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  • 51.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Sanden, Michael
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Marine geovetenskap.
    Asavanant, Jack
    Eyewitness reports of the impact of the 2004 tsunami in the Khao Lak area, Thailand2008In: International Geological Congress, 33, 2008Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    On December 26, 2004, an MW 9.1 earthquake which occurred off the western coast of Sumatra triggered tsunami waves which propagated across the Indian Ocean. We present data collected from 20 eyewitness reports , who experienced the tsunami on December 26, 2004 in the class IV impact region, near Khao Lak, Thailand. These data include (1) paths by which eyewitnesses were carried by the tsunami wave(s), (2) number of waves experienced, (3) relative strength and height of the wave(s), (4) the geometry of the first impacting wave front and (5) the time interval between the arrivals of subsequent waves. These data are broadly consistent with (1) tidal gauge measurements, (2) measured runup heights (Tsuji et al., 2006; Choi et al., 2006) and (3) numerical simulations (Ioulalen et al., 2007). Based on these data we make the following tentative interpretations:

    1. The tsunami impacted in an east-northeasterly to northeasterly direction.

    2. The sea began retreating rapidly at approximately 10.00 a.m. local time.

    3. The first wave front impacted at 10.30 a.m. local time.

    4. The wave period was 35-50 minutes. Shorter wave periods (21-27 and 16-30 minutes) were estimated from eyewitness data. These probably reflect the complex geometry of the wave maxima with multiple (2-4) crests, which may have been experienced as separate waves.

    6. The wavelength was between 15-20 and 6-8 km. Given the wave period (35-50 minutes), we note that this could reflect attenuation of the incoming wave from a velocity of 18-34 km/hour to a velocity of 7-14 km/hour.

    7. Eye witness reports suggest that the wave height was 5-12 m. These estimates are broadly consistent with tidal gauge measurements (7-8 m) and runup heights (8-11 m).

    8. Wave refraction off the peninsula, west of Taph Lamu, might have been responsible for a smaller and weaker “pre-wave” which was experienced by survivors in the central part of the Khao Lak area and evident both from tidal gauge measurements and the model simulations.

    Based on the broad consistency between eyewitness reports, tidal gauge and runup measurements and model simulations, we conclude that eyewitness reports can provide a robust source of both qualitative and quantitative data, which can be used to constrain numerical models of tsunami propagation.

  • 52.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Einarsson, Draupnir
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Orr, Tim
    Dimmuborgir: a rootless shield complex in northern Iceland2016In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 78, no 5, article id 40Article in journal (Refereed)
    Abstract [en]

    The origin of Dimmuborgir, a shield-like volcanic structure within the Younger Laxa lava flow field near Lake Myvatn, in northern Iceland, has long been questioned. New airborne laser mapping (light detection and ranging (LiDAR)), combined with ground-penetrating radar results and a detailed field study, suggests that Dimmuborgir is a complex of at least two overlapping rootless shields fed by lava erupting from the nearby Ludentarborgir crater row. This model builds upon previous explanations for the formation of Dimmuborgir and is consistent with observations of rootless shield development at Kilauea Volcano, Hawaii. The larger rootless shields at Dimmuborgir, 1-1.5 km in diameter, elliptical in plan view, similar to 30 m in height, and each with a 500-m-wide summit depression, were capable of storing as much as 2-3x10(6) m(3) of lava. They were fed by lava which descended 30-60 min lava tubes along a distance of 3 km from the crater row. The height difference generated pressure sufficient to build rootless shields at Dimmuborgir in a timescale of weeks. The main summit depressions, inferred to be drained lava ponds, could have emptied via a 30-m-wide x 5-m-deep channel, with estimated effusion rates of 0.7-7 m(3) s(-1) and minimum flow durations of 5-50 days. We argue that the pillars for which Dimmuborgir is famed are remnants of lava pond rims, at various stages of disintegration that formed during pond drainage.

  • 53.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Whitmarsh, R.
    Edinburgh Univ.
    Arghe, F.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Koyi, H.
    Uppsala Univ.
    Constraining the rate and extent of mantle serpentization from seismic and petrological data: Implications for chemosynthesis and tectonic processes.2005In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 5, no 3, p. 153-164Article in journal (Refereed)
    Abstract [en]

