New data on speleothem deposition and palaeoclimate in Britain over the last forty thousand years     

T. C. Atkinson  T. J. Lawson  P. L. Smart  R. S. Harmon  J. W. Hess 《第四纪科学杂志》1986,1(1):67-72

²³⁰Th/²³⁴U dates are presented for 26 speleothems from three areas in Britain: Assynt, N.W. Scotland; N.W. Yorkshire; Mendip Hills, Somerset. These dates suggest that speleothem growth was widespread but rare in the period from 40 to 26 Ka, absent from 26 to 15 Ka, and abundant from 15 Ka to the present. The absence of speleothems between 26 and 15 Ka is attributed to glaciation in Yorkshire and Assynt and, tentatively, to continuous permafrost development in the Mendip Hills. The occurrence of speleothems before 26 Ka places restrictions on the extent of any ice sheet which may have been present in Scotland, and indicates that N.W. Scotland was not subjected to continuous permafrost at that time.  相似文献   

Radar observations of Comet P/2005 JQ5 (Catalina)     

J.K. Harmon  M.C. Nolan  D.B. Campbell  J.D. Giorgini 《Icarus》2006,184(1):285-288

Arecibo radar observations of Comet P/2005 JQ5 (Catalina) have produced the first delay-Doppler images of a comet nucleus and the first radar detection of large-grain ejection from a Jupiter-family comet. The nucleus is small (1.4 km diameter), rough, and rapidly rotating. The large (>cm) grains have low velocities (∼1 m/s) and a low production rate.  相似文献   

Dual-polarization radar observations of Mars: Tharsis and Environs     

John K. Harmon  Donald B. Campbell  Steven J. Ostro 《Icarus》1982,52(1):171-187

Observations of the Tharsis region of Mars with the 12.6-cm radar at Arecibo Observatory have yielded radar echoes which are highly depolarizes and which are, in terms of total echo power, dominated by diffuse rather than quasi-specular backscattering. The observations were made on February 7, 8, and 9, 1980, and the subradar track extended from 77 to 126°W Longitude at 22°N Latitude. Dual-polarized reception was employed, i.e., the echo was received in the same sense of circular polarization as transmitted (“depolarized” sense) as well as in the opposite (“polarized”) sense. The disk-integrated ratio of depolarized power to polarized power averages 0.37 and the ratio of diffuse power to quasi-specular power averages 3.2. The depolarized spectra are dominated by a broad “enhancement” identified primarily with echoes from the Tharsis Ridge, implying that extensive areas of Tharsis are rough on decimeter scales. The major Tharsis shield volcanoes are candidates for sources of strong depolarization, although they alone cannot account for the entire depolarization enhancement.  相似文献   

Predictions of MESSENGER Neutron Spectrometer measurements for Mercury's north polar region     

David J. Lawrence  John K. Harmon  William C. Feldman  John O. Goldsten  David A. Paige  Patrick N. Peplowski  Edgar A. Rhodes  Christina M. Selby  Sean C. Solomon 《Planetary and Space Science》2011,59(13):1665-1669

From radar images of Mercury's poles and MESSENGER Neutron Spectrometer (NS) measurements obtained during the spacecraft's flybys of Mercury, predictions of neutron count rates and their uncertainties are calculated for Mercury's north polar region as of the end of the MESSENGER primary orbital mission. If Mercury's poles contain large amounts of water ice, as has been suggested on the basis of the radar data, then during the one-year-long orbital mission the NS should detect signals indicative of excess polar hydrogen with a significance of at least 4σ, where σ is the standard deviation derived from Poisson counting statistics. If the polar deposits are not enriched with hydrogen, but are dominated by other elements, such as sulfur, then the MESSENGER neutron measurements should be able to confirm the absence of deposits having surface concentrations in excess of 50 wt% H₂O on permanently shadowed floors of craters near Mercury's north pole. Because of the large spatial footprint of the NS data, individual polar deposits will not be spatially resolved, but longitudinal asymmetries may be detected if residual systematic uncertainties are sufficiently low.  相似文献   

Estimates of live-tree carbon stores in the Pacific Northwest are sensitive to model selection     

Susanna L Melson  Mark E Harmon  Jeremy S Fried  James B Domingo 《Carbon balance and management》2011,6(1):2

Background  

Estimates of live-tree carbon stores are influenced by numerous uncertainties. One of them is model-selection uncertainty: one has to choose among multiple empirical equations and conversion factors that can be plausibly justified as locally applicable to calculate the carbon store from inventory measurements such as tree height and diameter at breast height (DBH). Here we quantify the model-selection uncertainty for the five most numerous tree species in six counties of northwest Oregon, USA.  相似文献   

Diverse mantle and crustal components in lavas of the NW Cerros del Rio volcanic field,Rio Grande Rift,New Mexico     

K. E. Duncker  J. A. Wolff  R. S. Harmon  P. T. Leat  A. P. Dickin  R. N. Thompson 《Contributions to Mineralogy and Petrology》1991,108(3):331-345

