The GCC is a critical part of how Earth’s climate system regulates temperature and is an important area of study for understanding the impacts of human activity, including rising atmospheric CO2 on the planet. One particular element of interest for scientists is the marine carbon cycle: the role that the oceans play in absorbing and releasing CO2 between the water and the atmosphere.
When studying the marine carbon cycle, it has been thought that the surface-water concentration of dissolved inorganic carbon (DIC) – the total carbon in its inorganic forms present in seawater – is regulated primarily by the exchange of CO2 between the surface ocean and the atmosphere, with any excess carbon then being exported to the deep ocean. Changes in the ocean carbon content – such as rising levels of dissolved CO2 caused by human-induced emissions – were assumed to mostly affect the upper layers of the ocean.
However, this understanding is based on present-day conditions and might not hold over longer timescales. The ocean also absorbs CO2 from the atmosphere during the long-term process of the weathering and erosion of silicate minerals on land. This CO2 is transported into the ocean by rivers, and once in ocean waters, it is gradually exported via the deep ocean to be deposited on the ocean floor, where it may be stored for millions of years.
Changes in the amount of CO2 absorbed by the oceans over geological time can be determined from the carbon isotope composition of ancient marine carbonate fossils, especially those of marine planktonic foraminifera, which give insight into the dissolved CO2 available in seawater when the foraminifera were living.
Previous studies based on the foraminiferal carbon isotope record have suggested that, over tens to hundreds of millions of years, the long-term storage of carbon on land or on the sea floor has been linked to changes in atmospheric carbon dioxide concentrations, with warm climates (high atmospheric CO2) corresponding to high rates of carbon storage on land and vice versa.
For the first time, a new study, led by University of Southampton scientists in partnership with Chinese and Portuguese colleagues, has directly linked changes in the inorganic carbon content of the oceans, including the deep reservoirs, to changes in sea level using well-dated marine carbonate fossils from oceanic sediments deposited over the last 75 million years. The carbon content reconstruction was conducted using a novel method – calcium carbonate clumped isotope palaeothermometry – which determines the temperature at which ancient marine carbonates precipitated, which is then used to estimate the inorganic carbon content of the deep ocean.
They found that when sea level was low, the inorganic carbon content of the ocean decreased and vice versa. The correlation was found to be most distinct during periods of long-term global warming, indicating that warmer climates, higher atmospheric CO2, increased rates of carbon storage in continental rocks and lower sea level tend to occur together, reflecting a strong linkage among multiple components of the Earth system under greenhouse conditions.
Dr Lei Cheng from Ocean and Earth Sciences at the University of Southampton, who led the study said: “Our findings reveal that the Earth’s carbon-cycle feedbacks over geological timescales act differently depending on the global climate background, and these feedbacks were most pronounced during past warm periods. This has implications for future climate change because under global warming, the Earth system is likely to shift to a state of high climatic sensitivity.”
