Double-diffusive convection in Lake Nyos, Cameroon
01 Aug 2004-Vol. 51, Iss: 8, pp 1097-1111
TL;DR: In this article, a temperature time series measured at 62 m depth indicates that the double-diffusive convection started in the second half of March 2002, when a set of 26 well-mixed layers with thicknesses of 0.2-2.1 m and sharp interfaces in between were discovered at 53-74 m depth.
Abstract: Since the catastrophic CO2 eruption in 1986, Lake Nyos has been investigated in detail by several research groups. However, no signs of double-diffusive convection were observed before December 2002, when a set of 26 well-mixed layers with thicknesses of 0.2-2.1 m and sharp interfaces in between were discovered at 53-74 m depth. Such pronounced steps are a characteristic feature of double-diffusive convection of the diffusive regime. A temperature time series measured at 62 m depth indicates that the double-diffusive convection started in the second half of March 2002. The trigger was most probably the cooling at the top of this layer caused by relatively strong seasonal convective mixing down to 52.5 m depth during the dry season in February 2002. The heat fluxes calculated by the heat budget method and the thicknesses of the layers agree within the uncertainties with the values expected from the double-diffusive flux laws. The heat fluxes increased by an order of magnitude since the establishment of the double-diffusive convection and reached values comparable to the heat input by a source of warm and CO2-enriched water to the deepest zone of the water column. In contrast, the CO2 fluxes caused by double diffusion Lire negligible compared to the input by this source. Because the double-diffusive heat fluxes were higher in the upper layers of the staircase compared to the lower ones, the temperature gradient between 60 and 75 m depth approximately doubled from March 2002 to December 2002, whereas the total dissolved solids gradient remained almost constant during this period. Consequently, this process is reducing the stability of the staircase and could potentially lead to a complete homogenization of this zone within a few years. It cannot be excluded that a similar double-diffusive event could have been the trigger of the CO2 eruption in 1986. (C) 2004 Elsevier Ltd. All rights reserved.
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TL;DR: In this article, the authors showed that the vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers in the East African Rift Lake Kivu.
Abstract: [1] The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years Turbulent transport was regarded as the main pathway of vertical exchange Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800–1000 years Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release With the estimated current CH4 production, the gas concentrations could approach saturation within this century
145 citations
Cites background or methods from "Double-diffusive convection in Lake..."
...…JOURNAL OF THE EARTH SCIENCES Research Letter Volume 6, Number 7 26 July 2005 Q07009, doi:10.1029/2004GC000892 ISSN: 1525-2027 Copyright 2005 by the American Geophysical Union 1 of 11 could be initiated by a sufficiently large volcanic eruption within the deep water [Schmid et al., 2004b]....
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...The probability of a gas release triggered by a magmatic eruption within the lake increases much faster than the CH4 concentration due to the nonlinear relation between gas concentrations and the heat input needed to produce a rising plume that reaches saturation depth [Schmid et al., 2004b]....
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...CH4 concentrations are approximately 21% of CO2, but because of the low solubility of CH4, its contribution to the total gas pressure exceeds that of CO2 [Tietze, 1978; Schmid et al., 2004b]....
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...From this time series and the background temperature gradient, vertical displacements due to internal waves and conse- 4 of 11 quently their energy content was determined with the same method as for Lake Nyos [Schmid et al., 2004a]....
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...…the semiempirical flux law for double-diffusion by Kelley [1990], which successfully predicted heat fluxes in the comparable case of Lake Nyos [Schmid et al., 2004a], and (3) from observed temperature fluctuations within convectively mixed layers by applying the convective scaling relation…...
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TL;DR: In this article, the isotopic signatures of dissolved Fe(II) and settling Fe(III) particles were combined with the concentration profiles and settling fluxes of the Fe particles in the lake.
79 citations
Additional excerpts
...vertical transport was enhanced due to double diffusive convection ( Schmid et al., 2004 )a ndKZ was set to 3·10 �7 m2 s�1....
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TL;DR: In this article, the authors performed an ad hoc lake survey of the stability of the water column stratification and double-diffusive mixing phenomena in Lake Kivu after the Nyiragongo volcano eruption.
