Residual Plume Research Articles

The upper-mantle density variation beneath the South Atlantic Ocean and its implications for hotspots and large igneous provinces

The South Mid-Atlantic Ridge (SMAR), the most typical slow-spreading ridge on the earth, is a significant area to study the oceanic crust-mantle evolution. We used gravity data and the S-wave velocity model to derive a 3D density model of the upper-mantle beneath the South Atlantic Ocean to better understand the relationship between ridges, hotspots, and large igneous provinces. The density variations in the upper-mantle are related to thermal and compositional changes. The segment near the Agulhas-Falkland Fracture Zone (AFFZ) is characterized by low-density but inconspicuous low-velocity anomalies reflecting the depleted composition. Its composition is different from the northern part of the SMAR. A low-density layer at 100 km that extends 200 km from the ridge to the Discovery hotspot and that disappears near the AFFZ, is also present. The higher degree of mantle melting beneath the segment near the AFFZ is most probably influenced by the residual plume materials from the Discovery hotspot. Deep low densities beneath the Rio Grande Rise (RGR) and the Walvis Ridge (WR) indicate a higher degree of melting and their formation is probably influenced by the Tristan da Cunha plume. When the RGR is driven away from the plume, its original hot materials cool down and present as cold and dense uppermost mantle deposits. As the plume is located in the African plate, the WR was continuously formed as a seamount chain by intraplate volcanism of mantle plume.

Dynamics of sediment-laden plumes in the ocean

We present a series of experiments to illustrate the dynamics of positively or negatively buoyant particle-laden plumes in a cross-flow, with relevance for the discharge of sediment into the ocean during deep-sea mining operations. In an unstratified ambient fluid, our experiments identify three different regimes, corresponding to (i) a dense particle-laden plume, host to relatively dense saline fluid, in which the particles separate from the descending plume as the flow speed falls below the particle settling speed; (ii) a dense particle-laden plume, host to buoyant fluid, in which the fluid gradually rises from the sinking plume of particles, to form a second rising plume of source fluid; and (iii) a buoyant particle-laden plume, host to buoyant fluid, which rises from the discharge pipe, and from which particles gradually sediment. Classical models of single-phase plumes describe the initial motion of the plumes in cases (i) and (iii), but as the flow speed falls below the particle fall speed, sedimentation leads to a change in the averaged buoyancy, and, hence, the plume speed. Our data also suggest that the sedimentation leads to a reduction in the rate of entrainment of ambient fluid, compared with the classical single-phase plumes. We also show that with a density stratified ambient fluid, the stratification may arrest the plume prior to significant particle sedimentation, and in this case, the plume tends to spread downstream at the level of neutral buoyancy where particle sedimentation proceeds. The bulk density of the residual plume fluid may then remain intermediate between the density of the upper and lower layer fluid, or may become less dense than the upper layer fluid, in which case, following sedimentation, the plume fluid rises through the upper layer. While the dynamics of deep-sea mining plumes in the ocean are more complex, for example, owing to background turbulence and mixing, the results of our new laboratory experiments highlight the range of flow processes which may influence the initial dispersion and sedimentation of particles in such plumes following release into the water, depending on the initial conditions, the ambient density and the particle fall speed. We also discuss the relevance of our work in the context of ash dispersal by volcanic plumes.

Crustal structure and lateral variations in Moho beneath the Delhi fold belt, NW India: Insight from gravity data modeling and inversion

The Proterozoic Delhi fold belt (DFB) in NW India shows a prominent NNE-SSW trending gravity high, but its northward extension beneath Gangetic alluvium lack understanding. We model the gravity anomaly using radially averaged power spectrum, depth from extreme points (DEXP) method, wavelength filtering, 3D structural inversion to constrain crustal structure of the DFB. The radially averaged power spectrum analysis suggests three interfaces and the DEXP method indicates presence of a horizontal cylindrical source below the DFB. The 3D structural inversion shows depth variation from 39 to 46 km in the study region and presence of an upwarp below the DFB and Sandmata complex. Our modeling suggests that an underplated mafic material possibly a residual plume head produces gravity high. The transverse structure of DFB is absent beneath Gangetic alluvium and show notable westward deflection in line with the NW-SE trending Delhi-Sargodha ridge. The transverse DFB attains westward deflection possibly due to NW indentation and counterclockwise rotation of Indian plate after Himalayan collision in Eocene. With continued convergence, this DFB structural element at the leading edge of Indian peninsula got involved in flexural upwarp of underthrusting Indian plate beneath the Himalaya.

