Joseph Victor Hell

Eruptive history of the Barombi Mbo Maar, Cameroon Volcanic Line, Central Africa: Constraints from volcanic facies analysis

Abstracthis study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U1, U2 and U3. Interpretation of these features indicates that U1 consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U2, a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith- and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under “dry” and “wet” conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U2 and U3, would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar’s polygenetic origin.

Disaster prevention, disaster preparedness and local community resilience within the context of disaster risk management in Cameroon

Cameroon was one of the 57 countries that participated in the Global Network of Civil Society Organizations for Disaster Reduction (GNDR) Views from the Frontline (VFL) 2013 project on everyday disasters, community resilience and disaster preparedness. Working with 6 other civil society organizations, Geotechnology, Environmental Assessment and Disaster Risk Reduction administered 400 questionnaires to frontliners in 7 administrative regions of the country on 14 disaster indicators that assessed the underlying causes of disasters and the level of preparedness and resilience of the communities. Scores from the 89% of informants who responded show that Cameroon occupied the 43rd position globally, was 15th out of the 23 African countries, and was 7th out of the 9 West African countries surveyed. Cameroon average scores for all 14 indicators were lower (poorer) than the West African average, suggesting that a lot more effort is needed in managing disaster risks in the country, i.e., reducing vulnerabilities and increasing preparedness and resilience. At the national level, the Center and Adamaoua Regions recorded the lowest scores of the survey. Above-average scores recorded for some indicators in the Southwest, Northwest and Far North Regions are interpreted to be due to disaster prevention activities like monitoring via early warning systems, resilience building and outreach exercises carried out for disasters like landslides, floods, gas explosions from lakes, and volcanic eruptions, in these areas. Cameroon presently has many laws relating to disaster risk management matters, but an analysis of how the laws are applied shows that the expected results have not been attained, mainly because of over-centralization and a reactive, rather than a proactive approach to disaster risk management. Given her current disaster risk profile, Cameroon has to increase research, better manage, and make disaster risks a central tenet in her development project decision-making, if the goal earmarked in her development vision to become a newly industrialized country by 2035 has to be realized. We propose the creation of an autonomous statutory National Disaster Risk Management Agency which will have a local community-driven bottom-top approach to disaster risk management, and disseminate appropriately tailored disaster risk information to promote a proactive community-based resilience and disaster prevention framework. This will fulfill the post-2015 Sendai framework priority of action No. 2 (strengthening disaster risk governance to manage disaster risk) and appropriately prepare Cameroon to face the challenges of the Sustainable Development Goals (SDGs).

Decreasing removal rate of the dissolved CO2 in Lake Nyos, Cameroon, after the installation of additional degassing pipes

Abstract The amount of dissolved CO2 (CO2aq) in Lake Nyos, Cameroon, has been measured annually since January 2011. First, concentration was estimated by a chemical analysis with the assumption of equilibria between CO2aq, and . The estimated was correlated with the observed electrical conductivity of lake water. The profile of [CO2aq] was generated from the electrical conductivity, pH and temperature profiles. The [CO2aq] profile was integrated over the water column to obtain the total amount of CO2aq. The amounts were 8.4, 6.8, 5.5, 4.4 and 3.9 Gmol (=109 mol) in 2011, 2012, 2013, 2014 and 2015, respectively. The fast reduction between 2011 and 2013 was attributed to the installation of two additional degassing pipes in April 2011. However, the rate of reduction decreased between 2014 and 2015 due to the reduced [CO2aq] at the inlet of the degassing pipes.

