While photographing fossil pollen, spores and marine algae embedded in ancient, organic-rich sedimentary rock, Sam Slater and colleagues noticed something curious — many appeared to have tiny holes scattered across their surface.
Upon closer evaluation, and with the help of a powerful scanning electron microscope, the scientists discovered that the "holes" contained intricate structures within them, and it quickly became clear just what these features were. Slater and his team had discovered impression or "ghost" fossils of coccolithophore nannoplankton flawlessly preserved in the surfaces of the larger organic particles.
"I had never seen this style of fossil preservation before and finding them was a total surprise," said Slater. "This discovery was completely unexpected."
Coccolithophores are a group of unicellular phytoplankton (nannoplankton) that clad themselves in hard calcium carbonate scales called coccoliths. Although tiny — many times smaller than the width of a human hair — these organisms play an important role in today's oceans. They produce much of the oxygen we breathe, support marine food webs and lock carbon in seafloor sediments.
During intervals when Earth's climate was warmer, like those that occurred during the Jurassic and Cretaceous Periods, coccolithophores seemingly vanish from the fossil record, suggesting that these key organisms were negatively impacted by warming oceans and accompanying acidification — a conclusion that does not bode well considering our present climate situation.
However, what made Slater and the team's unexpected discovery notable was that the abundant imprints they found were left behind in rocks dating back to these past warm periods, contradicting these previously held assumptions.
Coccolithophores never disappeared from the fossil record — they were there all along — it's just that no one thought to look for their ghosts.
The findings, published in the May 20 issue of Science, reveal a previously hidden fossil record of nannoplankton abundance, suggesting that the tiny but crucial organisms were far more resilient to Earth's past periods of prolonged ocean warming than traditional fossil evidence suggests.
The Biocalcification Crisis of Warmer Oceans
Despite their microscopic size, coccolithophores are widely abundant in the modern ocean and occasionally form massive cloud-like blooms visible from space. When they die, their coccoliths sink to the seafloor in vast quantities, becoming fossilized as large beds of nannofossils.
However, as the planet warms and levels of atmospheric carbon dioxide increase, resulting ocean acidification and declining concentrations of seawater carbonite are expected to make it more difficult for many calcifying marine organisms to produce their calcium carbonite shells or skeletons.
This is particularly true for coccolithophores, which are the most productive marine calcifiers and which play a central role in crucial global biogeochemical processes. Even so, predicting their response to future environmental change remains a challenge, one that relies heavily on the fossil record of nannoplankton during Earth's past warming events.
"Throughout several global warming events in the geological record, abundance declines and disappearances of coccoliths have been observed, suggesting that these plankton were severely impacted by ocean acidification and climate changes," said Slater, a paleobiologist at the Swedish Museum of Natural History and lead author of the study.
Declines in coccolithophore fossils during previous ocean warming events have been interpreted as a "biocalcification crisis," whereby ocean acidification and related environmental factors compromised the survival of these foundational marine organisms, suggesting drastic changes in marine biodiversity and carbon cycle function.
An Absence of Evidence Is Not Evidence of Absence
However, the new "ghost" fossil record reveals that nannoplankton were abundant and diverse throughout previous warming events, said Slater, indicating that they were more resilient than the previous nannofossil record would suggest.
"This imprint fossil record has remained almost entirely overlooked," said Slater, "in part, because the fossils are extremely small and cryptically preserved, but also because the methods applied [in our study] are not used to study nannofossils."
The ghost fossils are a type of imprint fossil and likely formed while coccolith-rich sediments were being buried at the seafloor. As the pressure mounted, hard coccoliths were pressed into the soft surfaces of pollen, spores and other organic matter. Later, acidic waters dissolved away the coccoliths themselves, leaving behind only their impressions like ghosts.
Slater and the researchers discovered coccolith imprint nannofossils preserved in rocks from the United Kingdom, Germany, Japan and New Zealand that were deposited during global warming events 94, 120 and 183 million years ago.
"Our study has shown that nannofossil imprints can be found in rocks where normal nannofossils are missing," said Slater. "This new record therefore has the potential to provide a new window to examine nannoplankton during intervals where an absence of normal nannofossils has hampered such research."
[Credit for additional images: Sam Slater/ Swedish Museum of Natural History]