Optimal Remediation Scheme for a Wastewater Recharge Site: Contaminants Fate and Transport Model.

A 3-dimensional hydrologic groundwater flow and contaminant transport model was developed to address the feasibility and relevant time-scales for remediation of a large soil aquifer treatment (SAT) site for domestic wastewater. The model accounted for a range of organic pollutant retardation factors and biodegradation kinetics. In order to detect the effect of the most retained and the most refracted/recalcitrant pollutants, it was crucial to model fractions of different properties rather than with average set properties. Four aquifer remediation scenarios, including two active washing procedures and two involving natural attenuation, were evaluated. The remediation efficiencies, in terms of residual plume size and mass of pollutant washout, were compared to each other and to the current operation scheme. According to the results, remediation mainly via natural attenuation rather than some form of intense pump and treat technology, is feasible, unlike the situation with hazardous waste sites. Enforced washout becomes less efficient with time due to increasing retardation of the residual pollutants. Therefore, pollution containment with natural biodegradation, which removed 95% of the emerging contaminants within 30 to 40 years was found to be the most valuable remediation scheme.

Changes in domestic heating fuel use in Greece: effects on atmospheric chemistry and radiation

Abstract. For the past 8 years, Greece has been experiencing a major financial crisis which, among other side effects, has led to a shift in the fuel used for residential heating from fossil fuel towards biofuels, primarily wood. This study simulates the fate of the residential wood burning aerosol plume (RWB smog) and the implications on atmospheric chemistry and radiation, with the support of detailed aerosol characterization from measurements during the winter of 2013–2014 in Athens. The applied model system (TNO-MACC_II emissions and COSMO-ART model) and configuration used reproduces the measured frequent nighttime aerosol spikes (hourly PM10 > 75 µg m−3) and their chemical profile (carbonaceous components and ratios). Updated temporal and chemical RWB emission profiles, derived from measurements, were used, while the level of the model performance was tested for different heating demand (HD) conditions, resulting in better agreement with measurements for Tmin < 9 °C. Half of the aerosol mass over the Athens basin is organic in the submicron range, of which 80 % corresponds to RWB (average values during the smog period). Although organic particles are important light scatterers, the direct radiative cooling of the aerosol plume during wintertime is found low (monthly average forcing of –0.4 W m−2 at the surface), followed by a minor feedback to the concentration levels of aerosol species. The low radiative cooling of a period with such intense air pollution conditions is attributed to the timing of the smog plume appearance, both directly (longwave radiation increases during nighttime) and indirectly (the mild effect of the residual plume on solar radiation during the next day, due to removal and dispersion processes).

Simulation of Van Allen Probes plasmapause encounters

AbstractWe use anE×B‐driven plasmapause test particle (PTP) simulation to provide global contextual information for in situ measurements by the Van Allen Probes (Radiation Belt Storm Probes (RBSP)) during 15–20 January 2013. During 120 h of simulation time beginning on 15 January, geomagnetic activity produced three plumes. The third and largest simulated plume formed during enhanced convection on 17 January, and survived as a rotating, wrapped, residual plume for tens of hours. To validate the simulation, we compare its output with RBSP data. Virtual RBSP satellites recorded 28 virtual plasmapause encounters during 15–19 January. For 26 of 28 (92%) virtual crossings, there were corresponding actual RBSP encounters with plasmapause density gradients. The mean difference in encounter time between model and data is 36 min. The mean model‐data difference in radial location is 0.40 ± 0.05 RE. The model‐data agreement is better for strong convection than for quiet or weakly disturbed conditions. On 18 January, both RBSP spacecraft crossed a tenuous, detached plasma feature at approximately the same time and nightside location as a wrapped residual plume, predicted by the model to have formed 32 h earlier on 17 January. The agreement between simulation and data indicates that the model‐provided global information is adequate to correctly interpret the RBSP density observations.