Products of Pliocene (2–4 Ma) mafic to intermediate volcanism in the northwestern Cerros del Rio, a dominantly mafic volcanic field in the Española Basin of the Rio Grande Rift (RGR), range from 49% to 63% SiO₂ and exhibit diversity in silica saturation, trace-element patterns, and isotopic compositions. Tholeiites, which are largely confined to west of the Rio Grande, have trace-element abundances that resemble those of oceanic basalts, but with mild depletions in Nb and Ta, and high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd, and high δ¹⁸O compared to typical OIB. They are regarded as asthenospherically-derived magmas contaminated with continental crust. Alkali basalts and hawaiites erupted from vents east of the Rio Grande are geochemically distinct, having generally higher overall incompatible-element abundances, but with pronounced depletions in K, Rb, Nb and Ta with respect to Th and LREE. Spatially-associated benmoreites, mugearites and latites (collectively termed “evolved” lavas) have similar trace-element characteristics to the mafic mildly-alkaline compositions, but are typically not as depleted in K. Hawaiites and evolved lavas exhibit a good negative correlation of ¹⁴³Nd/¹⁴⁴Nd with SiO₂, due to interaction with lower continental crust. The most silicic “evolved” lavas carry the highest proportions of crustal material, and consequently have higher K/Th than the related hawaiites. Several (mostly mafic) lavas contain abundant crustally-derived resorbed quartz xenocrysts in O-isotope disequilibrium with the host magma. The δ¹⁸O values of xenocrystic quartz range over 4‰, indicating a variety of quartz-bearing crustal contaminants beneath the Española Basin. The hawaiites, with their unusual combination of trace-element enrichments and depletions, cannot be generated by any process of fractionation or crustal contamination superposed on a common mantle source type (oceanic or arc-source). It is a regional mantle source type, inasmuch as it was also present beneath NW Colorado during the mid-late Cenozoic. We argue that the hawaiite source must have originally existed as arc-source mantle enriched in LILE, generated during Mesozoic to early Cenozoic subduction at the western margin of North America. This arc-source mantle lost K, Rb and Ba, but not Th or LREE, prior to magmagenesis. Selective element loss may have occurred during lithospheric thinning and uprise of hydrated phlogopitebearing peridotite-possibly as a thermal boundary layer between lithosphere and asthenosphere — to shallow mantle depths, with consequent conversion of phlogopite to amphibole (an inferior host for K, Rb and Ba). We suggest that this occurred during the early extensional phase of the northern RGR. Further extension was accompanied by partial melting and release of magma from this source and the underlying asthenosphere, which by the Pliocene was of oceanic type. The hawaiite source mantle is the product of a long history of subduction succeeded by lithospheric extension of the formerly overriding plate. Similar chemical signatures may have developed in the mantle beneath other regions with comparable histories.  相似文献   

Evaluation of gases,condensates, and SO2 emissions from Augustine volcano,Alaska: the degassing of a Cl-rich volcanic system     

Robert B Symonds  William I Rose  Terrence M Gerlach  Paul H Briggs  Russell S Harmon 《Bulletin of Volcanology》1990,52(5):355-374

After the March–April 1986 explosive eruption a comprehensive gas study at Augustine was undertaken in the summers of 1986 and 1987. Airborne COSPEC measurements indicate that passive SO₂ emission rates declined exponentially during this period from 380±45 metric tons/day (T/D) on 7/24/86 to 27±6 T/D on 8/24/87. These data are consistent with the hypothesis that the Augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption. Gas samples collected in 1987 from an 870°C fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation (and loss) of H₂O in the intake tube of the sampling apparatus. Thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases. Although not conclusive, this restoration is consistent with the idea that the gases were in equilibrium at 870°C with an oxygen fugacity near the Ni–NiO buffer. These restored gas compositions show that, relative to other convergent plate volcanoes, the Augustine gases are very HCl rich (5.3–6.0 mol% HCl), S rich (7.1 mol% total S), and H₂O poor (83.9–84.8 mol% H₂O). Values of D and ¹⁸O suggest that the H₂O in the dome gases is a mixture of primary magmatic water (PMW) and local seawater. Part of the Cl in the Augustine volcanic gases probably comes from this shallow seawater source. Additional Cl may come from subducted oceanic crust because data by Johnston (1978) show that Cl-rich glass inclusions in olivine crystals contain hornblende, which is evidence for a deep source (>25km) for part of the Cl. Gas samples collected in 1986 from 390°–642°C fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible. H and O isotope data suggest that these gases are variable mixtures of seawater, FMW, and meteoric steam. These samples are much more H₂O-rich (92%–97% H₂O) than the dome gases, possibly due to a larger meteoric steam component. The 1986 samples also have higher Cl/S, S/C, and F/Cl ratios, which imply that the magmatic component in these gases is from the more degassed 1976 magma. Thus, the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the Augustine magma reservoir, even though they were collected a year later and contain a significant seawater component. Future gas studies at Augustine should emphasize fumaroles on active lava domes. Condensates collected from the same lava-dome fumarole have enrichments ot 10⁷–10² in Cl, Br, F, B, Cd, As, S, Bi, Pb, Sb, Mo, Zn, Cu, K, Li, Na, Si, and Ni. Lower-temperature (200°–650°C) fumaroles around the volcano are generally less enriched in highly volatile elements. However, these lower-termperature fumaroles have higher concentration of rock-forming elements, probably derived from the wall rock.  相似文献   

