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Climatic Cooling (climatic + cooling)
Selected AbstractsPalaeoclimate indicators (clay minerals, calcareous nannofossils, stable isotopes) compared from two successions in the late Jurassic of the Volga Basin (SE Russia)GEOLOGICAL JOURNAL, Issue 1 2002A. H. Ruffell Abstract A study of clay mineral and calcareous nannofossil abundances in late Jurassic,early Cretaceous sediments from the Volga Basin, SE Russia, is presented. From these results, we are able to compare some general patterns of mineralogical and palaeontological change for the Volga Basin to the palaeoclimate models developed for northern Europe and beyond. The two successions examined comprise calcareous mudstones with black organic-rich shale horizons, overlain by a series of phosphatic silty sands. Clay mineralogical results show a progressive decrease in kaolinite and the concomitant increase of smectite and illite through the middle Volgian, followed by an abrupt increase in kaolinite in the late Volgian. The clay mineral evidence suggests increasing aridity at the end of the Jurassic, similar, in part, to many western European successions. Because of differential settling of clay minerals, superimposed upon this possible climatic signature is likely to be the effect of relative sea-level change. Calcareous nannofossil analysis from a single section reveals a shift through the middle Volgian from low nutrient, warm water assemblages dominated by Watznaueria to cooler surface water and high nutrient assemblages dominated by Biscutum constans. These observations suggest that increased aridity is also associated with climatic cooling. Black shales are associated with increased productivity, higher sea levels and increases in smectite content. Hence, periods of low (chemical) hinterland weathering during semi-arid conditions are paradoxically associated with relatively nutrient-rich waters, and organic-rich shales. Comparison of published carbon and oxygen stable isotope results from this and other sections to the clay mineral and nannofossil data confirms the palaeoclimatic interpretation. This study significantly improves the published biostratigraphically constrained clay mineral database for this time period, because other European and North American successions are either non-marine (and thus poorly dated), absent (through penecontemporaneous erosion) or condensed. Copyright © 2002 John Wiley & Sons, Ltd. [source] Post-Hypsithermal plant disjunctions in western Alberta, CanadaJOURNAL OF BIOGEOGRAPHY, Issue 3 2003W. L Strong Abstract Aim, Evaluate the hypothesis that nine disjunct vascular plant species along the eastern slopes of the Rocky Mountains and in the Peace River District of west-central Alberta represent remnants of more southerly vegetation that occupied these areas during the Holocene Hypsithermal (9000,6000 yr bp). Alternatively, these plants represent populations that became established because of independent chance dispersal events. Location, This study focuses on the area east of the Rocky Mountain Continental Divide in the Province of Alberta and the State of Montana in western Canada and USA, respectively. Methods, Disjunct species were identified and their distributions mapped based on a review of occurrence maps and records, botanical floras and checklists, herbaria specimens, ecological and botanical studies, and field surveys of selected species. A disjunct species was defined as a plant population separated from its next nearest occurrence by a distance of > 300 km. Evaluation of the hypothesis was based on a review of published and unpublished pollen stratigraphy and palaeoecological studies. The potential geographical distribution of Hypsithermal vegetation was based on modern regional-based ecosystem mapping and associated monthly temperature summaries as well as future climatic warming models. Results, The hypothesis was compatible with Holocene pollen stratigraphy, Hypsithermal permafrost and fen occurrence, and palaeosol phytolith analyses; and future global climatic warming models. Modelled regional Hypsithermal vegetation based on a 1 °C increase in July temperatures relative to current conditions, indicated that much of the boreal forest zone in Alberta could have been grassland, which would explain the occurrence of Prairie species in the Peace River District. This amount of latitudinal vegetation shift (6.5°) was similar to an earlier Hypsithermal permafrost zone location study. An equivalent shift in vegetation along the eastern Cordillera would have placed south-western Montana-like vegetation and species such as Boykinia heucheriformis (Rydb.) Rosend. and Saxifraga odontoloma Piper within the northern half of the Rocky Mountains and foothills in Alberta, which represents the location of modern-day disjunct populations of these species. Main conclusions, Warmer and drier climatic conditions during the Holocene Hypsithermal resulted in the northward displacement of vegetation zones relative to their current distribution patterns. Most of Alberta was probably dominated by grasslands during this period, except the Rocky Mountains and northern highlands. Modern-day species disjunctions within the Rocky Mountains and Peace River District as well as more northerly areas such as the Yukon Territory occurred when the vegetation receded southward in response to climatic cooling after the Hypsithermal. Wind dispersal was considered an unlikely possibility to explain the occurrence of the disjunct species, as most of the plants lack morphological adaptations for long distance transport and the prevailing winds were from west to east rather than south to north. However, consumption and transport of seeds by northward migrating birds could not be excluded as a possibility. [source] Surface exposure dating of the Great Aletsch Glacier Egesen moraine system, western Swiss Alps, using the