Reevaluation of the lake-sediment chronology in the Dead Sea basin, Israel, based on new Th/U dates     

Aaron Kaufman  Yoseph Yechieli  Michael Gardosh 《Quaternary Research》1992,38(3)

The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of ²³⁰Th/²³⁴U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with ¹⁴C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin.  相似文献   

Aerosol optical depth, physical properties and radiative forcing over the Arabian Sea     

S. K. Satheesh  K. Krishna Moorthy  Y. J. Kaufman  T. Takemura 《Meteorology and Atmospheric Physics》2006,91(1-4):45-62

Summary The Arabian Sea region (4° N–20° N to 50° E–78° E) has a unique weather pattern on account of the Indian monsoon and the associated winds that reverse direction seasonally. The aerosol data, collected using ship-borne and island platforms (for 8 years from 1995 to 2002) along with MODIS (onboard TERRA satellite) data (from 2000 to 2003) have been used to evolve a comprehensive characterisation of the spatial and temporal variation in the physical, chemical, and radiative properties of aerosols over the Arabian Sea. The aerosol optical depth (AOD) was found to increase with latitude between the equator and 12° N. Over the northern Arabian Sea (regions lying north of 12° N), AODs do not show significant latitudinal variations; the average aerosol optical depth for this region was 0.29±0.12 during winter monsoon season (WMS; November to March) and 0.47±0.14 during summer monsoon season (SMS; April/May to September). The corresponding Angstrom exponents were 0.7±0.12 and 0.3±0.08, respectively. The low values of the exponent during SMS indicate the dominance of large aerosols (mainly dust particles >1 μm). The latitudinal gradient in AOD in the southern Arabian Sea is larger during SMS compared to WMS. The size distribution of aerosols shows two well-defined modes, one in the accumulation size regime and the other in the coarse size regime. During WMS, a third mode (nucleation) also appears in the sub micron range below ∼0.1 μm. The single scattering albedo does not show significant seasonal variations (remains within ∼0.93 to 0.98 through out the year). During WMS (SMS), top of the atmosphere diurnally averaged aerosol forcing remains around −6.1 (−14.3)W m⁻² over the northern Arabian Sea up to around 12° N and decreases southwards till it attains a value of −3.8 (−3.4)W m⁻² at the equator. The surface forcing remains around −16.2(−15.2)W m⁻² over the northern Arabian Sea up to 12° N and decreases southwards to a value of −5.5 (−3.5)W m⁻² at the equator. Over the north Arabian Sea, instantaneous forcing (flux change) at the surface can be as high as −50 W m⁻². The instantaneous forcing decreases with latitude in the southern Arabian Sea at a rate of ∼3 W m⁻²deg⁻¹.  相似文献   

Erratum     

G. G. Baxter  S. M. Cargill  A. H. Chidester  P. E. Hart  G. M. Kaufman  F. Urquidi-Barrau  D. D. Picklyk 《Mathematical Geosciences》1979,11(3):343-343

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Complex Archean lower crustal structure revealed by COCORP crustal reflection profiling in the Wind River Range,Wyoming     

Scott B. Smithson  Jon A. Brewer  S. Kaufman  J.E. Oliver  R.L. Zawislak 《Earth and Planetary Science Letters》1980,46(2):295-305

A COCORP deep crustal reflection profile across the Wind River uplift crosses exposed Archean rocks and resolves an unusual complex deep crustal structure at a depth of 24–31 km in an area where depth relations in Precambrian rocks can be inferred. The different levels of exposure across the beveled plunge of the Wind River uplift reveal supracrustal rocks at shallower levels with migmatites and pyroxene granulites at deeper levels. For the first time, deep crustal structure from reflection profiling may be interpreted in terms of exposed basement geology. A folded, multilayered deep structure shown by relfection data resembles multiply folded pyroxene granulite interlayered with granitic gneiss exposed in the central Wind River uplift; isoclinal folding is suggested in the folded layered seismic structure. Earlier seismic reflection studies suggested a simpler lower crust. These data indicate that lower crustal structure may have a complexity similar to deeply eroded Precambrian granulite-facies rocks. If this seismic feature represents folded metamorphic rocks, it seems unlikely that this Archean crust could have been thickened by underplating after 2.7 b.y. B.P. and the crust would have to be at least 30 km thich when this structure was formed.  相似文献   

Why is risk aversion unaccounted for in environmental policy evaluations?     

