Den dynamiske istid

Intet resum

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

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. (C) 2020 The Authors. Published by Elsevier Ltd.Peer reviewe

Rekonstruktion av paleomiljön för området kring den Mesolitiska stenåldersboplatsen vid Sammakko, Norrbotten

A large number of quartz fragments from knapping andburnt bone, discovered during reconnaissance (NorrbottensMuseum) of forest-cleared areas north of the villageSammakko, c. 40 km SE of Gällivare, have been interpretedto indicate a short-term used dwelling site. Th esite, located c. 100 meters north of the Linaälven Riverand at the edge of an almost overgrown pond, underwentarchaeological preliminary investigation in the summerof 2019. Collected material of burnt bone and charcoalshows that the settlement was used c. 8800 - 8900 yearsago by Mesolithic hunters. Th e present report includesa landscape analysis and vegetation reconstruction, thelatter carried out through pollen analysis of sedimentarylayers from two nearby lake basins, of the area into whichthese early Holocene hunters migrated. Th e settlement islocated in a so-called Veikimorain area, a landscape withhigh and broad plateaus with depressions in between.Th is landscape was formed during the fi nal phase of aprevious glaciation, a landscape morphology that wasonly very slightly aff ected by the most recent glaciationDating of the lowest sediment layers in the lake basins(silt, clay) shows that current lake basins became completelyice-free c. 9200 years ago while surrounding higher-situated areas became ice-free at least 600 years earlier- stagnant ice residues lingered in the terrain lows whilethe gradually ice-free landscape in higher locations wasoccupied by a vegetation of the Arctic heathland type.When the hunter-gatherer settlement at Sammakko wasutilized, the landscape had been completely ice-free forc. 300–400 years and it constituted an inland settlementat c. 20 km distance from the coastline of the LittorinaSea further east. From the pollen analyzes we can see thateven a little earlier the vegetation had changed from arcticheath to an open birch forest (Betula) with elements ofpine (Pinus) and in more humid locations there was alder(Alnus). Dwarf shrubs, including dwarf birch (Betulanana), willow (Salix) and juniper (Juniperus) were commonand so were various herbs, grasses and sedges in thesemi-open fi elds. Only after c. 8500 BP does the forestbecome a more closed birch-pine forest

Native or Introduced? Fossil Pollen and Spores may Say. An Example from the Azores Islands

Aim: Among the various possible approaches to assess whether a species is native or introduced, the analysis of subfossil pollen and spores from natural archives is a valuable tool. Requirements include a sufficiently high taxonomic resolution of the pollen morphology, as well as sufficient temporal and spatial resolution. The aim of this study is to show the usefulness of this method in solving whether the taxon is native or introduced. Location: The results are derived from two islands of the Azores: Flores and Pico. Methods: Analysis of pollen and spores in lake sediments; radiocarbon dating of the studied sediments. Results: Selaginella kraussiana (Kunze) A. Braun has occurred on the Azores Islands at least for the last 6000 years. Main conclusions: Different authors had assigned three different types of status to Selaginella kraussiana: native, introduced (invasive), or uncertain. High numbers of spores of this taxon were present in the sediment cores from the two studied lakes already several thousand years before the Portuguese discovery and the Flemish settlement in the 15th century. This proves that the species is native on the Azores Islands. Pollen and spore analysis can therefore contribute to historical biogeography not only regarding questions about pre-Quaternary plate tectonics but also about early human impact

Warm summers during the Younger Dryas cold reversal

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.Peer reviewe

Antarctic Glacial History Since the Last Glacial Maximum: An Overview of the Record on Land

This overview examines available circum-Antarctic glacial history archives on land, related to developments after the Last Glacial Maximum (LGM). It considers the glacial-stratigraphic and morphologic records and also biostratigraphical information from moss banks, lake sediments and penguin rookeries, with some reference to relevant glacial marine records. It is concluded that Holocene environmental development in Antarctica differed from that in the Northern Hemisphere. The initial deglaciation of the shelf areas surrounding Antarctica took place before 10000 C-14 yrs before present(sp), and was controlled by rising global sea level. This was followed by the deglaciation of some presently ice-free inner shelf and land areas between 10000 and 8000 yr sp. Continued deglaciation occurred gradually between 8000 yr sp and 5000 yr sp. Mid-Holocene glacial readvances are recorded from various sites around Antarctica. There are strong indications of a circum-Antarctic climate warmer than today 4700-2000 yr sp. The best dated records from the Antarctic Peninsula and coastal Victoria Land suggest climatic optimums there from 4000-3000 yr sp and 3600-2600 yr sp, respectively. Thereafter Neoglacial readvances are recorded. Relatively limited glacial expansions in Antarctica during the past few hundred years correlate with the Little Ice Age in the Northern Hemisphere

Circum-Antarctic Coastal Environmental Shifts During the Late Quaternary Reflected by Emerged Marine Deposits

This review assesses the circumpolar occurrence of emerged marine macrofossils and sediments from Antarctic coastal areas in relation to Late Quaternary climate changes. Radiocarbon ages of the macrofossils, which are interpreted in view of the complexities of the Antarctic marine radiocarbon reservoir and resolution of this dating technique, show a bimodal distribution. The data indicate that marine species inhabited coastal environments from at least 35000 to 20000 yr sp, during Marine Isotope Stage 3 when extensive iceberg calving created a \u27meltwater lid\u27 over the Southern Ocean. The general absence of these marine species from 20000 to 8500 yr sp coincides with the subsequent advance of the Antarctic ice sheets during the Last Glacial Maximum. Synchronous re-appearance of the Antarctic marine fossils in emerged beaches around the continent, all of wh ich have Holocene marine-limit elevations an order of magnitude lower than those in the Arctic, reflect minimal isostatic rebound as relative sea-level rise decelerated. Antarctic coastal marine habitat changes around the continent also coincided with increasing sea-ice extent and outlet glacial advances during the mid-Holocene. in view of the diverse environmental changes that occurred around the Earth during this period, it is suggested that Antarctic coastal areas were responding to a mid-Holocene climatic shift associated with the hydrological cycle. This synthesis of Late Quaternary emerged marine deposits demonstrates the application of evaluating circum-Antarctic phenomena from the glacial-terrestrial-marine transition zone

Geochemistry of Hyltemossen Bog peat core from Sweden

Geochemistry data of a high-resolution peat core from the Past Global Changes - Carbon in Peat on EArth through Time (PAGES_C-PEAT) Project