Mantle plume,large igneous province and continental breakup——Additionally discussing the Cenozoic and Mesozoic mantle plume problems in East China

Based on the former workers study results such as numerical simulation of fluid mechanics,seismic tomography of the whole earth and igneous rocks,the basic characteristics of mantle plumes are summarized in detail,namely the mantle plume,from the D″layer near the core-mantle bouundary(CMB)of 2900 km deep,is characterized by the spape of large head and thin narrow conduit,by the physical property of high temperature and low viscosity.The LIP(large igneous province)is the best exhibition when the mantle plume ascends to the surface.According to the basic characteristics of the mantle plumes and the LIP,as well as the temporal-spatial relationships between the mantle plume and continental breakup,the detailed research on petrology,geochemistry,temporal-spatial distribution,tectonic background of the Cenozoic-Mesozoic igneous rocks and gravity anomaly distribution in East China has been done.As a result,the Mesozoic igneous rocks in Southeast China should not be regarded as an example of typical LIP related to mantle plumes.for their related characteristics are not consitent with those of the typical LIPs related to mantle plumes.The Cenozoic igneous rocks in Northeast China have no the typical characteristics of mantle plumes and hotspots,so the Cenozoic volcanism in Northeast China might have no the direct relationships with the activity of mantle plumes.     

Monitoring of the plume from the basaltic phreatomagmatic 2004 Grímsv?tn eruption—application of weather radar and comparison with plume models

The Grímsv?tn eruption in November 2004 belongs to a class of small- to medium-sized phreatomagmatic eruptions which are common in Iceland. The eruption lasted 6?days, but the main phase, producing most of the 0.02?km³ of magma erupted, was visible for 33?h on the C-band weather radar of the Icelandic Meteorological Office located in Keflavík, 260?km to the west of the volcano. The plume rose to 8–12?km high over sea level during 33?h. The long distance between radar and source severely reduces the accuracy of the plume height determinations, causing 3.5-km steps in recorded heights. Moreover, an apparent height overestimate of ~1.5?km in the uncorrected radar records occurs, possibly caused by wave ducting or super-refraction in the atmosphere. The stepping and the height overestimate can be partly overcome by averaging the plume heights and by applying a height adjustment based on direct aircraft measurements. Adjusted weather radar data on plume height are used to estimate the total mass erupted using empirical plume models mostly based on magmatic eruptions and to compare it with detailed in situ measurements of the mass of erupted tephra. The errors arising because of the large radar plume distance limit the applicability of the data for detailed comparisons. However, the results indicate that the models overestimate the mass erupted by a factor of three to four. This supports theoretical models indicating that high steam content of phreatomagmatic (wet) plumes enhances their height compared to dry plumes.     

Geochemical anomalies of hydrothermal plume at EPR 13°N

During the DY105-12, 14cruise (R/V DAYANG YIHAO, November 2003) on East Pacific Rise (EPR) 12- 13°N, the submarine hydrothermal activity was investigated and the CTD hydrocast was carried out at EPR12°39′N-12°54′N. From the temperature anomalies and the concentrations of magnesium, chlorine, bromine in seawater samples, we discover that magnesium depletes 9.3%-22.4%, chlorine and bro- mine enrich 10.3%-28.7% and 10.7%-29.0% respectively relative to normal seawater at the stations which have chemistry anomalies, moreover temperature and chemistry anomalies are at the same layer. The depletion of magnesium in the plume may be caused by a fluid lacking of magnesium which rises after the hydrothermal fluid reaches the equilibrium with ambient seawater, the enrichment of chlorine and bromine might be the result of inputting later brine which is generated by phase separation due to hydrothermal activity. In addition, the Br/Cl ratio in the abnormal layers at the survey area is identical to that in seawater, which implies that halite dissolution (or precipitation) occurs neither when the fluid is vented nor when hydrothermal fluid entraining ambient seawater rises to form plume. From the ab- normal instance at E55 station, it is very possible that there might exist a new hydrothermal vent site.     

