摘要:

The study of a c. 18 m thick Late Weichselian‐ Early Holocene (isotope stage 2/1) marine succession (original water depth 100–150 m) from the Skagen 3 borehole, northern Denmark, has led to a better understanding of the palaeoenvironmental changes during the last deglaciation. The palaeoenvironmental interpretation is based on benthic foraminifera, stable isotope composition and lithology, whereas the chronostratigraphy and sedimentation rates are based on AMS radiocarbon dates. Marine conditions were established in the area at between 15 000 and 14 500 BP (reservoir corrected14C years), and the first influence of Atlantic water masses imported through the Norwegian Channel is registered from 13 100 BP to 10 900BP (the Bølling‐ Allerrad interstadial complex). This was followed by the Younger Dryas cooling event between 10 900 and 10 100 BP. The hydrographic change at the Pleistocene‐Holocene transition after the Younger Dryas occurred in two stages. Arctic/subarctic deep‐water assemblages persisted continuously at Skagen after the first stage at 10 200 BP, while cold boreal assemblages existed in shallower water environments in the Kattegat‐Skagerrak basin during the same period of time. The slight warming in the early Holocene seems to have been interrupted by a short‐term cooling at about 9700–9600 BP. Finally, at 9600 BP the arrival of warm Atlantic water masses created full‐interglacial conditions i

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00836.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Mineral magnetic measurements were carried out on twelve sediment cores from the northwestern part of the Baltic Proper. The purpose was to use magnetic properties to correlate sediment sequences and to investigate changes in depositional conditions (e.g. rate of sedimentation and redox conditions). A general lithostratigraphy consisting of five units was established based mainly on mineral magnetic properties. Sediment has been deposited from the time of the Baltic Ice Lake (before 10 30014C years BP) to the present, i.e. since the Late Weichselian deglaciation. Five of the cores were also investigated by means of biostratigraphical methods and three by14C dates. Based on the general stratigraphy and hiatuses, variations in sedimentation rate were identified. The occurrence of authigenically formed ferrimagnetic greigite (Fe3S4), mainly in sediments deposited during the brackish Yoldia Sea and Litorina Sea stages, is implied. These sequences are characterized by low S‐ratios and high SIRM/z ratios. Glacial clay, deposited in freshwater during the Baltic Ice Lake stage and the early freshwater phase of the Yoldia Sea stage, seems to be unaffected by diagenetic processes. Gyttja clay, deposited in the Litorina Sea, has significantly lower susceptibility and SIRM than in the underlying clay. Dissolution of magnetic iron oxides (e.g. magnetite and hematite) in an anoxic environment was suggested as an explanatio

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00837.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

A study of two piston cores and a 3.5 kHz seismic profile from the Franz Victoria Trough provides new stratigraphic, stable isotopic and foraminiferal AMS14C data that help constrain the timing of ice‐sheet retreat in the northern Barents Sea and the nature of the deglacial marine environment. Silty diamicton at the base of each core, interpreted as till or ice‐marginal debris flow, suggests that the Barents ice sheet was grounded at the core sites (470 m water depth). Eight AMS14C dates on sediment overlying the diamicton indicate that the ice sheet retreated from both core sites by 12.9 ka and that postglacial sedimentation began 10 ka ago. These dates, combined with a recently published14C date from a nearby core, suggest that the Franz Victoria Trough may not have been deglaciated untilc. 13 ka, 2000 years later than modeled ice‐sheet reconstructions indicate. In the trough, oxygen isotopic ratios in planktonic foraminiferaN.pachyderma(sinistral) were 0.5–0.750, lower during deglaciation than after, probably as a result of ice‐sheet and/or iceberg melting. Foraminiferal assemblages suggest that Atlantic‐derived intermediate water may have begun to penetrate the troughc

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00838.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

At the Dänischer Wohld Peninsula coastal sections (North West Germany), subglacial deformation was found at three scales. At the smallest scale, features typical of deforming bed tills were found, i.e. small boudins, tectonic laminations and low fabric strength till. At an intermediate scale, large lenses of glaciolacustrine sediments were found within subglacially deformed till. At the largest scale, there were large (over 5 m high) subglacial folds. We suggest that these styles of sedimentation/deformation were associated with a series of readvances during overall glacial retreat: subglacial deformation occurred during each advance and glaciolacus trine sedimentation occurred during each retreat. This led to glaciolacustrine sediments and deforming bed tills being folded together during subsequent readvances. Where the rheology was relatively weak, the lacustrine sediments were totally incorporated into the diamicton and lost their previous identity. However, where the glaciolacustrine sediments were relatively strong, they survived. We suggest that this style of deformation is typical of the conditions just upglacier from the ice margin and is associated with a relatively thick deforming layer and a high input of subglacial sediment. We conclude that the evidence found at this site provides further indications that the southern margins of the Fenno‐Scandinavian ice sheet were coupled with the glacier bed and underwent deforming bed conditio

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00839.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Data from heavy mineral analysis, X‐ray fluorescence spectrometry (XRF) and X‐ray diffractrometry (XRD) are compared with those obtained from mineral magnetic analysis for a range of glacial diamicton samples taken from the Quaternary sequence in the Isle of Man. These data show that the mineral magnetic approach offers a useful means of characterizing glacial sediments for purposes of lithostratigraphic correlation and provenance indication and that the magnetic data are of a comparable quality to those obtained from standard applications of the more traditional techniques. The heavy mineral and XRD data indicate that little post‐depositional chemical alteration of the detrital composition of the sediments has taken place. The mineral magnetic, heavy mineral and XRF analysis suggests both similar lithostratigraphic relationships between the various diamicton units and consistent indications of their likely provenance. These are consistent with published, field‐based, stratigraphic relationships of the same sediments and ice‐flow directions in the Irish Sea Basin during the

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00840.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The changing palaeogeographical pattern of Alpine deposits across the European forelands can be traced by identifying mineral assemblages and establishing the chronology of Pliocene‐Pleistocene deposits in Alpine foreland. In the late Miocene, the upper courses of the Rhine and the Aar flowed east from the Swiss molasse plain towards the Danube. In the early Pliocene (Brunssumian, 5‐3.2 Ma), these same rivers headed north wards towards the Rhine Graben of Alsace. In the early Reuverian, these streams were captured south of the Rhine Graben by the Doubs. They ceased their northward flow and headed west to feed the Bresse Graben. This phase is dated to the Lower and Middle Reuverian (3.2‐2.6 Ma). From the Upper Reuverian (2.6 Ma) to the present day, the Rhine has adapted approximately its present course towards the North Sea, south to north along the Rhine Graben and across the Rhine Schist Massif to feed the Dutch Grabens. This changing pattern of capture and alteration of the hydrographic system of the upper reaches of the Rhine and the Aar can be explained by local tectonic move

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00841.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1996.tb00842.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY