Journal of African Earth Sciences | 2019
Lago Albano, the “anti-Nyos-type” lake: The past as a key for the future
Abstract
Abstract On 21 August 1986 almost 1800 people were asphyxiated by a CO 2 cloud violently released by Lake Nyos, Cameroon. Post-1986 monitoring of Lake Nyos revealed that CO 2 steadily accumulates in bottom waters through recharge from soda-rich aquifers. The 1986 lake roll-over event triggered scientific research on volcanic lakes, creating “Nyos biased” interpretations: the search for dissolved CO 2 in other lakes might have led to over-interpretations regarding hazard assessment. In this study, a thorough review of the historical literature on Lago Albano over the past approx. 2800 years shows no clear evidence of Nyos-type bursts, contrary to previous ideas. In 1989 Lago Albano was affected by a large CO 2 pulse, concomitant with a seismic swarm below Colli Albani volcano. Tracing back in historical literature, at least two similar anomalous degassing episodes occurred out of five seismic crises between 1829 and 1927. Partial temperature- and density-driven roll-over of the top-9 m of Lago Albano commonly releases accumulated CO 2 each winter (Chiodini et al., 2012). This degassing dynamics avoids long-term CO 2 accumulation in bottom waters, as is the case at permanently stratified lakes in the tropics, such as Lake Nyos. Here we show that Lago Albano is an “anti-Nyos-type lake: sudden recharge and regularly periodic release of CO 2 (Lago Albano) vs. steady recharge and sudden release of CO 2 (Lake Nyos). Despite past evidence of hazardous events, (1) the intensive well pumping from the Albano aquifer might lower lake level, and hence decrease the CO 2 saturation pressure threshold, and (2) the absence of cold winters, hence avoiding yearly lake roll-over favoring CO 2 accumulation at bottom layers, are modern factors that rise the need to revise hazard assessment and future monitoring strategies. Despite the fact that CO 2 concentration in bottom waters was far from saturation conditions during the last survey (May 2010; Chiodini et al., 2012), making a limnic eruption highly unlikely, it is necessary to know the saturation state of CO 2 in bottom waters and physical lake stability at any time, in order to be prepared for a next anomalous co-seismic CO 2 degassing event of unknown quantity near Lago Albano.