Geomagnetic field intensity: How high can it get? How fast can it change? Constraints from Iron Age copper slag     

Ron Shaar  Erez Ben-Yosef  Hagai Ron  Lisa Tauxe  Amotz Agnon  Ronit Kessel 《Earth and Planetary Science Letters》2011,301(1-2):297-306

The intensity of the geomagnetic field varies over different time scales. Yet, constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record. The purpose of this study is to place firm limits on these fundamental properties by constructing a high-resolution archaeointensity record of the Levant from the 11th century to the early 9th century BCE, a period over which the geomagnetic field reached its maximum intensity in Eurasia over the past 50,000 years. We investigate a ¹⁴C-dated sequence of ten layers of slag material, which accumulated within an ancient industrial waste mound of an Iron Age copper-smelting site in southern Israel. Depositional stratigraphy constrains relative ages of samples analyzed for paleointensity, and ¹⁴C dates from different horizons of the mound constrain the age of the whole sequence. The analysis yielded 35 paleointenisty data points with accuracy better than 94% and precision better than 6%, covering a period of less than 350 years, most probably 200 years. We construct a new high-resolution quasi-continuous archaeointensity curve of the Levant that displays two dramatic spikes in geomagnetic intensity, each corresponding to virtual axial dipole moment (VADM) in excess of 200 ZAm². The geomagnetic spikes rise and fall over a period of less than 30 years and are associated with VADM fluctuations of at least 70 ZAm². Thus, the Levantine archaeomagnetic record places new constraints on maximum geomagnetic intensity as well as for its rate of change. Yet, it is not clear whether the geomagnetic spikes are local non-dipolar features or a geomagnetic dipolar phenomenon.  相似文献   

Precipitation variations and possible forcing factors on the Northeastern Tibetan Plateau during the last millennium     

《Quaternary Research》2014,81(3):508-512

Understanding precipitation variation, drought and flood history, and their associated forcing mechanisms are important to human society. In this study, five moisture-sensitive tree-ring width chronologies are used to represent variations in precipitation over the past millennium on the Northeastern Tibetan Plateau (NETP). We find a strong coherency between chronologies in the NETP, indicating a common response to regional climate during the last millennium. The first principal component of the five chronologies (PC1) correlates significantly with regional precipitation and can thus be used as an indicator of regional precipitation variations. Dry spells, even more severe than the 1920s drought, occurred during AD 1139–1152, 1294–1309, 1446–1503 and 1708–1726. Previous studies in this area using other proxies also identified these droughts. Multi-Taper spectral analysis demonstrates significant periodicities at 205 yr and 73 yr, plus a range of ~ 2 yr cycles, suggesting possible linkage with solar variation and the Pacific Decadal Oscillation (PDO). PC1 also shows coherent patterns with solar irradiance variation: the precipitation tends to reach low values during the well-known solar minimum.  相似文献   

Combined records from a stalagmite from Barbados and from lake sediments in Haiti reveal variable seasonality in the Caribbean between 6.7 and 3 ka BP     

《Quaternary Science Reviews》2007,26(9-10):1332-1343

The growth of a well-dated stalagmite from Barbados records high infiltration rates into the karst aquifer and hence increased rainfall intensity between 6.7 and 3 ka BP in agreement with records from Lake Miragoane, Haiti [Hodell et al., 1991. Reconstruction of the Caribbean climate change over the past 10,500 years. Nature 352, 790–793], mainly reflecting the insolation maximum of the Northern Hemisphere. Both the lake record and the stable isotope record of the stalagmite reveal additional centennial variability of recharge. High oxygen isotope values in stalagmite calcite, corresponding to reduced recharge, are synchronous with periods of lower stable isotope values recorded in Lake Miragoane ostracods, previously attributed to enhanced precipitation. Accordingly, periods of increased recharge in Barbados correspond to ¹⁸O-enriched isotope values of ostracods, which were attributed to higher evaporation/precipitation ratios in the lakes. We ascribe this apparent discrepancy to changes in seasonality, i.e., winter periods of reduced temperature and relative humidity following summer months of increased precipitation. At present, such climate conditions occur during periods of enhanced Northern Atlantic Oscillation (NAO⁺). If enhanced seasonality is a consequence of a NAO⁺ situation, the apparent discrepancy of high isotope values in lakes (previously attributed to droughts) can be reconciled with lower winter temperatures in the lakes. Further, the correlation of solar intensity (derived from ¹⁴C and ¹⁰Be) with the isotopic signals recorded in the lacustrine sediments suggests a solar forcing of the NAO during the mid Holocene.  相似文献   

