白令海盆17ka以来的古海洋与古气候记录

通过白令海盆08BR02钻孔(57°0.06'N,174°29.97'E,水深3 805 m)的AMS14C测年数据、颜色反射率b*与NGRIP(North Greenland Ice Core Project)氧同位素曲线及白令海4个钻孔颜色反射率b*记录作对比,建立该孔的地层年代框架,顶、底部沉积物年龄分别为4.2 ka和17.0 ka。该孔自17 ka以来,IRD(ice-rafted detritus:冰筏碎屑)、粒度的砂组分以及敏感组分中的33—63μm组分百分含量在全球性冷事件H1(Heinrich event 1)、B/A(Blling/Allerd)中期和YD(Younger Dryas)期间明显增加。IRD和有孔虫丰度在H1、B/A和YD期间同步增加,可能反映海冰融化为表层水提供营养物质,促使表层钙质生产力勃发并将有机质进一步输送至海底,为底栖生物提供营养物质。     

Deglaciation and catchment ontogeny in coastal south‐west Greenland: implications for terrestrial and aquatic carbon cycling

Here we present Holocene organic carbon, nitrogen, sulphur, carbon isotope ratio and macrofossil data from a small freshwater lake near Sisimiut in south‐west Greenland. The lake was formed c. 11 cal ka BP following retreat of the ice sheet margin and is located above the marine limit in this area. The elemental and isotope data suggest a complex deglaciation history of interactions between the lake and its catchment, reflecting glacial retreat and post‐glacial hydrological flushing probably due to periodic melting of local remnant glacial ice and firn areas between 11 and 8.5 cal ka BP. After 8.5 cal ka BP, soil development and associated vegetation processes began to exert a greater control on terrestrial–aquatic carbon cycling. By 5.5 cal ka BP, in the early Neoglacial cooling, the sediment record indicates a change in catchment–lake interactions with consistent δ¹³C while C/N exhibits greater variability. The period after 5.5 cal ka BP is also characterized by higher organic C accumulation in the lake. These changes (total organic carbon, C/N, δ¹³C) are most likely the result of increasing contribution (and burial) of terrestrial organic matter as a result of enhanced soil instability, as indicated by an increase in Cenococcum remains, but also Sphagnum and Empetrum. The impact of glacial retreat and relatively subdued mid‐ to late Holocene climate variation at the coast is in marked contrast to the greater environmental variability seen in inland lakes closer to the present‐day ice sheet margin. Copyright © 2012 John Wiley & Sons, Ltd.     

Exploring the extent to which fluctuations in ice-rafted debris reflect mass changes in the source ice sheet: a model–observation comparison using the last British–Irish Ice Sheet

The British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore ice-rafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity iceberg–climate model, with varying BIIS catchment-level iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000-year time steps during the height of the last BIIS glaciation (31–6 ka bp ). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances.     

CLIMATE-CONTROLLED GLACIAL EROSION IN THE UNCONSOLIDATED SEDIMENTS OF NORTHWESTERN EUROPE,BASED ON A GENETIC MODEL FOR TUNNEL VALLEY FORMATION

The development of large erosive subglacial forms in unconsolidated sediments is generally attributed to the eroding power of subglacial meltwater flowing under high pressure conditions. Most explanations, however, differ in the source of meltwater and the speed at which it erodes the subglacial bed. Based on the geometry of deep tunnel valleys and glacial basins in northwestern Europe, a reconstruction of subglacial hydrological conditions during the development of subglacial depressions is made. It is demonstrated that the flow of subglacial meltwater in subglacial channels under high glaciostatic pressures is only capable of eroding large volumes of sediment as long as there is imminent glaciohydrological instability. For the thick aquifers in northwestern Europe, this instability is achieved when large quantities of supraglacial meltwater are available. Furthermore, a theoretical definition is given for maximum depression depth to be reached by subglacial erosion. It is shown that this maximum depth is strongly related to average air temperatures during deglaciation and that glacier bed lowering is to be expected during any final phase of glaciations. The theoretical framework presented enables a tentative comparison between large-scale glacial morphology of different glaciations in northwestern Europe.     

A high‐resolution seismic stratigraphy from a Scottish sea loch and its implications for Loch Lomond Stadial deglaciation

A detailed high‐resolution seismic stratigraphy, calibrated by core data and terrestrial geomorphological mapping, has been constructed for Loch Ainort, Isle of Skye. This study has provided a palaeoenvironmental history of the area as well as important corroborative evidence for the stepped deglaciation of the Loch Lomond Stadial ice‐field on Skye. The Ainort Glacier reworked pre‐Loch Lomond glacial deposits terminating in a grounded tidewater ice‐front potentially 800 m beyond the previously extrapolated limit. The first stage of deglaciation was characterised by the formation of De Geer moraines indicative of a period of interrupted retreat. The second phase, by contrast, produced hummocky relief with sporadic linear moraines suggesting periods of uninterrupted retreat with occasional stillstands/readvances. Paraglacial reworking of terrestrial slopes resulted in the deposition of thick, subaqueous, debris flows which graded into fluvioglacial dominated sediments and ultimately modern fjordic deposits. The identification of an initial period of active retreat punctuated by numerous readvances correlates directly with the terrestrial record. However, the offshore stratigraphy suggests that although the second phase was dominated by uninterrupted retreat, occasional stillstands/ readvances did occur. Copyright © 2000 John Wiley & Sons, Ltd.     

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.     