Space–time variability of the Plata plume inferred from ocean color

Satellite ocean color and surface salinity data are used to characterize the space–time variability of the Río de la Plata plume. River outflow and satellite wind data are also used to assess their combined effect on the plume spreading over the Southwestern South Atlantic continental shelf. Over the continental shelf satellite-derived surface chlorophyll-a (CSAT) estimated by the OC4v4 SeaWiFS retrieval algorithm is a good indicator of surface salinity. The log (CSAT) distribution over the shelf presents three distinct modes, each associated to: Subantarctic Shelf Water, Subtropical Shelf Water and Plata Plume water. The log (CSAT) 0.4–0.8 range is associated with a sharp surface salinity transition across the offshore edge of the Plata plume from 28.5 to 32.5. Waters of surface salinity <31, derived from mixtures of Plata waters with continental shelf waters, are associated to log (CSAT)>0.5. In austral winter CSAT maxima extend northeastward from the Plata estuary beyond 30°S. In summer the high CSAT waters along the southern Brazil shelf retreat to 32°S and extend south of the estuary to about 37.5°S, only exceeding this latitude during extraordinary events. The seasonal CSAT variations northeast of the estuary are primarily controlled by reversals of the along-shore wind stress and surface currents. Along-shore wind stress and CSAT variations in the inner and mid-shelves are in phase north of the estuary and 180° out of phase south of the estuary. At interannual time scales northernmost Plata plume penetrations in winter (∼1200 km from the estuary) are associated with more intense and persistent northeastward wind stress, which in the period 2000–2003, prevailed over the shelf south of 26°S. In contrast, in winter 1999, 2004 and 2005, characterized by weaker northeastward wind stress, the plume only reached between 650 and 900 km. Intense southwestward plume extensions beyond 38°S are dominated by interannual time scales and appear to be related to the magnitude of the river outflow. The plume response to large river outflow fluctuations observed at interannual time scales is moderate, except offshore from the estuary mouth, where outflow variations lead CSAT variations by about 2 months.     

Subduction tectonics vs.plume tectonics——Discussion on driving forces for plate motion

Plate tectonics describes the horizontal motions of lithospheric plates,the Earths outer shell,and interactions among them across the Earths surface.Since the establishment of the theory of plate tectonics about half a century ago,considerable debates have remained regarding the driving forces for plate motion.The early"Bottom up"view,i.e.,the convecting mantledriven mechanism,states that mantle plumes originating from the core-mantle boundary act at the base of plates,accelerating continental breakup and driving plate motion.Toward the present,however,the"Top down"idea is more widely accepted,according to which the negative buoyancy of oceanic plates is the dominant driving force for plate motion,and the subducting slabs control surface tectonics and mantle convection.In this regard,plate tectonics is also known as subduction tectonics."Top down"tectonics has received wide supports from numerous geological and geophysical observations.On the other hand,recent studies indicate that the acceleration/deceleration of individual plates over the million-year timescale may reflect the effects of mantle plumes.It is also suggested that surface uplift and subsidence within stable cratonic areas are correlated with plumerelated magmatic activities over the hundred-million-year timescale.On the global scale,the cyclical supercontinent assembly and breakup seem to be coupled with superplume activities during the past two billion years.These correlations over various spatial and temporal scales indicate the close relationship and intensive interactions between plate tectonics and plume tectonics throughout the history of the Earth and the considerable influence of plumes on plate motion.Indeed,we can acquire a comprehensive understanding of the driving forces for plate motion and operation mechanism of the Earth's dynamic system only through joint analyses and integrated studies on plate tectonics and plume tectonics.     

Algal δ15N values detect a wastewater effluent plume in nearshore and offshore surface waters and three-dimensionally model the plume across a coral reef on Maui, Hawai'i, USA

The coral reef at Kahekili, Maui is located ~300 m south of the Lahaina Wastewater Reclamation Facility which uses four Class V injection wells to dispose of 3-5 million gallons of wastewater effluent daily. Prior research documented that the wastewater effluent percolates into the nearshore region of Kahekili. To determine if the wastewater effluent was detectable in the surface waters offshore, we used algal bioassays from the nearshore region to 100 m offshore and throughout the water column from the surface to the benthos. These algal bioassays documented that significantly more wastewater effluent was detected in the surface rather than the benthic waters and allowed us to generate a three-dimensional model of the wastewater plume in the Kahekili coastal region. Samples located over freshwater seeps had the highest δ(15)N values (~30-35‰) and the effluent was detected in surface samples 500 m south and 100 m offshore of the freshwater seeps (~8-11‰).     