Palynology,sedimentology and palaeoecology of the late Holocene Dead Sea     

《Quaternary Science Reviews》2007,26(11-12):1476-1498

Palynological and sedimentological studies were performed at two Holocene profiles in erosion gullies (Ze’elim and Ein Feshkha) which dissect the retreating western shore of the Dead Sea. The aim of the project was to analyse possible links between climate, lithology, and vegetation development. The section in Ze’elim shows both lacustrine and fluvial sediments, whereas sedimentation at Ein Feshkha is predominantly lacustrine. The Ze’elim profile, previously used for paleo-lake reconstruction provides an opportunity to compare climate triggered lake levels as paleo-hydrological indicators and vegetation history by use of palynology. The vegetation development in Ze’elim and Ein Feshkha is influenced by both climate and human impact. The pollen record of Ze’elim begins in the Pottery Neolithic, the section of Ein Feshkha in the Late Bronze Age, both records end in the Middle Ages. The Ze’elim section is characterized by sedimentary hiati between the beginning of the Chalcolithic Period until the Middle Bronze Age and within the Late Bronze Age. Settlement periods during the Middle Bronze Age, Iron Age and Hellenistic–Roman–Byzantine Period are indicated by high values of anthropogenic indicators and/or Mediterranean trees. Collapses of agriculture, which can be related to climate effects, are evident during the Late Bronze Age, during the Iron Age and at the end of the Byzantine Period when the lake level curve indicates arid conditions. A comparison of the two pollen records, from different environments, illustrates a more prominent influence of Mediterranean vegetation and cultivated plants in the pollen diagram of Ein Feshkha. The southern Dead Sea region (at the desert fringe) is more vulnerable to regional climate change.  相似文献   

Late Pleistocene dust deposition in the Patagonian steppe - extending and refining the paleoenvironmental and tephrochronological record from Laguna Potrok Aike back to 55 ka     

Torsten Haberzettl  Flavio S. Anselmetti  Sabine W. Bowen  Michael Fey  Christoph Mayr  Bernd Zolitschka  Daniel Ariztegui  Barbara Mauz  Christian Ohlendorf  Stephanie Kastner  Andreas Lücke  Frank Schäbitz  Michael Wille 《Quaternary Science Reviews》2009,28(25-26):2927-2939

Paleoenvironmental records extending well into the last glacial period are scarce in the steppe regions of southern South America. Here, we present a continuous record for the past 55 ka from the maar lake Laguna Potrok Aike (51°58′ S, 70°23′ W, southern Patagonia, Argentina). Previous studies on a sedimentary core from a lake level terrace near the northern margin of the lake covered parts of Oxygen Isotope Stage (OIS) 3 (59–29 ka) whereas a second core from the centre of the basin comprised the last 16 ka. Tephrostratigraphical constraints and OSL ages from a third core located below the lake level terrace provide the crucial piece to close the gap between the previous coring sites. High-resolution XRF and magnetic susceptibility as well as grain size data indicate a positive hydrological balance alongside with relatively high aeolian activity during the glacial which is contemporaneous with increased dust fluxes in Antarctica. This is therefore the first evidence for contemporaneity of aeolian deposition in both the target area (Antarctica) and in the major source area of Patagonia. During the Holocene climatic conditions driving sediment deposition seem to have been more variable and less dominated by wind compared to glacial times. The identification of a minor lake level lowering at approximately 4 cal ka BP allows to refine earlier paleoenvironmental reconstructions for the Holocene. Within error margins the OSL ages are consistent with published radiocarbon-dated records offering hence a valuable tool for further studies of the sediments from Laguna Potrok Aike. The new chronology confirms the age of three tephra layers up to now only found in Laguna Potrok Aike sediments and ascribed to OIS 3.  相似文献   

