Climatic changes in areas adjacent to the North Atlantic during the last glacial-interglacial transition (14-9 ka BP): A contribution to IGCP-253

This paper presents a summary of the evidence for climatic changes during the last glacial-interglacial transition (14-9 ka BP) in land areas adjacent to the North Atlantic. It is a synthesis of the results of the 12 regional summaries compiled by contributors to the North Atlantic Seaboard Programme of IGCP-253 that are published separately in this issue of Journal of Quaternary Science. Eighteen palaeotemperature curves are compared and arranged in three transects, one from southern Europe to Spitsbergen, a second from Ireland to Poland, and the third from southern New England to the Labrador-Baffin Shelf area. Ten maps are presented that summarise the synoptic climatic conditions of the region in 500-year time periods. The purpose of the exercise is to examine the extent to which any emerging patterns support the Ruddiman and Mclntyre model of large-scale shifts in the position of the oceanice Polar Front during the last glacial-interglacial transition. Some broad agreement emerges, although the influence of oceanic changes is diminished in areas that lay in close proximity to the Laurentide and Fennoscandian ice sheets. Attention is drawn to limitations in the compilations and to the potential for improved models in the future.     

Late-glacial climatic oscillations as recorded in Swiss lake sediments

Regional pollen assemblage zones for the late-glacial period of the Swiss Plateau are introduced and defined. They include four major zones (Artemisia, Juniperus—Hippophaë, Betula, Pinus PAZ) with several subzones. Pollen and oxygen-isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000-9500 yr BP. The first oscillation, termed the Aegelsee fluctuation, occurs shortly before 12 000 yr BP and varve counts suggest its duration was ca. 100 yr. It is characterised by a short decrease in the oxygen isotopes as well as a short increase in NAP associated with a depression in birch pollen values. The second oscillation, which occurs in the δ¹⁸O record shortly before the deposition of the Laacher See Tephra (ca. 11 000 yr BP), is termed the Gerzensee fluctuation. It occurs during a pine-dominated phase and its vegetational effects cannot be determined palynologically. The most prominent regressive phase is the Younger Dryas biozone (ca. 10 700-10 000 yr BP) characterised by an increase in heliophilous NAP and low δ¹⁸O values. The Younger Dryas biozone can often be subdivided palynologically into two parts: a first part rich in grasses and juniper and a second part with higher Filipendula and birch values. During the Preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios. Comparison of the Swiss oxygen isotope profiles with the Greenland Dye 3 record suggests that not only the three major shifts in the δ¹⁸O curves but also the minor ones are closely comparable, suggesting some common climatic control.     

MIS3中期以来北黄海中部陆架古环境演化

通过对北黄海中部陆架DLC70-2孔的岩性、粒度、微体古生物以及AMS14C年代学的分析,探讨了研究区MIS3中期以来的古环境演化。研究表明,北黄海中部陆架MIS3中期以来的沉积环境演化与海平面变化存在良好的对应关系,具有明显的阶段性特征:岩芯22.89~18.50 m段(43 639~42 558 cal yr B.P.)对应MIS3中期海平面上升过程中形成的河口湾相,与上覆地层之间存在明显的沉积间断;随着海平面的逐渐上升,海水自14 602 cal yr B.P. 开始侵入研究区,形成了18.50 m以上的沉积地层,自下而上可划分为末次冰盛期以来海平面上升过程中的河口充填相(18.50~10.02 m,14 602~12 602 cal yr B.P.)—新仙女木事件中晚期海平面停滞期间的河流泛滥平原相(10.02~5.10 m,12 602~10 357 cal yr B.P.)—全新世早中期海平面上升过程中的滨海、浅海相(5.10~0 m,10 357~4 913 cal yr B.P.)。岩芯10.02~5.10 m段(12 602~10 357 cal yr B.P.)记录的源自河流泛滥平原沉积的硬质黏土层与新仙女木事件的全球效应密切相关,可作为新仙女木事件在北黄海陆架响应的一个重要证据。     

