Biogeomorphology in the marine landscape: Modelling the feedbacks between patches of the polychaete worm Lanice conchilega and tidal sand waves

Tidal sand waves are dynamic bedforms found in coastal shelf seas. Moreover, these areas are inhabited by numerous benthic species, of which the spatial distribution is linked to the morphological structure of sand waves. In particular, the tube-building worm Lanice conchilegais of interest as this organism forms small mounds on the seabed, which provide shelter to other organisms. We investigate how the interactions between small-scale mounds (height ∼dm) and large-scale sand waves (height ∼m) shape the bed of the marine environment. To this end, we present a two-way coupled process-based model of sand waves and tube-building worm patches in Delft3D. The population density evolves according to a general law of logistic growth, with the bed shear stress controlling the carrying capacity. Worm patches are randomly seeded and the tubes are mimicked by small cylinders that influence flow and turbulence, thereby altering sediment dynamics. Model results relate the patches with the highest worm densities to the sand wave troughs, which qualitatively agrees with field observations. Furthermore, the L. conchilegatubes trigger the formation of sandy mounds on the seabed. Because of the population density distribution, the mounds in the troughs can be several centimetres higher than on the crests. Regarding sand wave morphology, the combination of patches and mounds are found to shorten the time-to-equilibrium. Also, if the initial bed comprised small sinusoidal sand waves, the equilibrium wave height decreased with a few decimetres compared to the situation without worm patches. As the timescale of mound formation (years) is shorter than that of sand wave evolution (decades), the mounds induce (and accelerate) sand wave growth on a similar spatial scale to the mounds. Initially, this leads to shorter sand waves than they would be in an abiotic environment. However, near equilibrium the wavelengths tend towards their abiotic counterparts again. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Subaqueous geothermal activity revealed by lacustrine sediments of the acidic Nakadake crater lake, Aso Volcano, Japan

Lacustrine sediments were sampled from the inaccessible acidic (pH = 0.43) Nakadake crater lake of Aso Volcano, Japan by a simple method. The sediments contain an extremely high content (74 wt.%) of sulfur, which exits as elemental sulfur, gypsum and anhydrite. The abundant elemental sulfur is likely formed by the reaction of SO₂ and H₂S gases and by the SO₂ disproportionation reaction in magmatic hydrothermal system below the crater lake. Based on the sulfur content of sediments and measurements of elevation change of the crater bottom, the sulfur accumulation rate at the Nakadake crater lake was calculated as 250 tonne/day, which is comparable with the SO₂ emission rate (200–600 tonne/day) from the Nakadake crater. The sediments include a small amount (9%) of clear glass shards that are apparently not altered in spite of the high reactivity of hyperacid lake water. This finding suggests that the clear glass shards are fragments of recently emitted magmas from fumaroles on the bottom of the crater lake and the magma emissions continuously occur even in quiescent periods.     

2001年3月2日磁通量传输事件特性的研究

2001年3月2日11:00 至11:15 UT 期间,Cluster Ⅱ在南半球极尖区晨侧附近磁鞘内探测到3个通量传输事件（简称FTEs）. 本文利用Cluster Ⅱ星簇4颗卫星观测到的磁场和等离子体资料研究了这些通量传输事件的磁场形态和粒子特征. 并利用它们探测到的空间磁场梯度资料由安培定律直接求出星簇所在区域的电流分布. 结果指出：（1）BY占优势的行星际磁场结构在磁层顶的重联可以在极尖区附近发生;（2）FTEs通量管形成初期内外总压差和磁箍缩应力不一定平衡,达到平衡有一发展过程;（3）FTEs通量管截面在L M平面内的线度约为1.89RE;（4）FTEs通量管中等离子体主要沿轴向场方向流动,整个通量管以慢于背景等离子体的速度沿磁层顶向南向尾运动;（5）FTEs通量管中不仅有轴向电流,也存在环向电流. 轴向电流基本沿轴向磁场方向流动. 轴向和环向电流在管内均呈体分布,因而轴向电流产生的环向磁场接近管心时不断减小到零,而环向电流生成的轴向场则不断增大到极值;（6）在通量管的磁鞘部分观测到磁层能量粒子流量的增强,这表明通量管通过磁层顶将磁鞘和磁层内部连通起来了.     

