Global Temperatures Rise When Ancient Reefs Encounter Subduction Zones

When the movement of tectonic plates pushes buried ancient reefs into contact with subduction zones, it liberates the carbon in the reef, which escapes through volcanoes. These interactions send carbon dioxide into the atmosphere, where it impacts the global climate, and may have contributed to warm spells during Earth’s history.

Carbonate platforms are ancient reefs that build up over millions of years, composed of the carbon-rich skeletons of coral and other tiny sea creatures. When these reefs get swallowed up by a subduction zone, where one tectonic plate sinks beneath another, the buried carbon can be remobilized and released to the atmosphere through volcanoes lining the subduction zone.

A new study by DCO members Jodie Pall, Sabin Zahirovic, Sebastiano Doss, and Dietmar Müller (all at University of Sydney, Australia), and colleagues, finds that the remobilization of reef carbon through volcanoes can have a measurable impact on global climate. The researchers modeled the intersection of carbonate platforms and subduction zones during Earth’s history since the Devonian period, 410 million years ago. By comparing the total length of these intersections to estimates of historic atmospheric carbon dioxide levels, they could pick out periods when liberated reef carbon contributed to warmer temperatures, including during the Paleocene–Eocene Thermal Maximum (PETM), about 55 million years ago. They report their findings in a new paper in the journal Climate of the Past [1].

carbonate platforms and subduction zones
Researchers modeled the intersection of carbonate platforms and subduction zones during the last 410 million years of Earth’s history to see if the release of carbon dioxide through volcanoes impacted global climate. Credit: Image courtesy of Pall et. al

Reefs can be both a source and sink for carbon dioxide in the atmosphere. Corals capture carbon dioxide and incorporated into reef structures over time, but that carbon can be rapidly released again to the atmosphere via volcanoes when it collides with subduction zones. “The most notable example is Mt. Etna,” said Pall. “The volcano intersects with a huge carbonate platform and currently emits 20 percent of global volcanic carbon dioxide emissions each year.”

The researchers wanted to know if volcanic release of carbon dioxide from carbonate platforms had impacted Earth’s climate in the past. They assembled previously published data on the location of carbonate platforms since the Devonian period, mapped them onto tectonic plates, and estimated how much carbon the platforms accumulated over time. The researchers could then see how often a carbonate platform came within 500 kilometers of a subduction zone, using the software program GPlates. This open-source software tool, developed by Müller’s EarthByte group and several international collaborators, reconstructs the movement of tectonic plates throughout Earth’s history.

Next, the researchers calculated the total length of carbonate-intersecting subduction zones (CISZs) and compared them to estimates of atmospheric carbon dioxide levels over 410 million years. They used wavelet analysis, which is a statistical test that identifies relationships between two data sets over time. Through the analysis, the researchers could pick out certain periods when peaks in CISZs corresponded to upticks in atmospheric carbon dioxide. “This suggests that there might have been tectonic-forced climate behavior,” said Pall. 

Their analysis showed that these interactions played a role in the temperature spike during the PETM, when global surface temperatures increased by 5 to 9 degrees Celsius within a few thousand years. Carbonate platforms also may have contributed to the warmer Cretaceous-Jurassic climate that occurred about 200 to 100 million years ago. Pall notes, however, that it was difficult to tell if CISZs caused this temperature increase, or if they were simply part of a global increase in subduction zones that occurred at that time.

All of the models and data from this research are open-source and available to the public. This work is part of the “subduction zone analysis toolkit” developed by researchers at EarthByte, which also includes a model of carbon accumulation in carbonate platforms, the lengths of all subduction zones, and the carbon dioxide content of ocean crusts throughout the past 410 million years. The EarthByte group hopes that these resources will be useful to other researchers interested in modeling the paleoclimate and the global carbon cycle.

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