Tracking African Killer Lakes

NDSU Geosciences Spring Seminar 2004

Fargo, North Dakota

By Damion Knudsen

 

Figure 1.) Satellite photograph of Lake Kivu. Goma and Gisenyi are ~4 Km apart while Mt. Nyiragongo is ~18 Km from the shores of Lake Kivu.

http://svs.gsfc.nasa.gov/vis/a000000/a002300/a002353/lakekivu.tif  (14mb)

 

Abstract

 

Many rift and volcanic lakes are known to achieve great depths. With these great depths and hydrothermal activity they can trap and dissolve magmatic derived gases. This will happen only if the conditions are right forming a chemocline or zone where the waters ionic content increases in relation to the water stratified above it. The chemocline generally marks the last of the stratified layers near the bottom of a lake. These lakes are considered meromictic meaning that some of the stratigraphic layers are not mixing completely and the layer containing the highest concentrations of gas is the monimolimnion. The reason for this stratification is because of the seasonal invariability of annual lake temperature. The lakes in question Nyos, Monoun, and Kivu have strong chemoclines. Lakes Nyos and Monoun have recently received international attention for their gaseous explosions which released CO₂ and other gases into the air. Unfortunately CO₂ is more dense than air so it will seek a lower base level following valleys and land surfaces. Nyos and Monouns CO₂ eventually found their way to villages killing roughly 1800 people. Lake Nyos’s expulsed gases traveled approximately 23 Km. This was some what of a wake up call for scientist’s world wide. After this they focused their attention on trying to predict if it would happen again as also ways to release the gases in these lakes. At this time they have the two lakes under control by controlled gas release also called gas-lift but are there other lakes in the this region that could under go similar gas expulsion?

 

It is known that the rift Lake Kivu in mid western Africa contains massive amounts of CO₂ and methane. A brewery on the shores of Lake Kivu actively extracts methane from the lake floor and is where the idea of gas-lift removal of CO₂ from Nyos and Monoun came from. This lake was not really given much consideration regarding its monimolimnion gas content in relation to a Lake Nyos type catastrophe until January 2002 when Mount Nyiragongo started erupting. Lava from this volcano flowed some 15 miles through Goma, Democratic Republic Of The Congo (Zaire) and into Lake Kivu coming within 150 meters of the lakes chemocline thwarting a possible lake turn over. Lake Kivus shores are home to roughly 1,000,000 people and the lake contains a considerable amount of methane which could be flammable if released. Evidence has supported that the lake has turned over five times in the last 5,000 years killing most to all aquatic life in it and forming a dark layer in the stratigraphic record.

 

Figure 2.) Cross section through a volcanic or rift lake

Figure 3.) Rift zones in Eastern Africa.

Figure 4.) Location of Lakes Monoun, Nyos, and Kivu.

                             

 

Outline

 

 

 

1.)    Geology

·        Oku Volcanic Field located in Cameroon, West Africa

§         Lake Nyos

§         Approximately 2,000 people live on its shores.

·         Bambouto Volcanic Field

§         Lake Monoun

·        Albertine Rift System. Zone of high tectonism and volcanism.

§         Lake Kivu in Rwanda and Congo

§         Approximately 1,000,000 people live on its shores.

·        Lake Nyos and Monoun

§         Lava fountain deposits and basaltic flows are exposed along the lakes, and pyroclastic deposits that include abundant peridotite xenoliths are on the N and E flanks of Lake Nyos. These craters are located on the Cameroon Volcanic Line (CVL), a zone of crustal weakness containing Triassic to Recent, generally alkaline volcanoes. Large amounts of Siderite precipitate at these lake bottoms producing H⁺.

 

2.)    Geochemistry

·        The volcanic Lakes Nyos and Monoun have similar chemistry in that they have very little methane vs. CO₂ in them which possibly indicates little biological activity in its basin.

·        The rift Lake Kivu has similar chemistries but a greater ratio of methane vs. CO₂ possibly indicating much more lake bottom biological activity. Lake Kivu has active methane extraction from the lake bottom which is used to heat brewery vats.

·        Helium to Carbon isotope ratios confers magmatic origins for the CO₂ in these rift and volcanic lakes.

·        Criterion for the formation of a stratified lake with gaseous pressurized monimolimnion.

1.) Lake must be meromictic

2.) A source of gas such as fractured rift or caldera.

3.) Must be approximately 80m or deeper but may occur in shallower lakes.

4.) Temperatures do not appear to matter as much as long as they are uniform throughout the lakes stratification.

·        Crater and Yellowstone Lake in the US both are actively geothermal and bubbles of CO₂ rise to the surface in Yellowstone Lake but these lakes are constantly turning themselves over thus what happened at Lake Nyos is highly unlikely to happen there.

·        It would appear that warm bodies of water work pretty well at trapping methane as long as the water fits the above criterion.