Comparing detection and location performance of perpendicular and parallel broadside GPR antenna orientations

GPR (Ground Penetrating Radar) results are shown for perpendicular broadside and parallel broadside antenna orientations. Performance in detection and localization of concrete tubes and steel tanks is compared as a function of acquisition configuration. The comparison is done using 100 MHz and 200 MHz center frequency antennas. All tubes and tanks are buried at the geophysical test site of IAG/USP in São Paulo city, Brazil. The results show that the long steel pipe with a 38-mm diameter was well detected with the perpendicular broadside configuration. The concrete tubes were better detected with the parallel broadside configuration, clearly showing hyperbolic diffraction events from all targets up to 2-m depth. Steel tanks were detected with the two configurations. However, the parallel broadside configuration was generated to a much lesser extent an apparent hyperbolic reflection corresponding to constructive interference of diffraction hyperbolas of adjacent targets placed at the same depth. Vertical concrete tubes and steel tanks were better contained with parallel broadside antennas, where the apexes of the diffraction hyperbolas better corresponded to the horizontal location of the buried target disposition. The two configurations provide details about buried targets emphasizing how GPR multi-component configurations have the potential to improve the subsurface image quality as well as to discriminate different buried targets. It is judged that they hold some applicability in geotechnical and geoscientific studies.     

Trace and major elements distribution and transfer within a benthic system: Polychaete Chaetopterus variopedatus, commensal crab Polyonyx gibbesi, worm tube,and sediments

Samples of the polychaete Chaetopterus variopedatus, worm tubes, commensal crab Polyonyx gibbesi and sediments were collected in eight sites in Todos os Santos Bay, Brazil, in order to evaluate the potential use of the polychaetes and crabs as biomonitors and to assess the relationships and accumulation of trace and major elements in different benthic compartments. Trace and major elements were determined by ICP OES. Organic carbon, total nitrogen and sulfur were determined by CNS elemental analyser. Tubes, crabs and polychaetes were important in the retention of trace and major elements. Metals that presented the highest accumulation in polychaetes (i.e. Mg > Al > Fe > Zn > Mn > Co > Cu > Ba > Cr) where the same for crabs (i.e. Mg > Al > Fe > Mn > Co > Zn > Cu > Ba > Cr). High concentrations of Al, Ba, Cr, Mn and Fe, from terrigenous sources, were observed in tubes, which presented accumulation factors up to 81.5 for Mn. Sedentary polychaetes are seen as good biomonitor alternatives for metal contamination studies, because they are one of the most abundant taxon in the benthic system, live in direct contact with sediments, are present in broad distributions and can also handle relatively high concentrations of metals ensuring chronic exposition. The possibility to work with not only the polychaete but also its tube offers advantages compared to bivalves that generally do not accumulate certain metals in very high levels.     

Origin of a native sulfur chimney in the Kueishantao hydrothermal field, offshore northeast Taiwan

Analyses of rare earth and trace element concentrations of native sulfur samples from the Kueishantao hydrothermal field were performed at the Seafloor Hydrothermal Activity Laboratory of the Key Labo-ratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. Using an Elan DRC II ICP-MS, and combining the sulfur isotopic compositions of native sulfur samples, we studied the sources and formation of a native sulfur chimney. The results show, when comparing them with native sulfur from crater lakes and other volcanic areas, that the native sulfur content of this chimney is very high (99.96%), the rare earth element (REE) and trace element constituents of the chimney are very low (ΣREE<21×10?9), and the chondrite-normalized REE patterns of the native sulfur samples are similar to those of the Kueishantao andesite, implying that the interaction of subseafloor fluid-andesite at the Kueishantao hydrothermal field was of short duration. The sulfur isotopic compo-sitions of the native sulfur samples reveal that the sulfur of the chimney, from H2S and SO2, originated by magmatic degassing and that the REEs and trace elements are mostly from the Kueishantao ande-site and partly from seawater. Combining these results with an analysis of the thermodynamics, it is clear that from the relatively low temperature (<116 ℃ ), the oxygenated and acidic environment is favorable for formation of this native sulfur chimney in the Kueishantao hydrothermal field.     