Origin of major ions in monthly rainfall events at the Bamenda Highlands, North West Cameroon

Rainwater characteristics can reveal emissions from various anthropogenic and natural sources into the atmosphere. The physico-chemical characteristics of 44 monthly rainfall events (collected between January and December 2012) from 4 weather stations (Bamenda, Ndop plain, Ndawara and Kumbo) in the Bamenda Highlands (BH) were investigated. The purpose was to determine the sources of chemical species, their seasonal inputs and suitability of the rainwater for drinking. The mean pH of 5 indicated the slightly acidic nature of the rainwater. Average total dissolved solids (TDS) were low (6.7 mg/L), characteristic of unpolluted atmospheric moisture/air. Major ion concentrations (mg/L) were low and in the order K(+) > Ca(2+) > Mg(2+) > Na(+) for cations and NO3(-)≫HCO3(-)>SO4(2-)>Cl(-)>PO4(3-)>F(-) for anions. The average rainwater in the area was mixed Ca-Mg-SO4-Cl water type. The Cl(-)/Na(+) ratio (1.04) was comparable to that of seawater (1.16), an indication that Na(+) and Cl(-) originated mainly from marine (Atlantic Ocean) aerosols. High enrichments of Ca(2+), Mg(2+) and SO(2-)4 to Na(+) ratios relative to seawater ratios (constituting 44% of the total ions) demonstrated their terrigenous origin, mainly from Saharan and Sahelian arid dusts. The K(+)/Na(+) ratio (2.24), which was similar to tropical vegetation ash (2.38), and NO3(-) was essentially from biomass burning. Light (< 100 mm) pre-monsoon and post-monsoon convective rains were enriched in major ions than the heavy (> 100 mm) monsoon rains, indicating a high contribution of major ions during the low convective showers. Despite the acidic nature, the TDS and major ion concentrations classified the rainwater as potable based on the WHO guidelines.

Numerical assessment of the potential for future limnic eruptions at lakes Nyos and Monoun, Cameroon, based on regular monitoring data

Abstract We assessed the potential for limnic eruptions at lakes Nyos and Monoun, Cameroon on the basis of numerical modelling and CO2 profiles obtained by regular monitoring of the lakes. The change through time of the profiles suggests one particular scenario for producing an eruption: a supply of CO2-undersaturated fluid from the lake bottom that induces upwards growth of the CO2-rich bottom layer, leading eventually to CO2 saturation at mid-depths of the lake. By using a numerical model for the ascent of a plume of CO2 bubbles, we found that under realistic conditions (e.g. a profile of CO2 as deduced from the regular monitoring data), a bubble plume generated from the middle depths of the lake can reach the lake surface with a high flux of CO2, which corresponds to a limnic eruption. In addition, we developed a numerical model to investigate how changes in the CO2 concentration at the lake bottom affect the dynamics of a two-phase flow in the controlled degassing pipe, using the recently observed CO2 profiles. This model enables us to estimate the CO2 concentrations at the lake bottom from the heights of fountains that are observable at the lake surface.

Vertical distribution of dissolved CO2 in lakes Nyos and Monoun (Cameroon) as estimated by sound speed in water

Abstract Vertical profiles of sound speed (SS) in lake water were measured at lakes Nyos and Monoun (Cameroon) in 2012 and 2014. A significant linear correlation with Pearsons r of 0.987 to 0.995 between total CO2 concentration ([CO2]=[CO2(aq)]+[HCO3−]) and SS excess (Δv) in water was found. Based on this correlation, we propose the SS method as a new simple tool to monitor CO2 concentration in lakes Nyos and Monoun. We applied this method to multipoint measurements of CO2 in lakes Nyos and Monoun, and found horizontal differences between CO2 distributions in the two lakes. The results indicated that Lake Nyos water was stably stratified, and the total CO2 decreased from 2012 to 2014. At Lake Monoun, which consists of three basins (the western basin, the central basin, and the main basin), waters in the western and central basins exhibited a higher dissolved CO2 content than those in the main basin at the same depth. An increase in CO2 was also detected in the main basin from 2012 to 2014.

