Comment on Fofonoff's mode

Abstract

Recently Merkine, Mo and Kalnay (1985) have re-examined the possible existence of Fofonoff's (1954) steady free inertial solution and the role of Fofonoff's mode in numerical circulation models after Veronis (1966). Merkine et al. conclude that the Fofonoff mode does not occur, that such a mode would be barotropically unstable and that resemblances between numerical circulations and Fofonoff's mode are more dependent upon the natures of forcing and dissipation. On the contrary, I suggest that Fofonoff's mode very naturally does emerge and that forcing and dissipation only impede the full realization of Fofonoff's mode. Moreover, statistical mechanical arguments from Salmon, Holloway and Hendershott (1976) show that the Fofonoff mode is expected to co-exist with a transient eddy field whose statistics are in equilibrium with the mode; thus bartotropic instability does not argue for non-realization of Fofonoff's mode.     

Mechanisms for incompatible-element enrichment on the Moon deduced from the lunar basaltic meteorite Northwest Africa 032

The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb-Sr and Sm-Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb-Sr and Sm-Nd ages. Rubidium-Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial ⁸⁷Sr/⁸⁶Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an ⁸⁷Rb/⁸⁶Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium-neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial ε_Nd of +9.71 ± 0.74 corresponding to a source region with ¹⁴⁷Sm/¹⁴⁴Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts.Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.     

HARP/ACSIS: a submillimetre spectral imaging system on the James Clerk Maxwell Telescope

This paper describes a new Heterodyne Array Receiver Program (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope. The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are ∼120 K across the whole band, and system temperatures of ∼300 K are reached routinely under good weather conditions. The system includes a single-sideband (SSB) filter so these are SSB values. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully sampled maps of     size can be observed in under 1 h.
The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies and produce the essential preparatory work for submillimetre interferometers such as the Submillimeter Array (SMA) and Atacama Large Millimeter/Submillimeter Array (ALMA).     

Inner Solar System dynamical analogs of plutinos

By studying orbits of asteroids potentially in 3:2 exterior mean motion resonance with Earth, Venus, and Mars, we have found plutino analogs. We identify at least 27 objects in the inner Solar System dynamically protected from encounter through this resonance. These are four objects associated with Venus, six with Earth, and seventeen with Mars. Bodies in the 3:2 exterior resonance (including those in the plutino resonance associated with Neptune) orbit the Sun twice for every three orbits of the associated planet, in such a way that with sufficiently low libration amplitude close approaches to the planet are impossible. As many as 15% of Kuiper Belt objects share the 3:2 resonance, but are poorly observed. One of several resonance sweeping mechanisms during planetary migration is likely needed to explain the origin and properties of 3:2 resonant Kuiper Belt objects. Such a mechanism likely did not operate in the inner Solar System. We suggest that scattering by the next planet out allows entry to, and exit from, 3:2 resonance for objects associated with Venus or Earth. 3:2 resonators of Mars, on the other hand, do not cross the paths of other planets, and have a long lifetime. There may exist some objects trapped in the 3:2 Mars resonance which are primordial, with our tests on the most promising objects known to date indicating lifetimes of at least tens of millions of years. Identifying 3:2 resonant systems in the inner Solar System permits this resonance to be studied on shorter timescales and with better determined orbits than has been possible to date, and introduces new mechanisms for entry into the resonant configuration.     

Large differential land subsidence and earth fissures in Jiangyin,China

Jiangyin County is in the infamous Su–Xi–Chang land subsidence area caused by excessive groundwater withdrawal in Jiangsu province, China. The maximum accumulated land subsidence reached 1,310 mm near the centre of the subsiding trough in 2006 in southern Jiangyin, and earth fissures of significant vertical offsets have been observed at Changjing, Hetang and Wenlin which form an arc towards the subsidence trough. An ancient Yangtze River course is found underlying and passing through the depression in southern Jiangyin, forming a local basin surrounded by outcropped bedrock ridges in the north and south. The Quaternary stratigraphy demonstrates significant heterogeneities in the basin; the second confined aquifer is much thicker and deeper and encapsulated inside the basin and absent above the ridges. The development of earth fissures along the Changjing–Hetang–Wenlin arc might be a combination of an inward rotation of sediments due to a large differential subsidence, an inward movement driven by seepage force and a steeper slope along the south-eastern shoulder of the basin that facilitates the development of horizontal tensile strain and/or shear strain necessary for fissuring. The land subsidence has slowed down and no new earth fissure zone has occurred in the area after the banning of deep groundwater extraction was enacted in 2001.     