Sulfur geochemistry of hydrothermal waters in Yellowstone National Park, Wyoming, USA. II. Formation and decomposition of thiosulfate and polythionate in Cinder Pool

Cinder Pool is an acid-sulfate-chloride boiling spring in Norris Geyser Basin, Yellowstone National Park. The pool is unique in that its surface is partially covered with mm-size, black, hollow sulfur spherules, while a layer of molten sulfur resides at the bottom of the pool (18 m depth). The sulfur speciation in the pool was determined on four different days over a period of two years. Samples were taken to evaluate changes with depth and to evaluate the importance of the sulfur spherules on sulfur redox chemistry. All analyses were conducted on site using a combination of ion chromatography and colorimetric techniques.Dissolved sulfide (H₂S), thiosulfate (S₂O₃²⁻), polythionates (S_xO₆²⁻), and sulfate were detected. The polythionate concentration was highly variable in time and space. The highest concentrations were found in surficial samples taken from among the sulfur spherules. With depth, the polythionate concentrations dropped off. The maximum observed polythionate concentration was 8 μM. Thiosulfate was rather uniformly distributed throughout the pool and concentrations ranged from 35 to 45 μM. Total dissolved sulfide concentrations varied with time, concentrations ranged from 16 to 48 μM. Sulfate was relatively constant, with concentrations ranging from 1150 to 1300 μM. The sulfur speciation of Cinder Pool is unique in that the thiosulfate and polythionate concentrations are significantly higher than for any other acid-sulfate spring yet sampled in Yellowstone National Park. Complementary laboratory experiments show that thiosulfate is the intermediate sulfoxyanion formed from sulfur hydrolysis under conditions similar to those found in Cinder Pool and that polythionates are formed via the oxidation of thiosulfate by dissolved oxygen. This last reaction is catalyzed by pyrite that occurs as a minor constituent in the sulfur spherules floating on the pool's surface. Polythionate decomposition proceeds via two pathways: (1) a reaction with H₂S, yielding thiosulfate and elemental sulfur; and (2) by disproportionation to sulfate and thiosulfate.This study demonstrates that the presence of a subaqueous molten sulfur pool and sulfur spherules in Cinder Pool is of importance in controlling the pathways of aqueous sulfur redox reactions. Some of the insights gained at Cinder Pool may be relevant to acid crater lakes where sulfur spherules are observed and variations in polythionate concentrations are used to monitor and predict volcanic activity.     

Development of lava tubes in the light of observations at Mauna Ulu,Kilauea Volcano,Hawaii

During the 1969–1974 Mauna Ulu eruption on Kilauea's upper east rift zone, lava tubes were observed to develop by four principal processes: (1) flat, rooted crusts grew across streams within confined channels; (2) overflows and spatter accreted to levees to build arched roofs across streams; (3) plates of solidified crust floating downstream coalesced to form a roof; and (4) pahoehoe lobes progressively extended, fed by networks of distributaries beneath a solidified crust. Still another tube-forming process operated when pahoehoe entered the ocean; large waves would abruptly chill a crust across the entire surface of a molten stream crossing through the surf zone. These littoral lava tubes formed abruptly, in contrast to subaerial tubes, which formed gradually. All tube-forming processes were favored by low to moderate volume-rates of flow for sustained periods of time. Tubes thereby became ubiquitous within the pahoehoe flows and distributed a very large proportionof the lava that was produced during this prolonged eruption. Tubes transport lava efficiently. Once formed, the roofs of tubes insulate the active streams within, allowing the lava to retain its fluidity for a longer time than if exposed directly to ambient air temperature. Thus the flows can travel greater distances and spread over wider areas. Even though supply rates during most of 1970–1974 were moderate, ranging from 1 to 5 m³/s, large tube systems conducted lava as far as the coast, 12–13 km distant, where they fed extensive pahoehoe fields on the coastal flats. Some flows entered the sea to build lava deltas and add new land to the island. The largest and most efficient tubes developed during periods of sustained extrusion, when new lava was being supplied at nearly constant rates. Tubes can play a major role in building volcanic edifices with gentle slopes because they can deliver a substantial fraction of lava erupted at low to moderate rates to sites far down the flank of a volcano. We conclude, therefore, that the tendency of active pahoehoe flows to form lava tubes is a significant factor in producing the common shield morphology of basaltic volcanoes.     

