Painstaking study of over 50 years of sediment cores on the seafloor has paid off surprisingly – Zoo House News
An international team of scientists meticulously collected data from more than 50 years of scientific drilling missions at sea to conduct a unique study of organic carbon falling to the bottom of the ocean and being pulled deep into the planet.
Their study, published in Nature this week, suggests that global warming could decrease organic carbon storage and increase the amount of carbon being returned to the atmosphere because warmer ocean temperatures could increase bacterial metabolic rates.
Researchers from Rice University, Texas A&M University, the University of Leeds and the University of Bremen analyzed data from drill cores of muddy seafloor sediments collected during 81 of the more than 1,500 shipborne expeditions of the International Ocean Discovery Program (IODP) and its predecessors. Their study provides the most detailed account of organic carbon storage to date over the past 30 million years and suggests that scientists still have much to learn about the dynamics of Earth’s long-term carbon cycle.
“What we find is that organic carbon burial is very active,” said study co-author Mark Torres of Rice. “It’s changing a lot and is much more responsive to Earth’s climate system than scientists previously thought.”
The paper’s corresponding author, Texan A&M oceanographer Yige Zhang, said: “If our new records prove correct, they will greatly change our understanding of the organic carbon cycle. As we warm the ocean, it will make it harder for organic carbon to find its way to being buried in the marine sediment system.
Carbon is the main component of life, and carbon is constantly circulating between Earth’s atmosphere and the biosphere as plants and animals grow and decompose. Carbon can also travel through the earth on a journey that takes millions of years. It starts at tectonic subduction zones, where the relatively thin tectonic plates on oceans are pulled under thicker plates that sit on continents. The plunging oceanic crust warms as it sinks, and most of its carbon returns to the atmosphere as carbon dioxide (CO2) from volcanoes.
Scientists have long studied the amount of carbon buried in marine sediments. Seafloor cores contain layers of sediment deposited over tens of millions of years. Using radiometric dating and other methods, researchers can determine when specific sediments were deposited. Scientists can also learn a lot about past conditions on Earth by examining minerals and microscopic skeletons of organisms trapped in sediments.
“There are two isotopes of carbon — carbon-12 and carbon-13,” said Torres, an assistant professor in Rice’s Department of Earth, Environmental and Planetary Sciences. “The difference is just one neutron. So carbon-13 is just a little bit heavier.
“But life is lazy, and when something’s heavier — even just a little bit — it’s harder to move,” Torres said. “So life prefers the lighter isotope, carbon-12. And when you grow a plant and give it CO2, it actually prefers to take up the lighter isotope. That means the ratio of carbon-13 to -12 in the plant will be lower — containing less 13 — than in the CO2 you put into the plant.”
For decades, scientists have used isotopic ratios to study the relative amounts of inorganic and organic carbon spilled at specific times in Earth’s history. Based on these studies and computational models, Torres said, scientists largely assumed that the amount of carbon being buried has changed very little over the past 30 million years.
Zhang said, “We had this idea of taking the actual data and calculating their organic carbon spill rates to calculate global charring. We wanted to see if this ‘bottom-up’ method was consistent with the traditional method of calculating isotopes, which is more ‘top-down’.”
The task of compiling data from IODP expeditions fell to study lead author Ziye Li from Bremen, who was then a visiting student in Zhang’s lab at A&M.
Zhang said the results of the study were shocking.
“Our new results are very different – they are the opposite of what the isotope calculations suggest,” he said.
Zhang said this was particularly the case during a period called the Middle Miocene, about 15 million years ago. Conventional science held that a large amount of organic carbon was buried around this trend, as exemplified by the organic-rich “Monterey Formation” in California. The team’s findings instead suggest that this is the period during which the smallest amount of organic carbon was buried in the last 23 million years.
He described the team’s paper as the start of a potentially powerful new way to analyze data that could help understand and tackle climate change.
“It’s people’s curiosity, but I also want to make it more informative about what’s going to happen in the future,” Zhang said. “We’re doing several things very creatively to really use paleodata to inform us about the present and the future.”
The research was supported by the American Chemical Society’s Petroleum Research Fund (59797-DNI2). On behalf of the National Science Foundation, Texas A&M has served as the scientific operator of the IODP drillship JOIDES Resolution for 36 years on the largest federal research grant currently administered by the university.