Geochemical imaging of flow near an artificial recharge facility, Orange County, California

Critical for the management of artificial recharge operations is detailed knowledge of ground water dynamics near spreading areas. Geochemical tracer techniques including stable isotopes of water, tritium/helium-3 (T/3He) dating, and deliberate gas tracer experiments are ideally suited for these investigations. These tracers were used to evaluate flow near an artificial recharge site in northern Orange County, California, where approximately 2.5 x 10(8) m3 (200,000 acre-feet) of water are recharged annually. T/3He ages show that most of the relatively shallow ground water within 3 km of the recharge facilities have apparent ages < 2 years; further downgradient apparent ages increase, reaching > 20 years at approximately 6 km. Gas tracer experiments using sulfur hexafluoride and xenon isotopes were conducted from the Santa Ana River and two spreading basins. These tracers were followed in the ground water for more than two years, allowing subsurface flow patterns and flow times to be quantified. Results demonstrate that mean horizontal ground water velocities range from < 1 to > 4 km/year. The leading edges of the tracer patch moved at velocities about twice as fast as the center of mass. Leading edge velocities are important when considering the potential transport of microbes and other "time sensitive" contaminants and cannot be determined easily with other methods. T/3He apparent ages and tracer travel times agreed within the analytical uncertainty at 16 of 19 narrow screened monitoring wells. By combining these techniques, ground water flow was imaged with time scales on the order of weeks to decades.     

Phase relations of carbonate-bearing eclogite assemblages from 2.5 to 5.5 GPa: implications for petrogenesis of carbonatites

We have experimentally investigated the phase and melting relations of garnet + clinopyroxene + carbonate assemblages at 2.5–5.5 GPa, to assess the feasibility of carbonated eclogite as a source for some crustally emplaced carbonatites. The solidus of our composition was at 1,125 °C at 2.5 GPa, 1,225 °C at 3.5 GPa and 1,310 °C at 5.0 GPa. Melts were sodic calcio-dolomitic carbonatites, and were markedly more calcic than the dolomitic melts produced by partial melting of carbonated peridotite. Na contents of the experimental carbonatites decreased with increasing pressure when compared at similar degrees of melting, and SiO₂ contents increased with degree of melting. Experiments on a second composition with enhanced Na₂O demonstrated its strong effect in lowering melting temperatures in carbonate eclogite. Natural carbonated eclogite bodies in the peridotitic upper mantle will have a range of solidus temperatures. In many cases, carbonate will be molten in the upper 250 km. Carbonate melt would segregate from its source eclogite at very low melt fractions and infiltrate surrounding peridotitic wall rock. This would result in metasomatic enrichment of the peridotitic wall rock, but its exact nature will depend on the relative P–T positions of the eclogite + CO₂ and peridotite + CO₂ solidii. As a result of these inevitable metasomatic interactions, it is considered unlikely that carbonatite melts derived from carbonated eclogite in the upper mantle could be emplaced into the crust unmodified. However, they may have a role in metasomatically enriching and carbonating parts of the upper mantle, producing sources suitable for subsequent production of silica undersaturated silicate liquids and carbonatites ultimately emplaced in the crust.Editorial responsibility: J. Hoefs     

Removal of ammonia toxicity in marine sediment TIEs: a comparison of Ulva lactuca,zeolite and aeration methods

