Lessons from Tiny Fossils: Insights into Ancient Climate Change

By examining foram shells obtained from drilling cores, a study led by geologists connects significant climate shifts that resulted in thermal maxima around 50 million years ago to escalating CO₂ levels.
At the close of the Paleocene and the start of the Eocene epochs, occurring between 59 to 51 million years ago, the Earth underwent substantial warming events, featuring both prolonged eras lasting millions of years and sudden warming spikes termed hyperthermals.

The rise in global temperatures was fueled by major discharges of carbon dioxide (CO₂) and other greenhouse gases, alongside potential influences from tectonic movements.

Recent research from the University of Utah geoscientists aligns sea surface temperatures with atmospheric CO₂ levels from this epoch and reveals a tight relationship between the two. These results also offer case studies to evaluate carbon cycle feedback mechanisms and sensitivities that are essential in forecasting human-induced climate change, especially with the unprecedented levels of greenhouse gas emissions today.

“The primary interest in these global carbon release periods is that they can serve as parallels for future changes,” explained lead author Dustin Harper, a postdoctoral researcher in the Department of Geology & Geophysics. “We currently lack a perfect parallel event with identical background conditions and rates of carbon emissions.”

However, the study released on Monday in the Proceedings of the National Academy of Sciences, or PNAS, suggests that emissions from two ancient thermal maxima bear enough resemblance to current human-induced climate alterations to assist scientists in predicting possible outcomes.

The research team scrutinized microscopic fossils found in drilling cores from an underwater plateau in the Pacific to analyze surface ocean chemistry during the lifetimes of these shelled organisms. They employed advanced statistical models to reconstruct sea surface temperatures and atmospheric CO₂ levels over a 6-million-year span encompassing two hyperthermals, namely the Paleocene-Eocene Thermal Maximum (PETM) approximately 56 million years ago and Eocene Thermal Maximum 2 (ETM-2) around 54 million years ago.

The results demonstrate a strong correlation, indicating that as CO₂ in the atmosphere increased, global temperatures followed suit.

“There are various pathways through which our planet and atmosphere can be impacted by additions of CO₂, and regardless of the source, similar effects on the climate system are observed,” stated co-author Gabriel Bowen, a professor of geology & geophysics at the University of Utah.

“Our focus is on assessing the climate system’s responsiveness to these CO₂ fluctuations. The study reveals some level of variability—perhaps slightly reduced sensitivity or warming linked to a specific CO₂ change—when analyzing these long-term shifts. Yet, overall, a consistent range of climate sensitivities is observed.”

At present, human activities tied to fossil fuel usage are releasing carbon at a rate 4 to 10 times higher than what transpired during these ancient hyperthermal episodes. Nonetheless, the total carbon emissions from those ancient events are comparable to what is projected for human output, possibly providing researchers a glimpse into future scenarios for us and subsequent generations.

Initially, scientists must ascertain what transpired regarding climate and oceans during these historical warming events over 50 million years ago.

“These occurrences might serve as a mid- to worst-case scenario for study,” Harper remarked. “We can examine these to explore what environmental changes arise from such a carbon release.”

During the PETM, Earth experienced extremely high temperatures, lacking polar ice sheets, with ocean temperatures soaring into the mid-90s degrees Fahrenheit.

To assess oceanic CO₂ levels, the researchers utilized fossilized forms of foraminifera, single-celled organisms with shells similar to plankton. The study relied on cores previously extracted by the International Ocean Discovery Program at two sites in the Pacific.

The foram shells contain small quantities of boron, with isotopes serving as proxies reflecting historical CO₂ concentrations in the ocean when the shells were created, according to Harper.

“We analyzed the boron chemistry of the shells and translated those values using current observations to deduce past seawater conditions. This allowed us to derive seawater CO₂ and convert that into atmospheric CO₂ levels,” Harper elaborated. “The aim of our targeted study period was to generate new records of CO₂ and temperatures for the PETM and ETM-2, which stand as some of the best comparisons for modern changes and to offer a long-term context for these climatic events.”

The cores Harper examined were taken from Shatsky Rise in the subtropical North Pacific, an optimal setting to obtain ocean-bottom sediments that mirror ancient conditions.

Carbonate shells disintegrate when they sink into deep waters, thus scientists need to explore shallow underwater plateaus like Shatsky Rise. Although foraminifera thrived millions of years ago, their shells preserve records of sea surface conditions.

“They eventually die and settle on the ocean floor, which is about two kilometers deep,” Harper noted. “We can retrieve the entire sequence of fossilized remains. In these mid-ocean areas, sediment supply from continents is limited, enabling us to primarily analyze these fossils. This provides an excellent archive for our objectives.”