Upper Neogene stratigraphy and tectonics of Death Valley — a review     

《Earth》2006,74(1-4):245-270

New tephrochronologic, soil-stratigraphic and radiometric-dating studies over the last 10 years have generated a robust numerical stratigraphy for Upper Neogene sedimentary deposits throughout Death Valley. Critical to this improved stratigraphy are correlated or radiometrically-dated tephra beds and tuffs that range in age from > 3.58 Ma to < 1.1 ka. These tephra beds and tuffs establish relations among the Upper Pliocene to Middle Pleistocene sedimentary deposits at Furnace Creek basin, Nova basin, Ubehebe–Lake Rogers basin, Copper Canyon, Artists Drive, Kit Fox Hills, and Confidence Hills. New geologic formations have been described in the Confidence Hills and at Mormon Point. This new geochronology also establishes maximum and minimum ages for Quaternary alluvial fans and Lake Manly deposits. Facies associated with the tephra beds show that ∼3.3 Ma the Furnace Creek basin was a northwest–southeast-trending lake flanked by alluvial fans. This paleolake extended from the Furnace Creek to Ubehebe. Based on the new stratigraphy, the Death Valley fault system can be divided into four main fault zones: the dextral, Quaternary-age Northern Death Valley fault zone; the dextral, pre-Quaternary Furnace Creek fault zone; the oblique–normal Black Mountains fault zone; and the dextral Southern Death Valley fault zone. Post − 3.3 Ma geometric, structural, and kinematic changes in the Black Mountains and Towne Pass fault zones led to the break up of Furnace Creek basin and uplift of the Copper Canyon and Nova basins. Internal kinematics of northern Death Valley are interpreted as either rotation of blocks or normal slip along the northeast–southwest-trending Towne Pass and Tin Mountain fault zones within the Eastern California shear zone.  相似文献   

From straight to deeply incised meandering channels: Slope impact on sinuosity of confined streams     

Elad Dente  Nadav G. Lensky  Efrat Morin  Yehouda Enzel 《地球表面变化过程与地形》2021,46(5):1041-1054

Meandering channels and valleys are dominant landscape features on Earth. Their morphology and remnants potentially indicate past base-level fluctuations and changing regional slopes. The prevailing presence of meandering segments in low-slope areas somewhat confuses the physically based relationships between slope and channel meandering. This relationship underlies a fundamental debate: do incised sinuous channels actively develop during steepening of a regional slope, or do they inherit the planform of a preexisting sinuous channel through vertical incision? This question was previously explored through reconstructed evolution of meandering rivers, numerical simulations, and controlled, scaled-down laboratory experiments. Here, we study a rare, field-scale set of a dozen adjacent perennial channels, evolving in recent decades in a homogeneous erodible substrate in response to the Dead Sea level fall (> 30 m over 40 years). These channels are fed by perennial springs and have no drainage basin or previous fluvial history; they initiated straight and transformed into incising meandering channels following the emergence of the preexisting lake bathymetry, which resulted in increased channel lengths and regional slopes at different rates for each channel. This field setting allows testing the impact of changing regional slope on the sinuosity of a stream in the following cases: (a) relatively long and low-gradient shelf-like margins, (b) a sharp increase in the basinward gradient at the shelf-slope transition, and (c) gradually steepening slopes. Under a stable and low valley slope, the channels mainly incise vertically, inheriting a preexisting sinuous pattern. When the regional slope steepens, the channels start to meander, accompanying the vertical incision. The highest sinuosity evolved in the steepest channel, which also developed the deepest and widest valley. These results emphasize the amplifying impact of steepening regional slope on sinuosity. This holds when the flow is confined and chute cutoffs are scarce.  相似文献   

