Humanity's Carbon Signature Traced Deep Into the North Atlantic Ocean

Carbon released by burning fossil fuels has left a measurable chemical signature across much of the North Atlantic, reaching deep waters far from the atmosphere, a new study shows. Scientists traced it by measuring shifts in the ratio of two carbon forms, or isotopes, C-12 and C-13. Burning coal, oil, and natural gas lowers the atmospheric ratio of C-13 to C-12, a shift called the Suess effect that lets researchers distinguish fossil-fuel carbon from carbon already circulating in nature.

Emma Bavoux and colleagues at MARUM (Center for Marine Environmental Sciences), University of Bremen, analyzed carbon isotope ratios in samples collected aboard the research vessel MARIA S. MERIAN in 2017 and 2018, near 48 degrees North. The study, published in Geophysical Research Letters, also used earlier measurements of sulfur hexafluoride, a human-made gas that helps date water's last surface contact.

The team detected the signal in nearly every water mass, varying in strength with how recently it had touched the atmosphere. Young subsurface waters carried the clearest evidence, while it was weak or absent in old water such as Northeast Atlantic Deep Water, probably isolated from the atmosphere for several hundred years. "We were surprised by how significant the Suess effect already is in the deep-water masses of the North Atlantic," Bavoux said.

Western North Atlantic waters differed. Labrador Sea water and Denmark Strait currents sinking into the deep ocean showed Suess effect changes of about 0.3 to 0.6 per mille versus preindustrial levels. The team also tracked a large Labrador Sea eddy carrying an industrial signature down to 2,000 meters, or 6,562 feet.

The same shift is preserved in the shells of foraminifera, microscopic marine organisms that build shells from seawater carbon, as their remains accumulate on the seafloor. Co-author Stefan Mulitza said the ocean floor preserves signals of human origin marking the Anthropocene's start, adding that how clearly this boundary is defined depends mainly on future carbon dioxide emissions.