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SILVIO TSCHUDI SUSAN IVY‐OCHS CHRISTIAN SCHLÜCHTER PETER KUBIK HEIKKI RAINIO 《Boreas: An International Journal of Quaternary Research》2000,29(4):287-293
Boulders of the Younger Dryas Salpausselkä I (Ss I) formation west of Lahti, southern Finland, were sampled for surface exposure dating. The 10Be concentrations, determined by accelerator mass spectrometry, yield minimum exposure ages of 11 930 ± 950, 11 220 ± 890, 11 050 ± 910 and 11 540 ± 990 years, using recently published production rates scaled for latitude and elevation. This includes a correction to the production rate resulting from postglacial uplift of the Fennoscandian lithosphere (i.e. changing elevation) during the time of exposure. The error‐weighted mean exposure age of 11 420 ± 470 years of the analysed boulders agrees with previous varve dates of Ss I, which range from 11 680 to 11 430 calendar years BP. However, erosion has to be taken into account as a process affecting rock surfaces and therefore influencing exposure ages. Available information suggests an erosion rate of 5 mm/kyr, which increases the error‐weighted mean exposure age to a value of 11 610 ± 470 years. Within the errors, the formation of Ss I in the Vesala area west of Lahti falls into the Younger Dryas time bracket, as defined by the GRIP and GISP 2 ice core (Greenland). 相似文献
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PETER H. KRISTENSEN KAREN LUISE KNUDSEN 《Boreas: An International Journal of Quaternary Research》2006,35(2):349-366
A new investigation of the coastal cliff section at Mommark in southern Denmark has revealed a complete Eemian interglacial sequence for the first time in the southwestern Baltic area. Environmental changes through the lacustrine and marine interglacial deposits are discussed on the basis of foraminiferal assemblages and stable isotope composition as well as ostracods. In general, the assemblages indicate relatively high temperatures throughout the Eemian, and the Lusitanian foraminiferal species Pseudoeponides falsobeccarii Rouvillois has been reported for the first time from the Eemian of northwest Europe. A floating chronology of the deposits is based on a previously published correlation of the local pollen stratigraphy with annually laminated sequences in northern Germany. An initial early Eemian lacustrine phase, with ostracodal indication of deposition in a large freshwater lake, lasted until c. 300 years after the beginning of the interglacial, i.e. to the transition between the regional pollen zones E2 and E3. After that, marine conditions persisted almost throughout the interglacial, and the Cyprina Clay was deposited. The foraminiferal and ostracodal assemblages indicate that relatively deep water prevailed in the area until c. 6000 years after the beginning of the interglacial. However, both the foraminiferal assemblages and the oxygen isotope results show that a trend from relatively high salinity to lower salinity conditions had begun already at about 4000 years. After c. 6000 years the fauna indicates a gradual change to shallower water and further reduction in salinity, the latter also being reflected by a general decrease in the oxygen isotope values. The marine deposition ended at c. 10 600 years after the beginning of the Eemian, i.e. within the topmost part of pollen zone E7. This was succeeded by a late Eemian and early Weichselian freshwater phase. 相似文献
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PETER G. KNIGHT CARRIE E. JENNINGS RICHARD I. WALLER ZOE P. ROBINSON 《Geografiska Annaler: Series A, Physical Geography》2007,89(3):203-215
Advance of part of the margin of the Greenland ice sheet across a proglacial moraine ridge between 1968 and 2002 caused progressive changes in moraine morphology, basal ice formation, debris release, ice‐marginal sediment storage, and sediment transfer to the distal proglacial zone. When the ice margin is behind the moraine, most of the sediment released from the glacier is stored close to the ice margin. As the margin advances across the moraine the potential for ice‐proximal sediment storage decreases and distal sediment flux is augmented by reactivation of moraine sediment. For six stages of advance associated with distinctive glacial and sedimentary processes we describe the ice margin, the debris‐rich basal ice, debris release from the glacier, sediment routing into the proglacial zone, and geomorphic processes on the moraine. The overtopping of a moraine ridge is a significant glaciological, geomorphological and sedimentological threshold in glacier advance, likely to cause a distinctive pulse in distal sediment accumulation rates that should be taken into account when glacial sediments are interpreted to reconstruct glacier fluctuations. 相似文献
