Over the past 4.5 million years, the Earth’s climate cooled from the warmth of the Pliocene epoch into the recurring ice ages of the Pleistocene, driven by an accumulation of carbon deep within the Pacific Ocean, Advanced Science News reported.
Deep Pacific Carbon Accumulation Drives Long-Term Global Cooling
The Pliocene epoch, occurring roughly 4.5 million years ago, featured temperatures between 0.5°C and 6.0°C warmer than modern conditions. Ice sheets during this period were significantly smaller, even though atmospheric carbon dioxide levels matched today’s concentrations. These historical temperatures align closely with modern climate predictions driven by human activities.
Researchers have long understood that oceans act as major carbon reservoirs, absorbing carbon dioxide that would otherwise warm the atmosphere. While Quaternary period climate cycles over the last 2.6 million years are linked to this marine carbon draw, the exact mechanics shifting the planet from a warm Pliocene to the cooler Pleistocene remained uncertain.
“Because the deep ocean is the largest active reservoir of carbon on Earth—acting like a giant ‘carbon sponge’—we wanted to understand if changes in how the ocean stored carbon helped drive these long-term global cooling trends by pulling carbon dioxide (CO2) out of the atmosphere,” said Joseph Novak, a researcher at the University of California, Santa Cruz, in an email to Advanced Science News.
Microscopic Foraminifera Shells Reveal Ocean Chemistry
To reconstruct carbon storage from millions of years ago, Novak and his research team analyzed chemical proxies preserved in microscopic fossils. They examined benthic foraminifera from the genus Cibicidoides, single-celled organisms that build shells using carbon derived from sinking organic matter.
Deep Pacific waters originate primarily in the North Atlantic. As this water mass circulates, decaying marine life adds carbon to the current. By comparing carbon isotope signatures in fossils from the Atlantic and Pacific, scientists can track ocean circulation sluggishness and calculate extra carbon accumulation in the deep Pacific.
The team utilized carbon isotope datasets from Ocean Drilling Program sites in both oceans. The data showed that the deep North Pacific Ocean stored more carbon during the late Pleistocene than during the late Pliocene, when more carbon likely remained in the atmosphere.
Trace Metals and Shell Chemistry Confirm Carbon Sequestration
To corroborate the isotope data, the researchers examined trace metals like uranium and manganese preserved in marine sediments. Because decaying organic matter consumes dissolved oxygen and releases carbon dioxide, low oxygen levels signal increased respired carbon accumulation.
The team also analyzed shell chemistry to track seawater acidity. Carbonate ion concentrations in the deep Pacific dropped between 4 million and 2 million years ago, indicating rising carbon levels in marine waters.
These independent lines of evidence indicate that carbon sequestration in the deep Pacific steadily expanded, functioning as a mechanism for global cooling. Continental ice sheets reached their largest size during periods when oceanic carbon stores were largest, whereas the warm Pliocene featured low ice volume and significantly less carbon storage.
Positive Feedback Loops and Future Climate Risks
The researchers propose that this coupling between deep ocean carbon storage and global cooling operated as a positive feedback loop. Over time, initial cooling may have shifted Southern Hemisphere westerly winds toward the equator, restricting carbon dioxide release back into the atmosphere.
“Essentially, a slow but massive reorganization of ocean currents and biology progressively locked more CO2 away in the deep ocean, which reinforced the global cooling trend,” Novak added.
This historical climate driver also highlights potential modern risks. Anthropogenic warming could alter future ocean circulation, reducing the ocean’s efficiency at holding carbon and accelerating future warming if stored carbon leaks back into the atmosphere.
“The Earth’s past is the best laboratory we have for understanding its future,” Novak stated regarding the study published in Paleoceanography and Paleoclimatology.
Frequently Asked Questions About Deep Pacific Carbon Storage
What organisms did researchers use to track ancient ocean carbon?
Scientists examined microscopic fossils of benthic foraminifera from the genus Cibicidoides, which incorporate carbon from sinking organic matter into their shells over millennia.
How much warmer was the Pliocene compared to today?
During the Pliocene epoch 4.5 million years ago, global temperatures were between 0.5°C and 6.0°C warmer than modern temperatures, with significantly smaller continental ice sheets.
What role did trace metals play in the study?
Researchers analyzed trace metals such as uranium and manganese in marine sediments to measure dissolved oxygen changes, which reflect how much organic matter decomposed and released carbon into the deep ocean.
Worth a look