Deep Carbon Deposits

An unknown amount of carbon is trapped inside Earth, and is occasionally released as CO₂ by erupting volcanoes. By unlocking this mystery of the deep, scientists hope to understand the planet’s global carbon cycle from surface to core.

Scripps Institution of Oceanography at UC San Diego scientists David Hilton and Peter Barry analyzed CO₂ and other volatile chemicals collected in geothermal fluids in Costa Rica and throughout Central America. Their recent presentation at the American Geophysical Union Fall meeting in San Francisco addressed the region’s carbon flux – an input versus output estimate of carbon entering the deep earth. Carbon enters Earth’s interior through tectonic plates and their accompanying sediments as they descend into the mantle. It is released from submarine seeps and land-based volcanoes.

“A significant amount of carbon is locked away in minerals in Earth’s mantle,” said Hilton, a geochemistry professor at Scripps. “We are interested in understanding how much makes its way back to the surface and into the atmosphere.”

Scientists are studying crust- and mantle-derived carbon sources and sinks as well as those from deep microbial supplies to better estimate how they are linked to terrestrial carbon reservoirs and contribute to the global carbon cycle.

For nearly a decade, Hilton has been collecting rocks, groundwater and geothermal fluids at locations connected to the Pacific Ocean’s subducting Cocos tectonic plate to better understand the origin of land and ocean-based carbon in Central America.

Recently the team sampled two sites — submarine seeps off the Costa Rican coast and groundwater close to the coast — located between the trench, which is the entry point for the subducting plate into the mantle, and a chain of volcanoes extending along the coastal edge of Central America.

By analyzing the concentrations and isotopic composition of the CO₂, Hilton’s team estimated that the carbon flux in these coastal regions is significantly less than the output from land-based volcanoes.

“Volcanoes return only about 20 percent of the CO₂ subducted via the trench,” said Hilton, “which means that only a fraction of the total carbon dioxide budget is returned to the surface in locations such as Central America.”

Subduction zones, such as the Central America convergent margin, where tectonic plates collide and slip back inside Earth, provide an important pathway for input of carbon from Earth’s external reservoirs — crust, sediments, oceans — to the mantle. This along with outputs from volcanoes comprises the deep carbon cycle.

Hilton anticipates making future expeditions to Central America to study the geochemical signatures of fluids from ocean seeps and land-based groundwater from the coastal region where frequent large earthquakes are known to occur.

-- Annie Reisewitz