Plate motions and the viscosity structure of the mantle — Insights from numerical modelling

We use sulfur (S) isotope signatures within ancient sediments and a photochemical model of sulfur dioxide (SO₂) photolysis to interpret the evolution of the atmosphere over the first half of Earth's history. A decrease in mass-independent sulfur isotope fractionation has been reported in Archean rocks deposited between ~ 2.7 Ga and ~ 3.2 Ga, and is reinforced by new S isotope data that we report here. This pattern has been interpreted by some as evidence that atmospheric oxygen (O₂) was elevated during this time. In this paper, we argue against that conclusion, and show that it is inconsistent with other geochemical data. In its place, we propose a new model that can explain the sulfur isotope record that can also avoid conflicts with independent constraints on O₂ and account for concurrent glacial deposits. Specifically, we suggest that prior to the rise of O₂ excursions in the sulfur isotope record were modulated by the thickness of an organic haze. This haze would have blocked the lower atmosphere from the UV fluxes responsible for the anomalous sulfur photochemistry and would have caused an anti-greenhouse effect that triggered the glaciations. We used a photochemical model to verify that a haze could have affected the isotopic signal in this manner, and to examine how changes in atmospheric methane (CH₄) and carbon dioxide (CO₂) concentrations could have controlled haze thickness. Finally, we combined the resulting relationships with climate models and sulfur isotope and glacial records to deduce a new evolutionary sequence for Archean climate, surface chemistry, and biology.     

Plume chemistry provides insights into mechanisms of sulfur and halogen degassing in basaltic volcanoes

This paper deals with sulfur, chlorine and fluorine abundances in the eruptive volcanic plume of the huge October 2002-January 2003 eruption of Mount Etna, aiming at relating the relevant compositional variations observed throughout with changes in eruption dynamics and degassing mechanisms. The recurrent sampling of plume acidic volatiles by filter-pack methodology revealed that, during the study period, S/Cl and Cl/F ratios ranged from 0.1-6.8 and 0.9-5.6, respectively. Plume S/Cl ratios increased by a factor of ∼10 as volcanic activity drifted from paroxysmal lava fountaining (mid- and late November) to passive degassing and minor effusion (early January), and then decreased to the low values (S/Cl=0.1) typical of the final stages of the eruption. Parallel variations in chlorine to fluorine ratios were also observed. A theoretical model is proposed for quantitative interpretation of these changes in plume composition. The model calculates the composition of a volatile phase exsolving from an ascending Etna magma, based on knowledge of solubilities and abundances in the undegassed melt of sulfur and halogens [T.M. Gerlach, EOS 72 (1991), 249, 254-255]. According to this model, degassing of Etnean basaltic melt at high pressures and depths (>100 MPa, 3 km) is likely to release a CO₂+H₂O-rich vapor phase with S/Cl molar ratios ∼1. Extensive sulfur and chlorine degassing from the melt would take place at shallower depth (P<20 MPa, 0.6 km), with S/Cl ratios in the vapor phase increasing as pressure drops to 0.1 MPa. Comparisons between model compositions and volcanic plume data demonstrate that the chemical trends observed during the eruption may be explained by increased degassing due to depressurization of a basaltic magma batch ascending toward the surface.     

Sulfur and chlorine degassing from primitive arc magmas: temporal changes during the 1982–1983 eruptions of Galunggung (West Java, Indonesia)

The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to 2000 ppm sulfur, but concentrations decrease rapidly with increasing SiO₂ down to matrix glass values (50–100 ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (200 ppm S). Chlorine concentrations increase from 750 to 2200 ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2 Mt is not in accordance with a petrologic estimate of 0.09 Mt. Using a published value of 550 Mt of erupted material about 0.34 km³ fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26 km³), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3 Mt chlorine, which agrees well with an output of 0.47 Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.     

Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (~2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of △33S varies from -0.89‰ to 1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero △33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative △33S value, however, Superior-type BIFs which are far away from the volcanic center have positive △33S value. The δ 34S varies in a large range from -22.0‰ to 11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10-2―10-3 of the present atmospheric level (PAL).     

TOMS measurement of the sulfur dioxide emitted during the 1985 Nevado del Ruiz eruptions

