Evidence of "Lake Nyos-type" behavior in the geological record: A review

In this article, volcanic lakes that have shown sedimentological evidence of limnic eruptions (i.e., Nyos-type gas bursts) are reviewed. Indeed, to better assess "Nyos-type lakes" related hazards, paleolimnology offers a promising tool to trace the evidence of potential ancient Nyos-type gas explosions. After gas bursts from Lakes Monoun and Nyos in 1984 and 1986, respectively, multiple paleolimnological approaches have been applied to several lakes assumed to be Nyos-type Around the world,. Only 3 lakes in Europe (i.e. lakes Pavin in France, Albano and Monticchio in Italy and one in Africa (i.e. Lake Kivu in D.R. Congo) evidenced markers of limnic eruptions in their sedimentary archives. These features include reworked sediments with reversed ages, brown colors of sedimentary deposits, gas-rich sediments, iron hydroxide-rich sediments, strong Ti and Fe enrichments, sedimentary hiatuses, absence of seismic evidence in the sedimentary record, and significant change in geochemical signature. The dating of these sedimentary deposits has made it possible to determine the ages of the events and their recurrence. This has led to associating these markers with evidence of limnic eruptions, even though some lakes are in temperate climates that favor seasonal overturning of lake waters and thus gradual release of accumulated gas. There is still no agreement on the dynamics and causes, and the scientific debate remains open since there is no concrete reference event in historical time. Lakes Monoun and Nyos, the first and only lakes exploded in recent history, could therefore be considered as natural laboratories to better understand limnic eruptions in lakes around the world. Unfortunately, the well-studied aspects of these Cameroonian "killer lakes" are based more on the dynamics of the explosions, hazard assessment based on water chemistry, and gas releases, rather than on the possible similar behavior in the recent geologic past by applying a combination of old and new limnological approaches. In addition, as the first natural laboratory, Lake Monoun features several advantages, including smaller surface area, shallower depth favorable for coring, easy access, and negligible gas content after artificial degassing since the early 2000s.