Bjerkreim Late-glacial

The following shows our findings and conclusions of the work Bjerkreim was deglaciated around 16 ka BP. Estimated July mean temperatures 12-13℃ in Bølling, 9℃ in Older Dyas, between 9.5 and 11℃ in Allerød, between 6.5 and 9℃ in Younger Dryas, and 11.5 – 12.5℃ in the early Holocene. The chronology is based on the Vedde Ash and Saksunarvatn ash beds, chronohorizons of well-defined climatic boundaries, and AMS 14C-dated remains of terrestrial plants, chironomids, and Cladocera. Dominant vegetation was open pioneer vegetation in pre-Bølling, dwarf shrubs and snow-beds in Bølling, shrubs and open birch-forests in Allerød, snow beds in Younger Dryas, and Betula/Populus/Corylus-forests in the early Holocene. Elm and oak were probably present regionally around 11 ka BP. Through macrofossil finds, we present the first evidence of the LG presence of tree-birch in Norway. Its sparse presence in Bølling with temperatures well above the requirements for birch-forest development shows that factors such as shallow soils and drought restricted tree-birch growth. The Empetrum expansions in Rogaland with a lag towards the east (mid-Bølling vs early Allerød), reflect the development of acid humus-soils controlled by an erosion/outwash gradient decreasing towards the east. Compared to other LG studies in N Europe, the coastal position of Rogaland with surplus access of moisture/water, facilitated the early formation of dwarf-shrub heath. The three interstadial cooling events (Older Dryas/GI-1d, G1-1c2, GI-1b/Gerzensee) are distinctly signaled in the pollen and lithostratigraphical records as small-scale parallels to YD, showing the ecotonal position of Bjerkreim with low resilience to these changes. A tentative effort to distinguish local pollen from long-distance pollen from the continental N-Europe include comparison of plants present temperature demands vs estimated palaeotemperatures, and pollen records in SW Norway vs those in continental N-Europe. We also assess the high ecological variation of LG habitats at Bjerkreim, and plant features such as ecology, pollen production and dispersal capacities. Based on these considerations, we assume that Populus tremula, Fagopyrum, Hippohaë rhamnoides, Jasione montana, Alisma plantago-aquatica, Hedysarum hedysaroides, Sanguisorba officinalis, and S. minor belonged to the LG vegetation of Norway. This must be confirmed by macrofossils and/or DNA. The presence of the North Sea Continent/Doggerland was crucial for the rapid immigration of species into SW Norway.以下展示了本研究的发现与结论。 比约克雷姆（Bjerkreim）地区的冰消作用始于约16 ka BP（距今1.6万年）。博林阶（Bølling）的7月平均气温估算为12~13℃，老仙女木期（Older Dryas）为9℃，阿勒罗德期（Allerød）为9.5~11℃，新仙女木期（Younger Dryas）为6.5~9℃，全新世早期（early Holocene）则为11.5~12.5℃。 本次年代学框架基于维德火山灰层（Vedde Ash）、萨克萨瓦特纳火山灰层（Saksunarvatn ash beds）这两处具有明确气候边界的年代地层标志，以及加速器质谱碳14（AMS ¹⁴C）测年得到的陆生植物、摇蚊（chironomids）与枝角类（Cladocera）遗骸建立。 该区域的优势植被类型依次为：博林阶前以开阔先锋植被为主，博林阶以矮灌与雪床群落为主，阿勒罗德期以灌丛与开阔桦木林为主，新仙女木期以雪床群落为主，全新世早期则以桦木属（Betula）/杨属（Populus）/榛属（Corylus）林为主。约11 ka BP（距今1.1万年），区域内可能已出现榆属与栎属植物。 通过大化石（macrofossil）发现，我们首次证实了挪威存在晚冰期（Late Glacial，简称LG）的垂枝桦（tree-birch）种群。该物种在博林阶仅零星分布，而当时的气温已远超桦木林生长所需的温度阈值，这表明浅土层与干旱等因素限制了垂枝桦的生长扩张。 罗加兰（Rogaland）地区的岩高兰属（Empetrum）植物扩张呈现出向东滞后的特征（博林阶中期 vs 阿勒罗德早期），这反映了受侵蚀/冰水沉积梯度控制的酸性腐殖质土壤的发育规律——该梯度自西向东逐渐降低。与北欧其他晚冰期研究相比，罗加兰的沿海区位拥有充足的水分供给，这促进了矮灌石楠荒原的早期形成。 三次间冰阶降温事件（老仙女木期/GI-1d、GI-1c2、GI-1b/格尔岑湖期（Gerzensee））在孢粉与岩石地层记录中均有清晰体现，可作为新仙女木事件的小型类比事件，这表明比约克雷姆地区处于生态交错带，对这类气候变化的生态弹性较低。 我们尝试区分本地孢粉与来自北欧大陆的长距离传输孢粉，具体方法包括：对比植物的温度需求与估算的古气温，以及对比挪威西南部与北欧大陆的孢粉记录。此外，我们还评估了比约克雷姆地区晚冰期生境的高度生态异质性，以及植物的生态习性、孢粉产量与传播能力等特征。 基于上述考量，我们推测欧洲山杨（Populus tremula）、荞麦属（Fagopyrum）、沙棘（Hippophaë rhamnoides，原文笔误为Hippohaë）、山地钟花（Jasione montana）、泽泻（Alisma plantago-aquatica）、北岩黄耆（Hedysarum hedysaroides）、地榆（Sanguisorba officinalis）与小地榆（S. minor）属于挪威晚冰期植被的组成物种。该结论尚需通过大化石或DNA检测予以证实。 北海古大陆（North Sea Continent/多格兰（Doggerland））的存在，对于物种快速迁入挪威西南部起到了关键作用。