Limnic eruption at Kabuno Bay basin? A geochemical and isotopical perspective
F. Tassi, O. Vaselli. D. Tedesco, M.M. Yalire
Department of Earth Sciences, University of Florence, Italy Department of Environmental Sciences, University, Naples, Italy United Nations, Office for the Coordination of Humanitarian Affairs, Switzerland Goma Volcano Observatory, Goma, Democratic Republic of Congo Lake Kivu is constituted by 5 basins: Main, Kalehe, Ishungu, Bukavu and Kabuno Bay. Shallow sills separate these basins. Kabuno Bay 7 m
Main
Kalehe
Ishungu
Bukavu When does a limnic eruption occur?
Pre-requisite: presence of a gas reservoir
•Meromictic lakes (no complete mixing) •Gas discharges from lake bottom •Morphological features
Gas exolution from deep (?) lake strata Lake stability mainly depends on the distribution of density gradient along the vertical water column controlling the resistance to turbulent mixing.
Water density mainly depends on: o Water temperature o Water salinity o Dissolved gases Lake stability
Density gradient
Gas accumulation
“external” destabilizing phenomenon and/or Gas total pressure (Σpressure igas) > hydrostatic pressure Distribution of chemical and isotopic compositions of water and dissolved gases along the vertical profiles: the common approach
Different groups, different methods, same aim Parameters controlling lake stability: vertical distribution of the main dissolved gases (Main basin ) Parameters controlling lake stability: vertical distribution of the main dissolved gases (Kalehe basin ) Parameters controlling lake stability: vertical distribution of the main dissolved gases (Ishungu basin ) Parameters controlling lake stability: vertical distribution of the main dissolved gases (Bukavu basin ) Parameters controlling lake stability: vertical distribution of the main dissolved gases (Kabuno Bay basin ) Relative concentrations of the main dissolved gas species A first consideration
The risk of limnic eruption at Kabuno Bay is greater (100 times ? Tietze suggests) than at the Main basin, although the total gas content is much less than that of the Main basin.
How can this hazard be evaluated (quantified) ?... “A huge CO2 cloud was suddenly released from the lake…”
…and then ? Hazards related to a limnic eruption
“The Nyos limnic eruption in 1986 culminated with a roaring gas-water jet breaking the surface that reached 80-100 metres into the air and created surface waves up to 25 metres high.” Tsunami hazard ?? Hazard related to a limnic eruption …an area of limited water resources Rainwater has high F Water stored in banana’s concentrations caused trees is typically used during by the volcanic plume. dry season. However, during erutpions this water is stronlgy polluted by ashes.
Skeletal fluorosis Hazard related to a limnic eruption at Kabuno Bay
The lake is an important water resource, but…
Lake Nyos prior (1985) and after (1986) the liminc erutpion. The CO2 hazard inland Kabuno Bay
CO2 dry emissions (mazukus) and CO2- Exhalating rich springs are present in several fracture location close to both the Kabuno Bay and Main basins shoreline…
CO2-rich spring The CO2 hazard inland
At Sake village, CO2 concentrations up to several 10s% were measured indoor (1/2 m from the ground)… Hazard related to a limnic eruption
CO2 asphyxiating cloud
CO2 up to 15.000 ppmv (~ 40 times higher than in air) may cause respiration problems, acidosis in blood, a sense of heaviness in the chest, dizziness and nausea. Higher concentrations rapidly induce increased respiratory rate, tachycardia, cardiac arrhythmias and impaired consciousness. Carbon dioxide concentration above 10 %
by vol., coupled with the decrease of O2 may quickly provoke coma and death CO2 behavior in the atmosphere
CO2 in air at lethal concentrations even can be recognized even at great distance from the emitting source CO2 behavior in air
Sun exposure: Diurnal evolution
Turbolent motion
Slow wind causes
little CO2 cloud dilution with air, favouring the cloud transport CO2 behavior in air
Nocturnal evolution CO2 behavior in air
Wind and sun exposure: Weather conditions Where does the CO2 cloud escape ? Why so much CO2 at Kabuno Bay ?
…geochemical evidences lead to open questions CO2/CH4 ratios: similarities and differences among the basins
Main, Ishungu, Kalehe Kabuno Bay basins basin
CO2/CH4 = 6-10 (at the bottoms)
CO2/CH4 = 4-8 CO2/CH4 = 35 (at -140 m) (at the bottom)
CO2/CH4 = 15-20 CO2/CH4 = 85 (at -75) (at -75) Carbon isotopic signature
Main, Ishungu, Kalehe Kabuno Bay basins basin
13 13 δ C-CO2 between δ C-CO2 between -8.7 -3.9 and -5.5 ‰PBD and -11.2 ‰PBD ≠
13 d C-CO2 (mantle) ~ -3 and -7 ‰PBD Helium isotopic signature
Main, Ishungu, Kalehe Kabuno Bay basins basin
R/Ra ~ 2.5 R/Ra ~ 5.5
R/Ra (MORB) ~ 8 Two different domains Kabuno Bay and Sake Main Kivu and Goma area area low Mantle contribution (+ high Mantle contribution crustal) vs. vs. organic CO2 ? Less CH4 !! magmatic CO2 E W Lake Kivu Kabuno Bay Basement Basement 0 Seasonally mixed layer 100 signature? signature? different Permanently Fault system? 200 stratified CO Organic
300 High gas δ Depth (m) Depth (m)
concentration 13
400 C-CO CO2 CH4
Biomass Microbial 2 ? 500 sedimentation reduction 2
CH4 Corganic Microbial Magmatic CO2 and Magmatic CO (?) and fermentation 2 magmatic Helium magmatic Helium diluted by that of slightly affected by the basement that of the basement Two different feeding systems ?
Kabuno Bay and Sake area: Nyamuragira fluid circulating system High CO2 recharge rate
Main Kivu and Goma area: Nyiragongo fluid circulating system
Low CO2 recharge rate Concluding remarks
Admitting that Kabuno Bay is the “best”site for a limnic erutpion, “collateral effects” should be taken into severe account.
Meteorologic parameters, strongly control the CO2 behavior in the air, and the hazard related to a limnic erutpion.
The fluid circulating patterns needs to be defined to better constrain the CO2 recharge rate of the basins, especially that feeding the sublacustrine discharges at Kabuno Bay. …Helium and Carbon isotopes at Nyamuragira !!!
Definition of possible external perturbation and evaluation of their impact on lake (single basin ?) stability. Let’s go !!
High tech equipment (not trash) The Lake trio