Sedimentological and provenance response to Cambrian closure of the Clymene ocean: The upper Alto Paraguai Group,Paraguay belt,Brazil

Final Gondwana amalgamation was marked by the closure of the Neoproterozoic Clymene ocean between the Amazonia craton and central Gondwana. The events which occurred in the last stage of this closure were recorded in the upper Alto Paraguai Group in the foreland of the Paraguay orogen. Outcrop-based facies analysis of the siliciclastic rocks of upper Alto Paraguai Group, composed of the Sepotuba and Diamantino Formations, was carried out in the Diamantino region, within the eastern part of the Barra dos Bugres basin, Mato Grosso state, central-western Brazil. The Sepotuba Formation is composed of sandy shales with planar to wave lamination interbedded with fine-grained sandstone with climbing ripple cross-lamination, planar lamination, swaley cross-stratification and tangential to sigmoidal cross-bedding with mud drapes, related to marine offshore deposits. The lower Diamantino Formation is composed of a monotonous, laterally continuous for hundreds of metres, interbedded siltstone and fine-grained sandstone succession with regular parallel lamination, climbing ripple cross-lamination and ripple-bedding interpreted as distal turbidites. The upper part of this formation consists of fine to medium-grained sandstones with sigmoidal cross-bedding, planar lamination, climbing ripple cross-lamination, symmetrical to asymmetrical and linguoid ripple marks arranged in lobate sand bodies. These facies are interbedded with thick siltstone in coarsening upward large-scale cycles related to a delta system. The Sepotuba Formation characterises the last transgressive deposits of the Paraguay basin representing the final stage of a marine incursion of the Clymene ocean. The progression of orogenesis in the hinterland resulted in the confinement of the Sepotuba sea as a foredeep sub-basin against the edge of the Amazon craton. Turbidites were generated during the deepening of the basin. The successive filling of the basin was associated with progradation of deltaic lobes from the southeast, in a wide lake or a restricted sea that formed after 541 ± 7 Ma. Southeastern to east dominant Neoproterozoic source regions were confirmed by zircon grains that yielded ages around 600 to 540 Ma, that are interpreted to be from granites in the Paraguay orogen. This overall regressive succession recorded in the Alto Paraguai Group represents the filling up of a foredeep basin after the final amalgamation of western Gondwana in the earliest Phanerozoic.     

Characteristics of high-resolution marine reflection profiling sources

Midwater signatures of eight common types of marine seismic sources have been recorded and characteristics of importance for high-resolution reflection profiling have been analyzed. Analyzed characteristics include wavelet shape, peak frequency, bandwidth, repeatability and directivity. Digitization rates required to describe the signatures before any processing and after whitening deconvolution were determined.It was intended that most of the broad range of available source types be represented. Toward that end, both resonant and impulsive types were studied; the impulsive types being both electric-discharge and pneumatic. They included a conventional sonar transducer, a chirped sonar transducer, a boomer, a plasmagun, a multi tip sparker, a watergun, an airgun and three sizes of sleevegun.It was observed that the peak frequency of each conventional sonar is similar to its nominal value and that of the chirped sonar is about the midpoint of its nominal sweep bandwidth. The dominant frequencies of the electric-discharge impulsive sources are about an order of magnitude higher than those of the pneumatic impulsive sources. Among impulsive sources, the boomer has the highest peak frequency and the watergun has the broadest bandwidth. The sleevegun and airgun were found to be comparable, both having low frequencies, poor repeatability and weak directivity. The conventional sonar and the boomer are the most repeatable sources. The boomer shows the strongest directivity with the chip sonar and the watergun being the most nondirectional.Sample rates required to describe the unprocessed signatures vary from 2 to 88 times the peak frequency and from 1 to 4 times the bandwidth 40dB below peak power. The required rate depends largely on source type but can also be affected by energy level. Bandwidth seems to be a more stable indicator than peak frequency. Rates required to describe the results of applying whitening deconvolution to signatures were about 1.5–3 times higher than rates calculated for the unprocessed signatures. This leads to the conclusion that the digitization rate required in a specific situation can depend as much on the intended processing scheme as it does on source type.     

The impact of shale gas on the costs of climate policy

The use of shale gas is commonly considered as a low-cost option for meeting ambitious climate policy targets. This article explores global and country-specific effects of increasing global shale gas exploitation on the energy markets, on greenhouse gas emissions, and on mitigation costs. The global techno-economic partial equilibrium model POLES (Prospective Outlook on Long-term Energy Systems) is employed to compare policies which limit global warming to 2°C and baseline scenarios when the availability of shale gas is either high or low. According to the simulation results, a high availability of shale gas has rather small effects on the costs of meeting climate targets in the medium and long term. In the long term, a higher availability of shale gas increases baseline emissions of greenhouse gases for most countries and for the world, and leads to higher compliance costs for most, but not all, countries. Allowing for global trading of emission certificates does not alter these general results. In sum, these findings cast doubt on shale gas’s potential as a low-cost option for meeting ambitious global climate targets.

