Late Glacial stable isotope record, radiocarbon stratigraphy, pollen and mollusc analyses from the Geiseltal area, Central Germany     

TATJANA BÖTTGER  ACHlM HILLER  FRANK WOLFGANG JUNGE  THOMAS LITT  DIETRICH MANIA  NORBERT SCHEELE 《Boreas: An International Journal of Quaternary Research》1998,27(2):88-100

Combined investigations of isotopes, pollen, and molliisc shells were carried out on a Latc Glacial limnic high-resolution sediment sequence from the Geiseltal open-cast mine in Central Gcrmany. The dala confirm thc division of the Late Glacial biozones into two colder (Older and Younger Dryas) and two warmer periods (Bølling, Allcrød), which have already bccn cstablishcd lor the Central and Northern German area. Radiocarbon data, mainly based on wood material, cover a time span between c , 10 800 BP and 12 760 BP, indicating a mean sedimentation rate of c . 2 mm/yr. The stable isotope data (δ¹⁸O, δ¹³C) reflect the transition periods between the climatic phases as changes in relative air humidity (transition from dry, cold to warmer. more humid conditions). The dominance of evaporation effects, however, is superimposed on any ternpcratiirc aignal throughout the profile. Repeated conversions of the hydrologic regime (silting-up phases. lake-level variations) characterize the specific history of the lake development.  相似文献   

Syros Metasomatic Tourmaline: Evidence for Very High-{delta}11B Fluids in Subduction Zones     

MARSCHALL  HORST R.; LUDWIG  THOMAS; ALTHERR  RAINER; KALT  ANGELIKA; TONARINI  SONIA 《Journal of Petrology》2006,47(10):1915-1942

High-pressure (HP) metamorphic blocks enclosed in a mafic toultramafic matrix from a mélange on the island of Syrosare rimmed by tourmaline-bearing reaction zones (blackwalls).The B isotopic composition of dravitic tourmaline within theseblackwalls was investigated in situ by secondary ion mass spectrometry.Boron in these tourmalines is unusually heavy, with ¹¹B valuesexceeding +18 in all investigated samples and reaching an extremevalue of +28·4 in one sample. Blackwalls formed duringexhumation of the HP mélange at a depth of 20–25km at temperatures of 400–430°C, by influx of externalhydrous fluids. The compositions of the fluids are estimatedto be in the range of 100–300 µg/g B with ¹¹B valuesof +18 to +28. The high ¹¹B values cannot be explained by tourmalineformation from unmodified slab-derived fluids. However, suchfluids could interact with the material in the exhumation channelon their way from the dehydrating slab to the site of tourmalineformation in the blackwalls. This could produce exceptionallyhigh ¹¹B values in the fluids, a case that is modelled in thisstudy. The model demonstrates that subduction fluids may beeffectively modified in both trace element and isotopic compositionduring their migration through the material overlying the subductingslab. Blackwall tourmaline from Syros has a large grain size(several centimetres), high abundance, and an exceptionallyhigh ¹¹B value. The formation of tourmaline at the contact betweenmafic or felsic HP blocks and their ultramafic matrix involvedfluids released during dehydration reactions in the subductingslab. It forms a heavy-boron reservoir in hybrid rocks overlyingthe subducting slab, and may, thus, have a significant impacton the geochemical cycle of B and its isotopes in subductionzones. KEY WORDS: boron isotopes; tourmaline; subduction zone; fluid, high pressure  相似文献   

Supra-subduction Zone Pyroxenites from San Jorge and Santa Isabel (Solomon Islands)     

BERLY  THOMAS J.; HERMANN  JORG; ARCULUS  RICHARD J.; LAPIERRE  HENRIETTE 《Journal of Petrology》2006,47(8):1531-1555