The eruptions of Nevado del Ruiz in 1985 were unusually rich in sulfur dioxide. These eruptions were observed with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) which can quantitatively map volcanic sulfur dioxide plumes on a global scale. A small eruption, originally believed to be of phreatic origin, took place on September 11, 1985. However, substantial amounts of sulfur dioxide from this eruption were detected with TOMS on the following day. The total mass of SO₂, approximately 9 ± 3 × 10⁴ metric tons, was deposited in two clouds, one in the upper troposphere, the other possibly at 15 km near the stratosphere.The devastating November 13 eruptions were first observed with TOMS at 1150 EST on November 14. Large amounts of sulfur dioxide were found in an arc extending 1100 km from south of Ruiz northeastward to the Gulf of Venezuela and as an isolated cloud centered at 7°N on the Colombia-Venezuela border. On November 15 the plume extended over 2700 km from the Pacific Ocean off the Colombia coast to Barbados, while the isolated mass was located over the Brazil-Guyana border, approximately 1600 km due east of the volcano. Based on wind data from Panama, most of the sulfur dioxide was located at 10–16 km in the troposphere and a small amount was quite likely deposited in the stratosphere at an altitude above 24 km.The total mass of sulfur dioxide in the eruption clouds was approximately 6.6 ± 1.9 × 10⁵ metric tons on November 14. When combined with quiescent sulfur dioxide emissions during this period, the ratio of sulfur dioxide to erupted magma from Ruiz was an order of magnitude greater than in the 1982 eruption of El Chichon or the 1980 eruption of Mount St. Helens.     

A new method for sampling and analyzing volcanic sublimates — application to Merapi volcano, Java

A new method for the sampling of sublimates from high-temperature volcanic gases has been used at Merapi volcano, Java, in 1978. The sublimates were collected on the inner walls of silica tubes introduced into fumarolic vents. Volcanic gases were allowed to move freely through the tubes and as they cooled, a fraction of the volatile components condensed on the inner walls of the tubes along the temperature gradient. The sublimates were then analyzed by a combination of light microscopy, scanning electron microscopy, electron microprobe and X-ray diffraction.Six successive zones of different compositions and mineralogical associations have been identified along the covered range of temperatures (900° to around 400°C). From the high to the low temperatures, these zones are composed of: (1) cristobalite, magnetite, hercynite; (2) molybdenite; (3) acmite; (4) halite, sylvite; (5) sphalerite, pyrite; and (6) galena. Equilibrium calculations show that these crystalline phases are stable for pS₂, pC₁, and pO₂, values typical of magma-buffered gases that have not been contaminated by atmospheric oxygen.The deposits observed in the tubes may be useful in aiding the understanding of the mechanisms acting during the cooling of the gaseous phase on its way to the surface and before its emission into the atmosphere.     

Massive barite deposits and carbonate mineralization in the Derugin Basin, Sea of Okhotsk: precipitation processes at cold seep sites

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (∼1%) in the barite fabric imply the presence of H₂S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative δ¹³C values of these carbonates (>−43.5‰ PDB) indicate methane as major carbon source; δ¹⁸O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with ¹⁴C-ages ranging from 20?680 to >49?080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO₄^2- concentrations and a parallel increase of H₂S with depth. Additionally, S and O isotope data of barite sulfate (δ³⁴S: 21.0-38.6‰ CDT; δ¹⁸O: 9.0-17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with ‘normal’ seawater sulfate. While massive barite deposits are commonly assumed to be of hydrothermal origin, the assemblage of cheomautotrophic clams, methane-derived carbonates, and non-thermally equilibrated barite sulfate strongly implies that these barites have formed at ambient bottom water temperatures and form the features of a Giant Cold Seep setting that has been active for at least 49?000 yr.     

Sulfur,chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

 The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H₂SO₄ aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods. Received: 30 May 1995 / Accepted: 19 April 1996     

On the formation mechanism of the sporadic solar wind

In this paper, we examine the nature of the main source of the sporadic solar wind on the Sun: coronal mass ejections (CMEs). Analysis of data from Mark 3 and Mark 4, the Digital Prominence Monitor (MLSO), and STEREO (EUVI) spacecraft has revealed the existence of two types of CMEs: gradual and impulse. They differ in the place, velocity, and angular size at the instant of their emergence. The source of gradual CMEs is located in the corona, at a distance of 1.1 R ₀ < R ≤ 1.7 R ₀ from the center of the Sun. They start moving from a state of rest, having an angular size ≈15–65° (in the heliographic coordinate system). Impulse CMEs are probably formed under the Sun’s photosphere. This may be due to the supersonic emergence of magnetic tubes (ropes) from the convective zone. The possibility of this phenomenon has been demonstrated earlier in theory. The radial velocity of such tubes at the photospheric level may be 100 km/s or higher; the minimum angular size is ∼1°.     