The Younger Dryas and the Sea of Ancient Ice

We propose that prior to the Younger Dryas period, the Arctic Ocean supported extremely thick multi-year fast ice overlain by superimposed ice and firn. We re-introduce the historical term paleocrystic ice to describe this. The ice was independent of continental (glacier) ice and formed a massive floating body trapped within the almost closed Arctic Basin, when sea-level was lower during the last glacial maximum. As sea-level rose and the Barents Sea Shelf became deglaciated, the volume of warm Atlantic water entering the Arctic Ocean increased, as did the corresponding egress, driving the paleocrystic ice towards Fram Strait. New evidence shows that Bering Strait was resubmerged around the same time, providing further dynamical forcing of the ice as the Transpolar Drift became established. Additional freshwater entered the Arctic Basin from Siberia and North America, from proglacial lakes and meltwater derived from the Laurentide Ice Sheet. Collectively, these forces drove large volumes of thick paleocrystic ice and relatively fresh water from the Arctic Ocean into the Greenland Sea, shutting down deepwater formation and creating conditions conducive for extensive sea-ice to form and persist as far south as 60°N. We propose that the forcing responsible for the Younger Dryas cold episode was thus the result of extremely thick sea-ice being driven from the Arctic Ocean, dampening or shutting off the thermohaline circulation, as sea-level rose and Atlantic and Pacific waters entered the Arctic Basin. This hypothesis focuses attention on the potential role of Arctic sea-ice in causing the Younger Dryas episode, but does not preclude other factors that may also have played a role.     

Is the North Atlantic Oscillation reflected in Scandinavian glacier mass balance records?

The North Atlantic Oscillation (NAO) is one of the modes of climate variability in the North Atlantic region. The atmospheric circulation during the winter season in this region commonly displays a strong meridional (north–south) pressure contrast, with low air pressure (cyclone) centred close to Iceland and high air pressure (anticyclone) near the Azores. This pressure gradient drives the mean surface winds and the mid‐latitude winter storms from west to east across the North Atlantic, bringing mild moist air to northwest Europe. The NAO index is based on the difference of normalised sea‐level pressures (SLP) between Ponta Delgada, Azores and Stykkisholmur, Iceland. The SLP anomalies at these stations are normalised by division of each monthly pressure by the long‐term (1865–1984) standard deviation. Interannual atmospheric climate variability in northwest Europe, especially over Great Britain and western Scandinavia has, during the last decades, been attributed mainly to the NAO, causing variations in the winter weather over the northeast North Atlantic and the adjacent land areas. A comparison between the NAO index and the winter (December–March) precipitation between ad 1864 and 1995 in western Norway shows that these are strongly linked (correlation coefficient 0.77). Variations in the NAO index are also reflected in the mass balance records of glaciers in western Scandinavia. The NAO index is best correlated with mass balance data from maritime glaciers in southern Norway (e.g. Ålfotbreen R² = 0.51). The record of Holocene (last ca. 11 500 cal. yr) glacier variations of maritime glaciers in western Scandinavia is thus a proxy of pre‐instrumental NAO variations. Copyright © 2000 John Wiley & Sons, Ltd.     

西南印度洋罗德里格斯岛石笋记录的新仙女木事件

新仙女木（Younger Dryas,简称YD）事件是末次冰消期末期升温过程中北半球高纬一次剧烈的突变降温事件,主流机制为淡水注入北大西洋减缓了北大西洋翻转环流,使得北半球普遍降温、热带辐合带南移等全球范围的气候重组。虽然该事件研究程度较高,然而其在南半球、尤其在西南印度洋的气候变化模态尚不明朗。本研究利用来自罗德里格斯岛（罗岛）高分辨率、高精度定年的石笋氧同位素（δ18O）记录,重建了过去13.3~10.8 ka期间（对应石笋样品100~169 mm）西南印度洋亚热带地区的气候变化。结果表明,在YD事件期间罗岛石笋δ18O相对偏重,指示降水相对减少。与印度洋北岸的地质记录对比显示,YD时期罗岛石笋记录与众多亚洲季风石笋氧同位素记录共同展现出偏重的\"谷状结构\",并且显示出\"缓进快出\"的不对称特征。与印度洋东岸的地质记录对比结果显示,罗岛石笋δ18O与印太暖池南北边缘记录共同显示出偏干状态,而与近赤道记录的气候特征不同。与印度洋西岸的地质记录对比结果显示,东非热带赤道地区至南部亚热带地区呈现\"北干南湿\"的特征。罗岛与东非赤道地区的气候变化也具有一致性。YD时期,热带辐合带南移而罗岛降水却减少,可能原因是热带辐合带强度减弱,削弱了对流降水,使得亚洲季风从源区至下游地带水汽传输均发生减弱。     