Peridotites associated with pyroxenites (with rare olivine andspinel) are exposed on the islands of San Jorge and Santa Isabelin the Solomon Islands. Orthopyroxenite occurs in large outcrops(100 m²) whereas websterite and clinopyroxenite occur as layersand veins/dykes in peridotites. The bulk compositions of thepyroxenites are characterized by high Mg²⁺/(Mg²⁺ + Fe²⁺) (0·78–0·91)and low Al₂O₃ (<2·7 wt %). Low rare earth elementabundances are coupled with large ion lithophile element enrichmentsand positive Sr and Pb anomalies (primitive mantle-normalized)relative to adjacent rare earths. Temperatures of equilibrationfor the pyroxenites are between 950 and 1050°C. These relativelylow temperatures, combined with the occurrence of primary fluidinclusions, suggest that the pyroxenites formed by interactionof peridotite protoliths with an aqueous fluid. Bulk-rock andmineral compositions of the orthopyroxenites are similar tothose of mantle-derived pyroxenites, whereas the websteriteshave closer chemical affinity with crustal arc cumulates. Nevertheless,field relationships plus petrological, textural and geochemicalevidence are consistent with formation of all pyroxenite typesin supra-subduction zone mantle, resulting from metasomatismof peridotite by subducted Pacific Plate-derived fluid. Sucha setting for pyroxenite has not previously been reported indetail. We propose that these processes produce mantle pyroxenitewith compositions similar to crustal pyroxenite. KEY WORDS: mantle metasomatism; pyroxenite; supra-subduction zone  相似文献   

Regional Saharan dust modelling during the SAMUM 2006 campaign     

By BERND HEINOLD  INA TEGEN  MICHAEL ESSELBORN  KONRAD KANDLER  PETER KNIPPERTZ  DETLEF MÜLLER  ALEXANDER SCHLADITZ  MATTHIAS TESCHE  BERNADETT WEINZIERL  ALBERT ANSMANN  DIETRICH ALTHAUSEN  BENOIT LAURENT  REAS MASSLING  THOMAS MÜLLER  REAS PETZOLD  KERSTIN SCHEPANSKI  ALFRED WIEDENSOHLER 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):307-324

The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006 SAMUM field campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations. However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust source placement by the model. While simulated winds and dust distributions are well matched for dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.  相似文献   

New insights into Holocene atmospheric circulation dynamics in central Scandinavia inferred from oxygen‐isotope records of lake‐sediment cellulose     

NATALIE A. ST. AMOUR  DAN HAMMARLUND  THOMAS W.D. EDWARDS  BRENT B. WOLFE 《Boreas: An International Journal of Quaternary Research》2010,39(4):770-782

St. Amour, N. A., Hammarlund, D., Edwards, T. W. D. & Wolfe, B. B. 2010: New insights into Holocene atmospheric circulation dynamics in central Scandinavia inferred from oxygen‐isotope records of lake‐sediment cellulose. Boreas, Vol. 39, pp. 770–782. 10.1111/j.1502‐3885.2010.00169.x. ISSN 0300‐9843 Cellulose‐inferred lakewater oxygen‐isotope records have been obtained from two hydrologically open basins (Lake Spåime and Lake Svartkälstjärn), located on a west–east transect across central Sweden, to investigate changes in atmospheric circulation patterns during the Holocene. The Lake Spåime δ¹⁸O record is sensitive to changes in the seasonal distribution of precipitation in the Scandes Mountains of west‐central Sweden, and thus generally portrays variations in δ¹⁸O of precipitation (δ¹⁸O_P) that are governed predominantly by the influence of air masses originating from the North Atlantic. In contrast, the Lake Svartkälstjärn δ¹⁸O record appears to reflect the varying influence of air masses delivering moisture from the North Atlantic and the Baltic Sea. A comparison of inferred changes in δ¹⁸O_P over the Holocene between the two sites reveals systematic patterns of variability over widely different time scales. These include: (1) a previously recognized long‐term regional decline in δ¹⁸O_P, possibly in response to the declining vigour of Northern Hemisphere atmospheric circulation related to decreasing summer solar insolation; (2) newly identified inverse centennial‐ to millennial‐scale δ¹⁸O_P fluctuations at the two sites that may be linked to changes in modes of atmospheric circulation analogous to those described at interannual to multidecadal time scales by the North Atlantic Oscillation (NAO) index; and (3) a prolonged interval of apparent climatic stability in the mid‐Holocene (c. 6300–4200 cal. yr BP) characterized by persistently negative NAO‐like circulation.  相似文献   

Paper II – Dirt,dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia,and their implications for sedimentary ancient DNA studies     

LEE J. ARNOLD  RICHARD G. ROBERTS  ROSS D. E. MACPHEE  JAMES S. HAILE  FIONA BROCK  PER MÖLLER  DUANE G. FROESE  ALEXEI N. TIKHONOV  ALLAN R. CHIVAS  M. THOMAS P. GILBERT  ESKE WILLERSLEV 《Boreas: An International Journal of Quaternary Research》2011,40(3):417-445