Stable carbon isotope ratios and the existence of a gas phase in the evolution of carbonate ground waters     

Peter Deines  Donald Langmuir  Russell S. Harmon 《Geochimica et cosmochimica acta》1974,38(7):1147-1164

The chemical and stable isotope compositions of unpolluted ground waters in carbonate terranes are a function of the pH, , ¹³C content of the ground water recharge, the ¹³C content of the carbonate rock, and the manner in which the rock is dissolved or precipitated. Physico-chemical models show that significantly different relationships exist between Ca²⁺ plus Mg²⁺, HCO₃^?, pH and ¹³C content of unpolluted ground waters when carbonate solution occurs in the presence or absence of a gas phase. A study was made of these relationships in waters from 21 springs and 13 wells in Nittany Valley, Pennsylvania. Assuming that CO₂ in the recharge zone has a ¹³C contents below ?21%. vs PDB, the data indicate that ground water flow to wells and springs, and carbonate rock solution probably occurs chiefly in the absence of a gas phase. This is in spite of the fact that most of such flow is under water table conditions. ¹³C contents averaged ?12·3%. for the spring waters and ?13·3%. for the well waters. Five well waters polluted by septic tank or sewage effluents had carbon isotopic compositions from ?13·5 to ?16·4%.,vs ?11·3 to?12·7 %. for the eight well waters relatively free of organic wastes.  相似文献   

Mars: Dual-polarization radar observations with extended coverage     

John K. Harmon  Steven J. Ostro 《Icarus》1985,62(1):110-128

Thirteen-centimeter-wavelength radar observations of Mars made in 1982 at Arecibo Observatory yield accurate measurements of the full backscatter spectrum in two orthogonal polarizations. The data, which were obtained for several widely separated subradar longitudes at 24°N latitude, provide the first global view of the distribution of small-scale surface roughness on Mars. The diffuse component of the echo exhibits strong spatial variations. Areas of maximum depolarization correlate well with volcanic regions (Tharsis and Elysium), while the heavily cratered upland terrain yields relatively low depolarization. Parts of Tharsis give near-complete depolarization (polaziation ratio $\text{μ}{}_{\mathrm{<mtext>c</mtext>}}\text{? 1}$ when viewed at oblique angles of incidence). Northern Martian plains regions (Tharsis, Elysium, and Amazonis) may comprise the most extensive area of severe decimeter-scale surface roughness in the inner Solar System. On the average, the northern Martian tropics yield higher diffuse radar cross sections (σ^D = 0.05–0.12) and a higher of degree disk-integrated depolarization (μ_c = 0.1–0.4) than is found for the Moon, Mercury, and Venus. Comparisons between the Moon and Mars using radar data, ground truth, and simple scattering models suggest that Mars possesses a relatively high average coverage by decimeter-scale rocks. Also discussed are several of the more interesting quasispecular scattering results, the most unsual of which were obtained over the Olympus Mons aureole region.  相似文献   

Hydrologic fragmentation-induced eutrophication in Dove Sound,Upper Florida Keys,USA     

Thomas S. Harmon  Joseph M. Smoak  Matthew N. Waters  Christian J. Sanders 《Environmental Earth Sciences》2014,71(10):4387-4395

Anthropogenic impacts to island systems can have deleterious effects on coastal aquatic ecosystems. These effects can alter water quality, primary production as well as habitat. Land development often fragments hydrologic connectivity within aquatic ecosystems forcing alterations in nutrient transport and increases the potential for eutrophication. Dove Sound, a tidal lagoon located in the Upper Florida Keys on Key Largo, has been subjected to anthropogenic influences of land development during the last century. To investigate these influences a short sediment core was collected from within Dove Sound and investigated using ²¹⁰Pb dating, stable isotopes of carbon and nitrogen, and sedimentary pigments. Results indicated that Dove Sound has undergone eutrophication and the primary producer community structure has shifted from dominantly macrophytic to a system that supports substantial algal production. While septic waste was a possible source for eutrophication, low δ¹⁵N did not support this conclusion. However, the timing of the shifts in Dove Sound along with indicators of anoxia leads to the conclusion that fragmentation caused by the construction of a railroad was the root cause. The hydrologic fragmentation reduced the flushing rates, thereby enhancing anoxic conditions in the system and increasing the internal nutrient loading.  相似文献