In situ signatures of residual plasmaspheric plumes: Observations and simulation

AbstractWe compare in situ observations of the Los Alamos National Laboratory (LANL) Magnetospheric Plasma Analyzers with output of a dynamic, plasmapause test particle (PTP) simulation for the moderately disturbed interval 18–20 January 2000. In the model, weakly enhanced convection on 18 January creates a narrow drainage plume (plume A) that wraps completely around the main torus. Moderate convection on 19 January triggers significant plasmaspheric erosion, forming a second plume (B) that coexists with the narrow, wrapped, residual plume A. We fly three virtual LANL satellites through the simulation domain. The observations are globally consistent with the PTP simulation; LANL data contain several intervals of plume plasma in the model's predicted magnetic local time (MLT) sector. The modeled durations of plume sector transits are in good agreement with the LANL data. On a subglobal scale, the MLT widths and timings of the simulated plumes do not precisely agree with observations. However, several observation intervals exhibit good morphological agreement with virtual spacecraft signatures of two distinct, coexisting plumes (A and B). The fine‐scale structure in the PTP model arises from the merging of residual plume A with the newer plume B. Plume merging is one theoretical means of generating fine structure in the plasmasphere: during multiple cycles of erosion and recovery, successive layers of wrapped, residual plumes can merge with newer plumes, creating layers of filamentary density structure. The model‐data comparisons suggest that the plasmaspheric density distribution may preserve some memory of prior epochs of erosion and recovery.

Coupling between Marine Plankton and Freshwater Flow in the Plumes off a Small Estuary

AbstractFreshwater discharge from rivers is a powerful forcing agent in coastal ecosystems. It not only generates strong ecological effects in estuaries, but also drives the dynamics of nearshore marine waters where prominent river plumes form biogeochemical hot spots in coastal seas worldwide. Large plumes from major rivers exert important controls on pelagic processes. The majority of estuaries are smaller, however, and the importance of the smaller plumes they generate is unknown. We measured the degree of coupling between freshwater flow and inshore zooplankton in such a plume from a subtropical estuary on the east coast of Australia. Flow regimes encompassed long periods of low freshwater input, punctuated by pulsed freshets that initiated the formation of buoyant, lower‐salinity plumes in the nearshore marine zone. Plumes stimulated phytoplankton biomass in the receiving waters, and ultimately changes in zooplankton assemblages. Zooplankton responded strongly to river discharge: (1) in the absence of substantial freshwater flows and plumes, zooplankton was broadly similar in density and biomass across the estuarine‐marine gradient; (2) freshets that generated significant plumes strongly modified hydrological conditions and lowered zooplankton in the estuarine and nearshore waters, and (3) after the initial freshet, zooplankton in the residual plume was at a higher density in nearshore than shelf waters. We demonstrate that coupling between riverine and coastal pelagic systems operates in small plumes, but that there is substantial temporal variance linked to fluctuations in freshwater delivery. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

S-wave velocity structure beneath the High Lava Plains, Oregon, from Rayleigh-wave dispersion inversion