Arnold, L. J., Roberts, R. G., MacPhee, R. D. E., Haile, J. S., Brock, F., Möller, P., Froese, D. G., Tikhonov, A. N., Chivas, A. R., Gilbert, M. T. P. & Willerslev, E. 2010: Paper II – Dirt, dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia, and their implications for sedimentary ancient DNA studies. Boreas, Vol. 40, pp. 417–445. 10.1111/j.1502‐3885.2010.00181.x. ISSN 0300‐9483 The sedimentary ancient DNA (sedaDNA) technique offers a potentially invaluable means of investigating species evolution and extinction dynamics in high‐latitude environments. An implicit assumption of the sedaDNA approach is that the extracted DNA is autochthonous with the host deposit and that it has not been physically transported from older source deposits or reworked within the sedimentary profile by postdepositional mixing. In this paper we investigate whether these fundamental conditions are upheld at seven perennially frozen wetland sites across the Taimyr Peninsula and coastal lowlands of north‐central Siberia. Optically stimulated luminescence (OSL) and radiocarbon (¹⁴C) dating are used to constrain the ages of both the inorganic and organic fractions of perennially frozen deposits from which sedaDNA of extinct and extant species have been recovered. OSL and ¹⁴C age/depth profiles, as well as single‐grain equivalent dose (D_e) distribution characteristics, are used to assess the stratigraphic integrity of these sedaDNA sequences by (i) identifying the presence of primary or reworked organic and inorganic material, and (ii) examining the types of depositional and postdepositional processes that have affected specific sedimentary facies. The results of this study demonstrate that even though DNA preservation and stratigraphic integrity are commonly superior in perennially frozen settings, this does not, in itself, guarantee the suitability of the sedaDNA approach. The combined OSL and ¹⁴C chronologies reveal that certain perennially frozen sites may be poorly suited for sedaDNA analysis, and that careful site selection is paramount to ensuring the accuracy of any sedaDNA study – particularly for ‘latest appearance date’ estimates of extinct taxa.  相似文献   

Revision of the early Holocene lake sediment based chronology and event stratigraphy on Hochstetter Forland, NE Greenland   总被引：1，自引：1，他引：1  

SVANTE BJÖRCK  BARBARA WOHLFARTH  OLE BENNIKE  CHRISTIAN HJORT  THOMAS PERSSON 《Boreas: An International Journal of Quaternary Research》1994,23(4):513-523

The previously established and often debated lake sediment based chronology and event stratigraphy from Hochstetter Forland, NE Greenland, has been re-examined. These new studies show that the last deglaciation of the coastal area is several thousand years younger than previously described. The main reason behind the difference is the fact that the older chronology was based on ¹⁴C datings of bulk sediment samples, which are now shown to contain enough coal fragments to produce erroneous ages. The re-examination was performed on sediments from two lakes located within the Nanok moraine system: one is situated at or slightly above the marine limit around 65 m. the other at 21–22 m a.s.l. The combined stratigraphy from the two lakes shows that the area was deglaciated before 9000 BP, after which followed deposition of glaciomarine sediments, fining upwards. The first vegetation seems to have been dominated by grasses, Lycopodium and Polypodiaceae. At c . 8000 BP the limnic production increased significantly and a pioneer vegetation characterized the area. At this time the 'Artemisia' grains appear. A short but distinct climatic cooling occurs at c . 7500 BP causing a significant drop in lake productivity and possibly also producing coarser sediments in the (glacio)marine environment. About 200 years later the lake productivity again increased, very rapidly, and the marine sediments became finer and more rich in molluscs, as a response to the beginning of the climatic optimum. Because of the time-lag between climate and vegetation response it took another 300 years before Betulci nana immigrated, at the same time as the 'Artemisia' grains disappeared, and another several hundred years before a real dwarf-shrub vegetation developed in these parts of Hochstetter Forland. Before the lower lake was isolated from the sea at c . 6000 BP, coarse wave-washed sediments, followed by a typical isolation sequence, were deposited in it.  相似文献   

Planktonic foraminiferal and oxygen isotopic stratigraphy and paleoclimatology of Norwegian Sea deep-sea cores     

THOMAS B. KELLOGG  JEAN CLAUDE DUPLESSY  NICHOLAS J. SHACKLETON 《Boreas: An International Journal of Quaternary Research》1978,7(1):61-73