    We used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub-continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of kinetically limited transport of the serpentinization front yields a front displacement of 2197 ± 89 m, a time-integrated fluid flux of 1098 ± 45 m<sup>3</sup> m<sup>−2</sup> and a Damköhler number of 6.0 ± 0.2. Whether either surface reaction or chemical transport limit the rate of reaction, we calculate timescales for serpentinization of approximately 10<sup>5</sup>–10<sup>6</sup> years. This yields time-average fluid flux rates for H<sub>2</sub>O, entering and reacting with the mantle, of 60–600 mol m<sup>−2</sup> a<sup>−1</sup> and for CH<sub>4</sub>, produced as a by-product of oxidation of Fe<sup>++</sup> to magnetite and exiting the mantle, of 0.55–5.5 mol m<sup>−2</sup> a<sup>−1</sup>. This equates to a CH<sub>4</sub>-flux of 0.18–1.8 Tg a<sup>−1</sup> for coeval serpentinization of the mantle that was exhumed west of Iberia. This represents 0.03–0.3% of the present-day annual CH<sub>4</sub>-flux from all sources and a higher fraction of pre-anthropogenic (lower) CH<sub>4</sub> levels. CH<sub>4</sub> released by serpentinization at or beneath the seafloor could provide substrate for biological chemosynthesis and/or promote gas-hydrate formation. Finally, noting its volumetric extent and rapidity (<10<sup>6</sup> years), we interpret serpentinization to be a reckonable component of tectonic processes, contributing both diapiric and expansional forces and helping to ‘lubricate’ extensional processes. Given its anisotropic permeability, actively deforming serpentinite might impede melt migration which may be of interest, given the apparent lack of melt in some rifted margins.

  • 54.
    Stockmann, Gabrielle
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Balic-Zunic, Tonci
    Langhof, Jörgen
    Skogby, Henrik
    Karlsson, Andreas
    Control of a calcite inhibitor (phosphate) and temperature on ikaite precipitation in Ikka Fjord, southwest Greenland2018In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 89, p. 11-22Article in journal (Refereed)
    Abstract [en]

    Ikaite (CaCO3 center dot 6H(2)O) forms submarine tufa columns in Ikka Fjord, SW Greenland. This unique occurrence is thought to relate to aqueous phosphate concentration and low water temperatures (< 6 degrees C). Phosphate ions are well-known inhibitors of calcite precipitation and Ikka Fjord has a naturally high-phosphate groundwater system that when mixing with seawater leads to the precipitation of ikaite. In the study presented here, experiments simulating conditions of Ikka Fjord show that a) the formation of ikaite is unrelated to the aqueous phosphate concentration (0-263 mu mol/ kg PO43-) in 0.1 M NaHCO3/0.1 M Na2CO3 solutions mixing with seawater at 5 degrees C and pH 9.6-10.6, and b) ikaite forms at temperatures up to 15 degrees C without phosphate and in open beakers exposed to air. Instead, supersaturation of ikaite and the seawater composition are the likely factors causing ikaite to precipitate in Ikka Fjord. This study shows that adding Mg2+ to a NaHCO3/Na2CO3 - CaCl2 mixed solution leads to the formation of ikaite along with hydrated Mg carbonates, which points to the high Mg2+ concentration of seawater, another known inhibitor of calcite, as a key factor promoting ikaite formation. In experiments at 10 and 15 degrees C, increasing amounts of either nesquehonite (Mg(HCO3)(OH)center dot 2H(2)O) or an amorphous phase co-precipitate with ikaite. At 20 degrees C, only the amorphous phase is formed. In warming Arctic seawater, this suggests Mg carbonate precipitation could become dominant over ikaite in the future.

  • 55.
    Tollefsen, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lundqvist, Lena
    Sturkell, Erik
    Secondary alteration of the Gronnedal-Ika igneous complex and the genesis of ikaite, CaCO3 center dot 6H(2)O, SW Greenland2019In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 510, p. 18-30Article in journal (Refereed)
    Abstract [en]

    The mineral ikaite (CaCO3 center dot 6H(2)O) precipitates from a mixture of spring water and seawater as tufa columns which grow at a rate of up to 50 cm per year reaching heights of up to 18 m in Ikka Fjord, SW Greenland. In the fjord, column formation occurs only at the base of a nepheline syenite-carbonatite complex that flanks the fjord and an association has therefore been proposed. The spring water that seeps up at the bottom of the fjord is oversaturated in Na+ and HCO3-. In this study, we show that these ions were acquired by alteration reactions in the syenite-carbonatite complex: Na+ is released during replacement of nepheline by illite and analcime in nepheline-syenite rocks and HCO3- is released by oxidation of siderite to goethite in carbonatite rocks. The chemically charged groundwater mixes with seawater and gives rise to the formation of the tufa columns. We performed a mass balance to show that the mass of the carbonatite in the complex is more than sufficient to provide the CO2 needed to produce the observed mass of tufa columns. We estimated a time frame of similar to 600 years to produce the necessary CO2 to form the 700 ikaite columns in the fjord.

