Scientists believe carbon dioxide release into the atmosphere from Earth’s interior takes place mostly via degassing from active volcanoes, but carbon dioxide can also escape along faults away from active volcanic centers.
Scientists believe carbon dioxide (CO2) release into the atmosphere from Earth’s interior takes place mostly via degassing from active volcanoes. CO2 can also escape along faults away from active volcanic centers. However, such tectonic degassing was poorly constrained, and to date has been largely unmeasured. DCO’s Tobias Fischer (University of New Mexico, USA) and colleagues conducted research to effectively study carbon emissions through fault systems in the East African Rift (EAR) in an effort to understand carbon emissions from Earth’s interior and how it affects the atmosphere. Their work was published in Nature Geoscience, and was part of a continued effort to better quantify global emissions of CO2 from Earth’s interior [1].
These new measurements contribute to improving their understanding of how carbon moves from the mantle to the atmosphere, a main focus of DCO’s Deep Carbon Degassing (DECADE) effort. DECADE was an initiative within DCO to instrument currently active volcanoes for continuous CO2 flux measurements, and also to better quantify CO2 emissions from volcanic and active tectonic regions that have not yet been measured. DECADE recently supported expeditions to the Aleutians and South America to constrain volcanic carbon emissions. The East Africa work shows that continental rifts were major, previously largely unquantified, contributors to global carbon flux.
Led by UNM Ph.D. student Hyunwoo Lee, the lead author of the paper, the scientists set out to measure diffuse CO2 flux from the Magadi-Natron basin in the East African Rift (EAR) between Kenya and Tanzania.
“CO2 was the main source of the greenhouse effect,” said Lee. “Natural carbon emissions come from volcanoes and are derived from magma. Mostly, people have thought the major sources of magmatic emissions have come through active volcanic events. Their research was the first attempt to quantify magmatic CO2 gases from non-volcanic and continental rift regions.”
The EAR was the world’s largest active continental rift and was comprised through distinct western and eastern sectors. Several active volcanoes emit large volumes of CO2 including Nyiragongo in the Congo and Oldoinyo Lengai in Tanzania. Additionally, significant amounts of CO2 were stored in large anoxic lakes in this region.
Additional gas samples collected along fault zones in the Magadi-Natron basin showed elevated CO2 flux and provided further evidence that faults act as permeable pathways facilitating the ascent of deeply-derived CO2. This particular study area represented a conservative 10 percent of the entire Natron-Magadi region. The team then compared the data to gas data from the active volcano Oldoinyo Lengai, and found carbon isotope compositions indicating a strong magmatic contribution to the observed CO2 degassing.
“We found that about 4 megatonnes per year of mantle-derived CO2 was released in the Magadi-Natron Basin, at the border between Kenya and Tanzania,” Lee said. “Seismicity at depths of 15 to 30 kilometers detected during our project implies that extensional faults in this region may penetrate the lower crust.” Thus, the ultimate source of the CO2 was the lower crust or the mantle, consistent with the carbon isotopes measured in the gas.
The findings suggest that CO2 was transferred from upper mantle or lower crustal magma bodies along these deep faults. Extrapolation of the measurements to the entire Eastern branch of the rift system implies a huge CO2 flux of 71 megatonnes per year, comparable to emissions from the entire global mid-ocean ridge system of 53 to 97 megatonnes per year.
“It is often argued that large volcanic eruptions instantly transfer significant amounts of CO2 and other gases into the atmosphere where they affect the global climate over a few years,” Fischer said. “On human time-scales, continental rifting is extremely slow at spreading rates of mm’s per year but on geologic time-scales, rifting can be considered a catastrophic continental break-up event.”
Large-scale rifting events could play a previously unrecognized role in heating up the atmosphere and perhaps ending global ice ages.
Cindy Ebinger, a professor of earth and environmental sciences at the University of Rochester, USA, coordinated field activities near the Kenya-Tanzania border and analyzed earthquake patterns within the rift zone.
“The unique coupling of gas chemistry and earthquake studies made it possible to discover the escape of gas along permeable fault zones that serve as conduits to the surface,” said Ebinger. “The work also allowed us to document the process of crustal growth through the formation of igneous rocks from magma in early-stage continental rift zones.”
Lee says the scientists plan to measure diffuse CO2 flux and collect gas samples from other areas in the EAR to better constrain how much it releases deep carbon to try to better constrain how much deeply derived CO2 comes from natural systems.
“Because some geological settings, for example fault zones, have never been paid attention to, global CO2 flux from natural systems are obviously underestimated,” he said. “Although there are still many ongoing studies to find better ways to quantify CO2 flux from active volcanoes, we expect this study to trigger more research on CO2 output from non-volcanic areas.”
Additional scientists involved in the study included: James Muirhead (University of Idaho, USA), Zach Sharp (University of New Mexico, USA), Simon Kattenhorn (University of Idaho, USA), and Gladys Kianji (University of Nairobi, Kenya.