Flood-duration-integrated stream power and frequency magnitude of >50-year-long sediment discharge out of a hyperarid watershed     

Judith Lekach  Yehouda Enzel 《地球表面变化过程与地形》2021,46(7):1348-1362

Determining sediment discharge out of watersheds is a global, long-term challenge. In the vast, usually data-poor, hyperarid regions of the world, this is a greater challenge. Here, we present a unique, decades-long dataset of individual floods and their respective sediment discharge out of Nahal Yael, an experimental, well-instrumented, hyperarid (~25–30 mm year⁻¹) watershed in southern Israel. The high correlation between directly measured sediment yield by discrete individual floods and their respective total energy, represented by flood-integrated stream power (FISP), serves here as a rating curve. Using this rating curve, the 51-year-long series of FISP in Nahal Yael, calculated from the detailed individual flood hydrographs, was converted into a series of sediment yield by these floods. This, in turn, allows determining the long-term frequency-magnitude of sediment exported out of this hyperarid basin. This can assist in landscape evolution modeling, in testing impacts of flood frequency changes enforced by altered regional climatology, and hint at changes needed in forming the observed alluvial fans. We conclude that, at the decadal scale, moderate floods are the most effective in terms of total sediment transport. However, the recurrence intervals of these moderate hyperarid floods are longer than in temperate regions and reach 5–10 years.  相似文献   

Salt diapirs in the Dead Sea basin and their relationship to Quaternary extensional tectonics     

《Marine and Petroleum Geology》2001,18(7):779-797

Regional extension of a brittle overburden and underlying salt causes differential loading that is thought to initiate the rise of reactive diapirs below and through regions of thin overburden. We present a modern example of a large salt diapir in the Dead Sea pull-apart basin, the Lisan diapir, which we believe was formed during the Quaternary due to basin transtension and subsidence. Using newly released seismic data that are correlated to several deep wells, we determine the size of the diapir to be 13×10 km, its maximum depth 7.2 km, and its roof 125 m below the surface. From seismic stratigraphy, we infer that the diapir started rising during the early to middle Pleistocene as this section of the basin underwent rapid subsidence and significant extension of the overburden. During the middle to late Pleistocene, the diapir pierced through the extensionally thinned overburden, as indicated by rim synclines, which attest to rapid salt withdrawal from the surrounding regions. Slight positive topography above the diapir and shallow folded horizons indicate that it is still rising intermittently. The smaller Sedom diapir, exposed along the western bounding fault of the basin is presently rising and forms a 200 m-high ridge. Its initiation is explained by localized E–W extension due monoclinal draping over the edge of a rapidly subsiding basin during the early to middle Pleistocene, and its continued rise by lateral squeezing due to continued rotation of the Amazyahu diagonal fault.  相似文献   

Deduction of groundwater flow regime in a basaltic aquifer using geochemical and isotopic data: The Golan Heights, Israel case study     

Elad Dafny  Avi Burg  Haim Gvirtzman 《Journal of Hydrology》2006,330(3-4):506-524

Groundwater flow-paths through shallow-perch and deep-regional basaltic aquifers at the Golan Heights, Israel, are reconstructed by using groundwater chemical and isotopic compositions. Groundwater chemical composition, which changes gradually along flow-paths due to mineral dissolution and water–rock interaction, is used to distinguish between shallow-perched and deep-regional aquifers. Groundwater replenishment areas of several springs are identified based on the regional depletion in rainwater δ¹⁸O values as a function of elevation (−0.25‰ per 100 m). Tritium concentrations assist in distinguishing between pre-bomb and post-bomb recharged rainwater.

It was found that waters emerging through the larger springs are lower in δ¹⁸O than surrounding meteoric water and poor in tritium; thus, they are inferred to originate in high-elevation regions up to 20 km away from their discharge points and at least several decades ago. These results verify the numerically simulated groundwater flow field proposed in a previous study, which considered the geological configuration, water mass balance and hydraulic head spatial distribution.  相似文献