USGS Science⌕ Search
Home/Archive/USGS

USGS · 70017274

Six years of change in Lake Nyos, Cameroon, yield clues to the past and cautions for the future

William C. Evans·L. D. White·M. L. Tuttle·G.W. Kling·G. Tanyileke·R. L. Michel
View original source ↗Export metadata ↓

Abstract

The catastrophic release of gas from Lake Nyos, Cameroon, in 1986 caused substantial but incomplete mixing of the stratified water column. The post-release evolution of water-column structure has been monitored through April 1992. Changes began immediately after the event as rainfall and inflow brought dilute fluid into the surface layer. Inflow and surface mixing have gradually deepened the chemocline. The Total Dissolved Solids (TDS) values in the upper 40 m of the water column have dropped from a few hundred mg/kg just after the release to <100 mg/kg. The chemocline is presently strongest at 50 m depth; 5 m below this, the TDS = 570 mg/kg. From 55 to 150 m depth is a gentle gradient in which TDS reaches 920 mg/kg. Little change in water-column chemistry has occurred in this depth interval since the release. Between 150 m depth and lake bottom at 210 m depth, a strong secondary chemocline has formed. Temperature, CO 2 concentration ([CO 2 ]), and TDS have all increased in the deepest layer in response to recharge by warm, mineralized water, reaching values of 25.0°C, 320 mmol/kg, and 1800 mg/kg, respectively, 1 m above lake bottom. Considering all these changes in part as a “recovery” process, it is possible to construct a model of the pre-release water column. The data indicate that the pre-release chemocline was at least 50 m deep. Above the chemocline was a dilute layer containing a seasonal thermocline; below the chemocline was probably a gradient zone(s) with correlated increases in TDS and [CO 2 ] and a secondary chemocline near lake bottom. Maximum values of TDS and [CO 2 ] calculated for pre-release bottom water are 2400 mg/kg and 430 mmol/kg, respectively, based on tritium data. From this pre-release structure, a model of the gas release is proposed that is consistent with available chemical and observational data. An important feature of the model is that disruption of the pre-existing stratification was much more extensive than previously proposed, and even the deepest water layers were involved in the event. This model is not intended to limit possible gas release mechanisms, and thus complete re-establishment of pre-1986 water-column conditions is not a prerequisite for a future release. Spontaneous instability could occur at lake bottom in <20 yr if dissolved gas pressures continue to increase in this zone by 0.5–1 bar/yr as they have for the last 6 yr.

Explore related subjects

Geochemical JournalArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

William C. Evans, L. D. White, M. L. Tuttle, G.W. Kling, G. Tanyileke, R. L. Michel. 1994. Six years of change in Lake Nyos, Cameroon, yield clues to the past and cautions for the future. https://doi.org/10.2343/geochemj.28.139

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2008

Evolution of CO2 in Lakes Monoun and Nyos, Cameroon, before and during controlled degassing

Evolution of CO 2 in Lakes Monoun and Nyos (Cameroon) before and during controlled degassing is described using results of regular monitoring obtained during the last 21 years. The CO 2(aq) profiles soon after the limnic eruptions were estimated for Lakes Monoun and Nyos using the CTD data obtained in October and November 1986, respectively. Based on the CO 2(aq) profiles through time, the CO 2 content and its change over time were calculated for both lakes. The CO 2 accumulation rate calculated from the pre-degassing data, was constant after the limnic eruption at Lake Nyos (1986-2001), whereas the rate appeared initially high (1986-1996) but later slowed down (1996-2003) at Lake Monoun. The CO 2 concentration at 58 m depth in Lake Monoun in January 2003 was very close to saturation due to the CO 2 accumulation. This situation is suggestive of a mechanism for the limnic eruption , because it may take place spontaneously without receiving an external trigger. The CO 2 content of the lakes decreased significantly after controlled degassing started in March 2001 at Lake Nyos and in February 2003 at Lake Monoun. The current content is lower than the content estimated soon after the limnic eruption at both lakes. At Monoun the degassing rate increased greatly after February 2006 due to an increase of the number of degassing pipes and deepening of the pipe intake depth. The current CO 2 content is ∼40% of the maximum content attained just before the degassing started. At current degassing rates the lower chemocline will subside to the degassing pipe intake depth of 93 m in about one year. After this depth is reached, the gas removal rate will progressively decline because water of lower CO 2(aq) concentration will be tapped by the pipes. To keep the CO 2 content of Lake Monoun as small as possible, it is recommended to set up a new, simple device that sends deep water to the surface since natural recharge of CO 2 will continue. Controlled degassing at Lake Nyos since 2001 has also reduced the CO 2 content. It is currently slightly below the level estimated after the limnic eruption in 1986. However, the current CO 2 content still amounts to 80% of the maximum level of 14.8 giga moles observed in January 2001. The depth of the lower chemocline may reach the pipe intake depth of 203 m within a few years. After this situation is reached the degassing rate with the current system will progressively decline, and it would take decades to remove the majority of dissolved gases even if the degassing system keeps working continuously. Additional degassing pipes must be installed to speed up gas removal from Lake Nyos in order to make the area safer for local populations.

