In the depths of the ocean, ancient carbon is preserved within rocks.
In a recent study led by a researcher, the secrets of this carbon were deciphered, allowing scientists to redraw the history of life on Earth.
Carbon is the element of life the building block of every living organism, from proteins and sugars to fats and DNA.
Carbon is continuously released into the oceans: from living organisms that perform photosynthesis a process where carbon dioxide is absorbed and organic carbon compounds are produced, compounds made of carbon bound to hydrogen and sometimes other elements and also from other organisms that decompose after death, leaving their remains in the water.
A new study published in the journal Nature reveals that dissolved organic carbon (DOC) can be preserved for billions of years in natural “time capsules.”
The carbon binds to iron-containing minerals and is embedded in ocean rocks.
In this way, ocean rocks formed on the seafloor become a natural archive of biological processes preserved for more than a billion years.
Previously, it was difficult to isolate and measure the value of dissolved organic carbon accurately, but in the current study researchers developed a new method to reconstruct levels of organic carbon.
The objects where dissolved organic carbon is preserved are called ooids microscopic iron spheres formed in shallow water that trap organic carbon inside them.
Over time, the organic carbon decomposes, but the ratio of different carbon isotopes remains even after many years, serving as a marker for the original amount.
Developing the measurement method was not simple: researchers had to identify and isolate extremely small iron grains, up to two millimeters in size, from ancient rock layers, and then measure the carbon isotopes contained within with very high sensitivity.
Each iron sphere contains an “isotopic fingerprint” that allows reconstruction of the ocean’s condition more than a billion years ago.
The Story of the Sea
Since the study examined particularly ancient periods, samples were not collected from research ships at sea, but from ancient sedimentary rocks worldwide, exposed on land after processes related to tectonic plate movements and erosion.
Each sample represents the geological period in which it formed, and combining the measurements allowed researchers to reconstruct a comprehensive picture of changes in carbon levels over more than 1.5 billion years.
Researchers found that dissolved organic carbon concentrations in oceans varied significantly throughout Earth’s history.
The findings align with existing knowledge of life’s evolution and reflect its impact.
About 1.6 to 1 billion years ago, during the Paleoproterozoic era, mostly small single-celled organisms existed.
At this time, dissolved organic carbon levels were high and relatively stable.
The activity of single-celled organisms released carbon into the water, and oxygen-poor deep oceans allowed it to be preserved, as the lack of oxygen prevented decomposition.
Later, about 1 to 0.54 billion years ago, during the Neoproterozoic era, a sharp decline 90 to 99 percent in dissolved organic carbon is observed.
Oxygen levels in the water remained similar, but it appears that primitive cells evolving into more complex organisms significantly affected the organic carbon level in the water.
Likely, more carbon accumulated on the seafloor and sank into rock layers, leaving only a tiny amount dissolved in the water.
Finally, during the Cambrian period, about 540 million years ago, dissolved organic carbon concentrations increased again, alongside the emergence of more complex and diverse organisms that influenced both carbon and oxygen distribution in oceans.
More organisms performed photosynthesis, increasing oxygen levels, and with the proliferation of life, larger amounts of organic carbon were produced.
At this stage, the ocean became rich in oxygen, and the dissolved organic carbon level also increased significantly, stabilizing its concentration in the water.
Understanding dissolved organic carbon cycles is essential not only for studying ancient biological processes on Earth but also for understanding the global carbon cycle.
This component represents a massive reservoir, often overlooked compared to studied carbon cycles on Earth, although its quantity is similar to the carbon dioxide present in the atmosphere.
Significant changes in ocean carbon levels can trigger chain reactions affecting biodiversity and climate, which is directly influenced by oceanic processes.