UNIVERSITY OF HAWAII IN MANOA
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PICTURE: CLEVELAND VOLCANO, ALEUTIAN ISLANDS ERUPTION IN 2006. Volcanism is one of the most important sources of carbon dioxide in the long-term carbon cycle, which is offset by the weather
Predictions of future climate change require a clear and differentiated understanding of the earth’s past climate. In a study published today in Science Advances, oceanographers at the University of Hawaii (UH) at Mānoa fully reconciled climate and carbon cycle trends over the past 50 million years, resolving a controversy that has been debated in the scientific literature for decades.
Over the course of the earth’s history, the global climate and carbon cycle have changed significantly, some of which have challenged current understanding of the dynamics of the carbon cycle.
Less carbon dioxide in the atmosphere cools the earth and reduces the weathering of rocks and minerals on land over long periods of time. Less weathering should result in a shallower Calcite Compensation Depth (CCD). This is the depth in the ocean at which the rate at which carbonate material rains down equals the rate at which carbonate dissolves (also known as the “snow line”). The depth of the CCD can be followed through the geological history by examining the calcium carbonate content of sediment cores on the sea floor.
Former oceanography student Nemanja Komar and Professor Richard Zeebe, both at UH Mānoa School of Ocean and Geosciences and Technology (SOEST), used the most comprehensive computer model of ocean carbonate chemistry and CCD to date, making this the first study to do this did quantitatively connect all the important parts of the carbon cycle throughout the Cenozoic era (over the past 66 million years).
Contrary to expectations, the deep-sea carbonate records suggest that the global CCD deepened (not swarmed) with the decline in atmospheric carbon dioxide (CO2) over the past 50 million years, creating a mystery about the carbon cycle.
“The variable position of the paleo-CCD over time carries a signal of the combined dynamics of the carbon cycle of the past,” said Komar, lead author of the study. “Tracking CCD evolution throughout the Cenozoic Era and identifying mechanisms responsible for its fluctuations are therefore important in solving previous changes in atmospheric CO2, weathering and carbonate burial in the deep sea. As the CO2 and temperature dropped above the Cenozoic Era, the CCD should have swarmed, but the recordings show that it was indeed deepening. “
Komar and Zeebe’s computer model enabled them to study possible mechanisms responsible for the long-term trends observed and provide a mechanism to reconcile all observations.
“Surprisingly, we found that the CCD response was decoupled from changes in weathering rates of silicate and carbonate, challenging the long-standing hypothesis of buoyancy that attributes the CCD response to an increase in weathering rates due to the formation of the Himalayas and the contradicts our results, ”said Komar.
Their research suggests that the separation developed in part due to the increasing proportion of carbonate in the open ocean relative to the continental shelf due to the drop in sea level as the earth cooled and the formation of continental ice sheets. In addition, the marine conditions during this period caused the proliferation of carbonate-producing organisms in the open ocean.
“Our work provides new insights into the fundamental processes and feedback loops in the Earth system that are critical to predicting future climate and carbon cycle changes,” said Komar.
Researchers are currently working on new techniques to narrow the chronology of climate and carbon cycle changes over the past 66 million years.
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From EurekAlert!
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