Excess carbon dioxide emitted by human activities -- such as fossil fuel burning, land-use changes and deforestation -- is absorbed by the world's oceans. While this absorption helps mitigate global warming, it also has adverse effects on marine life, including fish and plants.
While the impact of what is known as anthropogenic carbon dioxide on the open oceans has been extensively studied, there has been limited observational data on its presence and sources in coastal oceans, the broad range of saltwater ecosystems, from estuaries to coral reefs, that link the land and sea.
A recent study from Wei-Jun Cai's lab at the University of Delaware, titled "The Source and Accumulation of Anthropogenic Carbon in the U.S. East Coast," published in Science Advances, addresses this gap.
The lead author, Xinyu Li, earned her doctorate from UD's School of Marine Science and Policy in 2023 and is now a postdoctoral researcher at the Pacific Marine Environmental Laboratory. Wei-Jun Cai, associate dean for research and the Mary A.S. Lighthipe Chair Professor of Earth, Ocean, and Environment, was Li's advisor and supervised the study. Co-authors include Zelun Wu, a dual-degree doctoral student at UD and Xiamen University, and Zhangxian Ouyang, a postdoctoral researcher at UD.
The researchers analyzed a high-quality carbonate dataset from five research cruises conducted between 1996 and 2018. This dataset covers the East Coast of the United States' Mid-Atlantic Bight, a coastal region stretching from Massachusetts to North Carolina.
The 1996 dataset, provided by Doug Wallace, a professor of oceanography at Dalhousie University, allowed the researchers to track changes in carbon dioxide levels over time. Except for the 1996 cruise, the data were collected by members of Cai's group under the Ocean Acidification Program of the National Oceanic and Atmospheric Administration (NOAA).
The researchers used this data to investigate where and how much anthropogenic carbon dioxide is entering coastal waters, which are crucial to the global carbon budget.
Surface water -- the top 200 meters of the ocean -- showed the highest increase in anthropogenic carbon dioxide due to its direct contact with the atmosphere, which leads to greater absorption of atmospheric CO2.
Cai noted that an intriguing aspect of the study was analyzing the proportions of natural versus anthropogenic CO2 in the water and how water age affects anthropogenic carbon accumulation.
Surface water, being newer and arriving via the Gulf Stream from the Gulf of Mexico, exhibited high levels of anthropogenic carbon dioxide but relatively low levels of naturally occurring carbon dioxide.
In contrast, the middle layer of water (below 200 meters) had high concentrations of natural carbon dioxide and lower levels of anthropogenic carbon dioxide.
"The surface water has very high anthropogenic carbon dioxide but the middle layer water, that water that comes from the Southern Ocean and is called the Antarctic Intermediate Water, that water travels a long time, maybe 100 years from the Southern Ocean to the East Coast," said Cai. "That water has a lot of natural carbon dioxide because of microbial decomposition but that water has very low amounts of anthropogenic carbon."
Below these layers lies the North Atlantic Deep Water, which sinks in winter and travels from the Labrador Sea to the East Coast over two decades. "This water has an intermediate level of anthropogenic carbon dioxide," Cai said. "Each water mass has a recorded level of carbon dioxide from its time of formation, and this gave us a history of these changes. It's interesting to see that the more recent waters had the highest levels of anthropogenic carbon."
Li described this distribution as a "sandwich structure," with high anthropogenic carbon on the surface, low anthropogenic carbon in the middle layers, and intermediate levels deeper down.
"This distribution is closely related to water age, when it comes in contact with the atmosphere on the surface and absorbs carbon dioxide from the atmosphere," Li said.
The study also found that anthropogenic carbon decreases from offshore to nearshore waters, correlating with lower salinity. This suggests that there is no net increase in the export of anthropogenic carbon dioxide from nearshore areas like estuaries and wetlands to the open ocean.
"When we extrapolate our results to low salinity waters, like the water coming out of the Delaware Bay and the Chesapeake Bay, we found that there is actually very little anthropogenic carbon dioxide increase in very low salinity waters," Cai explained. "That water has a lot of natural carbon dioxide but there's very little anthropogenic carbon dioxide there."
This finding supports previous research indicating that net anthropogenic carbon dioxide transport from estuaries and wetlands to the continental shelf is essentially zero, or even negative. Possible reasons include low buffer capacity and short residence times in estuarine waters, which limit their ability to absorb anthropogenic CO2. Additionally, the loss rate of North American wetlands is three times its growth rate, reducing the opportunity for carbon uptake and transport to coastal waters.
Cai highlighted the broader implications of these findings for the global carbon cycle. "This paper clarifies conflicting views from terrestrial studies," he said. "There is a big debate about whether there is an increase of transport of anthropogenic carbon dioxide from terrestrial systems to the coastal ocean. Our conclusion is that there is no natural transport of anthropogenic carbon and that anthropogenic carbon in the coastal waters is really all mixed in from the offshore water masses and comes locally from the atmosphere above it. A majority of the latter is then exported to the ocean."
