Post-glacial carbon stocks in two restricted Patagonian fjord basins (52°S) using acoustic profiles and sediment cores

Deep beneath the surface of the ocean, in the cold, sheltered waters of southern Patagonia, lies a vast, silent archive of the Earth's recent history. These underwater valleys, known as fjords, act as natural traps for sediment. As rivers carry soil, rock fragments, and organic matter from the land, the water slows down in these deep basins, allowing the material to settle and pile up over thousands of years. This process is not just a geological curiosity; it is a critical part of the global carbon cycle. Carbon, the fundamental building block of life, gets locked away in these layers of mud and sand, effectively removed from the atmosphere where it could otherwise contribute to warming the planet. Scientists have long known that fjords are efficient at burying carbon, but they have struggled to measure exactly how much is stored there. Most previous studies have relied on taking a single sample from the top of the seafloor or from one narrow tube of mud, then guessing how that small piece represents the entire basin. This approach often misses the bigger picture, failing to account for the complex shapes of the seafloor or the deep layers of sediment that lie far below the surface.

To solve this puzzle, a team of researchers turned their attention to two specific, restricted basins in the southern Patagonian fjord system: Bahía Caribe and Bahía Trampa. These are small, deep pockets of water surrounded by steep land, located in a region dominated by strong winds and heavy rainfall. The researchers wanted to know the total amount of carbon stored in the entire column of sediment in these basins, from the very top of the seafloor down to the base of the post-glacial deposits. They knew that looking only at the surface would give a misleadingly small number, so they devised a way to see the whole story. They combined two different methods: first, they used a specialized sonar device to map the shape of the seafloor and the bottom of the sediment layer, creating a three-dimensional model of the hidden valley. Second, they retrieved long, continuous tubes of sediment from the deepest parts of these basins to analyze the chemical makeup of the mud at different depths. By merging the shape of the basin with the chemical data from the cores, they could calculate the total weight of carbon hidden in the ground. Because the sediment layers were deeper than the cores in some areas, they used statistical modeling to estimate the carbon content in the deepest, un-sampled sections based on the patterns found in the deepest parts of their samples.

The results revealed a hidden reservoir of carbon that was far larger than surface measurements suggested. In the larger basin, Bahía Caribe, the team calculated that the sediment holds approximately 65,100 tonnes of organic carbon and 38,000 tonnes of inorganic carbon. In the smaller, shallower basin, Bahía Trampa, the total was about 13,400 tonnes of organic carbon and 4,600 tonnes of inorganic carbon. When the researchers compared these full-depth totals to the amount of carbon found in just the top ten centimeters of sediment, they found a staggering difference. The total carbon stored in the full column of sediment was roughly one hundred times greater than what would be estimated by looking only at the surface layer. This massive discrepancy highlights how dangerous it is to assume that the top of the mud represents the whole pile; the deep layers hold the vast majority of the stored carbon.

An unexpected discovery was the significant amount of inorganic carbon found in these basins. While organic carbon comes from living things like plants and animals, inorganic carbon in these sediments comes from the shells of tiny marine creatures and other mineral fragments. In Bahía Caribe, this inorganic carbon made up 37 percent of the total carbon stock, and in Bahía Trampa, it accounted for 25 percent. This is a crucial distinction because the way inorganic carbon is stored is different from organic carbon. The formation of these mineral shells actually releases a small amount of carbon dioxide into the surrounding water, meaning that while this carbon is buried in the sediment, it does not act as a net removal of carbon dioxide from the atmosphere in the same way organic matter does. The researchers noted that this inorganic carbon likely came from the shells of creatures like clams and microscopic foraminifera that lived in the water, rather than from the rocks on land, which contain no carbonate minerals.

The study also shed light on how the shape of the basin and the depth of the water influence what gets stored. The larger, deeper basin, Bahía Caribe, which connects to the open ocean through a deeper channel, contained a higher proportion of inorganic carbon compared to the smaller, shallower basin. This suggests that the marine environment in the deeper basin was more favorable for the growth of shell-building organisms. In contrast, the shallower basin was more influenced by fresh water and glacial runoff, resulting in a different mix of sediment. The researchers used a statistical method to simulate the shape of the sediment layers a thousand times to account for uncertainty, ensuring their final numbers were robust. They found that while they could not see the very bottom of the sediment in some areas, the layers they did sample suggested that the carbon density remained steady even at great depths.

Ultimately, this work provides a much clearer picture of how much carbon is hidden in the fjords of Patagonia. It shows that relying on surface samples alone vastly underestimates the true storage capacity of these underwater valleys. The total amount of carbon in these two basins is comparable to the carbon stored in the peatlands of the region, which are famous for their ability to hold carbon. However, the study also warns that not all carbon in the sediment is equal; the inorganic portion, while substantial, plays a different role in the climate system than the organic portion. By mapping the full depth of these basins, the researchers have provided a more accurate accounting of the Earth's carbon budget, showing that the deep, silent layers of the fjords are holding a significant, and previously overlooked, portion of the planet's carbon. This understanding is essential for accurately predicting how the Earth's climate will respond to changes in the future, as these underwater archives continue to grow and shift with the tides and the ice.