Development of the 1990 Kalapana Flow Field,Kilauea Volcano,Hawaii

The 1990 Kalapana flow field is a complex patchwork of tube-fed pahoehoe flows erupted from the Kupaianaha vent at a low effusion rate (approximately 3.5 m³/s). These flows accumulated over an 11-month period on the coastal plain of Kilauea Volcano, where the pre-eruption slope angle was less than 2°. the composite field thickened by the addition of new flows to its surface, as well as by inflation of these flows and flows emplaced earlier. Two major flow types were identified during the development of the flow field: large primary flows and smaller breakouts that extruded from inflated primary flows. Primary flows advanced more quickly and covered new land at a much higher rate than breakouts. The cumulative area covered by breakouts exceeded that of primary flows, although breakouts frequently covered areas already buried by recent flows. Lava tubes established within primary flows were longer-lived than those formed within breakouts and were often reoccupied by lava after a brief hiatus in supply; tubes within breakouts were never reoccupied once the supply was interrupted. During intervals of steady supply from the vent, the daily areal coverage by lava in Kalapana was constant, whereas the forward advance of the flows was sporadic. This implies that planimetric area, rather than flow length, provides the best indicator of effusion rate for pahoehoe flow fields that form on lowangle slopes.     

The sulfur flow fields of the Fossa di Vulcano

Sulfur flow deposits at the Fossa di Vulcano fumarole field (Italy) are dominated by thermal erosion features. These are characteristic of sulfur flows at this location, where most flows are emplaced in a combusting mode such that all flow sulfur is melted and consumed during the emplacement event. Further, thermal erosion during emplacement results in pits and channels that mark the passage of the combusting flow. These thermal erosion pits and channels are typically littered with non-combusted silicate blocks, show overhanging rims and an absence of sulfur. If activity remains confined to a source fumarole basin, then sulfur lake activity will result. Combustion of such a feature leaves thermally eroded pits, typically a few tens of centimeters to a few meters wide and long, and a few tens of centimeters deep. However, the increase in sulfur volume during melting and erosion of pit walls mean that overflow and breaching is common. This leads to capture of new sulfur encrusted fumarole basins and flow extension. Flow extension away from the lake results in thermal erosion channels as much as 1.7 m wide, 0.6 m deep and 23.5 m long. Flow direction is dictated by slope, cinder ejection and sources of new sulfur, thus flows are capable of moving down, across and/or up slope if that is the dominant source of new sulfur. We estimate that sulfur flow activity has combusted 2,000–5,000 m³, or 4,000–10,000 tons, of sulfur at Vulcano. Only one noncombusted unit could be found during seven fumarole-fieldwide surveys during 1998–2003; this was 7.3 m long and 0.3 m wide, and had a viscosity of 0.1–40 Pa s. This viscosity is consistent with emplacement temperatures of 165–180°C, which are lower than sulfurs combustion temperature. At Vulcano the commonality of thermal erosion features over noncombusted sulfur flow units indicates that combusting emplacement has been the main mode of flow emplacement at this volcano. The common occurrence of combustion is also evident from reference to the same phenomenon by Déodat de Dolomieu in 1783.Editorial responsibility: M. Carroll     

Fossil carotenoids and paleolimnology of meromictic Mahoney Lake,British Columbia,Canada

Vertical distribution of fossil carotenoids in a sediment core from meromictic Mahoney Lake was studied. Besides okenone and demethylated okenone, lutein and zeaxanthin and-carotene isomers were identified. No carotenoids typical for purple nonsulfur or green sulfur bacteria were detected. The ratio of zeaxanthin to lutein (above 1:1 in all samples) indicates a dominance of cyanobacteria over green algae in the phytoplankton assemblages of the past. Okenone, which is found exclusively in Chromatiaceae, was the dominating carotenoid in all sediment zones.The oldest sediment layers containing okenone were deposited 11 000 years ago. Between 9000 and 7000 and since 3000 years b.p., Chromatiaceae reached a considerable biomass in the lake. Vertical changes in okenone concentration were not related to changes of paleotemperatures. In contrast, okenone concentrations decreased during periods of volcanic ash input. During most of the lake history, however, mean okenone concentrations were positively correlated with sedimentation rates. This indicates that vertical changes of okenone concentration in the sediment reflect past changes of purple sulfur bacterial biomass in the lake.According to these results, the past limnology of Mahoney Lake resembled that of the present with a sulfide-containing monimolimnion and a well-developed population of okenone-bearing purple sulfur bacteria.