The High Lava Plains (HLP) “hotspot” track is a prominent volcanic lineament that extends from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. With the age of silicic volcanism decreasing along track to the northwest, the HLP and Newberry volcano are a rough mirror image to the Eastern Snake River Plain and Yellowstone but, in the case of the HLP, at an orientation strongly oblique to North American plate motion. Since this orientation is incompatible with plate motion over a fixed hotspot, other proposed origins for the HLP, such as asthenospheric inflow around a steepening slab, residual effects of a Columbia River/Steens plume, backarc spreading, and Basin and Range extension, relate it to various tectonic features of the Pacific Northwest. To begin distinguishing between these hypotheses, we image upper-mantle structure beneath the HLP and adjacent tectonic provinces with fundamental-mode Rayleigh waves recorded by stations of the USArray Transportable Array, the recently-initiated HLP seismic experiment, the United States National Seismograph Network, and the Berkeley seismic network. We estimate phase velocities along nearly 300 two-station propagation paths that lie within and adjacent to the HLP and cross the region along two azimuths, parallel to and perpendicular to the HLP track. The dispersion curves, which typically give robust results over the period range 16–171 seconds, are grouped by tectonic region, and the composite curves are inverted for S-wave velocity as a function of depth. We also fit a single plane wave to phase delays across the region to identify laterally homogeneous subregions. The resulting variations in upper-mantle structure correlate with variations in surface volcanism and tectonics. The lowest velocities (∼ 4.1 km/s) occur at ∼ 50 km depth in the SE corner of Oregon, where there has been extensive basaltic volcanism in the past 2–5 kyr, and suggest uppermost mantle temperatures sufficient to produce basaltic partial melting. While the seismic velocities of the uppermost mantle beneath the volcanic High Lava Plains are low relative to the standard Tectonic North America (TNA) model, they are only slightly lower than those found for the adjacent northern Great Basin and they appear to be significantly higher than upper-mantle velocities beneath the Eastern Snake River Plain. Our results provide no evidence for a residual plume signature beneath the HLP region, leaving open questions as to the origin of the HLP volcanic track itself.

Multiple mantle plume components involved in the petrogenesis of subduction‐related lavas from the northern termination of the Tonga Arc and northern Lau Basin: Evidence from the geochemistry of arc and backarc submarine volcanics

We report new geochemical data for boninites and backarc basin‐type basalts recovered from the northern termination of the Tonga trench and Lau Basin. Boninitic pillow lavas, ranging from high‐Mg compositions to andesites and dacites, have been erupted within large submarine volcanic edifices (calderas and volcanoes) associated with active rifting of both the northern end of the Tofua volcanic arc and in a backarc position relative to the arc volcanoes on the northern Tonga Ridge. The mantle sources in the area are a complex mixture of (1) the “normal” Tongan mantle wedge source that has “Pacific”‐type isotopic signature with (2) the plume‐related components (EMI, EMII, and HIMU) and (3) an “Indian”‐type source upwelling beneath the backarc spreading. Some of these sources, such as the “normal” mantle wedge and variably depleted residual plume mantle, are fluxed by subduction components from the slab, which produces boninites, tholeiites, and mixtures thereof. Other mantle sources, such as “Indian”‐type backarc mantle and also some of the plume mantle, produce melts due to adiabatic decompression. These melts are variably mixed with each other and with the slab‐fluid fluxed subduction‐related melts to form the observed spectrum of magma compositions.

Two-stage plume melting: A possible mechanism for the origin of martian magmatism

The origin of martian magmatism is investigated by geochemical modeling based on the isotopic and rare earth element (REE) compositions of the shergottite, nakhlite and chassignite (SNC) meteorites. The model proposed in this study consists of three processes; early mantle differentiation in the deepest mantle to produce a plume source region composed of hydrated ringwoodite and majorite, first-stage dehydration melting of the plume to produce nakhlite and chassignite magmas in the deep mantle, and finally second-stage decompression melting of the residual plume to produce shergottite magmas in the uppermost mantle. The production of light-REE-enriched nakhlite and chassignite magmas can be explained by the persistence of residual garnet through first-stage melting, and the depleted isotopic composition of the nakhlite and chassignite meteorites can be reasonably attributed to the presence of majorite in the plume source region. The strongly depleted REE and isotopic composition of the primitive shergottites can similarly be explained by the existence of majorite in the plume source region coupled with extraction of the nakhlite melt from the depleted source. The proposed two-stage plume melting model therefore reproduces the chemical and isotopic variations of the martian magmas well, suggesting a genetic relationship among the SNC magmas.

