Comparison of methods to model the gravitational gradients from topographic data bases

A number of methods have been developed over the last few decades to model the gravitational gradients using digital elevation data. All methods are based on second-order derivatives of the Newtonian mass integral for the gravitational potential. Foremost are algorithms that divide the topographic masses into prisms or more general polyhedra and sum the corresponding gradient contributions. Other methods are designed for computational speed and make use of the fast Fourier transform (FFT), require a regular rectangular grid of data, and yield gradients on the entire grid, but only at constant altitude. We add to these the ordinary numerical integration (in horizontal coordinates) of the gradient integrals. In total we compare two prism, two FFT and two ordinary numerical integration methods using 1" elevation data in two topographic regimes (rough and moderate terrain). Prism methods depend on the type of finite elements that are generated with the elevation data; in particular, alternative triangulations can yield significant differences in the gradients (up to tens of Eötvös). The FFT methods depend on a series development of the topographic heights, requiring terms up to 14th order in rough terrain; and, one popular method has significant bias errors (e.g. 13 Eötvös in the vertical–vertical gradient) embedded in its practical realization. The straightforward numerical integrations, whether on a rectangular or triangulated grid, yield sub-Eötvös differences in the gradients when compared to the other methods (except near the edges of the integration area) and they are as efficient computationally as the finite element methods.     

Subsidence in the super-deep Pattani and Malay basins of Southeast Asia: a coupled model incorporating lower-crustal flow in response to post-rift sediment loading

Two Early Cenozoic rifts in Southeast Asia (beneath the Pattani and Malay basins) experienced only limited upper-crustal extension (β≤1.5); yet very thick post-rift sequences are present, with 6–12 km of Late Cenozoic terrestrial and shallow-marine sediment derived from adjacent sources. Conventional post-rift backstripping requires depth-dependent lithospheric thinning by β=2–4 to explain these tremendous thicknesses. We assess an alternative explanation for this post-rift subsidence, involving lower-crustal flow from beneath these basins in response to lateral pressure-gradients induced by the sediment loads and the negative loads arising from the erosion of their sediment sources. We calculate that increased rates of erosion in western Thailand in the Early Miocene placed the crust in a non-steady thermal state, such that the depth (and thus, the pressure) at the base of the brittle upper crust subsequently varied over time. Following such a perturbation, thermal and mass-flux steady-state conditions took millions of years to re-establish. In the meantime, the lateral pressure-gradient caused net outflow of lower crust, thinning the crust beneath the depocentre by several kilometres (mimicking the isostatic effect of greater crustal extension having occurred beforehand) and thickening it beneath the sediment source region. The local combination of hot crust and high rates of surface processes, causing lower-crustal flow to be particularly vigorous and thus making its effects more readily identifiable, means that the Pattani and Malay basins represent a set of conditions different from basins in many other regions. However, lower-crustal flow induced by surface processes will also occur to some extent, but less recognisably, in many other continental crustal provinces, but its effects may be mistaken for those of other processes, such as larger-magnitude stretching and/or depth-dependent stretching.     

Carbon budget for a subtropical seagrass dominated coastal lagoon: How important are seagrasses to total ecosystem net primary production?

It has been assumed that because seagrasses dominate macrophyte biomass in many estuaries they also dominate primary production. We tested this assumption by developing three carbon budgets to examine the contribution of autotrophic components to the total ecosystem net primary production (TENPP) of Lower Laguna Madre, Texas. The first budget coupled average photosynthetic parameters with average daily irradiance to calculate daily production. The second budget used average photosynthetic parameters and hourly in situ irradiance to estimate productivity. The third budget integrated temperature-adjusted photosynthetic parameters (using Q₁₀=2) and hourly in situ irradiance to estimate productivity. For each budget TENPP was calculated by integrating production from each autotroph based on the producers’ areal distribution within the entire Lower Laguna Madre. All budgets indicated that macroalgae account for 33–42% of TENPP and seagrasses consistently accounted for about 33–38%. The contribution by phytoplankton was consistently about 15–20%, and the contribution from the benthic microalgae varied between 8% and 36% of TENPP, although this may have been underestimated due to our exclusion of the within bed microphytobenthos component. The water column over the seagrass beds was net heterotrophic and consequently was a carbon sink consuming between 5% and 22% of TENPP, TENPP ranged between 5.41×10¹⁰ and 2.53×10¹¹ g C yr⁻¹, depending on which budget was used. The simplest, most idealized budget predicted the highest TENPP, while the more realistic budgets predicted lower values. Annual production rates estimated using the third budget forHalodule urightii andThalassia testudinum compare well with field data. Macroalgae and microalgae contribute 50–60% of TENPP, and seagrass may be more important as three-dimensional habitat (i.e., structure) than as a source of organic carbon to the water column in Lower Laguna Madre.     

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.     

Changes in the mode of Southern Ocean circulation over the last glacial cycle revealed by foraminiferal stable isotopic variability

Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in δ¹³C, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the δ¹³C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the ‘Southern Ocean low δ¹³C problem’. One explanation for this glacial distribution of δ¹³C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age δ¹³C values. We show here, however, that glacial-interglacial δ¹³C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of δ¹³C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of δ¹³C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean δ¹³C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well.     

Optimized Seismic Threshold Monitoring – Part 2: Teleseismic Processing

v--vThis second paper (Part 2) pertaining to optimized site-specific threshold monitoring addresses the application of the method to regions covered by a teleseismic or a combined regional-teleseismic network. In the first paper (Part 1) we developed the method for the general case, and demonstrated its application to an area well-covered by a regional network (the Novaya Zemlya nuclear test site). In the present paper, we apply the method to the Indian and Pakistani nuclear test sites, and show results during the periods of nuclear testing by these two countries in May 1998. Since the coverage by regional stations in these areas is poor, an optimized approach requires the use of selected, high-quality stations at teleseismic distances.¶To optimize the threshold monitoring of these test sites, we use as calibration events either one of the nuclear explosions or a nearby earthquake. From analysis of the calibration events we derive values for array beamforming steering delays, filter bands, short-term averages (STA) lengths, phase travel times (P waves), and amplitude-magnitude relationships for each station. By applying these parameters, we obtain a monitoring capability of both test sites ranging from m_b 2.8-3.0 using teleseismic stations only. When including the nearby Nilore station to monitor the Indian tests, we show that the threshold can be reduced by about 0.4 magnitude units. In particular, we demonstrate that the Indian tests on 13 May, 1998, which were not detected by any known seismic station, must have corresponded to a magnitude (m_b) of less than 2.4.¶We also discuss the effect of a nearby aftershock sequence on the monitoring capability for the Pakistani test sites. Such an aftershock sequence occurred in fact on the day of the last Pakistani test (30 May, 1998), following a large (m_b 5.5) earthquake in Afghanistan located about 1100 km from the test site. We show that the threshold monitoring technique has sufficient resolution to suppress the signals from these interfering aftershocks without significantly affecting the true peak of the nuclear explosion on the threshold trace.     

Beyond world cities: Central America in a global space of flows

Central America is a region without a world city. Traditionally, the region's national projects have been based upon openness to the world economy: how do the region's contemporary transnational projects connect to the world economy under the new conditions of openness that is contemporary globalization? Focusing upon advanced producer services, three connections into the world city network are identified: global service firms operating directly in Central American cities, local service firms operating indirectly through extra-regional world cities and global service firms operating through Latin American regional offices. Miami is confirmed as the primary world city through which Central America connects into the world economy     

Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo

The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.