  • 56.
    Tollefsen, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dupraz, Christophe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Chemical controls on ikaite formation2018In: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, no 5, p. 1119-1129Article in journal (Refereed)
    Abstract [en]

    The hydrated carbonate mineral ikaite (CaCO3 center dot 6H(2)O) is thermodynamically unstable at all known conditions on Earth. Regardless, ikaite has been found in marine sediments, as tufa columns and in sea ice. The reason for these occurrences remains unknown. However, cold temperatures (<6 degrees C), high pH and the presence of Mg2+ and SO42 in these settings have been suggested as factors that promote ikaite formation. Here we show that Mg concentration and pH are primary controls of ikaite precipitation at 5 degrees C. In our experiments a sodium carbonate solution was mixed with seawater at a temperature of 5 degrees C and at a constant rate. To test the effect of Mg2+ and SO42 we used synthetic seawater which allowed us to remove these elements from the seawater. The pH was controlled by different ratios of Na2CO3 and NaHCO3 in the carbonate solution. We found that ikaite precipitated when both seawater and synthetic seawater from which SO4 had been removed were used in the experiments. However, ikaite did not precipitate in experiments conducted with synthetic seawater from which Mg had been removed. In these experiments, calcite precipitated instead of ikaite. By varying the Mg concentration of the synthetic seawater and the pH of the sodium carbonate solution, we constructed a kinetic stability diagram for ikaite and calcite as a function of Mg concentration and pH. One possible explanation of our finding is that Mg2+ inhibits calcite nucleation and thereby allows metastable ikaite to form instead.

  • 57.
    Wästeby, Niklas
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Liljedahl, Lillemor Claesson
    Sturkell, Erik
    Mörth, Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 7, p. 5727-5740Article in journal (Refereed)
    Abstract [en]

    Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200 m, was conducted over a period of 10 years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Husavik-Flatey Fault (HFF) near Husavik town, northern Iceland. After 10 weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15-20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8-10 years after the earthquake.

  • 58.
    Zhao, Zhihong
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    An assessment of the role of nonlinear reaction kinetics in parameterization of metamorphic fluid flow2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 8, p. 6249-6262Article in journal (Refereed)
    Abstract [en]

    Based on inverse modeling of reaction progress data using a numerical framework that considers coupled advection and diffusion, linear and nonlinear reaction kinetics, and with effective diffusivity given by Archie's law, we show that (1) choice of reaction order has little effect (<0.3 orders of magnitude) on estimates of time-integrated and time-averaged metamorphic fluid fluxes and metamorphic fluid flow durations based on reaction progress data, (2) reaction order must be known for robust determination of time-averaged net reaction rates based on reaction progress data and that underestimation of this term by more than 3 orders of magnitude can arise from assuming linear reaction kinetics, (3) differing reaction orders between laboratory experiments and natural metamorphic systems and/or a nonlinear dependence of effective diffusivity on porosity can explain order-of-magnitude discrepancies between field-based and laboratory-based estimates of time-averaged net reaction rates, and (4) parameterization of metamorphic fluid flow is limited to time-averaged values which fail to account for the possibility that metamorphism occurs in short-lived pulses during longer time periods of metamorphic quiescence.

  • 59.
    Zhao, Zhihong
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Simultaneous calculation of metamorphic fluid fluxes, reaction rates and fluid-rock interaction timescales using a novel inverse modeling framework2013In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 373, p. 217-227Article in journal (Refereed)
    Abstract [en]

    To study metamorphic carbonation at greenschist facies conditions in the SW Scottish Highlands, a novel inverse modeling framework, which combines solutions of the transport equation with a global optimization method of differential evolution, was developed. Using this framework, we calculated simultaneously time-integrated and time-averaged metamorphic fluid fluxes of 83.4 +/- 35.4 m(3) m(-2) and 10(-10.1) (+/-) (0.5) m(3) m(-2) s(-1), respectively, a time-averaged reaction rate constant of 10(-12.7--10.2) m s(-1) and comparable timescales for fluid flow and fluid-driven reaction of 10(4.3 +/- 0.5) yr and 10(2.7-5.2) yr, respectively. These parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity. Our estimates are (1) consistent with single pass flow of fluid released by metamorphic devolatilization, (2) within the range where heat is transported by conduction and matter is transported by advection, (3) in agreement with an emerging consensus that metamorphic events are relatively short-lived, and (4) supportive of applying laboratory-based estimates of kinetic parameters to metamorphic systems. Based on a sensitivity analysis, we show that (1) selecting the diffusion coefficient (rather than fluid velocity, reaction rate or flow duration) as an input parameter yields more robust estimates of metamorphic fluid flow parameters, and (2) ignoring reaction-dependent porosity and reaction rates can result in an order-of-magnitude uncertainty in best-fit flow parameters, evaluated from concentration profiles. Finally, similarity between our calculated time-averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the 'quasi-stationary state' assumption to quantify metamorphic fluid flow parameters.

12 51 - 59 of 59
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