Thermal venting to recover less-volatile hydrocarbons from the unsaturated zone, 2. Model applications

The first part (Kaluarachchi and Islam, this issue) of this two-part series of papers developed the theoretical framework of analysis describing thermal venting using nonisothermal gas flow, heat energy transport and multicomponent mass transport. The work presented in this paper demonstrated the applicability of thermal venting through a series of numerical simulations in one- and two-dimensional flow domains. In both cases, a four-component hydrocarbon mixture consisting of volatile benzene, moderately-volatile n-dodecane, and less-volatile naphthalene and n-hexylbenzene in equal mass fraction was used. The results from the one-dimensional laboratory-scale column simulations showed a total recovery of naphthalene, n-dodecane and n-hexylbenzene by thermal venting, where as corresponding removal by normal venting was < 33% for the same time period. Effect of thermal energy was negligible with volatile benzene due to high ambient vapor pressure. Similar results were obtained from the two-dimensional field-scale simulations too. The temperature distribution in the subsurface was affected by simultaneous evaporation of contaminants and moisture condensation from humid air. The simulations also considered the parameter sensitivity to overall recovery. The overall results of this theoretical analysis suggested that thermal venting may be a powerful remediation technique that is applicable to the unsaturated zone when normal soil venting fails to recover the less-volatile fraction of the residual plume. However, the results of this analysis need to be experimentally validated for complete evaluation of the overall technology.

Dredged Material Disposal at the Edge of the Florida Current

Abstract A field data collection project was undertaken to investigate the short-term fate of dredged material discharged in the designated Miami Ocean Dredged Material Disposal Site (ODMDS) before dredging of the Miami River and the Miami Harbor Turning Basin begins. the designated ODMDS is located in relatively deep water for discharge sites with a typical bottom depth of 150 metres and is also located in the western boundary region of the Gulf Stream current off Miami. Acoustical backscattering, current, particulate, temperature and salinity data were gathered over a three day period from April 24, 1990 through April 26, 1990. the major generic features of shallow-water discharge plumes were observed to be present: (a) the presence of a rapid convective descending plume portion; (b) impact of that plume portion with the ocean bottom and concomitant generation of a bottom surge; (c) rapid horizontal width growth of the descending plume through entrainment; and (d) retention of a residual plume portion within the water column. A well-mixed upper water column layer extending to a depth of 40 to 60 metres below the surface of the ocean permitted measurements of the plume entrainment coefficient free from bottom boundary, water column density gradient, and vertical current shear effects which are usually present in relatively shallow, e.g. less than 40 metres bottom depth, coastal ocean discharge studies. Entrainment coefficient estimates obtained in this study were between 0.5 to 0.7. the residual water plume material was tracked over one-half hour during each of eight discharge events and was transported in a north-northeast direction.

A two-layer model of the atmosphere indicating the effects of mixing between the surface layer and the air aloft

A two-layer air pollution model is developed for the case when inter-layer mixing is continuous, but slower than intra-layer vertical mixing. The model was designed to examine the implications of inter-layer transport for ground-level impacts. The model equations, consisting of two coupled partial differential equations, are solved numerically by the method of characteristics. In the case of first order chemical reactions, the zero and first order moments are computed in order to determine the locations of the centroids of pollutant mass. The model indicated that a pulse of pollutants emitted at ground level can result in bifurcation of a pollutant plume due to the wind directional shear between the two atmospheric layers. The residual concentrations due to the transport between layers can move upwind, downwind or remain stationary with respect to the ground level winds, depending on the speed and orientation of the winds aloft. Several specific cases are described, and, in each case, the behavior of the residual plume would not be expected from the surface layer winds that are normally used to predict the transport and dispersion of pollutants. In addition, it appears that the characteristic exchange rate between the air aloft and the surface mixing layer is large enough that the winds aloft must be considered when describing long range transport or the multi-day impacts of a pollutant.