cosmogenic nuclide 10BeJOURNAL OF QUATERNARY SCIENCE, Issue 5 2004Meredith A. Kelly Abstract Egesen moraines throughout the Alps mark a glacial advance that has been correlated with the Younger Dryas cold period. Using the surface exposure dating method, in particular the measurement of the cosmogenic nuclide 10Be in rock surfaces, we attained four ages for boulders on a prominent Egesen moraine of Great Aletsch Glacier, in the western Swiss Alps. The 10Be dates range from 10,460±1100 to 9040±1020,yr ago. Three 10Be dates between 9630±810 and 9040±1020,yr ago are based upon samples from the surfaces of granite boulders. Two 10Be dates, 10,460±1100 and 9910±970,yr ago, are based upon a sample from a quartz vein at the surface of a schist boulder. In consideration of the numerous factors that can influence apparently young 10Be dates and the scatter within the data, we interpret the weighted mean of four boulder ages, 9640±430,yr (including the weighted mean of two 10Be dates of the quartz vein), as a minimum age of deposition of the moraine. All 10Be dates from the Great Aletsch Glacier Egesen moraine are consistent with radiocarbon dates of nearby bog-bottom organic sediments, which provide minimum ages of deglaciation from the moraine. The 10Be dates from boulders on the Great Aletsch Glacier Egesen moraine also are similar to 10Be dates from Egesen moraines of Vadret Lagrev Glacier on Julier Pass, in the eastern Swiss Alps. Both the morphology of the Great Aletsch Glacier Egesen moraine and the comparison with 10Be dates from the inner Vadret Lagrev Egesen moraine support the hypothesis that the climatic cooling that occurred during the Younger Dryas cold episode influenced the glacial advance that deposited the Great Aletsch Glacier Egesen moraine. Because of the large size and slow response time of Great Aletsch Glacier, we suggest that the Great Aletsch Glacier Egesen moraine was formed during the last glacial advance of the multiphased Egesen cold period, the Kromer stage, during the Preboreal chron. Copyright © 2004 John Wiley & Sons, Ltd. [source] Stomatal evidence for a decline in atmospheric CO2 concentration during the Younger Dryas stadial: a comparison with Antarctic ice core recordsJOURNAL OF QUATERNARY SCIENCE, Issue 1 2002J. C. Mcelwain Abstract A recent high-resolution record of Late-glacial CO2 change from Dome Concordia in Antarctica reveals a trend of increasing CO2 across the Younger Dryas stadial (GS-1). These results are in good agreement with previous Antarctic ice-core records. However, they contrast markedly with a proxy CO2 record based on the stomatal approach to CO2 reconstruction, which records a ca. 70 ppm mean CO2 decline at the onset of GS-1. To address these apparent discrepancies we tested the validity of the stomatal-based CO2 reconstructions from Kråkenes by obtaining further proxy CO2 records based on a similar approach using fossil leaves from two independent lakes in Atlantic Canada. Our Late-glacial CO2 reconstructions reveal an abrupt ca. 77 ppm decrease in atmospheric CO2 at the onset of the Younger Dryas stadial, which lagged climatic cooling by ca. 130 yr. Furthermore, the trends recorded in the most accurate high-resolution ice-core record of CO2, from Dome Concordia, can be reproduced from our stomatal-based CO2 records, when time-averaged by the mean age distribution of air contained within Dome Concordia ice (200 to 550 yr). If correct, our results indicate an abrupt drawdown of atmospheric CO2 within two centuries at the onset of GS-1, suggesting that some re-evaluation of the behaviour of atmospheric CO2 sinks and sources during times of rapid climatic change, such as the Late-glacial, may be required. Copyright © 2002 John Wiley & Sons, Ltd. [source] Morphology and origin of major Cenozoic sequence boundaries in the eastern North Sea Basin: top Eocene, near-top Oligocene and the mid-Miocene unconformityBASIN RESEARCH, Issue 1 2001M. Huuse Unconformities in sedimentary successions (i.e. sequence boundaries) form in response to the interplay between a variety of factors such as eustasy, climate, tectonics and basin physiography. Unravelling the origin of sequence boundaries is thus one of the most pertinent questions in the analysis of sedimentary basins. We address this question by focusing on three of the most marked physical discontinuities (sequence boundaries) in the Cenozoic North Sea Basin: top Eocene, near-top Oligocene and the mid-Miocene unconformity. The Eocene/Oligocene transition is characterized by an abrupt increase in sediment supply from southern Norway and by minor erosion of the basin floor. The near-top Oligocene and the mid-Miocene unconformity are characterized by major changes in sediment input directions and by widespread erosion along their clinoform breakpoints. The mid-Miocene shift in input direction was followed by a marked increase in sediment supply to the southern and central North Sea Basin. Correlation with global ,18O records suggests that top Eocene correlates with a major long-term ,18O increase (inferred climatic cooling and eustatic fall). Near-top Oligocene does not correlate with any major ,18O events, while the mid-Miocene unconformity correlates with a gradual decrease followed by a major long-term increase in ,18O values The abrupt increases in sediment supply in post-Eocene and post-middle Miocene time correlate with similar changes worldwide and with major ,18O increases, suggesting a global control (i.e. climate and eustasy) of the post-Eocene sedimentation in the North Sea Basin. Erosional features observed at near-top Oligocene and at the mid-Miocene unconformity are parallel to the clinoform breakpoints and resemble scarps formed by mass wasting. Incised valleys have not been observed, indicating that sea level never fell significantly below the clinoform breakpoint during the Oligocene to middle Miocene. [source] | |||||||||||||