High‐frequency subbottom reflection types and lithologic and physical properties of sediments

Abstract

Eight types of reflections are interpreted from 3,800 km of 3.5 kHz profiles taken over a 25,000 km² area of the upper continental slope and shelf in the northeastern Gulf of Mexico off Panama City, Florida. The corresponding sediments in five of the reflection types were sampled in 77 piston cores from which data were obtained on in situ acoustic velocities (V), bulk densities (gr), sediment texture (mean grain size = Mz), CaCO₃ content (C), sedimentary structures, and gross sediment composition. A distinct bottom echo with numerous subbottom reflectors (Type I) is observed in deeper areas where terrigenous clay or lutite (Mgi = 9.9 to, gr = 1.4 g/cc, porosity (P) = 74 percent, C = 28 percent, and V (upper 2 m) = 1,435 m/s) predominates. Type I reflection grades upslope into Type IV, which shows a distinct bottom echo with fewer subbottom reflectors, and the corresponding sediment is a foraminiferal silty clay (mz = 9.4 to, gr = 1.43 g/cc, P = 73 percent, V = 1,447 m/s, and C = 37 percent). The uppermost slope gives indistinct, semiprolonged bottom echoes with faint subbottoms (Type VI) where calcareous silt (Mz = 6.6 to, gr = 1.57 g/cc, P = 65 percent, C = 70 percent, and V = 1,482 m/s) is the main sediment type. The shelf sediments (gr = 1.66 g/cc, P = 58 percent, V = sl1,530 m/ s), varying from coarse silt (Mz = 5.3 to) to very coarse sand (Mz = ‐0.3 to) and 25 to 100 percent carbonate, show indistinct, semiprolonged bottom echoes with intermittent or mushy subbottoms (Type VII). Prolonged echoes with no subbottoms (Type VIII) are observed in areas where algal sands of variable grain size (Mz ‐ ‐0.9 to 2.7 to, gr = 1.66 g/cc, P = 59 percent, V = 1,530 to 1,690 m/s) occur.

The major trends in reflection types (loss in depth of penetration, loss in number of reflectors, and prolongation of initial bottom reflections) follow gradients of sedimentary and physical properties of the sediments, which are increases in mean grain size, bulk density, in situ acoustic velocity, CaCO₃ content, and decrease in porosity. Increases in the reflection coefficient and attenuation of the sound energy in the shelf sediments are probably important factors in the observed decrease in the depth of penetration of the sound energy in the shelf sediments.     

Toward integrated assessment of environmental change: air pollution health effects in the USA

We demonstrate a method for integrating environmental effects into a computable general equilibrium model. This is a critical step forward toward the development of improved integrated assessment models of environmental change. We apply the method to examine the economic consequences of air pollution on human health for the US for the period from 1970 to 2000. The pollutants include tropospheric ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide, and particulate matter. We apply this method to the MIT Emissions Prediction and Policy Analysis (EPPA) model, a computable general equilibrium model of the economy that has been widely used to study climate change policy. The method makes use of traditional valuation studies, incorporating this information so that estimates of welfare change are consistent with welfare valuation of the consumption of market goods and services. We estimate the benefits of air pollution regulations in USA rose steadily from 1975 to 2000 from $50 billion to $400 billion (from 2.1% to 7.6% of market consumption). Our estimated benefits of regulation are somewhat lower than the original estimates made by the US Environmental Protection Agency, and we trace that result to our development of a stock model of pollutant exposure that predicts that the benefits from reduced chronic air pollution exposure will only be gradually realized. We also estimate the economic burden of uncontrolled levels of air pollution over that period. The uncertainties in these estimates are large which we show through simulations using 95% confidence limits on the epidemiological dose-response relationships     

Sensitivity of an atmospheric general circulation model to prescribed SST changes: feedback effects associated with the simulation of cloud optical properties

This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dumenil     

Capture of molybdenum in pyrite-forming sediments: role of ligand-induced reduction by polysulfides

Capture of Mo by FeS₂ is an important sink for marine Mo. X-ray spectroscopy has shown that Mo forms Mo-Fe-S cuboidal clusters on pyrite. Reduction of Mo^VI must occur to stabilize these structures. Sulfide alone is a poor reductant for Mo, producing instead a series of Mo^VI thioanions (MoO_xS_4−x²⁻, x = 0-3). In solutions that contain both H₂S and S⁰-donors (i.e. polysulfides; dissolved S₈), Mo is transformed to Mo^IV or Mo^V₂ polysulfide/sulfide anions. This intramolecular reduction requires no external reducing agent. Remarkably, an oxidizing agent (S⁰ donor), rather than a reducing agent, stabilizes the reducible Mo^VI complex. Thiomolybdates and their reduction products do not precipitate spontaneously; solutions supersaturated by 10⁹ with respect to molybdenite, MoS₂, produce no precipitate in 40 days. In 10-minute exposures, pyrite can scavenge MoOS₃²⁻ and MoS₄²⁻ weakly at mildly alkaline pH but can scavenge an unidentified product of the S⁰-induced reduction of MoOS₃²⁻ very strongly. On the basis of these observations, a reaction pathway for Mo capture by pyrite is proposed. Conditions that favor Mo capture by this pathway also favor pyrite growth. Ascribing Mo capture simply to low redox potential is too simplistic and neglects the likely role of oxidizing S⁰-donors. The aqueous speciation of Mo in anoxic environments will be a function of the activity of zero-valent sulfur as well as the activity of H₂S(aq).     

