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History of Geology

History of Geology

What rocks tell and how we came to understand it

Time for a new epoch? - the Anthropocene

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The notion that the influence on earth's systems by humankind is so great that this phase of earth's history needs a proper name is not new, already in 1873 the Italian geologist Antonio Stoppani suggested the term Anthropozoic, in 1879 the American geologist Joseph LeConte discusses in his textbook the Psychozoic and in 1927 the French philosoph Édouard Louis Emmanuel Julien Le Roy adopts the Noosphere from Russian mineralogist Vladimir Vernadsky to denote the third epoch of earth - after the inanimate geosphere and animate biosphere the epoch of human thought has begun. However considering the vastness of geologic time - "we find no vestige of a beginning, no prospect of an end" (James Hutton) - and apparently still untouched vast regions on the surface of the globe in the late 19th century the idea that humans can alter earth's history seemed negligible.

Fig.1. The "Appearance of Man", a historic depiction by L. FIGUIER 1872.

In the past 100 years with the advance o science and technology, with the increase of productivity of goods and food products the world population exploded and the impact of every single of us on earth's resources increased dramatically - becoming more and more evident.

In 2002 the chemist Paul Crutzen suggested that the effects of human population and culture on the environments are so pronounced, that they will leave a permanent geological marker in the stratigraphic record of the planet - he coined for the already started epoch the term Anthropocene, term that soon became informally incorporated in technical literature and experienced increased interest in the last decade - the GSA Meeting in 2011 even will discuss earth's history from the "Archean to the Anthropocene".

Most geological epochs were defined in historic times by observed geological changes in lithology, petrology, chemical composition and especially in the paleontological content - the extinction and replacement of species in the fossil record. Humans artefacts and fossils where recorded in the stratigraphic column since our first ancestors developed lithic industry, but only in the Holocene human activity and influence on environment is observable in the deposited sediments.

With the development of agriculture and pasture the pollen spectrum of plants recorded in bog sediments shifts significantly, tree species diminish, grass and cultivated plants increase. Clearing of forests modified the hydrology of river catchments and increased erosion, sand layers in lake sediments are often connected with human activity in the area surrounding the lake.

With the first civilisations, geochemical changes as results of environmental pollution by mining and handling of metals are observed in soil and lake sediments and even in the ice records of Greenland.

The impacts of humans increase during the second half of the Holocene as a result of increasing population and development of large civilisations, with many humans working together to realize projects that single individuals couldn't possible achieve by itself.

In the last 200 years, since the Industrial Revolution, humankind has surpassed all efforts to shape earth and nature for its needs compared to all previous generations. Humans use the majority of natural geo-resources, like minerals, rocks, soil and water. Water is stored or redirected, and soil and sediments are cultivated, excavated, transported and deposited, influencing sedimentation patterns and causing a lithological change in the geological record. The modifications of the landscape and the construction of buildings of rocks and concrete are effects and artefacts that can become fossilized, producing a recognizable layer in the stratigraphic record. The mining of Uranium and other fissionable materials and especially the detonation of atomic bombs introduced large quantities of radioactive substances in the earth's atmosphere, producing a sedimentary layer with a geochemical signature.

The burning of fossil fuels has altered the chemistry of the atmosphere, that on its own will influence temperature and precipitation, again influencing erosion (for example improved chemical weathering) and deposition. The climate change will have also effects on the basic erosion-sedimentation level of earth - the sea level, where all rivers transport their sediment load and where carbonate and siliciclastic rocks are deposited. The increase of carbon dioxide in the atmosphere alters also the chemistry of the oceans, modifying possibly the chemical precipitation and dissolution of rocks and the production of carbonate by organisms.

The use of fire to clear land and the hunt for animals maybe has forced the distribution and finally the extinction of the Pleistocene Megafauna - the Pleistocene extinction proceeds still today with increasing speed on land and in the seas: active hunt, destruction of habitats, relocation of animals and plant species, spread of pathogens, climate change have all catastrophic effects of the local native fauna and flora. The results are permanent in the fossil record, extinct species are no longer available to evolve, and future evolution will take place on surviving (and frequently anthropogenic relocated) stocks. BARNOSKY et al. 2011 compared the actual extinction rate with the five great extinctions of the geologic past and concluded that the extinction rate today is already significantly higher then the common background rate and depending on the scenario (assuming that the today threatened species will be extinct in the next 100 years) an extinction rate comparable to the past big mass extinction could be reached in the next centuries to millennia.

Despite the effects of humans on earth, the many ways that human's influence sedimentation makes it difficult to choose a single event or process to define official the base of the Anthropocene. Geologic epochs are defined by events or layers that are universally recognizable - the base of the Palaeocene is for example recognizable almost worldwide by the presence of a layer of clay with a pronounced Iridium-anomaly and the mass extinction of specific species of foraminifers.

Fig.2. The famous transition between the Cretaceous (lower part) and Paleocene (upper part) epoch in the Italian formation of the Scaglia Rossa in the gorge of Gubbio (image rotated by 90 degrees).

Many effects of humans are however local or increase only slowly over time, producing a fuzzy boundary in the stratigraphy - which effect should be used - should the Anthropocene start with the first changes observed in the vegetation during the agricultural revolution, or only with the heavy effects caused by the later industrial revolution (preferred by most supporters of the new stage). Critics of the Anthropocene also insinuate that the Holocene is already subdivided in regional pollen and vegetation zones and more importantly cultural phases, not to mention the written history of the last centuries.

Fig.3. Simplified schema of the European subdivision of the last 1,2 million years (click to enlarge, modified after the International Stratigraphic Chart 2009).

The proposal and discussion of the Anthropocene is therefore regarded often by the public as somehow superfluous exercise - however researches see the introduction and use of the term also as opportunity - by accepting our profound impact on earth we maybe will also accept our responsibility to the planet and toward ourselves.

Bibliography:

BARNOSKY, D.A.; MATZKE, N.; TOMIYA, S.; WOGAN, G.O.U.; SWARTZ, B.; QUENTAL, T.B.; MARSHALL, C.; McGUIRE, J.L.; LINDSEY, E.L.; MAGUIRE, K.C.; MERSEY, B. & FERRER, E.A. (2011): Has the Earth's sixth mass extinction already arrived? Nature Vol. 471: 51-57

CRUTZEN, P.J. (2002): Geology of mankind. Nature Vol.415: 23

FIGUIER, L. (1872): The World before the deluge. Cassel, Petter, Galpin & Co.: 518

ZALASIEWICZ, J. & WILLIAMS, M. (2008): Are we now living in the Anthropocene? GSA Today Vol.18 (2): 4-8

ZALASIEWICZ, J. & WILLIAMS, M.; STEFFEN, W. & CRUTZEN, P. (2010): The New World of the Anthropocene. Environ. Sci. Technol. 44: 2228-2231

Online Resources:

CBC (24.04.2010): The Anthropocene - the Epoch of Humans. (Accessed 22.07.2011)

The views expressed are those of the author and are not necessarily those of Scientific American.

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