Lake Monoun, Lake Nyos↗
USGS1997

Determination of lead, cadmium, indium, thallium and silver in ancient ices from Antarctica by isotope dilution-thermal ionization mass spectrometry

The concentrations of five chalcophile elements (Pb, Cd, In, Tl and Ag) and the lead isotope ratios in ancient ices from the Taylor Dome near coastal Antarctica, have been determined by the isotope dilution-thermal ionization mass spectrometry (ID-TIMS), with ultra-clean laboratory techniques. The samples were selected from segments of cores, one of which included a visible ash layer. Electric conductivity measurement (ECM) or dielectric properties (DEP) gave distinctive sharp peaks for some of the samples chosen. Exterior portions of the sample segments were trimmed away by methods described here. Samples were evaporated to dryness and later separated into fractions for the five elements using an HBr-HNO 3 anion exchange column method. The concentrations are in the range 2.62-36.7 pg Pb/g of ice, 0.413-2.83 pg Cd/g, 0.081-0.34 pg ln/g, 0.096-2.8 pg Tl/g and 0.15-0.84 pg Ag/g, respectively. The dispersions in duplicate analyses are about ±1% for lead and cadmium, ±2% for indium, ±4% for thallium and ±6% for silver, respectively. The concentrations of lead obtained are commonly higher than those in the present-day Antarctic surface snows, but the isotope ratios are distinctively higher than those of the present-day snows and close to those of the other ancient ice collected from a different Antarctic area.

Geochemical Journal↗
USGS1993

Scanning electron microscope observations of sublimates from Merapi Volcano, Indonesia

Sublimates were sampled from high-temperature (up to 800°C) fumaroles at Merapi volcano, Indonesia in January 1984. Sampling is accomplished by inserting silica tubes into high-temperature vents. Volcanic gas flows through the tubes and sublimates precipitate on the inner walls in response to the temperature gradient. With decreasing temperature (800–500°C) in the tubes, there are five sublimate zones: 1) cristobalite, magnetite, and halite; 2) halite, sylvite, K-Ca sulfate (K 2 Ca[SO 4 ] 2 ), acmite, wollastonite, and pyrite; 3) halite, sylvite, galena, Pb-Bi sulfide, aphthitalite, sphalerite, and Cs-K sulfate; 4) halite, sylvite, galena, PbKCl 3 , aphthitalite, and Na-K-Fe sulfate ([Na, K] 2 Fe[SO 4 ]Cl 2 ); and 5) halite, sylvite, galena, PbKCl 3 , and various sulfates of Pb, Cu, and Zn. Texturally, the sublimate phases grade from large, well-formed crystals at their highest-temperature occurrence to more numerous, smaller crystals that are less perfect at lower temperatures. These changes imply that the crystal nucleation and growth rates increase and decrease, respectively, as temperature decreases. Several of the sublimate phases also exhibit highly anisotropic morphologies (whiskers, platelets, spheres), especially below their highest temperature of deposition. Overall, the textural data suggest that the gas is saturated or slightly supersaturated with the phases at their hottest occurrence, but that the gas becomes increasingly supersaturated with the phases at lower temperatures. The anisotropic morphologies probably form because the crystals grow toward more supersaturated conditions in the center of the tube. The gas fails to maintain equilibrium with the precipitating sublimates because the high velocity of the carrier gas prevents complete mass transfer from the gas stream to the tube walls.

Merapi Volcano↗