Implementation of a wetting-and-drying model in simulating the Kennebec–Androscoggin plume and the circulation in Casco Bay

A high-resolution coastal ocean model was developed to simulate the temporal/spatial variability of the Kennebec–Androscoggin (K–A) river plume and the circulation in Casco Bay. The model results agree favorably with the moored and shipboard observations of velocity, temperature, and salinity. The surface salinity gradient was used to distinguish the plume from the ambient coastal water. The calculated plume thickness suggests that the K–A plume is surface trapped. Its horizontal scales correlate well with Q ^0.25, where Q is the volume discharge of the rivers. Directional spreading is affected by the wind with the upwelling favorable wind transporting the plume water offshore. Both the wind and the tide also enhance mixing in the plume. The inclusion of a wetting-and-drying (WAD) scheme appears to enhance the mixing and entrainment processes near the estuary. The plume becomes thicker near the mouth of the estuary, the outflow velocity of the plume is weaker, and the radius of the river plume shrinks. The flow field in the model run with the WAD is noisier, not only in shallow areas of Casco Bay but also in the plume and even on the shelf. We speculate that the WAD processes can affect much larger areas than the intertidal zones, especially via a river plume that feeds into a coastal current.     

Indications for control of the Iceland plume on the Eocene–Oligocene “greenhouse–icehouse” climate transition

The Eocene/Oligocene boundary, at about 33.5 Myr ago, marks the transition from ‘greenhouse-’ to ‘icehouse-world’, accompanied by a sudden cooling of ocean bottom-water. We show that this global event is simultaneous with a deep rooted mantle process: an abrupt suppression of the Iceland plume triggered rapid deepening of the Greenland–Scotland Ridge (GSR) — the sill moderating deep circulation between the Nordic seas and North Atlantic. Striking coincidence of several sets of events reflects the abrupt suppression of the Iceland plume and a rapid removal of its influence on the nearby Reykjanes Ridge (RR): 1) A sudden segmentation of the paleo-RR seen on seafloor magnetic anomalies, 2) a drop in spreading rate of the North Atlantic, 3) a transition from thick to normal oceanic crust, and 4) a rapid deepening and accelerated subsidence of the GSR, inferred from the sedimentary record of DSDP site 336. The plume suppression and the concomitant GSR deepening coincide with the initiation of North Atlantic Deep Water (NADW) at the Eocene/Oligocene (E/O) transition, attested by onset of drift sedimentation in the Faroe–Shetland Channel (FSC), the deepest spill-point on the GSR, and in the North Atlantic, the Feni Drift. These processes have influenced global deepwater composition and temperature as indicated by the striking correlation with the jump in global δ¹⁸O (> 1‰) measured on benthic foraminifers that reflects the E/O global cooling, and with enrichment of unradiogenic Nd isotopes in the southeastern Atlantic and Southern Ocean. The initiation of Atlantic thermohaline circulation at that time is inferred from the abrupt split between planktonic and benthic δ¹⁸O, indicating the building of ocean-water stratification. This scenario is further corroborated by a reversal in benthic δ¹⁸O at the late Oligocene, coincident with the renewal of vigorous Iceland plume some 25 Myr ago, causing a considerable retardation in NADW fluxes. The plume renewal is inferred from the emergence of the Iceland plateau, the transition to oblique-unsegmented RR axis, the cessation in deepening of the GSR, and rapid increase in spreading rate of the North Atlantic. These events coincide with decreasing difference in planktonic–benthic in global δ¹⁸O by the late Oligocene. All these inferences suggest the role of the NADW sourced at the Nordic seas to form background cooler conditions in the long time scale since the early Oligocene, or to form permanent conditions of invigorated thermohaline circulation that forces CO₂ trap in the oceans.     

Hf–Nd isotope constraints on the origin of the Cretaceous Caribbean plateau and its relationship to the Galápagos plume

Formation of the Cretaceous Caribbean plateau, including the komatiites of Gorgona, has been linked to the currently active Galápagos hotspot. We use Hf–Nd isotopes and trace element data to characterise both the Caribbean plateau and the Galápagos hotspot, and to investigate the relationship between them. Four geochemical components are identified in the Galápagos mantle plume: two ‘enriched’ components with ϵHf and ϵNd similar to enriched components observed in other mantle plumes, one moderately enriched component with high Nb/Y, and a fourth component which most likely represents depleted MORB source mantle. The Caribbean plateau basalt data form a linear array in Hf–Nd isotope space, consistent with mixing between two mantle components. Combined Hf–Nd–Pb–Sr–He isotope and trace element data from this study and the literature suggest that the more enriched Caribbean end member corresponds to one or both of the enriched components identified on Galápagos. Likewise, the depleted end member of the array is geochemically indistinguishable from MORB and corresponds to the depleted component of the Galápagos system. Enriched basalts from Gorgona partially overlap with the Caribbean plateau array in ϵHf vs. ϵNd, whereas depleted basalts, picrites and komatiites from Gorgona have a high ϵHf for a given ϵNd, defining a high-ϵHf depleted end member that is not observed elsewhere within the Caribbean plateau sequences. This component is similar, however, in terms of Hf–Nd–Pb–He isotopes and trace elements to the depleted plume component recognised in basalts from Iceland and along the Reykjanes Ridge. We suggest that the Caribbean plateau represents the initial outpourings of the ancestral Galápagos plume. Absence of a moderately enriched, high Nb/Y component in the older Caribbean plateau (but found today on the island of Floreana) is either due to changing source compositions of the plume over its 90 Ma history, or is an artifact of limited sampling. The high-ϵHf depleted component sampled by the Gorgona komatiites and depleted basalts is unique to Gorgona and is not found in the Caribbean plateau. This may be an indication of the scale of heterogeneity of the Caribbean plateau system; alternatively Gorgona may represent a separate oceanic plateau derived from a completely different Pacific plume, such as the Sala y Gomez.     