BOREAS Kellogg, T. B., Duplessy, J. C. & Shackleton, N. J. 1978 03 01: Planktonic foraminiferal and oxygen isotopic stratigraphy and paleoclimatology of Norwegian Sea deep-sea cores. Boreas. Vol. 7, pp. 61–73. Oslo. ISSN 0300–9483.
Three Norwegian Sea deep-sea cores, which penetrate to sediments at least 200,000 years old, were analyzed for oxygen isotope content, total calcium carbonate, and planktonic foraminifera. The oxygen isotopic stratigraphy was used to refine the time control for paleoclimatic and paleo-oceanographic events previously described for the region. Two pulses of relatively warm subpolar water entered the region between 124,000 B.P. and 115,000 B.P. (the last interglacial), and since about 13,000 B.P. The remaining portion of the last 150,000 years was characterized by extensive ice cover. The magnitude of the change in isotopic composition between peak glacial and peak interglacial conditions is larger than can be explained by the changing isotopic content of the oceans alone suggesting that large temperature and salinity effects are recorded in isotope curves from Norwegian Sea isotope curves. The magnitude of the isotopic change from substage 5e to 5d (greater than 1%) is attributed to a combination of changing oceanic isotopic composition combined with a large temperature effect due to a sudden sea-surface temperature decrease of about 6^oC. The persistence of heavy isotope values throughout substages 5d through 5a may be related to the sea-ice cover which prevented dilution of the isotopically heavy waters by isotopically light run-off. Sedimentation rates calculated for each of the isotope stages show large changes from one stage to another with some tendency for odd numbered stages to have higher rates.  相似文献   

Early Holocene environment on Bjørnøya (Svalbard) inferred from multidisciplinary lake sediment studies     

BARBARA WOHLFARTH  GEOFFREY LEMDAHL  SIV OLSSON  THOMAS PERSSON  IAN SNOWBALL  JONAS ISING  VIV JONES 《Polar research》1995,14(2):253-275

Bjørnøya, a small (178 km²) island situated between the mainland of Norway and southern Spitsbergen, provides the opportunity for the reconstruction of early Holocene terrestrial and limnic palaeoenvironments in the southwestern Barents Sea. The AMS ¹⁴C dating technique, geochemical, mineral magnetic, micro and macrofossil analyses were applied to sediments recovered from lake Stevatnet and the results are interpreted in terms of palaeoenvironmental conditions between 9800 and 8300 ¹⁴C bp. After the disappearance of local glaciers before ca 9800¹⁴C BP, the lake productivity increased rapidly at the same time as pioneer plant communities developed on soils which gradually became more stable. Insect data indicates that strong seasonal contrasts with mean July temperatures around 9°C and mean January temperatures around −12°C prevailed between 9500 and 8300 ¹⁴C BP. These high summer temperatures, possibly as much as 4-5°C higher than the present, favoured the development of a flora including Dryas and Angelica cf. archangelica . The enhanced freeze/thaw processes led to an increased erosion of minerogenic and organic material. After 8000 ¹⁴C BP the temperatures may have gradually declined. The environmental reconstruction derived from our data set supports the conceptual insolation model which proposes maximum Holocene seasonality for the Northern Hemisphere at ca 9000 ¹⁴C BP.  相似文献   

Berm development on a monsoon-influenced microtidal beach     

K. V. THOMAS  M. BABA 《Sedimentology》1986,33(4):537-546

The mechanisms of berm development along a microtidal-high energy beach is examined. Such a beach with medium-sized sand and monsoon wave-controlled profile at Valiathura, south-west coast of India, is selected for this study. The waves which very rarely fall below 1 m, often exceed 4 m during the monsoon period of May to October. The erosion-accretion pattern of the beach shows a cyclicity and the berm development is mainly due to the onshore migration and welding of longshore bars on to the beach following the monsoon rough season. The stages of berm development in the present microtidal beach are more or less similar to the model presented by Hine for a mesotidal case, except for the following intermediate additional stages. The longshore bar develops due to the erosion of beach when the wave steepness was above 0·04, gets flattened when it falls below 0·04, and then reforms nearer to the shoreline as a swash bar. This reformed bar gets divided and the inner bar gets welded on to the beach, followed by the outer bar developing the berm. During the onshore migration of the longshore bar and berm development the beach face becomes partially reflective with the surf scaling parameter, ε_b between 2·5 and 33. The inshore is dissipative with the inshore surf scaling parameter, ε_s?33. The offshore side of the longshore bar is partially reflective with its surf scaling parameter, ε_bar between 2·5 and 33. The breakers are spilling or plunging. Vertical growth of the berm is mainly due to the changes in swash-limit caused by the variations in wave steepness, breaker height and type. Vertical growth stops when the beach-face attains equilibrium with the grain size-wave energy relationship, and a wave steepness below 0·02 helps to sustain this state.  相似文献