When 9,000 square kilometres of rock got their own geological era named after them

A geological event with a name

In the geological time scale, most of Earth's history is divided into eras and periods named for places that happen to expose particularly clean rock from those intervals. The Cambrian comes from Wales (the Roman name for which was Cambria). The Devonian comes from Devon in southern England. The Permian comes from the Russian city of Perm. The Jurassic comes from the Jura mountains.

A few intervals, however, have been named not for a region but for a single specific event: the Cretaceous-Paleogene boundary for the asteroid impact that ended the dinosaurs, the Cambrian Explosion for the rapid diversification of animal body plans 540 million years ago, the Great Oxidation Event for the planet's first sustained accumulation of atmospheric oxygen.

In 2009, a similar honour was given to a single rock formation in Karelia. A team led by Victor Melezhik of the Geological Survey of Norway and Anthony Fallick, working from cores drilled by the FAR-DEEP project (Fennoscandia Arctic Russia Drilling Project) of the International Continental Scientific Drilling Program, named the event the Shunga Event.

The name comes from the Karelian village of Shunga. The event the rock records is one of the most consequential biogeochemical episodes in the entire history of life on Earth.

What the rock contains

The Karelian shungite deposit is, depending on how you measure it, the largest single body of pre-Cambrian organic carbon ever identified. Melezhik, Filippov, and Romashkin's 2004 paper in Ore Geology Reviews put the figure at approximately 25 × 10^10 tonnes of organic carbon, spread across roughly 9,000 square kilometres of the Karelian Onega structure. Twenty-five with ten zeros after it. About 250 billion tonnes of carbon, in one rock.

For comparison, all the carbon in all of Earth's atmosphere today, in all the carbon dioxide and methane combined, is about 870 billion tonnes. The shungite layer represents on the order of a quarter of one Earth-atmosphere of organic carbon, deposited 2 billion years ago and still sitting in Karelia.

How it got there

The Shunga Event sits inside a longer episode called the Lomagundi-Jatuli isotope excursion, which ran from approximately 2.22 to 2.06 billion years ago. During that 160-million-year window, the carbon-13 to carbon-12 ratio in marine carbonates worldwide rose to extraordinary positive values. The standard interpretation, supported by the modern synthesis (Bachan and Kump 2015, PNAS; Prave et al. 2022, J. Geol. Soc. London), is that bacterial photosynthesis was producing oxygen at a sustained rate large enough to push atmospheric O2 above modern levels for the first time in Earth's history. The corresponding burial of the photosynthetic organic carbon, into rocks that no longer reach the atmosphere, is what produces the isotopic signal.

Most of the buried organic carbon from the Lomagundi excursion is dispersed thinly through marine carbonates around the world. The Shunga Event is the place where it concentrated: a 9,000-square-kilometre patch of seafloor where conditions were exactly right for massive concentrated organic deposition to occur and be preserved. Methanotroph biomarkers identified by Qu, Črne, Lepland, and Van Zuilen (2012, Geobiology) confirm that the carbon source was bacterial: the bottom of this Karelian basin, two billion years ago, was carpeted with methane-eating microbial mats whose accumulated bodies became the rock.

The "Earth's first oil field" interpretation

Melezhik, Fallick, Filippov, and Larsen's 1999 paper in Earth-Science Reviews went further. They argued that the original deposit was structurally analogous to a modern oil field: organic-rich source rocks (the future shungite layers) buried beneath a thick sedimentary cover, with metamorphic alteration over the next two billion years driving petroleum-like products toward the surface. The hydrocarbons themselves are long gone, lost during exhumation. What remains is the residual high-carbon black slate that we now call shungite. The 1999 paper's title is "Karelian shungite: an indication of 2.0 Ga old metamorphosed oil-shale and generation of petroleum in 2 Ga, 9 km thick basin".

In other words: shungite is the fossilised exhaust of one of the largest ancient algal blooms in Earth's history, frozen in stone for two billion years, in the place where the conditions of the early Earth most directly fingerprinted the rock.

A note on the FAR-DEEP project

The drilling project that produced much of the modern Karelian-shungite work, FAR-DEEP (Fennoscandia Arctic Russia Drilling Early Earth Project), is one of the most important Earth-history scientific drilling programmes ever conducted. Its three-volume synthesis, Reading the Archive of Earth's Oxygenation (Springer, 2013), edited by Melezhik and others, runs to nearly 2,000 pages and is the standard modern reference for Paleoproterozoic Earth-system science. The Shunga Event chapter in volume 3 is the deep-source paper for everything in this thread.

Sources

- Melezhik VA, Fallick AE, Filippov MM, Larsen O 2009, "Palaeoproterozoic petrified oil field (Shunga Event)", Paleontological Journal 43(:866-871, DOI 10.1134/S0031030109080152
- Melezhik VA, Filippov MM, Romashkin AE 2004, "A giant Palaeoproterozoic deposit of shungite in NW Russia: genesis and practical applications", Ore Geology Reviews 24(1-2):135-154, DOI 10.1016/S0169-1368(03)00043-X
- Melezhik VA, Fallick AE, Filippov MM, Larsen O 1999, "Karelian shungite, an indication of 2.0-Ga-old metamorphosed oil-shale and generation of petroleum in 2 Ga, 9 km thick basin", Earth-Science Reviews 47(1-2):1-40, DOI 10.1016/S0012-8252(99)00027-6
- Melezhik VA et al. (eds.) 2013, Reading the Archive of Earth's Oxygenation (3 volumes), Springer Frontiers in Earth Sciences
- Qu Y, Črne AE, Lepland A, Van Zuilen MA 2012, "Methanotrophy in a Paleoproterozoic oil field ecosystem, Zaonega Formation, Karelia, Russia", Geobiology 10(6):467-478, DOI 10.1111/gbi.12007
- Bachan A, Kump LR 2015, "The rise of oxygen and siderite oxidation during the Lomagundi Event", PNAS 112:6562-6567, DOI 10.1073/pnas.1422319112
- Prave AR, Kirsimäe K, Lepland A, Fallick AE et al. 2022, "The grandest of them all: the Lomagundi-Jatuli Event and Earth's oxygenation", J. Geol. Soc. London 179, DOI 10.1144/jgs2021-036
- ICDP FAR-DEEP project: icdp-online.org

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