Multiple sea-ice states and abrupt MOC transitions in a general circulation ocean model     

Yosef Ashkenazy  Martin Losch  Hezi Gildor  Dror Mirzayof  Eli Tziperman 《Climate Dynamics》2013,40(7-8):1803-1817

Sea ice has been suggested, based on simple models, to play an important role in past glacial–interglacial oscillations via the so-called “sea-ice switch” mechanism. An important requirement for this mechanism is that multiple sea-ice extents exist under the same land ice configuration. This hypothesis of multiple sea-ice extents is tested with a state-of-the-art ocean general circulation model coupled to an atmospheric energy–moisture-balance model. The model includes a dynamic-thermodynamic sea-ice module, has a realistic ocean configuration and bathymetry, and is forced by annual mean forcing. Several runs with two different land ice distributions represent present-day and cold-climate conditions. In each case the ocean model is initiated with both ice-free and fully ice-covered states. We find that the present-day runs converge approximately to the same sea-ice state for the northern hemisphere while for the southern hemisphere a difference in sea-ice extent of about three degrees in latitude between the different runs is observed. The cold climate runs lead to meridional sea-ice extents that are different by up to four degrees in latitude in both hemispheres. While approaching the final states, the model exhibits abrupt transitions from extended sea-ice states and weak meridional overturning circulation, to less extended sea ice and stronger meridional overturning circulation, and vice versa. These transitions are linked to temperature changes in the North Atlantic high-latitude deep water. Such abrupt changes may be associated with Dansgaard–Oeschger events, as proposed by previous studies. Although multiple sea ice states have been observed, the difference between these states is not large enough to provide a strong support for the sea-ice-switch mechanism.  相似文献   

Modelling the galaxy bimodality: shutdown above a critical halo mass     

A. Cattaneo  A. Dekel  J. Devriendt  B. Guiderdoni  J. Blaizot 《Monthly notices of the Royal Astronomical Society》2006,370(4):1651-1665

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From nomadic herder-hunters to sedentary farmers: The relationship between climate change and ancient subsistence strategies in south-eastern Arabia     

《Journal of Arid Environments》2012

Despite the present hyper-aridity, archaeological investigations in South-east Arabia have demonstrated that the region supported extensive human communities throughout the Neolithic and Bronze Age. These early populations utilised the region’s natural environment in a variety of ways, ranging from the exploitation of coastal resources to practicing pastoral and agrarian lifestyles in the interior. Palaeoclimate data suggests the corresponding period was characterised by considerable climatic variability yet, to date, few studies have attempted to investigate the relationship between climate, the environment and early human populations in the region. This paper combines new high-resolution palaeoclimate data from Awafi palaeolake, United Arab Emirates (UAE), with the region’s archaeological record from the Neolithic through to the onset of the Bronze Age. The evidence presented in this paper suggests that the environment of South-east Arabia offered different constraints and opportunities for early human occupation and subsistence. In particular, abrupt phases of aridity are demonstrated to have had a profound impact. Most notable is the change which occurred following the onset of climatic aridity at 5900 cal yr BP, when the region’s semi-nomadic, herder-gatherer populations abandoned much of the landscape and concentrated in selected environmental refugia, such as along the northern Omani coast. Human repopulation during the Bronze Age coincided with a return to more pluvial conditions under which a network of oasis agricultural settlements appeared along the piedmont zone of the northern Hajar Mountains.  相似文献   

Cluster mass functions in the quintessential universe     

Ewa L. &#;okas  Paul Bode  Yehuda Hoffman 《Monthly notices of the Royal Astronomical Society》2004,349(2):595-602

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Objective classification of galaxy spectra using the information bottleneck method   总被引：1，自引：0，他引：1  

Noam Slonim  Rachel Somerville  Naftali Tishby  Ofer Lahav 《Monthly notices of the Royal Astronomical Society》2001,323(2):270-284

A new method for classification of galaxy spectra is presented, based on a recently introduced information theoretical principle, the information bottleneck . For any desired number of classes, galaxies are classified such that the information content about the spectra is maximally preserved. The result is classes of galaxies with similar spectra, where the similarity is determined via a measure of information. We apply our method to ∼6000 galaxy spectra from the ongoing 2dF redshift survey, and a mock-2dF catalogue produced by a cold dark matter (CDM) based semi-analytic model of galaxy formation. We find a good match between the mean spectra of the classes found in the data and in the models. For the mock catalogue, we find that the classes produced by our algorithm form an intuitively sensible sequence in terms of physical properties such as colour, star formation activity, morphology, and internal velocity dispersion. We also show the correlation of the classes with the projections resulting from a principal component analysis.  相似文献   

Early afterglow emission from a reverse shock as a diagnostic tool for gamma-ray burst outflows     

Ehud Nakar  Tsvi Piran 《Monthly notices of the Royal Astronomical Society》2004,353(2):647-653

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Hydrological response of a dryland ephemeral river to southern African climatic variability during the last millennium     

G. Benito  V.R. Thorndycraft  M.T. Rico  Y. Snchez-Moya  A. Sopea  B.A. Botero  M.J. Machado  M. Davis  A. Prez-Gonzlez 《Quaternary Research》2011,75(3):471-482

