Geological Processes Ensure Earth Remains Habitable Says Study
21.03.2014 06:14 Age: 128 days
Scientists explain in a new Nature paper that geological processes are responsible for ensuring that the Earth remains capable of supporting life and does not turn into a hot Venus or cold Mars.
Researcher Josh West treks through a valley in Peru in search of evidence of chemical weathering of rocks as they erode. Courtesy: Mark Torres
by Robert Perkins, University of Southern California
Researchers from the University of Southern California (USC) and Nanjing University in China have documented evidence suggesting that part of the reason that the Earth has become neither sweltering like Venus nor frigid like Mars lies with a built-in atmospheric carbon dioxide regulator — the geologic cycles that churn up the planet’s rocky surface.
Scientists have long known that “fresh” rock pushed to the surface via mountain formation effectively acts as a kind of sponge, soaking up the greenhouse gas CO2. Left unchecked, however, that process would deplete atmospheric CO2 levels to a point that would plunge the Earth into an eternal winter within a few million years during the formation of large mountain ranges like the Himalayas — which has clearly not happened.
And while volcanoes have been pointed to as a source of carbon dioxide, alone they cannot balance out the excess uptake of carbon dioxide by large mountain ranges. Instead, it turns out that “fresh” rock exposed by uplift also emits carbon through a chemical weathering process, which replenishes the atmospheric carbon dioxide at a comparable rate.
“Our presence on Earth is dependent upon this carbon cycle. This is why life is able to survive,” said Mark Torres, lead author of a study disclosing the findings that appear in Nature on March 20. Torres, a doctoral fellow at the USC Dornsife College of Letters, Arts and Sciences and a fellow at the Center for Dark Energy Biosphere Investigations (C-DEBI), collaborated with Joshua West, professor of Earth sciences at USC Dornsife, and Gaojun Li of Nanjing University.
While human-made atmospheric carbon dioxide increases are currently driving significant changes in the Earth’s climate, the geologic system has kept things balanced for million of years.
“The Earth is a bit like a big, natural recycler,” West said.
Torres and West studied rocks taken from the Andes mountain range in Peru and found that weathering processes affecting rocks released far more carbon than previously estimated, which motivated them to consider the global implications of CO2 release during mountain formation.
The researchers noted that rapid erosion in the Andes unearths abundant pyrite — the shiny mineral known as “fool’s gold” because of its deceptive appearance — and its chemical breakdown produces acids that release CO2 from other minerals. These observations motivated them to consider the global implications of CO2 release during mountain formation.
Like many other large mountain ranges, such as the great Himalayas, the Andes began to form during the Cenozoic period, which began about 60 million years ago and happened to coincide with a major perturbation in the cycling of atmospheric carbon dioxide.
Using marine records of the long-term carbon cycle, Torres, West and Li reconstructed the balance between CO2 release and uptake caused by the uplift of large mountain ranges and found that the release of CO2 release by rock weathering may have played a large, but thus far unrecognized, role in regulating the concentration of atmospheric carbon dioxide over the last roughly 60 million years.
The observed stability of Earth’s climate over millions of years is thought to depend on the rate of carbon dioxide (CO2) release from the solid Earth being balanced by the rate of CO2 consumption by silicate weathering. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium suggest that extensive uplift of mountain ranges may have stimulated CO2 consumption by silicate weathering, but reconstructions of sea-floor spreading do not indicate a corresponding increase in CO2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO2 within a few million years. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO2 to Earth’s atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO2 during the Cenozoic. A variety of other hypotheses have been put forward to explain the ‘Cenozoic isotope-weathering paradox’, and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes, atmospheric CO2 partial pressure and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.
Sulphide oxidation and carbonate dissolution as a source of CO2 over geological timescales by Mark A. Torres, A. Joshua West & Gaojun Li and published in Nature 507, 346–349 (20 March 2014) doi:10.1038/nature13030
Read the abstract and get the paper here.
University of Southern California news release here