The dominant, and yet least explored, way that carbon returns to Earth’s interior is through the accumulation and eventual subduction of layers of sedimentary carbonate rocks. These layers build up on the seafloor from the skeletons of dead marine plankton and later become buried by other sediments as they move across ocean basins due to seafloor spreading. Most of what we know about carbonate deposits comes from rock and sediment cores collected from 50 years of deep ocean drilling programs. But even these expeditions rarely drill deep enough to give a clear picture of where and how much carbonate lies buried in ocean basins.
To understand how much carbon is stored away in sedimentary carbonates found in the ocean basins, DCO members Adriana Dutkiewicz, Dietmar Müller, Sabin Zahirovic (all at the University of Sydney, Australia), and colleagues, developed a model of carbonate accumulation in deep-sea sediments spanning the last 120 million years. In a new paper in Geology, the researchers use the model to look at the impact of carbonate accumulation on global climate . The work provides a preliminary estimate of the size of the deep-sea sedimentary carbonate reservoir and its role in the deep carbon cycle over geological timescales.
The new work is part of an effort by the EarthByte group to model all of the major components of the mantle-lithosphere carbon budget through time. “Deep-sea carbonates represent a huge volume, so even small changes in the sequestration of carbonate carbon into this enormous sink are quite important for understanding net changes in atmospheric carbon dioxide and climate,” said Dutkiewicz.
Exactly how much sedimentary carbonate builds up in the deep ocean depends on three factors: the productivity of marine plankton, the depth of the ocean, and a line called the carbonate compensation depth (CCD). Marine plankton that build shells from calcium carbonate first appeared on the scene about 220 million years ago, but their numbers have grown over time. Their calcium carbonate shells accumulate above the CCD, which averages about 4.5 kilometers below the ocean surface. Since carbonate dissolves below this depth, deep-sea clay or the remains of plankton with silica shells dominate the deeper sediments.
Seafloor spreading operates like a conveyor belt, so carbonate ooze that initially accumulates along mid-ocean ridges, will eventually move to deeper regions below the CCD and become buried under carbonate-poor sediment such as deep-sea clay. This process takes millions of years. The CCD varies between ocean basins and has fluctuated considerably over geologic time, but as oceans have deepened over time, so has the CCD, increasing the area on the seabed where carbonate can accumulate.
The researchers explored a range of models to estimate the thickness of carbonate layers in Earth’s ocean basins, based on plankton evolution and the history of the carbonate compensation depth. They picked the best model by comparing their predictions with measured thicknesses of carbonate deposits from 38 ocean drilling sites that reached ocean crust.
By combining plate tectonic models and records of carbonate layers from scientific deep ocean drilling, the EarthByte group has made estimates of how carbonate sediments have accumulated on the seafloor since the Cretaceous Period, during the past 120 million years. The animation also illustrates where and when substantial accumulations of carbonate sediments were subducted. Credit: Adriana Dutkiewicz
According to their model, the amount of carbon stored in carbonate layers on the seafloor has increased tremendously over time. About 80 million years ago, only one megaton of carbon ended up in carbonate layers annually, which grew to about 30 megatons about 35 million years ago, and 200 megatons today. At the same time, carbonates forming in shallow waters, such as inland seas and on continental shelves, have decreased. But the rise in deeper deposits was far greater, creating a net increase in the total volume of carbonate sediments in the oceans in the last 80 million years.
The growth of this significant carbon sink, driven by the evolution of plankton with carbonate shells and a deepening of the CCD, may be responsible for the removal of carbon dioxide from the atmosphere that led to global cooling and the transition from a hothouse to an icehouse climate around 35 million years ago.
The new work also shows when and where deep-sea carbonate deposits encountered subduction zones, where they sinking beneath another tectonic plate. The researchers plugged the carbonate estimates into their open-source pyGPlates software, which can track the interplay of the evolution of the seafloor with recycling via subduction. The model quantifies how much carbonate has been subducted over time and pinpoints locations where significant amounts of deep sedimentary carbon likely escaped to the surface through volcanoes along subduction zones.
Next, the group plans to develop the model further so that it more accurately estimates changes in the CCD in individual ocean basins, leading to a much more tightly constrained model. But first they will have to comb through deep ocean drilling reports and databases to extract additional data on the locations and sizes of carbonate deposits. Dutkiewicz urges funding agencies and the scientific community to devote more resources to synthesizing the incredible amount of data collected over 50 years of ocean drilling expeditions.
“This enormous ocean drilling investment and data set should be used much more extensively for understanding Earth’s deep carbon cycle,” said Dutkiewicz. “Once you have coherent databases, a wide array of questions could be addressed.”
Main image: The white cliffs of Dover are spectacular carbonate cliffs composed of remains of fragments of shells from plankton deposited on the seafloor of the Cretaceous Ocean. Credit: Wikimedia, Immanuel Giel.