6 resultados para 1930s,1940s and 1950s

em Publishing Network for Geoscientific


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The surf clams Mesodesma mactroides Reeve, 1854 and Donax hanleyanus Philippi, 1847 are the two dominating species in macrobenthic communities of sandy beaches off northern Argentina, with the latter now surpassing M. mactroides populations in abundance and biomass. Before stock decimation caused by exploitation (during the 1940s and 1950s) and mass mortality events (1995, 1999 and 2007) M. mactroides was the prominent primary consumer in the intertidal ecosystem and an important economic resource in Argentina. Since D. hanleyanus was not commercially fished and not affected by mass mortality events, it took over as the dominant species, but did never reach the former abundance of M. mactroides. Currently abundance and biomass of both surf clams are a multiple smaller than those of forty years ago, indicating the conservation status of D. hanleyanus and M. mactroides as endangered. Therefore the aim of this study is to analyse the population dynamics (population structure, growth and reproductive biology) of D. hanleyanus and M. mactroides, and to compare the results with historical data in order to detect possible differences within surf clam populations forty years ago and at present.

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Presented are physical and biological data for the region extending from the Barents Sea to the Kara Sea during 158 scientific cruises for the period 1913-1999. Maps with the temporal distribution of physical and biological variables of the Barents and Kara Seas are presented, with proposed quality control criteria for phytoplankton and zooplankton data. Changes in the plankton community structure between the 1930s, 1950s, and 1990s are discussed. Multiple tables of Arctic Seas phytoplankton and zooplankton species are presented, containing ecological and geographic characteristics for each species, and images of live cells for the dominant phytoplankton species.

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The climate during the Cenozoic era changed in several steps from ice-free poles and warm conditions to ice-covered poles and cold conditions. Since the 1950s, a body of information on ice volume and temperature changes has been built up predominantly on the basis of measurements of the oxygen isotopic composition of shells of benthic foraminifera collected from marine sediment cores. The statistical methodology of time series analysis has also evolved, allowing more information to be extracted from these records. Here we provide a comprehensive view of Cenozoic climate evolution by means of a coherent and systematic application of time series analytical tools to each record from a compilation spanning the interval from 4 to 61 Myr ago. We quantitatively describe several prominent features of the oxygen isotope record, taking into account the various sources of uncertainty (including measurement, proxy noise, and dating errors). The estimated transition times and amplitudes allow us to assess causal climatological-tectonic influences on the following known features of the Cenozoic oxygen isotopic record: Paleocene-Eocene Thermal Maximum, Eocene-Oligocene Transition, Oligocene-Miocene Boundary, and the Middle Miocene Climate Optimum. We further describe and causally interpret the following features: Paleocene-Eocene warming trend, the two-step, long-term Eocene cooling, and the changes within the most recent interval (Miocene-Pliocene). We review the scope and methods of constructing Cenozoic stacks of benthic oxygen isotope records and present two new latitudinal stacks, which capture besides global ice volume also bottom water temperatures at low (less than 30°) and high latitudes. This review concludes with an identification of future directions for data collection, statistical method development, and climate modeling.

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During the early 2000s the Greenland Ice Sheet experienced the largest ice-mass loss of the instrumental record, largely as a result of the acceleration, thinning and retreat of large outlet glaciers in West and southeast Greenland. The quasi-simultaneous change in the glaciers suggests a common climate forcing. Increasing air and ocean temperatures have been indicated as potential triggers. Here, we present a record of calving activity of Helheim Glacier, East Greenland, that extends back to about AD 1890, based on an analysis of sedimentary deposits from Sermilik Fjord, where Helheim Glacier terminates. Specifically, we use the annual deposition of and grains as a proxy for iceberg discharge. Our record reveals large fluctuations in calving rates, but the present high rate was reproduced only in the 1930s. A comparison with climate indices indicates that high calving activity coincides with a relatively strong influence of Atlantic water and a lower influence of polar water on the shelf off Greenland, as well as with warm summers and the negative phase of the North Atlantic Oscillation. Our analysis provides evidence that Helheim Glacier responds to short-term fluctuations of large-scale oceanic and atmospheric conditions, on timescales of 3-10 years.

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Intercomparison of three new chemical ice core records from northern Greenland (covering the time span from approximately 1500 A.D. to present) with previously published records for southern and central Greenland reveals a uniform timing of anthropogenic changes in sulfate and nitrate firn concentrations over the entire ice sheet. The anthropogenic sulfate increase started around 1890, was interrupted by a transient decrease in the 1930s, and has resumed a major increase since 1950. Since the late 1970s though, a significant 30% decline in Greenland sulfate firn levels can be documented. The maximum anthropogenic increase in northern Greenland sulfate firn concentrations (up to 200-230 ppb) is 2-3 times larger than in southern and central Greenland. Nitrate records show an essentially steady increase since 1950 and, documented for the first time, a slight reduction during most recent years. Maximum nitrate firn levels of 100-130 ppb exceed the preindustrial background by 100% all over the Greenland ice sheet. Comparison with anthropogenic SO2 and NO x emission records indicates that the major increase in sulfate firn concentrations since 1950 can be attributed to Eurasian sources, while firn levels during the first half of this century appear to be dominated by North American emissions. A stronger North American source contribution is indicated over the entire 20th century in the case of nitrate. Applying a macroscopic deposition model separate time series for wet and dry deposition were derived which revealed a close correspondence of wet deposited sulfate with the timing of U.S. emissions, while the temporal evolution of Eurasian emissions is mainly reflected in the dry sulfate deposition record. During this century wet sulfate deposition increased by a factor of two while the total dry sulfate deposition flux increased by more than 500%. Wet and dry nitrate deposition both increased by 100% during the same period.

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A continuous time series of annual soil thaw records, extending from 1994 to 2009, is available for comparison with the records of thaw obtained from the Biocomplexity Experiment (BE) for the period 2006-2009. Discontinuous records of thaw at Barrow from wet tundra sites date back to the 1960s. Comparisons between the longer records with the BE observations reveal strong similarities. Records of permafrost temperature, reflecting changes in the annual surface energy exchange, are available from the 1950s for comparison with results from measurement programs begun in 2002. The long-term systematic geocryological investigations at Barrow indicate an increase in permafrost temperature, especially during the last several years. The increase in near-surface permafrost temperature is most pronounced in winter. Marked trends are not apparent in the active-layer record, although subsidence measurements on the North Slope indicate that penetration into the ice-rich layer at the top of permafrost has occurred over the past decade. Active-layer thickness values from the 1960s are generally higher than those from the 1990s, and are very similar to those of the 2000s. Analysis of spatial active-layer observations at representative locations demonstrates significant variations in active-layer thickness between different landscape types, reflecting the influence of vegetation, substrate, microtopography, and, especially, soil moisture. Landscape-specific differences exist in the response of active-layer thickness to climatic forcing. These differences are attributable to the existence of localized controls related to combinations of surface and subsurface characteristics. The geocryological records at Barrow illustrate the importance and effectiveness of sustained, well organized monitoring efforts to document long-term trends.