Sea level as a metronome of Earth's history
by Staff Writers
Geneva, Switzerland (SPX) May 30, 2017
Sedimentary layers record the history of the Earth. They contain stratigraphic cycles and patterns that precisely reveal the succession of climatic and tectonic conditions that have occurred over millennia, thereby enhancing our ability to understand and predict the evolution of our planet.
Researchers at the University of Geneva (UNIGE), Switzerland, - together with colleagues at the University of Lausanne (UNIL) and American and Spanish scientists - have been working on an analytical method that combines observing deep-water sedimentary strata and measuring in them the isotopic ratio between heavy and light carbon.
They have discovered that the cycles that punctuate these sedimentary successions are not, as one might think, due solely to the erosion of mountains that surround the basin, but are more ascribable to sea level changes. This research, which you can read in the journal Geology, paves the way for new uses of isotopic methods in exploration geology.
The area south of the Pyrenees is particularly suitable for studying sedimentary layers. Rocks are exposed over large distances, allowing researchers to undertake direct observation. Turbidites can be seen here: large sediment deposits formed in the past by underwater avalanches consisting of sand and gravel.
"We noticed that these turbidites returned periodically, about every million years. We then wondered what the reasons for this cyclicity were", explains Sebastien Castelltort, professor in the department of earth sciences in UNIGE's faculty of sciences.
The ups and downs of oceans regulate sedimentation cycles
The variations in the ratio helped us explore the possible link with the sea level". The research team found that the turbidite-rich intervals were associated with high 12C levels, and almost always corresponded to periods when the sea level was low. It seems that sedimentary cycles are mainly caused by the rise and fall of the sea level and not by the episodic growth of mountains.
When the sea level is high, continental margins are flooded under a layer of shallow water. Since the rivers are no longer able to flow, they begin to deposit the sediments they carry there. This is why so little material reaches the deep basins downstream. When the sea level is low, however, rivers erode their beds to lower the elevation of their mouth; they transfer their sediment directly to the continental slopes of the deep basins, creating an avalanche of sand and gravel.
Consequently, if the variations of the sea level are known, it is possible to predict the presence of large sedimentary accumulations created by turbidites, which often contain large volumes of hydrocarbons, one of the holy grails of exploration geology.
Measuring stable carbon isotopes: a new indicator of reservoir rocks
In addition, this measurement is relatively simple to perform and it provides accurate data - a real asset for science and mining companies. The study also highlights the importance of sea levels, which are a real metronome for the Earth's sedimentary history.
"Of course," concludes Honegger, "tectonic deformation and erosion are important factors in the formation of sedimentary layers; but they play a secondary role in the formation of turbidite accumulations, which are mainly linked to changes in the sea level".
Logan UT (SPX) May 28, 2017
Focusing on the management of carbon stores within vegetated coastal habitats provides an opportunity to mitigate some aspects of global warming. Trisha Atwood from Utah State University's Watershed Sciences Department of the Quinney College of Natural Resources and the Ecology Center has collaborated with several co-authors from Australia, including lead author Peter Macreadie from Deakin Unive ... read more
Universite de Geneve
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