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 Observatory, Goma, Democratic Republic of Congo 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 (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 o Water salinity o Dissolved gases Lake stability

Density gradient

Gas accumulation

“external” destabilizing phenomenon and/or Gas total (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.” 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. 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 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 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