A long-term flood record from the Buffels River, the largest ephemeral river of NW South Africa (9250 km²), was reconstructed based on interpretation of palaeoflood, documentary and instrumental rainfall data. Palaeoflood data were obtained at three study reaches, with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr. Geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence (OSL) dating. Annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records. Large floods coincided in the past within three main hydroclimatic settings: (1) periods of regular large flood occurrence (1 large flood/~30 yr) under wetter and cooler prevailing climatic conditions (AD 1600–1800), (2) decreasing occurrence of large floods (1 large flood/~100 yr) during warmer conditions (e.g., AD 1425–1600 and after 1925), and (3) periods of high frequency of large floods (~ 4–5 large floods in 20–30 yr) coinciding with wetter conditions of decadal duration, namely at AD 1390–1425, 1800–1825 and 1915–1925. These decadal-scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns, as inferred when comparing their onset and distribution with temperature proxies in southern Africa.  相似文献   

Determining the composition of C–H–O liquids following high-pressure and high-temperature diamond-trap experiments     

O. Dvir  A. Angert  R. Kessel 《Contributions to Mineralogy and Petrology》2013,165(3):593-599

Here, we present the first analytical technique (the quartz tube system technique—QTS) to directly analyze H₂O and CO₂ contents in liquids following high-pressure, high-temperature experiments in capsules containing mantle minerals and a diamond layer serving as a fluid/melt trap. In this technique, the capsule is frozen prior to opening; the diamond trap is cut out of the capsule and placed inside a N₂-filled quartz tube. The diamond trap is heated up to 900 °C to release the gases to an Infrared Gas Analyzer, which determines the CO₂ and H₂O contents. Three sets of experiments containing SiO₂ and CaCO₃ powders were performed at 6 GPa and 1,000 °C in order to calibrate and validate the technique. These experiments demonstrated that when samples are prepared in a N₂ environment, CO₂ and H₂O can be directly measured with an accuracy and precision of 2–3 and 3–4 %, respectively. The QTS technique (for H₂O and CO₂ determination) together with the cryogenic technique (total dissolved solids content) can be applied to diamond-trap capsules following HP–HT experiments in order to provide direct and complete liquid compositions coexisting with mantle material. The principal advantage of the QTS technique of direct analysis of volatile content in liquids over the indirect approach of mass balance calculations is the possibility of studying carbonated and hydrous liquid compositions in equilibrium with mantle material regardless of chemistry and pressure–temperature experimental conditions.  相似文献   

New insights into peri-Gondwana paleogeography and the Gondwana super-fan system from detrital zircon U–Pb ages     

Guido Meinhold  Andrew C. Morton  Dov Avigad 《Gondwana Research》2013,23(2):661-665

We present a synopsis of detrital zircon U–Pb ages of sandstones from North Africa and neighboring Israel and Jordan, which allows us to identify zones with characteristic sediment provenance along the northern Gondwana margin (in present-day coordinates) in Cambrian–Ordovician times, and helps us to unravel the peri-Gondwana jigsaw puzzle. A special feature of the early Paleozoic cover sequence of North Africa is the eastward increase of 1.1–0.95 Ga detrital zircons, which become ubiquitous in the early Paleozoic sandstones of the Saharan Metacraton. Detrital zircons aged about 2.7–2.5, 2.15–1.75 and 0.75–0.53 Ga are also present. Early Paleozoic sandstones with similar provenance are known from peri-Gondwana terranes in the Eastern and Western Mediterranean and from NW Iberia. These terranes need not be transported from western Gondwana (Amazonia) as suggested previously. They were likely located to the north of the Saharan Metacraton during the early Paleozoic before they rifted off from Gondwana. Furthermore, we recognize an increase, as stratigraphic ages get younger, of ca. 1.0 Ga detrital zircons at some point between the Late Cambrian and late Middle Ordovician. We speculate that this might be linked to far-field tectonics and regional uplift in central Gondwana related to plate-tectonic reorganization along the Gondwana margin, leading to erosion of ca. 1.0 Ga basement and country rocks of the Transgondwanan supermountain and fluvial dispersal of detritus toward the Gondwana margin.  相似文献   

Response of cyanobacterial carbon concentrating system to light intensity: a simulated analysis     

付翔  韩博平 《中国海洋湖沼学报》2010,28(3):478-488

Cyanobacteria possess a delicate system known as the carbon concentrating mechanism (CCM), which can efficiently elevate the intracellular inorganic carbon (Ci) concentration via active transportation. The system requires energy supplied by photosystems; therefore, the activity of the Ci transporter is closely related to light intensity. However, the relationship between CCM and light intensity has rarely been evaluated. Here, we present an improved quantitative model of CCM in which light is incorporated, and developed a CCM model that modified after Fridlyand et a1. in 1996. Some equations used in this model were inducted to describe the relationship between transport capacity and light intensity, by which the response of the CCM to light change is simulated. Our results indicate that the efficiency of the carbon concentrating system is sensitive to light intensity. When the external Ci concentration was low, CO₂ uptake dominated the total Ci uptake with increasing light intensity, while under high external Ci concentrations HCO₃⁻ uptake primarily contributed to the total Ci uptake. Variations in the ratio of energy allocated between the transport systems could markedly affect the operation of CCM. Indeed, our simulations suggest that various combinations of Ci fluxes can provide a possible approach to detect the way by which the cell distributes energy produced by the photosystems to the two active Ci transport processes. The proportion of the energy consumed on CCM to the total energy expenditure for the fixation of one CO₂ molecule was determined at 18%–40%.  相似文献