The Dingle Dell meteorite: A Halloween treat from the Main Belt

We describe the fall of the Dingle Dell (L/LL 5) meteorite near Morawa in Western Australia on October 31, 2016. The fireball was observed by six observatories of the Desert Fireball Network (DFN), a continental-scale facility optimized to recover meteorites and calculate their pre-entry orbits. The 30 cm meteoroid entered at 15.44 km s⁻¹, followed a moderately steep trajectory of 51° to the horizon from 81 km down to 19 km altitude, where the luminous flight ended at a speed of 3.2 km s⁻¹. Deceleration data indicated one large fragment had made it to the ground. The four person search team recovered a 1.15 kg meteorite within 130 m of the predicted fall line, after 8 h of searching, 6 days after the fall. Dingle Dell is the fourth meteorite recovered by the DFN in Australia, but the first before any rain had contaminated the sample. By numerical integration over 1 Ma, we show that Dingle Dell was most likely ejected from the Main Belt by the 3:1 mean motion resonance with Jupiter, with only a marginal chance that it came from the ν₆ resonance. This makes the connection of Dingle Dell to the Flora family (currently thought to be the origin of LL chondrites) unlikely.     

Neo-boreal climatic influences on the late prehistoric agricultural groups in the upper Mississippi Valley

Archaeological research in the Upper Mississippi River valley between 43°EN and the St. Croix River confluence (44.7°EN) has produced a series of large Oneota villages from which corn (Zea mays) has been recovered. Radiocarbon determinations indicate that the sites in the northern reach of this study area date from AD 1010 to 1440. These sites are characterized by large village areas and associated mound groups. Unlike villages in the northern region, sites in the LaCrosse area are more extensive, and burial mound complexes are absent. the southern villages have ¹⁴C dates ranging from AD 1030 to 1520. Based on the differences in community plans and artifact assemblages, it is suggested that the northern Oneota groups are not ancestors of the neighboring LaCrosse variant. Historical documents for Europe indicate that at similar latitudes, the climate began to deteriorate after AD 1300. With the onset of cooler summers characteristic of the Neo-Boreal climatic episode, corn agriculture became unreliable in the northern portion of the Mississippi basin after AD 1400. In response to the unfavorable climatic conditions, large semi-permanent villages were abandoned and an outward migration began. the peak cold at ca. AD 1550 (the “Little Ice Age”) caused the collapse of agriculture in the LaCrosse region as well. Historical documents, archaeological evidence, and palynological data indicate that the climatic “recovery” did not ensue in this region for more than two centuries. After AD 1750 climatic conditions were again favorable for the cultivation of aboriginal corn.     

Oceanic thunderstorms

Summary The oceanic thunderstorm has long been a difficult phenomenon to investigate. The diurnal variation of these storms together with that of land based storms is essential in providing the statistical correlation between global thunderstorm activity and oceanic potential gradients upon which the classical explanation of the maintenance of the earth's electric charge is based. A data base of over seven million synoptic observations by ships throughout the world ocean has been analyzed to give a fresh approach and insight to the universal time variation of ocean areas affected by thunderstorms. According to these data, the annual average of this ocean area has a mean of 202×10⁴ km² with a diurnal peak of 240×10⁴ km² in the interval of 1800 to 2400 GMT. A diurnal minimum is observed to be 167×10⁴ km² in the interval of 0600 to 1200 GMT.

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Zusammenfassung Es war lange Zeit schwierig, die ozeanischen Gewitter zu untersuchen. Die klassische Erklärung für die Aufrechterhaltung der elektrischen Erdladung gründet sich auf den Zusammenhang zwischen dem luftelektrischen Potentialgefälle auf See und der weltweiten Gewittertätigkeit, wofür die Kenntnis der Tagesgänge der Gewitter über Land und über den Ozean eine wichtige Voraussetzung bilden. Beobachtungsmaterial mit mehr als sieben Millionen synoptischen Beobachtungen — gewonen von Schiffen auf allen Ozeanen — wurde ausgewertet um auf eine neue Weise die von Gewittern bedeckten ozeanischen Flächengrösse in ihrer nach Weltzeit verlaufenden Variation zu untersuchen. Es stellt sich heraus dass das Jahresmittel dieser Flächengrösse bei 202·10⁴ km² liegt, mit einem tageszeitlichen Höchstwert von 240·10⁴ km² in der Zeit zwischen 1800 und 2400 GMT. Ein tageszeitlicher Mindestwert wurde zu 167·10⁴ km² gefunden, welcher zwischen 0600 und 1200 GMT auftritt.

     

GRS evidence and the possibility of paleooceans on Mars

The Gamma Ray Spectrometer (Mars Odyssey spacecraft) has revealed elemental distributions of potassium (K), thorium (Th), and iron (Fe) on Mars that require fractionation of K (and possibly Th and Fe) consistent with aqueous activity. This includes weathering, evolution of soils, and transport, sorting, and deposition, as well as with the location of first-order geomorphological demarcations identified as possible paleoocean boundaries. The element abundances occur in patterns consistent with weathering in situ and possible presence of relict or exhumed paleosols, deposition of weathered materials (salts and clastic minerals), and weathering/transport under neutral to acidic brines. The abundances are explained by hydrogeology consistent with the possibly overlapping alternatives of paleooceans and/or heterogeneous rock compositions from diverse provenances (e.g., differing igneous compositions).