Noah Kaufman 《Climatic change》2014,125(2):127-135

U.S. environmental regulations are increasingly influenced by cost-benefit analyses that are performed based on the guidance of the Office of Management and Budget (OMB). The OMB’s Circular A-4 directs Federal agencies to assume “risk neutrality” in conducting regulatory analysis, and in important instances, this guidance is not supported by economic theory. Risk neutrality is computationally convenient, and it can be justified when only the costs and benefits of regulations themselves are uncertain, because these risks are spread across a large population. However, the Circular A-4 does not distinguish between regulations that cause uncertainty and those that reduce pre-existing (i.e. baseline) uncertainty, such as the potential for catastrophic climate change. Basic economic theory shows that risk aversion should be incorporated into evaluations of policies that reduce pre-existing environmental uncertainty. Regulatory analyses generally ignore these risk-reduction benefits, leading to misinformed policymaking. Quantifying risk premiums is difficult and controversial, but no more so than discounting future costs and benefits to present value terms. Similar to how OMB has established discount rates for use in regulatory analyses, a method for when and how to incorporate risk aversion into policy evaluations should replace the blanket guidance for risk neutrality.  相似文献   

Primeval galaxies: Predicted luminosities     

Michele Kaufman 《Astrophysics and Space Science》1976,40(2):369-384

Absorption by gas and dust in circumstellar Hii regions within primeval galaxies could seriously depress the far-ultraviolet continuum radiation emitted by primeval galaxies. This effect might account for the failure of Partridge (1974) and Davis and Wilkinson (1974) to detect the redshifted radiation from primeval galaxies at optical and near-infrared wavelengths. A primeval galaxy becomes very bright only during the final stages of contraction. Provided that dust can form by the time the primeval galaxy reaches peak luminosity, a significant fraction of the stellar far-ultraviolet radiation is converted into far-infrared. Thus an appropriate spectral region to search for the redshifted integrated background from primeval galaxies lies between 350 , where the 2.7 K microwave background radiation becomes important, and 150 , where other extragalactic discrete sources, such as nearby galactic nuclei, may contribute. The expected IR flux is calculated with Kaufman's (1975) model for the star formation rate in the contracting galaxy. Letz _p be the redshift andT _g the grain temperature when the primeval galaxy becomes very bright. Unlessz _p10 orT _g is fairly high, the intensity of the far-infrared radiation from primeval galaxies would be dominated by the high frequency tail of the 2.7 K microwave background. On the other hand, if dust is unimportant, we determine the spectral energy distribution of a primeval galaxy emitted in the range 912 Å to 2050 Å; we find that the luminosities are not very sensitive to the dependence of effective temperatures on metal abundance.  相似文献   

A multi-proxy record of the Last Glacial Maximum and last 14,500?years of paleoenvironmental change at Lone Spruce Pond, southwestern Alaska     

Darrell S. Kaufman  Yarrow Axford  R. Scott Anderson  Scott F. Lamoureux  Daniel E. Schindler  Ian R. Walker  Al Werner 《Journal of Paleolimnology》2012,48(1):9-26

Sediment cores from Lone Spruce Pond (60.007°N, 159.143°W), southwestern Alaska, record paleoenvironmental changes during the global Last Glacial Maximum (LGM), and during the last 14,500 calendar years BP (14.5?cal?ka). We analyzed the abundance of organic matter, biogenic silica, carbon, and nitrogen, and the isotope ratios of C and N, magnetic susceptibility, and grain-size distribution of bulk sediment, abundance of alder shrub (Alnus) pollen, and midge (Chironomidae and Chaoboridae) assemblages in a 4.7-m-long sediment sequence from the depocenter at 22?m water depth. The basal unit contains macrofossils dating to 25?C21?cal?ka (the global LGM), and is interpreted as glacial-lacustrine sediment. The open water requires that the outlet of the Ahklun Mountain ice cap had retreated to within 6?km of the range crest. In addition to cladocerans and diatoms, the glacial-lacustrine mud contains chironomids consistent with deep, oligotrophic conditions; several taxa associated with relatively warm conditions are present, suggestive of relative warmth during the global LGM. The glacial-lacustrine unit is separated from the overlying non-glacial lake sediment by a possible disconformity, which might record a readvance of glacier ice. Non-glacial sediment began accumulating around 14.5?cal?ka, with high flux of mineral matter and fluctuating physical and biological properties through the global deglacial period, including a reversal in biogenic-silica (BSi) content during the Younger Dryas (YD). During the global deglacial interval, the ??¹³C values of lake sediment were higher relative to other periods, consistent with low C:N ratios (8), and suggesting a dominant atmospheric CO₂ source of C for phytoplankton. Concentrations of aquatic faunal remains (chironomids and Cladocera) were low throughout the deglacial interval, diversity was low and warm-indicator taxa were absent. Higher production and air temperatures are inferred following the YD, when bulk organic-matter (OM) content (LOI 550?°C) increased substantially and permanently, from 10 to 30?%, a trend paralleled by an increase in C and N abundance, an increase in C:N ratio (to about 12), and a decrease in ??¹³C of sediment. Post-YD warming is marked by a rapid shift in the midge assemblage. Between 8.9 and 8.5?cal?ka, Alnus pollen tripled (25?C75?%), followed by the near-tripling of BSi (7?C19?%) by 8.2?cal?ka, and ??¹⁵N began a steady rise, reflecting the buildup of N and an increase in denitrification in soils. Several chironomid taxa indicative of relatively warm conditions were present throughout the Holocene. Quantitative chironomid-based temperature inferences are complicated by the expansion of Alnus and resulting changes in lake nutrient status and production; these changes were associated with an abrupt increase in cladoceran abundance and persistent shift in the chironomid assemblage. During the last 2,000?years, chironomid-assemblage changes suggest cooler temperatures, and BSi and OM values were generally lower than their maximum Holocene values, with minima during the seventh and eighth centuries, and again during the eighteenth century.  相似文献   