A new method to determine dissolved CO2 concentration of lakes Nyos and Monoun using the sound speed and electrical conductivity of lake water

Abstract A new method to measure the CO2 concentration in lakes Nyos and Monoun using sound speed (SS) and electrical conductivity (EC) of the lake water was developed. Limnic eruptions at lakes Monoun and Nyos in Cameroon, characterized by a risk related to possible sudden release of magmatic CO2 gas dissolved in the lake water, occurred in 1984 and 1986 respectively, killing about 1800 residents. CO2 monitoring of the lake waters was initiated to prevent further disasters. Classical methods of CO2 measurement, which require chemical analysis of water samples, are not suitable for frequent and multi-point monitoring. Inspired by the method of measuring salt concentration using SS, we obtained data for dissolved CO2 (CO2(aq)) assuming the following formula , where Δv is additional SS due to dissolved ions and k1 and k2 are coefficients determined through experiments. The coefficient k1 was determined with temperature T (°C) dependence as 0.0323–0.000523T [l m s−1 mmol−1]. k2 was estimated to be 0.132 at Lake Nyos and 0.0930 at Lake Monoun. Once k1 and k2 values are obtained, [CO2(aq)] and are calculated using the SS, T and EC.

Rate of siderite precipitation in Lake Nyos, Cameroon

Abstract A large amount of CO2 is stored in the deep waters of Lake Nyos, a volcanic crater lake in Cameroon. The lake is meromictic and thus anoxic in the deeper areas, where dissolved iron exists as Fe2+. Since 2001, a controlled degassing of the lake has been underway. The degassing brings deep water containing Fe2+ to the lake surface as a fountain. This resulted in the formation of Fe(OH)3 precipitates and turned the lake surface red-brown. This coloration was accelerated after the addition of two further degassing pipes in 2011. The Fe(OH)3 precipitates sink to deeper parts of the lake, re-dissolve and are reduced back to Fe2+, which is thought to be precipitating as siderite. The rates of dissolution and precipitation of siderite in Lake Nyos were examined. Fragments of siderite crystals were covered with gel (Epikote) and placed in lake water for 54 h at several depths characterized by chemical features. The change in the thickness of the crystal surface before and after the reaction was analysed by an interferometer in the laboratory. The siderite dissolves at a rate of −0.09 µm a−1 for samples placed at a depth of 50 m, whereas precipitation took place in deeper waters. The precipitation rate of siderite increased by 0.29 µm a−1 with an increase in depth from 100 to 200 m, and decreased to 0.25 and 0.09 µm a−1 at depths of 208 and 210 m, respectively. However, the calculated saturation indices of siderite in the lake waters increased with depth. The objective of this study is to examine the observed rates of dissolution and precipitation of siderite and to compare them with the saturation index of siderite calculated from the chemistry of the lake water. We also discuss the change in Fe species from the oxidation of Fe2+ to Fe3+ at the surface and reduction to Fe2+ and consequent precipitation as siderite in deeper parts of the lake by degassing of CO2 from bottom water in Lake Nyos.

Decreasing capability of the degassing systems at lakes Nyos and Monoun (Cameroon): a new gas removal system applied to Lake Monoun to prevent a future limnic eruption

Abstract The degassing systems installed in the early 2000s at lakes Nyos and Monoun, Cameroon, have been working well, resulting in significant removal of dissolved gas. However, the systems of both lakes started losing their capability due to a reduced CO2 partial pressure in the bottom waters, especially after installation of additional pipes in 2011. After initiation of a degassing operation, changes in CO2 profiles in the bottom layer of Lake Nyos over time showed that gas-poor shallow water has descended to the bottom, leaving little CO2 in the bottom water. The degassing system at Lake Monoun has completely lost its gas self-lift capability as the reduced CO2 partial pressure in the bottom water is too low to sustain the gas-lift system, and all the degassing pipes stopped working in 2010. This situation led to the accumulation of CO2 due to continued natural recharge of magmatic CO2. To compensate this recharge of gas, we installed a new deep water removal system that is driven by solar power at Lake Monoun in December 2013. This system does not need power lines, fuel or complicated maintenance, thus it is convenient for remote lakes such as lakes Monoun and Nyos.