A biogenic-silica δO record of climatic change during the last glacial–interglacial transition in southwestern Alaska

Despite growing evidence for environmental oscillations during the last glacial–interglacial transition from high latitude, terrestrial sites of the North Pacific rim, oxygen-isotopic records of these oscillations remain sparse. The lack of data is due partially to the paucity of lakes that contain carbonate sediment suitable for oxygen-isotopic analysis. We report here the first record of oxygen-isotopic composition in diatom silica (δ¹⁸O_Si) from a lake in that region. δ¹⁸O_Si increases gradually from 19.0 to 23.5‰ between 12,340 and 11,000 ¹⁴C yr B.P., reflecting marked climatic warming at the end of the last glaciation. Around 11,000 ¹⁴C yr B.P., δ¹⁸O_Si decreases by 1.7‰, suggesting a temperature decrease of 3.5–8.9 °C at the onset of the Younger Dryas (YD) in southwestern Alaska. Climatic recovery began ca. 10,740 ¹⁴C yr B.P., as inferred from the increase of δ¹⁸O_Si to a maximum of 23.9‰ near the end of the YD. Our data reveal that a YD climatic reversal in southwestern coastal areas of Alaska occurred, but the YD climate did not return to full-glacial conditions.     

Rapid Ice Margin Fluctuations during the Younger Dryas in the Tropical Andes

Radiocarbon dated lacustrine sequences in Perú show that the chronology of glaciation during the late glacial in the tropical Andes was significantly out-of-phase with the record of climate change in the North Atlantic region. Fluvial incision of glacial-lake deposits in the Cordillera Blanca, central Perú, has exposed a glacial outwash gravel; radiocarbon dates from peat stratigraphically bounding the gravel imply that a glacier advance culminated between 11,280 and 10,990 ¹⁴C yr B.P.; rapid ice recession followed. Similarly, in southern Perú, ice readvanced between 11,500 and 10,900 ¹⁴C yr B.P. as shown by a basal radiocarbon date of 10,870 ¹⁴C yr B.P. from a lake within 1 km of the Quelccaya Ice Cap. By 10,900 ¹⁴C yr B.P. the ice front had retreated to nearly within its modern limits. Thus, glaciers in central and southern Perú advanced and retreated in near lockstep with one another. The Younger Dryas in the Peruvian Andes was apparently marked by retreating ice fronts in spite of the cool conditions that are inferred from the ∂¹⁸O record of Sajama ice. This retreat was apparently driven by reduced precipitation, which is consistent with interpretations of other paleoclimatic indicators from the region and which may have been a nonlinear response to steadily decreasing summer insolation.     

Chironomid-based Lateglacial climatic reconstruction for southeast Scotland

This paper presents the first chironomid-based climatic reconstruction for the UK Lateglacial, obtained from a lake basin in southeast Scotland, Whitrig Bog. Comparison of chironomid, lithological, geochemical, and pollen data reveals a marked vegetation lag behind the warming following deglaciation; warm-water chironomid taxa (e.g. chironomus) had replaced cold-water chironomid taxa (e.g. Paracladius) prior to the arrival of shrubs and trees and when local soils were still poorly developed. In addition to clear evidence for the Younger Dryas Stadial, chironomid data also reveal two short-term cold episodes that punctuate the Lateglacial Interstadial, both of which are also reflected in pollen and lithological data as reversals from open birch scrub/woodland to open herbaceous tundra coinciding with inwash of minerogenic matter into the basin. © 1997 John Wiley & Sons, Ltd.