A decline in molluscan carbonate production driven by the loss of vegetated habitats encoded in the Holocene sedimentary record of the Gulf of Trieste     

Adam Toma&#x;ových  Ivo Gallmetzer  Alexandra Haselmair  Darrell S. Kaufman  Borut Mavri   Martin Zuschin 《Sedimentology》2019,66(3):781-807

Carbonate sediments in non‐vegetated habitats on the north‐east Adriatic shelf are dominated by shells of molluscs. However, the rate of carbonate molluscan production prior to the 20th century eutrophication and overfishing on this and other shelves remains unknown because: (i) monitoring of ecosystems prior to the 20th century was scarce; and (ii) ecosystem history inferred from cores is masked by condensation and mixing. Here, based on geochronological dating of four bivalve species, carbonate production during the Holocene is assessed in the Gulf of Trieste, where algal and seagrass habitats underwent a major decline during the 20th century. Assemblages of sand‐dwelling Gouldia minima and opportunistic Corbula gibba are time‐averaged to >1000 years and Corbula gibba shells are older by >2000 years than shells of co‐occurring Gouldia minima. This age difference is driven by temporally disjunct production of two species coupled with decimetre‐scale mixing. Stratigraphic unmixing shows that Corbula gibba declined in abundance during the highstand phase and increased again during the 20th century. In contrast, one of the major contributors to carbonate sands – Gouldia minima – increased in abundance during the highstand phase, but declined to almost zero abundance over the past two centuries. Gouldia minima and herbivorous gastropods associated with macroalgae or seagrasses are abundant in the top‐core increments but are rarely alive. Although Gouldia minima is not limited to vegetated habitats, it is abundant in such habitats elsewhere in the Mediterranean Sea. This live–dead mismatch reflects the difference between highstand baseline communities (with soft‐bottom vegetated zones and hard‐bottom Arca beds) and present‐day oligophotic communities with organic‐loving species. Therefore, the decline in light penetration and the loss of vegetated habitats with high molluscan production traces back to the 19th century. More than 50% of the shells on the sea floor in the Gulf of Trieste reflect inactive production that was sourced by heterozoan carbonate factory in algal or seagrass habitats.  相似文献   

Mixing volume calculations,sources and aging trends of Floridan aquifer water by uranium isotopic methods     

J.K. Osmond  M.I. Kaufman  J.B. Cowart 《Geochimica et cosmochimica acta》1974,38(7):1083-1100

The activity ratio (A.R.) of two naturally occurring isotopes of uranium, U-238 and U-234, varies significantly in aqueous environments. The A.R., used in conjunction with the uranium concentration, provides a ‘fingerprint’ of groundwater masses and, by use of these parameters, mixing volumes and flow patterns can be determined. This method was applied to two large limestone springs in central Florida; the mixing volumes and flow patterns calculated are notably similar to those derived by standard hydrologic methods.In deeper parts of the aquifer, the waters are found to have low uranium concentration and a pattern of higher A.R.'s suggestive of an aging effect.  相似文献   

Chevron Ridges and Runup Deposits in the Bahamas from Storms Late in Oxygen-Isotope Substage 5e     

Paul J. Hearty  A.Conrad Neumann  Darrell S. Kaufman 《Quaternary Research》1998,50(3):309-322

Landward-pointing V-shaped sand ridges several kilometers long are common along the windward margin of the Bahama Islands. Their axes share a northeast–southwest trend. Internally, the ridges contain low-angle oolitic beds with few erosional truncations. Commonly interbedded are tabular, fenestrae-rich beds such as those formed by the sheet flow of water over dry sand. Defined here as “chevron ridges,” these landforms appear to have originated in the rapid remobilization of bank margin ooid bodies by the action of long-period waves from a northeasterly source. Deposits along adjacent coastlines also preserve evidence of the impact of large waves. Reworked eolian sand bodies preserve beach fenestrae and hydraulic scour traces up to +40 m on older ridges. On cliffed coasts, 1000-ton boulders have been thrown well inland, recording the impact of large waves. Amino acid ratios confirm a correlation of the ridges across the archipelago, while stratigraphy, spacing, and cross-cutting relationships indicate emplacement as sea level fell rapidly from the substage 5e maximum at or above +6 m.  相似文献