Toxicity Identification Evaluations (TIEs) can be used to determine the specific toxicant(s), including ammonia, causing toxicity observed in marine sediments. Two primary TIE manipulations are available for characterizing and identifying ammonia in marine sediments: Ulva lactuca addition and zeolite addition. In this study, we compared the efficacy of these methods to (1) remove NH(x) and NH(3) from overlying and interstitial waters and (2) reduce toxicity to the amphipod Ampelisca abdita and mysid Americamysis bahia using both spiked and environmentally contaminated sediments. The utility of aeration for removing NH(x) and NH(3) during a marine sediment TIE was also evaluated preliminarily. In general, the U. lactuca and zeolite addition methods performed similarly well at removing spiked NH(x) and NH(3) from overlying and interstitial waters compared to an unmanipulated sediment. Toxicity to the amphipod was reduced approximately the same by both methods. However, toxicity to the mysid was most effectively reduced by the U. lactuca addition indicating this method functions best with epibenthic species exposed to ammonia in the water column. Aeration removed NH(x) and NH(3) from seawater when the pH was adjusted to 10; however, very little ammonia was removed at ambient pHs ( approximately 8.0). This comparison demonstrates both U. lactuca and zeolite addition methods are effective TIE tools for reducing the concentrations and toxicity of ammonia in whole sediment toxicity tests.     

Groundwater-dependent ecology of the shoreline of the subtropical Lake St Lucia estuary

The ecology of the St Lucia estuary in South Africa is of unique international importance. During droughts the estuary experiences high salinities, with values above that of seawater. Ion-poor groundwater flowing into the estuary from prominent sand aquifers along its eastern shoreline forms low-salinity habitats for salt-sensitive biota. During droughts, plants and animals can take refuge in the groundwater discharge zone until the condition in the estuary regains tolerable salinity. Simulations of the groundwater discharge indicate that the flow can persist during droughts over at least a decade, and be of great important for the resilience of the estuary. Anthropogenic activities have reduced the river inflow and made the St Lucia estuary more sensitive to droughts. The groundwater has thereby become increasingly important for the estuary’s ecology. Protection of the groundwater discharge along the shoreline itself and actions to increase the groundwater recharge are therefore important management tasks.     

An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO2 fluxes and bottom water hypoxia

We investigated seasonal variability in organic carbon (OC) budgets using a physical-biological model for the Mississippi River turbidity plume. Plume volume was calculated from mixed layer depth and area in each of four salinity subregions based on an extensive set of cruise data and satellite-derived suspended sediment distributions. These physical measurements were coupled with an existing food web model to determine seasonally dependent budgets for labile (reactive on time scales of days to weeks) OC in each salinity subregion. Autochthonous gross primary production (GPP) equaled 1.3×10¹² g C yr⁻¹ and dominated labile OC inputs (88% of the budget) because riverine OC was assumed mostly refractory (nonreactive). For perspective, riverine OC inputs amounted to 3.9×10¹² g C yr⁻¹, such that physical inputs were 3 times greater than biological inputs to the plume. Annually, microbial respiration (R) accounted for 65% of labile OC losses and net metabolism (GPP—R) for the entire plume was, autotrophic, equaling 5.1×10¹¹ g C yr⁻¹. Smaller losses of labile OC occurred via sedimentation (20%), advection (10%), and export to higher trophic levels (5%). In our present model, annual losses of labile OC are 10% higher than inputs, indicating future improvements are required. Application of our model to estimate air-sea carbon dioxide (CO₂) fluxes indicated the plume was a net sink of 2.0×10⁹ mol CO₂ yr⁻¹, of which 90% of the total drawdown was from biotic factors. In all seasons, low salinity waters were a source of CO₂ (pCO₂=560–890 μatm), and intermediate to high salinity waters were a sink of CO₂ (pCO₂=200–370 μatm). Our model was also used to calculate O₂ demand for the development, of regional hypoxia, and our spring and early summer budgets indicated that sedimentation of autochthonous OC from the immediate plume contributed 23% of the O₂ demand necessary for establishment of hypoxia in the region.