Late quaternary chronology of the Taitao Peninsula,southern Chile

Six new radiocarbon ages and a pollen sequence are provided for Laguna Stibnite, a small lake on the Taitao Peninsula, Chile (latitude 46°25'S, longitude 74°24'W). The sediments record a late-glacial to present sequence, with a basal age of 14 335 ± 145 yr BP (Q-2840). The radiocarbon ages provide the basis for a sound chronology in a region of Chile where few palynological studies have been made and where previous radiocarbon ages are ambiguous. The chronology from Laguna Stibnite supports the acceptance of a radiocarbon chronology based on wood fragments (rather than bulk analysis) at a site near Puerto Edén further south in the Chilean Channels (49°08'S). These data from Laguna Stibnite provide evidence for an early deglaciation (before 14 000 yr BP) in this region of southern Chile. The sequence provides no evidence for a climatic reversal between 11 000 yr BP and 10 000 yr BP, the so-called Younger Dryas chron.     

Seismic stratigraphy records the deglacial history of Jakobshavn Isbræ, West Greenland

Jakobshavn Isbræ is one of the largest ice streams in the Greenland Ice Sheet, presently draining c. 6.5% of the Inland Ice. Here we present high‐resolution Chirp and Sparker sub‐bottom profiles from a seismic survey conducted just outside of the Jakobshavn Isfjord, which provides detailed insight into the glacimarine sedimentary history of the Jakobshavn ice stream during the Holocene. We observe acoustically stratified and homogeneous sediments that drape an irregular substratum and were deposited between ～10 and c. 7.6k cal a BP. The stratified lower units are interpreted as the product of ice‐proximal glacimarine sedimentation deposited rapidly when the grounded ice margin was located close to depositional basins on topographic highs. The upper acoustically homogenous units reflect suspension settling of fine‐grained material and gravitational flows that were extruded from an increasingly unstable ice margin as the ice retreated into the fjord. Proximity to the ice margin and bedrock topography were the dominant controls on sediment accumulation during deglaciation although the 8.2‐ka cooling event probably influenced the position of the ice margin at the fjord mouth. The post‐glacial sedimentary record is characterized by glacimarine and hemipelagic rainout with an increased ice‐rafted detritus fraction that records sedimentation following ice stream retreat into Jakobshavn Isfjord sometime after c. 7.8k cal a BP. Copyright © 2011 John Wiley & Sons, Ltd.     

菲律宾海北部19cal.kaBP以来表层海水温度变化

对菲律宾海北部E-2岩心柱(29.62°N、131.14°E,水深3 877m)进行了浮游有孔虫δ18 O分析、AMS14 C测年以及U3k7表层海水温度(U3k7-SST)的测试。结果表明,E-2岩心记录了约19cal.kaBP以来U3k7-SST变化过程,其末次冰消期整体变化过程与北大西洋高纬地区相似,记录了Heinrich 1(H1)、Blling/Allerd(B/A)和Younger Dryas(YD)等快速气候事件,同时也识别出发生于19.4～19.2和18.5～18.0cal.kaBP的2次变冷事件,说明了研究区与北大西洋气候存在遥相关;在B/A和YD阶段中发现的百年尺度气候波动及变化过程,可能反映了热带过程对研究区域的影响;E-2岩心柱末次冰消期变暖存在阶段性,其变暖时间开始于19.4cal.kaBP,而大幅升温阶段则从15.3cal.kaBP开始。     

Landscape and Coast Development of A Lowland Fjord Margin Following Deglaciation, East Greenland

The landscapes of western Jameson Land bordering Hall Bredning fjord comprise upper river basins, glacial landscapes, lower river basins and a near‐shore zone. The upper river basins are incised into bedrock and display no cover of young sediments whilst the glacial landscapes, located closer to the coast, are dominated by Pleistocene deposits and an irregular topography with hills and ridges. The lower river basins, dissecting the glacial landscapes, are connected to the upper river basins and contain well‐defined Holocene delta terraces. The near‐shore zone, which includes the present coast, displays a few raised shorelines. Geomorphological observations combined with stratigraphic work and ¹⁴C dates provide a chronological framework for the development of landscape and shoreline, as presented by a four‐stage reconstruction. The first stage covers the deglaciation of western Jameson Land at the Weichselian‐Holocene transition after a collapse of the main fjord glacier in Hall Bredning. The sea inundated the low‐lying areas on Jameson Land forming small side‐entry fjord basins that possibly follow the track of older valleys. This was followed by a second stage, the paraglacial period, when large meltwater production and sediment transport resulted in a fast infilling of the side‐entry fjord basins by deltas. These are now exposed in terraces in the lower river basins at 70–80 m a.s.l. During a third stage, the relaxation period, fluvial activity decreased and the land surface was increasingly occupied by a cover of tundra vegetation. A glacio‐isostatic rebound resulted in a relative sea level fall and fluvial incision. During stages two and three the coast was exposed to shallow marine processes that aided the alignment of the coast. Stages one to three presumably lasted for less than 2000 years. During stage four, the stable period, lasting for several thousand years till the present, there were minor adjustments of shoreline and landscape. The four‐step reconstruction describes the sedimentary response of a lowland fjord margin to dramatic changes in climate and sea level. The distribution of erosion and sedimentation during this development was mainly controlled by topography. The reconstruction of the latest environmental development of Jameson Land puts new light on Jameson Land's long and complex Quaternary stratigraphic record. The reconstruction may also be used as a model for the interpretation of deposits in similar areas elsewhere.