 

3.) Concerns/Problems

·        Lakes Nyos and Monoun have erupted killing around 1800 people. These are small lakes though but build up CO₂ very rapidly.

·        Lake Kivu has lower concentrations of CO₂, but its methane ratio and levels is much higher.

·        It was thought for along time that Lake Kivu was a quiet lake unlike Nyos or Monoun but going back through the paleo-record there is now evidence that the much larger Lake Kivu has possibly turned over at least five times in the last 5,000 years. 5,000 yr. BP is when volcanism and hydrothermal activity started in Lake Kivu. There are periods of time where there is no record of life in the paleo-record of the lake coinciding with layers of dark sediment for this 5,000 year time frame.

·        Early in 2002 the volcano Mount Nyiragongo 14 km north of Lake Kivu erupted spilling lava into its bays. The lava never reached the lower chemo cline where the gas is held suspended but it came within 150m of it. Definitely a cause for concern since the lava flowed through Goma, D.R. Congo with a population of 400,000 and Gisenyi, Rwanda with a population ~600,000 was only 4km away from Goma.

·        Lake Nyos released 1Km³ of gas in 1986 which was 2/3 of its stores and presently with the degassing pipes its levels are around 0.3 Km³. Lake Kivu has 310Km³ of built up pressurized gases ranging from 81% CO₂, and 18% CH₄ (methane) and N₂ ~1%. CO₂ is at 8% saturation but methane is at 57% being much less soluble than CO₂ in H₂O.

 

 

References Cited

 

Aescbach-Hertig, W.; Hofer, M.; Kipfer, R.; Imboden, D.M.; Wieler, R. 1999, Accumulation of mantle gases in a permanently stratified volcanic Lake (Lac Pavin, France). Geochimica et Cosmochimica Acta, v. 63, No. 19/20, 3357–3372.

 

Haberyan, K. A. and Hecky, R. E., 1987, The Late Pleistocene and Holocene Stratigraphy and Paleolimnology of Lakes Kivu and Tanganyika. Palaeogeography, Palaeoclimatology, Paaleolimnology, v.61, 169-197.

 

Halbwachs, M., Tietze, K., Lorke, A., Mudaheranwa, C. 2002,  Investigations in Lake Kivu (East Central Africa) After the Nyiragongo eruption of January 2002, Specific study of the impact of the sub-water lava inflow on the lake stability. Final report to SOLIDARITES, Aide Humanitaire d’Urgence, Paris,

http://www.uni-konstanz.de/limnologie/ags/Peeters/Uphys_people/ Andreas/publication_lakekivu2.pdf

 

Lockwood, J.P., Rubin, M., 1989, Origin and age of the Lake Nyos maar, Cameroon. Journal of Volcanology and Geothermal Research, v. 39, 117-124.

 

Kusakabe, M., Tanyileke,G. Z., McCord, S. A.,  Schladow, S. G., 2000, Recent pH and CO₂ profiles at Lakes Nyos and Monoun, Cameroon: Implications for the degassing strategy and its numerical simulation. Journal of Volcanology and Geothermal Research, v.97, 241-260.

 

Nordin, R. 2003, Dangerous and Nasty Lakes, www.nalms.org/bclss/volume5issue3.pdf

 

Rice, A. 2000, Rollover in volcanic crater lakes: a possible cause for Lake Nyos type disasters. Journal of Volcanology and Geothermal Research, v.97, 233-239.

 

Pasternack, G.B. and Varekamp J.C., 1997, Volcanic lake Systematics. I. Physical constraints. Bulletin of Volcanology, v.58, 528-538.

 

Phillips, J.C., Woods, A.W., 1999, Turbulent bubble plumes and CO₂ driven lake eruptions. Journal of Volcanology and Geothermal Research, v.92 259–270.

 

Schmid, M., Lorke, A., Wuest, A., Halbwachs,M., G. Tanyileke 2003, Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing. Ocean Dynamics, v.53, 288–301.

 

Varekamp, J.C. Pasternack, G.B. Rowe, Jr., G.L. 2000, Volcanic lake Systematics II. Chemical constraints. Journal of Volcanology and Geothermal Research, v.97 161-179.

 

Zhang, Y. 2000, Energetics of gas-driven limnic and volcanic eruptions. Journal of Volcanology and Geothermal Research, v.97, 215-231.

 

Figure 1.) NASA Goddard Space Flight Center (14mb TIFF)

http://svs.gsfc.nasa.gov/vis/a000000/a002300/a002353/lakekivu.tif

 

Figure 2.) The Science of Volcanic Lakes

http://lawr.ucdavis.edu/faculty/gpast/sublakesys.jpg

 

Figure 3.) Faults of Eastern Africa

http://styx.esrin.esa.it/sogha/nyiragongo/Crack%20faults.gif

 

Figure 4.) African political map

http://www.lib.utexas.edu/maps/africa/africa_pol_2003.jpg

 

Presentation in pdf format here.