If they told you that several million years ago huge volcanoes potentially caused the extinction of both terrestrial and marine animal and plant species, where would you look for evidence of this crime? Numerous clues are contained in the sedimentary rocks, which enclose the fossils of extinct species and bear evidence of major chemical changes in the atmosphere and biosphere in those times of profound global crises.
The vision of the MAPLES project (MAgma PLays with sEdimentary rockS) funded by Norwegian Research Council and led by the PI Dr. Sara Callegaro is to look for clues of this disruption within old magmatic rocks, or at their contact with the sediments. The project is structured around three case-studies, in which magmas intruded sedimentary rocks of different compositions ? evaporites: rocks produced by evaporation of ancient seas or lakes and now rich in sulfate and carbonate minerals; clastic sediments: rocks similar to beach sands, dominated by quartz and other silicate minerals; and shales: rocks rich in organic matter and potentially petroleum.
During the first year of MAPLES, the team concentrated on the study of tabular magmatic layers (called sills) intruded in evaporites in the Tunguska Basin (Siberia, Russia) and in shales of the Amazonas Basin (Brazil). We demonstrated that the Tunguska Basin sills are geochemically correlated, and thus arguably coeval, with the lava piles of the Siberian Traps Large Igneous Province, responsible of the most severe mass extinction in the relatively recent history of Earth, about 252 million years ago, at the end of the Paleozoic era. These sills show clear evidence of thermal and chemical interaction with the surrounding evaporites, and we will bring the study further by studying how and how much chlorine and sulfur were mobilized and released to the atmosphere during this volcanic event. Instead, sills from the Amazonas Basin (201 million years old, end of the Triassic period) were emplaced in shales. Post-doc Manfredo Capriolo found that in these sills tiny bubbles (geologists call them fluid inclusions) inside some quartz crystals contain methane. Released to the atmosphere, methane is a very powerful greenhouse gas, and its involvement in the late Triassic Earth crisis had already been theoretically postulated and demonstrated by numerical models, but this is the first time that methane is found directly in late Triassic sills. These magmas played thermally and chemically with the surrounding sedimentary rocks, rich in organic matter, and produced methane from the organic matter. Is this something that happens regularly when magma intrudes shales? We will try to look for similar inclusions in rocks from the Oslo Rift (Bile island) and from the Karoo Basin (South Africa).
There are two important messages that we can take home from this research. The first: that sometimes you have to focus on very small and apparently unessential details to understand the causes of very large (even global!) events. The second: as the latest IPCC report on climate points out, Anthropogenic greenhouse gas emissions are dramatically changing the composition of our atmosphere. What happened at the end of the Paleozoic or at the end of the Triassic is dangerously similar to what is happening today, and the study of these past volcanic events can help us forecast the risky consequences of human impact on the Earth´s climate and ecosystems.
The intrusive parts of Large Igneous Provinces (LIPs), huge magmatic episodes from the geologic past, have been shown to contribute enormously to the gas budgets that such events emit to the atmosphere, and hence on their impact on the global climate. At the roots of LIPs, we find large sill intrusions, a stack of nearly horizontal magmatic bodies, feeding the eruptions. Sills interact over a vast area with host sedimentary rocks in volcanic basins and generate a number of gas species and trace metals, which are released into the atmosphere by venting and explosive episodes. Current estimates of gas and toxic elements released from these dynamic systems are based solely on the metamorphism affecting the host-rocks, without any quantification of ‘within-sill’ reactions that occur in the magma bodies themselves. Can such reactions inside the cooling and crystallizing magma add further to the gas loads, or can they conversely result, even locally, in sinks, trapping gas and metal species within the system? MAPLES addresses this knowledge shortfall, through a focused geochemical and petrologic study of the processes occurring within the magmatic system. To provide a balanced understanding, MAPLES targets three host-rock lithologies: sills in evaporites and carbonates (Obj.1), in clastic lithologies (Obj.2), and sill-petroleum systems (Obj.3). After focusing on the reactions and their scales occurring in the intrusions within each lithology, a synthesis will be obtained by a balanced evaluation and comparison of the end-member scenarios (Obj.4). MAPLES will improve our understanding of the processes occurring during LIP emplacement. Also, MAPLES will clarify the boundaries between melt-stage and sub-solidus processes of sill-sediment interaction, defining new geochemical tracers to detect and quantify these interactions. LIP-related severe forcing on the atmosphere is the best analog case-study for the changes we are witnessing in the Anthropocene.