POLICY RELEVANCE

Many countries with a large shale gas resource base consider the expansion of local shale gas extraction as an option to reduce their GHG emissions. The findings in this article imply that a higher availability of shale gas in these countries might actually increase emissions and mitigation costs for these countries and also for the world. An increase in shale gas extraction may spur a switch from coal to gas electricity generation, thus lowering emissions. At the global level and for many countries, though, this effect is more than offset by a crowding out of renewable and nuclear energy carriers, and by lower energy prices, leading to higher emissions and higher mitigation costs in turn. These findings would warrant a re-evaluation of the climate strategy in most countries relying on the exploitation of shale gas to meet their climate targets.     

Assessment of episodic acidification in Sierra Nevada,California

Monte-Carlo simulations were used to assess the extent of shortterm alkalinity depressions occuring in Sierra Nevada lakes due to acidic deposition events. The Episodic Event Model (EEM) was used to simulate spring snowmelt events. Snow course data, precipitation data and lake acidification surveys were used to derive values for the EEM parameters. Spring snowmelt events were shown to have great impacts on the water quality of Sierran lakes. Lakes are likely to be most affected by the early-spring snowmelt event because the epilimnion depth is at a minimum, which indicates minimum dilution. Under annual average loading conditions, no Sierran lake has been reported as acidic although 29% of the lakes have alkalinities less than 40 µeq/L indicating a sensitivity to acidification. In simulations of early-spring snowmelt events, using present-day acidic loading conditions, it was estimated 79% ± 9% of the lakes would experience shortterm lake alkalinity depressions to levels less than 40 µeq/L. The results provided by the model simulations are valuable in establishing upper and lower limits on the extent of possible episodic acidification to lake-resources-at-risk. The most critical parameters controlling the magnitude of lake alkalinity depressions during snowmelt episodic events are a) the lake area to watershed area ratio — a measure of input loading, and b) the epilimnion volume — a measure of dilution and mixing.     

Tidally incised valleys on tropical carbonate shelves: An example from the northern Great Barrier Reef, Australia

The formation of incised valleys on continental shelves is generally attributed to fluvial erosion under low sea level conditions. However, there are exceptions. A multibeam sonar survey at the northern end of Australia's Great Barrier Reef, adjacent to the southern edge of the Gulf of Papua, mapped a shelf valley system up to 220 m deep that extends for more than 90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the maximum depth of fluvial incision that could have occurred under a − 120 m, eustatic sea level low-stand, as what occurred on this margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour, probably in relation to a sea level at around 30–50 m below its present position.

Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are consistent with a Pleistocene age and relict origin of the valley.

Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tidal erosion on meso- to macro-tidal, rimmed carbonate shelves is enhanced during sea level rise and fall when a tidal, hydraulic pressure gradient is established between the shelf-lagoon and the adjacent ocean basin. Tidal flows attain a maximum, and channel incision is greatest, when a large hydraulic pressure gradient coincides with small channel cross sections. Our tidal-incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the adjacent ocean basins during intermediate (rather than last glacial maximum) low-stand, sea level positions. The model may apply to other rimmed shelves, both modern and ancient.     

Mesoscale and submesoscale mechanisms behind asymmetric cooling and phytoplankton blooms induced by hurricanes: a comparison between an open ocean case and a continental shelf sea case

Right-side bias in both sea surface cooling and phytoplankton blooms is often observed in the wake of hurricanes in the Northern Hemisphere. This idealized hurricane modeling study uses a coupled biological-physical model to understand the underlying mechanisms behind hurricane-induced cooling and phytoplankton bloom asymmetry. Both a deep ocean case and a continental shelf sea case are considered and contrasted. Model analyses show that while right-side asymmetric mixing due to inertial oscillations and restratification from strong right-side recirculation cells contributes to bloom asymmetry in the open ocean, the well-mixed condition in the continental shelf sea inhibits formation of recirculation cells, and the convergence of water onto the shelf is a more important process for bloom asymmetry.     

A Review of “Pesticide Drift and the Pursuit of Environmental Justice”

Measured by growing student numbers and new programs, master's education is one of the most dynamic areas of U.S. higher education. We focus here on the current state of programs that award the master's as their highest degree, so-called master's only departments, a distinct institutional type in the United States, which has received little research attention. Focusing on sixty-four geography programs, considerable differences are apparent in terms of size (both student and faculty numbers), mission, and curriculum; this includes a number of new programs and hybrid degrees that have recently been developed. The findings raise questions about levels of program staffing, program structure, student recruiting, and overall program quality and viability. Given the changing nature of master's education, further attention should be focused on helping universities create, strengthen, and expand such programs in the future.