Abstract: During the eruption of Nyiragongo Volcano in January 2002 about 10 6 m 3 of lava entered Lake Kivu. The high concentrations of CO2 and CH4 dissolved in the deep waters of Lake Kivu raised serious concerns about a potential gas outburst with catastrophic consequences for the population in the Kivu-Tanganyika region. Therefore, 3 weeks after the volcanic eruption, we performed an ad hoc lake survey of the stability of the water column stratification. Vertical profiles of temperature and turbidity revealed signatures of the lava, which had penetrated to 100 m depth; however, there was no substantial warming or destratification of the gas-containing deep layers below. The deep double-diffusive structures also remained unaltered. Based on these observations, we conclude that a thermally driven gas outburst in Lake Kivu is not to be expected from future eruptions of comparable dimensions. In addition, the recent measurements allowed for an update and gave new insight into the stratification and double-diffusive mixing phenomena in Lake Kivu. A comparison with former measurements revealed a warming of the upper part of the lake of up to 0.58C within the last 30 yr, which could be attributed to climate variability.
77 citations
Cites methods from "Double-diffusive convection in Lake..."
...The ionic carbon species (HCO and CO ) are2 223 3 included in S, and [H2CO3] was estimated following Schmid et al. (2003), whereas [CH4] was taken from Tietze et al. (1980)....
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TL;DR: In this paper, double-diffusive staircases with a total of 230-350 mixed layers and sharp interfaces were observed in nine microstructure temperature profiles measured during February 2004 in Lake Kivu.
Abstract: Double-diffusive staircases with a total of 230-350 mixed layers and sharp interfaces were observed in nine microstructure temperature profiles measured during February 2004 in Lake Kivu. The presence of these staircases at depths. 120 m indicates that diapycnal turbulent mixing is weak and vertical diffusive transport is dominated by double diffusion. Contrary to previously investigated natural or laboratory double-diffusive systems, the dissolved gases CO(2) and CH(4) contribute significantly to the density stratification, thereby influencing the formation and the structure of the staircases. The density ratio (i.e., the ratio of the stabilizing effect of dissolved substances to the destabilizing effect of temperature) ranges between 2.0 and 4.5 in large sections of the deep waters, implying a high susceptibility to the formation of staircases. The mixed layers (average thickness 0.48 m) are shown to be in a state of active convection. The average thickness of the interfaces (0.18 m) is surprisingly constant and independent of the large-scale stratification. The vertical heat fluxes correlate well with the temperature steps across the interfaces. Lake Kivu receives inflows from subaquatic springs at several depths that maintain the large-scale structure of the density stratification and disturb the staircases. In comparison to earlier observations from 1972, the double-diffusive heat fluxes appear to have been reduced, leading to a heat accumulation in the deep waters. Conversely, the strengthening of the main chemocline indicates an increased discharge of the subaquatic springs that could be responsible for recent changes in the nutrient cycling and methane production in the lake.
72 citations
01 Jan 2009
TL;DR: Gammons et al. as mentioned in this paper summarized the literature of mining pit lakes, with a particular focus on issues that are likely to be of special relevance to the creation and management of pit lakes in northern climates.
Abstract: Gammons, C. H., Harris, L.N., Castro J.M., Cott, P.A., and Hanna, B.W. 2009. Creating lakes from open pit mines: processes and considerations with emphasis on northern environments. Can. Tech. Rep. Fish. Aquat. Sci. 2826: ix + 106 p. This document summarizes the literature of mining pit lakes (through 2007), with a particular focus on issues that are likely to be of special relevance to the creation and management of pit lakes in northern climates. Pit lakes are simply waterbodies formed by filling the open pit left upon the completion of mining operations with water. Like natural lakes, mining pit lakes display a huge diversity in each of these subject areas. However, pit lakes are young and therefore are typically in a non-equilibrium state with respect to their rate of filling, water quality, and biology. Separate sections deal with different aspects of pit lakes, including their morphometry, geology, hydrogeology, geochemistry, and biology. Depending on the type and location of the mine, there may be opportunities to enhance the recreational or ecological benefits of a given pit lake, for example, by re-landscaping and re-vegetating the shoreline, by adding engineered habitat for aquatic life, and maintaining water quality. The creation of a pit lake may be a regulatory requirement to mitigate environmental impacts from mining operations, and/or be included as part of a closure and reclamation plan. Based on published case studies of pit lakes, large-scale bio-engineering projects have had mixed success. A common consensus is that manipulation of pit lake chemistry is difficult, expensive, and takes many years to achieve remediation goals. For this reason, it is prudent to take steps throughout mine operation to reduce the likelihood of future water quality problems upon closure. Also, it makes sense to engineer the lake in such a way that it will achieve its maximal end-use potential, whether it be permanent and safe storage of mine waste, habitat for aquatic life, recreation, or water supply.
