka) recovered from Lake Peten Itza, Guatemala, to assess the magnitude and velocity of community responses. Ostracodes are sensitive to changes in water temperature and conductivity. Vegetation responds to shifts in temperature and the ratio of evaporation to precipitation. Ostracodes display larger and more rapid community changes than does vegetation. Heinrich Stadial 5-1 (HS5-1) was cold and dry and is associated with lower ostracode and vegetation species richness and diversity. In contrast, the slightly warmer and dry conditions during HS6 and HS5a are reflected in higher ostracode species richness and diversity. Our paleoecological study revealed the greatest ecological turnover for ostracodes occurred from 62.5 to 51.0 ka; for pollen, it was at the Pleistocene/Holocene transition. Future studies should use various climate and environmental indicators from lake and marine sediment records to further explore late glacial paleoclimate causes and effects in the northern neotropics.

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