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A series of sudden and extreme global warming events caused by massive releases of greenhouse gases punctuated Earth’s climate during the late Paleocene and early Eocene. Of all the past warm periods, the Paleocene-Eocene Thermal Maximum (PETM), about 56 million years ago, is intensely studied because it most closely resembles the current rate of atmospheric carbon dioxide (CO₂) release and offers clues to our climate response. 



During the PETM, the earth warmed very quickly, ocean acidification was widespread, and marine ecosystems were severely disrupted or suffered mass extinction. New work by USGS researchers and their colleagues shows that the PETM was immediately preceded by a smaller episode of warming and ocean acidification caused by a shorter release of carbon into the atmosphere. The precursor event released an amount of carbon similar to what has so far been released by the burning of fossil fuels and other human activities. In a sense, it shows us the consequences of our actions if our current emissions were to stop today. The PETM, on the other hand, represents what could happen if our carbon emissions continue.

This precursor event was documented in USGS sediment cores collected from southern Maryland (figure 1) by studying the chemical composition of ancient marine microfossils buried in the sediment. During the PETM, ocean levels were higher, and this region was an underwater shallow continental shelf. The calcium carbonate shells (called tests) of foraminifera (figure 2) that lived in the surface layer of the ocean accumulated in the shelf sediments over time as foraminifera died and their tests sank. These marine microfossils can now be used to reconstruct environmental changes of the past since changes in atmospheric carbon and ocean chemistry during past events were recorded and preserved in the calcium carbonate tests.

The precursor event has not been seen in sediment cores collected from the deep ocean, and this may be explained by the higher sedimentation rates on the shelf and the distance from the deep-sea center of ocean acidification, which dissolved the foraminifera tests before they were able to be preserved on the deep ocean floor. The geochemical analyses used on the Maryland cores included novel analytical methods to reconstruct ocean carbon content and pH from boron isotopes found in the foraminiferal calcium carbonate mineral lattice.

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The researchers found that the degree of ocean acidification and warming during these two distinct carbon pulses was profoundly different. The smaller, precursor event pulse triggered a fast carbon cycle response; the excess atmospheric carbon was mixed into the deep ocean, and the recovery took place within a few centuries to a few thousand years. The larger PETM pulse had a much slower recovery because the capacity of the deep ocean to absorb carbon from the atmosphere was exhausted, leaving only long-term processes to remove the excess carbon, like the weathering of silicate rocks, that takes tens of thousands of years.

Comparing these two events helps us to think about the consequences of rapidly curbed carbon emissions versus continued, business as usual, rates of carbon emission, with climate recovery estimates ranging from a few hundred years to several thousand. The paper, “Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum”, was published in Science Advances.

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