Digging deep to set the base of the Geological time

Just before 4.6 billion years ago, only a dense mass of #hydrogen gas and dust known as Solar nebula existed in the place that lately became our solar system. At that time, neither gaseous and rocky planets or their curious moons were already accreted. These celestial objects formed only after the collapse of the Solar nebula, an event that consumed much of the preexisting materials and finally gave place to our planet in the Universe. Residual materials of the proto-planetary disk (Fig. 1) that did not agglutinated to form planets survived as asteroids, comets and meteoroids. Some of these bodies hit the #Earth surface from time to time as #meteorites, after being captured by the planet’s gravity field.

Investigating the earliest evolutionary stages of the Solar System and Earth is a difficult task. It has been possible only through direct and indirect observations of the inner and outer space, theoretical approaches and the study of a few meteorites found in Earth along the last decades. These bolides testified the formation of our star system and preserve a small part of its oldest known solid particles. Aiming to set the base of the Geological time, we used these rare microscopic remnants to define and ratify the lower boundary of the #Hadean eon. This work is part of the most recent Subcommission on PreCryogenian Stratigraphy ( International Union of Geological Sciences (IUGS) ) efforts to improve the International Chronostratigraphic Chart in the time interval older than c. 720 million years.

The definition of a rock-based criterion to improve the oldest time boundaries in the International Chronostratigraphic Chart has been a major challenge of current and previous members of the International Commission on Stratigraphy. Besides representing only a small fraction of the rock exposures on Earth, most of the known Precambrian record (> c. 540 Ma) has been affected by multiple crustal reworking events through time leading to the strong obliteration of their original characteristics. It makes almost impossible replicating the same rock- and fossil-based criteria used for the Phanerozoic eon to this time interval, specially the Archean and #Hadean eons (older than 2.5 billion years ago). Nevertheless, after an intense debate and a hard work, we published last week a new paper formalizing the oldest time interval of the chronostratigraphic chart (i.e., #Hadean eon) and ratifying its lower time boundary at 4567 Ma.

No rocks older than 4.1 billion years have been found in the planet so far. For this reason, we used millimeters-sized CAI's and chondrules preserved in bolides that hit the Earth surface to define the base of the Hadean eon. CAI's are calcium-aluminum-rich microscopic inclusions formed through the condensation of gases in the Solar nebula and preserved in a special type of primitive meteorites known as carbonaceous chondrites (Fig. 2). These meteorites are thought to preserve the closest composition to the early Solar System. The inclusions are found together chondrules, which in turn represent primitive solids formed from the aggregation of the Solar nebula dust and lately accreted to generate planetesimals, planetary embryos, planets and other celestial bodies (Fig 3).

CAI's and chondrules found in the famous Allende and Efremovka meteorites (Fig. 2) were precisely dated at 4567 Ma in different independent laboratories of the world and using robust analytical techniques (Amelin et al. 2010, Connely et al. 2012, 2017). The precise age set the age of the Solar nebula collapse (i.e. Solar System formation) and the maximum age for the accretion of its planets. By consequence, it also marks a maximum age for #Earth and a pin point for the beginning of the Geological time. Combined with a previous proposal of the Subcommission on PreCryogenian Stratigraphy, we defined the Hadean eon as the time interval between the age of the oldest knwon solids in the Solar System (as dated by Amelin et al. 2010, Connely et al. 2012, 2017) and the appearence of the first rocks in the geological record (4.03 billion years-old Acasta gneiss of Canada; Bowring et al. 1999). This eon would, thus, enclose the planetary differentiation into core and mantle, the formation of the Moon and important events that led the emergence of the proto-crust, oceans, atmosphere and, probably, biosphere.

Defining the time boundaries of the International Chronostratigraphic Chart is a time-consuming work that demands the collaboration of researchers from the entire world. Usually, we take into account the physical geological record (minerals and rocks) to set time boundaries. This time, however, the only reference available to define the base of the Geological Time came from space. Curious... don't you think?

Be ready for more coming news!

Click in the links below to access the complete version of our paper entitled "Ratification of the base of the ICS Geological Time Scale: The Global Standard Stratigraphic Age (GSSA) for the Hadean lower boundary" by Halla et al.

https://www.researchgate.net/publication/379268644_Ratification_of_the_base_of_the_ICS_Geological_Time_Scale_the_Global_Standard_Stratigraphic_Age_GSSA_for_the_Hadean_lower_boundary

https://www.episodes.org/journal/view.html?doi=10.18814/epiiugs/2024/024002

I have been a voting member of the Subcommission on PreCryogenian Stratigraphy since 2016, which is composed by Precambrian specialists from all over the world.

Jaana Halla Matheus Kuchenbecker Evelyn Sanchez Jaana Halla @Nora Noffke Flavia Callefo Douglas Galante Stanley Awramik Andrey Bekker Alex Brasier @adrita choudhury Dr. Jan-Peter Duda @christopher fedo @jessica haddock Peter Haines @linda hinnov Alex Hofmann @alex kovalick Juha K?ykk? @donald lowe Martin Homann David Huston Simon Johnson Linda Kah @alan kaufman Noah Nhleko, PhD Barry Reno @yogmaya shukla @albertus smith Mark van Zuilen Frances Westall Martin Whitehouse

Reference list

Amelin, Y., Kaltenbach, A., Iizuka, T., Stirling, C. H., Ireland, T. R., Petaev, M., and Jacobsen, S. B., 2010, U–Pb chronology of the Solar System’s oldest solids with variable 238U/235U. Earth and Planetary Science Letters, v. 300, pp. 343–350. doi:10.1016/j.epsl.2010.10.015

Bowring, S., and Williams, I., 1999, Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, pp. 3–16. doi:10.1007/s004100050465

Connelly, J.N., Bollard, J., and Bizzarro, M., 2017, Pb–Pb chronometry and the early Solar System. Geochimica et Cosmochimica Acta, v. 201, pp. 345–363. doi:10.1016/j.gca.2016.10.044

Connelly, J.N., Bizzarro, M., Krot, A.N., Nordlund, ?., Wielandt, D., and Ivanova, M.A., 2012, The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk. Science, v. 338, pp. 651– 655. doi:10.1126/science.1226919

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