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IntroductionItisthefirsttimeintheworldtogetthestrongmotionrecordsofadestructiveearthquakewhentheLongBeachearthquakeonMarch10,... 相似文献
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Subduction Style Magmatism in a Non-subduction Setting: the Colville Igneous Complex, NE Washington State, USA 总被引:1,自引:0,他引:1
The Colville Igneous Complex is located within the Eocene MagmaticBelt of the North American Cordilleran interior. It straddlesthe USCanadian border in northeast Washington and southernBritish Columbia. The complex consists of three intrusive andtwo extrusive phases, the first extrusive phase being contemporaneouswith the latter two intrusive phases. As a consequence of sub-solidusre-equilibration in the plutonic rocks, this study concentrateson the two extrusive phases, the Sanpoil Volcanic Formationand the Klondike Mountain Formation. The Sanpoil Volcanic Formationconsists of andesites, dacites and rare trachyandesites (SiO2= 5570 wt %) exhibiting a slight decrease in total alkalis(Na2O + K2O) with increasing silica. The Klondike Mountain Formationconsists of basalts, basaltic andesites, andesites, dacitesand rhyolites (SiO2 = 5175 wt %) with total alkalis increasingwith increasing silica. The calc-alkaline affinity of the rocksof the Colville Igneous Complex, coupled with the presence ofa subduction signature of enriched large ion lithophileelements (LILE) and depleted high field strength elements (HFSE),has traditionally been attributed to petrogenesis in a subduction-relatedmagmatic arc, the Challis Arc. New trace and rareearth element and isotopic data (87Sr/86Sri, 相似文献
230.
PETER NIELSEN PHILIPPE MUCHEZ WOUTER HEIJLEN A. E. TONY FALLICK† EDDY KEPPENS‡ DOMINIQUE WEIS§ RUDY SWENNEN 《Sedimentology》2005,52(5):945-967
In topographic flat areas, sedimentary settings may vary from one outcrop to another. In these settings, calcite precipitates may yield macroscopically similar columnar features, although they are products of different sedimentary or diagenetic processes. Three columnar calcite crystal fabrics, i.e. rosettes, palisade crusts and macro-columnar crystal fans, have been differentiated near and at the contact between Upper Tournaisian dolomites and limestones along the southern margin of the Brabant-Wales Palaeohigh. Their petrographic characteristics, and geochemical and fluid inclusion data provide information on the (dia)genetic processes involved. Rosettes composed of non-luminescent columnar calcite crystal fans (1–5 cm in diameter) developed on top of one another, forming discrete horizons in repetitive sedimentary cycles. The cycles consist of three horizons: (I) a basal horizon with fragments from the underlying horizon, (II) a micrite/microspar horizon with incipient glaebules, (III) an upper horizon consisting of calcite rosettes, with desiccation features. The petrographical features and δ18O signatures of −10·0 to −5·5‰ and δ13C values of −5·5 to −3·2‰ support either evaporative growth, an evaporative pedogenic origin, or overprinting of marine precipitates. Palisade crusts, composed of a few to 10 mm long non-luminescent calcite crystals, coat palaeokarst cavities. Successive palisade growth-stages occur which are separated by thin laminae of micrite or detrital quartz, displaying a geopetal arrangement. Palisade crusts are interpreted as intra-Mississippian speleothems. This interpretation is supported by their petrographic characteristics and isotopic signature (δ18O = −8·7 to −6·5‰ and δ13C = −4·8 to −2·5‰). Macro-columnar crystals, 1–50 cm long, developed mainly perpendicular to cavity walls and dolomite clasts. Crystal growth stages in the macro-columnar crystals are missing. δ18O values vary between −16·4 and −6·8‰ and δ13C values between −5·2 and −0·9‰. These features possibly support a late diagenetic high temperature precipitation in relation to hydrothermal karstification. 相似文献