When scientists want to study Earth’s very ancient geological past — typically greater than 100 million years ago — they often turn to rocks called carbonates.
Calcium carbonates, the most ubiquitous forms of carbonate, are minerals that precipitate from seawater and form layered sedimentary deposits on the seafloor. They are commonly known as limestone. More than 3.5 billion years of Earth’s history are chronicled in carbonate rocks. Many scientists use them to reconstruct histories of changes in climate and the past global carbon cycle — that is, the process through which carbon travels between the oceans, the atmosphere, the biosphere and solid rock.
“You can learn a lot from carbonates,” said Emily Geyman, a 2019 Princeton graduate in geosciences and the lead author of a paper published Nov. 8 in the Proceedings of the Natural Academy of Sciences (PNAS). The paper was the result of Geyman’s senior thesis research in which she investigated the chemical composition of carbonates and how these carbonates record the carbon cycle.
“The thing that makes carbonates especially useful as opposed to something like a sandstone,” Geyman said, “is that the carbonate is precipitated directly from the sea water, so the idea is that the chemistry of the carbonates, which we can measure, will tell us something about the ancient ocean.”
But not all carbonates get preserved in the geologic record. Deep sea carbonates, for example, typically get subducted, which is why scientists often turn to carbonates that accumulate on shallow continental shelves. The problem, however, is that scientists still don’t know enough about how properties such as ocean chemistry, ocean temperature, wave energy and water depth get translated into the shallow carbonate record.
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