Estimation and propagation of volcanic source parameter uncertainty in an ash transport and dispersal model: application to the Eyjafjallajokull plume of 14–16 April 2010

Data on source conditions for the 14 April 2010 paroxysmal phase of the Eyjafjallaj?kull eruption, Iceland, have been used as inputs to a trajectory-based eruption column model, bent. This model has in turn been adapted to generate output suitable as input to the volcanic ash transport and dispersal model, puff, which was used to propagate the paroxysmal ash cloud toward and over Europe over the following days. Some of the source parameters, specifically vent radius, vent source velocity, mean grain size of ejecta, and standard deviation of ejecta grain size have been assigned probability distributions based on our lack of knowledge of exact conditions at the source. These probability distributions for the input variables have been sampled in a Monte Carlo fashion using a technique that yields what we herein call the polynomial chaos quadrature weighted estimate (PCQWE) of output parameters from the ash transport and dispersal model. The advantage of PCQWE over Monte Carlo is that since it intelligently samples the input parameter space, fewer model runs are needed to yield estimates of moments and probabilities for the output variables. At each of these sample points for the input variables, a model run is performed. Output moments and probabilities are then computed by properly summing the weighted values of the output parameters of interest. Use of a computational eruption column model coupled with known weather conditions as given by radiosonde data gathered near the vent allows us to estimate that initial mass eruption rate on 14 April 2010 may have been as high as 10⁸?kg/s and was almost certainly above 10⁷?kg/s. This estimate is consistent with the probabilistic envelope computed by PCQWE for the downwind plume. The results furthermore show that statistical moments and probabilities can be computed in a reasonable time by using 9⁴?=?6,561 PCQWE model runs as opposed to millions of model runs that might be required by standard Monte Carlo techniques. The output mean ash cloud height plus three standard deviations??encompassing c. 99.7?% of the probability mass??compares well with four-dimensional ash cloud position as retrieved from Meteosat-9 SEVIRI data for 16 April 2010 as the ash cloud drifted over north-central Europe. Finally, the ability to compute statistical moments and probabilities may allow for the better separation of science and decision-making, by making it possible for scientists to better focus on error reduction and decision makers to focus on ??drawing the line?? for risk assessment.     

Continuous soil CO<Superscript>2</Superscript> and discrete plume SO<Subscript>2</Subscript> measurements at Mt. Etna (Italy) during 1997–2000: a contribution to volcano monitoring

Continuous monitoring of soil CO₂ dynamic concentration (which is proportional to the CO₂ flux through the soil) was carried out at a peripheral site of Mt. Etna during the period November 1997–September 2000 using an automated station. The acquired data were compared with SO₂ flux from the summit craters measured two to three times a week during the same period. The high frequency of data acquisition with both methods allowed us to analyze in detail the time variations of both parameters. Anomalous high values of soil CO₂ dynamic concentration always preceded periods of increased flux of plume SO₂, and these in turn were followed by periods of summit eruptions. The variations were modeled in terms of gas efflux increase due to magma ascent to shallow depth and its consequent depressurization and degassing. This model is supported by data from other geophysical and volcanological parameters. The rates of increase both of soil CO₂ dynamic concentration and of plume SO₂ flux are interpreted to be positively correlated both to the velocity of magma ascent within the volcano and to lava effusion rate once magma is erupted at the surface. Low rates of the increase were recorded before the nine-month-long 1999 subterminal eruption. Higher rates of increase were observed before the violent summit eruption of September-November 1999, and the highest rates were observed during shorter and very frequent spike-like anomalies that preceded the sequence of short-lived but very violent summit eruptions that started in late January 2000 and continued until late June of the same year. Furthermore, the time interval between the peaks of CO₂ and SO₂ in a single sequence of gas anomalies is likely to be controlled by magma ascent velocity.Editorial responsibility: H. Shinohara