Anomalous 13C enrichment in modern marine organic carbon

Marine organic carbon is heavier isotopically (13C enriched) than most land-plant or terrestrial organic C1. Accordingly, δ 13C values of organic C in modern marine sediments are routinely interpreted in terms of the relative proportions of marine and terrestrial sources of the preserved organic matter2,3. When independent geochemical techniques are used to evaluate the source of organic matter in Cretaceous or older rocks, those rocks containing mostly marine organic C are found typically to have lighter (more-negative) δ 13C values than rocks containing mostly terrestrial organic C. Here we conclude that marine photosynthesis in mid-Cretaceous and earlier oceans generally resulted in a greater fractionation of C isotopes and produced organic C having lighter δ 13C values. Modern marine photosynthesis may be occurring under unusual geological conditions (higher oceanic primary production rates, lower P CO2) that limit dissolved CO2 availability and minimize carbon isotope fractionation4

Are limnic eruptions in the CO2-CH4-rich gas reservoir of Lake Kivu (Democratic republic of the Congo and Rwanda) possible? Insights from physico-chemical and isotopic data

An overturn of Lake Kivu was seriously considered after the January 2002 Nyiragongo volcanic eruption, which erupted 20 × 106 m3 of lava from a NS-oriented fissure on the southern flank of the Congolese volcano. Part of this silica-undersaturated magma produced a lava flow that entered Lake Kivu down to a depth of 70-100 m. The possibility of a rollover comes from the fact that Lake Kivu is meromictic and below ≈250 m a CO2-CH4-rich gas reservoir is present. Thus, the riparian population (≈2,500,000 people) is endangered by a possible limnic eruption. During last 30 years several vertical profiles carried out by several researchers have evidenced a relatively pronounced vertical physico-chemical and isotopical variation. Nevertheless, saturation of CO2 and CH4 appears to be far from critical values, indicating that presently the hydrostatic pressure cannot presently be overcome. Recent studies have suggested an increase in dissolved gases (particular CH4) at depth and the uprising of the chemocline, a limnic eruption could possibly occur within 80-200 years from now. More studies are needed to follow up the lake evolution with time. Simulations will shed light on possible internal and external factors able to provoke the release a suffocating and, possibly, inflammable killer cloud. In this paper, we investigate using recent historical eruptive events of Nyiragongo volcano, we discuss the possibility that a limnic eruption may occur in a near future, although the stability of Lake Kivu is presently high: only exceptionally high magnitude events appear to be able to destabilize the 560 × 109 m3 water volume contained in its basin