56 citations
Cites background from "Double-diffusive convection in Lake..."
...Lakes or ocean waters undergoing double-diffusion convection exhibit a characteristic “staircase” vertical profile in salinity and temperature (Schmid et al., 2004) (Fig....
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...Although double-diffusion convection has been documented from some natural meromictic lakes (e.g., Lake Nyos, Cameroon, see Schmid et al., 2004), the authors are not aware of a mining pit lake where this has been observed....
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TL;DR: In this article, the authors introduce linear internal waves and herar flows in a stratified fluid and double-diffusive convection in stably stratified fluids, and show that the shear flows can produce turbulence.
Abstract: Preface 1. Introduction and preliminaries 2. Linear internal waves 3. Finite amplitude motions in stably stratified fluids 4. Instability and the production of turbulence 5. Turbulent shear flows in a stratified fluid 6. Buoyant convection from isolated sources 7. Convection from heated surfaces 8. Double-diffusive convection 9. Mixing across density interfaces 10. Internal mixing processes Bibliography and author index Recent publications Subject index.
2,722 citations
"Double-diffusive convection in Lake..." refers background in this paper
...Such stratification, with a stabilizing and a destabilizing component, can lead to convective layers by double-diffusion (Turner, 1973), if the effect of the stabilizing component is not more than approximately 10 times as large as the destabilizing effect of temperature (Kelley et al....
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...Double-diffusive staircases have been observed in systems with Rrp10; but the susceptibility to double diffusion increases with decreasing Rr until the density ratio approaches 1, where the stratification destabilizes (Kelley et al., 2003; Turner, 1973)....
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...(16)), which is based on a scaling relation (Turner, 1973), and the semi-empirical Eq....
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...The susceptibility of a water column to doublediffusive convection is usually expressed by the density ratio Rr (Turner, 1973) Rr 1⁄4 bS @S=@z a @T=@z : ð9Þ...
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TL;DR: In this article, two models for the source of oceanic turbulence are considered; namely, production by the Reynolds stress working against a time variable mean shear, and the gravitational collapse of Kelvin-Helmholtz instabilities.
Abstract: Scaling of the turbulent energy equation suggests the balance of terms in the ocean is between turbulent production, dissipation and the loss to buoyancy. In this paper two models for the source of oceanic turbulence are considered; namely, production by the Reynolds stress working against a time variable mean shear, and the gravitational collapse of Kelvin-Helmholtz instabilities. Both of these shear instabilities are believed to be important in the ocean. Using values for the critical flux Richardson number and the measurements from studies of Kelvin-Helmholtz instabilities, the efficiency of turbulent mixing is shown to be comparable for the two models. Therefore, a general relationship between the dissipation rate and the buoyancy flux due to the small-scale turbulent velocity fluctuations is derived. The result is expressed as an upper bound on the value of the turbulent eddy coefficient for mass Kρ ⩽ 0.2ϵ/N2. Values of Kρ are calculated from recent oceanic measurements of energy dissipation...
1,874 citations
"Double-diffusive convection in Lake..." refers background in this paper
...15 (Osborn, 1980; W . uest and Lorke, 2003), the resulting turbulent diffusivities in the double-diffusive zone are ca....
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01 Jan 1997
814 citations
"Double-diffusive convection in Lake..." refers methods in this paper
...0 10 m s , calculated with diffusion coefficients given by Boudreau (1997)) is lower....
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TL;DR: In this article, small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior.
Abstract: Recent small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior. The surface layer consists of an energetic wave-affected thin zone at the very top and a law-of-the-wall layer right below, where the classical logarithmic-layer characteristic applies on average. Short-term current and dissipation profiles, however, deviate strongly from any steady state. In contrast, the quasi-steady bottom boundary layer behaves almost perfectly as a logarithmic layer, although periodic seiching modifies the structure in the details. The interior stratified turbulence is extremely weak, even though much of the mechanical energy is contained in baroclinic basin-scale seiching and Kelvin waves or inertial currents (large lakes). The transformation of large-scale motions to turbulence occurs mainly in the bottom boundary and not in the interior, where the local shear remains weak and the Richardson numbers are generally large.
519 citations