Geologists Have a New Tool for Reconstructing the Ancient Climate

The early Paleozoic era was an action-packed stretch when it came to the diversity of life on Earth.

First came the Cambrian explosion, when most of the major animal groups first burst onto the scene. The great Ordovician biodiversification event followed—species richness skyrocketed, and life spread from shallow seafloors and shorelines across entire oceans. Then, around 445 million years ago, the Late Ordovician mass extinction became the first of the “big five” mass extinctions on record. Some 85% of all marine species vanished forever.

New research published in the Proceedings of the National Academy of Sciences of the United States of America details the climatic conditions during this period of boom and bust between 541 million and 443 million years ago using an often-overlooked material: ancient seafloor mud, in the form of limestone. The findings showed a strong link between changes in climate and changes in biodiversity.

Microbes flourished when temperatures were hotter during the early part of the study period, the authors found. When it cooled down during the Ordovician, animal life took off. An unstable climate and glaciation led up to the Late Ordovician mass extinction.

“The Cambrian-Ordovician is such an exciting interval…it really felt like a worthwhile period to go in and do a very high resolution study on,” said Kristin Bergmann, a professor in the Department of Earth, Atmospheric and Planetary Sciences at Massachusetts Institute of Technology (MIT) and a coauthor on the new study.

Paleoclimatologists can’t make direct measurements of what temperatures were like thousands or millions of years ago. Instead, they rely on “paleothermometers,” or proxies that contain preserved physical records of past conditions. Examples of climate proxies include tree rings, ice cores, shells, and sediments. Each type of proxy has its particular uses depending on the time period and conditions in question.

A lot can happen in a span of hundreds of millions of years, though, and these physical records have the potential to become distorted over time. Shells from marine animals are the gold standard for many geologists reconstructing ancient climates because they tend to be more physically resistant to later alteration than other proxy materials. As an added bonus, their structure makes it relatively straightforward for scientists to determine whether they’ve been altered. Shells are composed of calcium carbonate that precipitates out from seawater, and the oxygen isotope ratio of a given shell contains information about the water temperature at which it precipitated.

There’s just one catch: Shells haven’t been around forever. That’s a problem for geologists interested in the early Paleozoic or earlier. “As you go too far back in time in evolution, animals had not yet evolved the ability to make shells,” said Sam Goldberg, a Ph.D. student in geology at MIT and the lead author of the study. “Only really in the past 500 million years do these shells even exist.” Even then, shells from the earliest days of their evolution tend to be thin and poorly preserved.

Out of the Mud, a Clearer Climate Record Emerges

Ancient mud, on the other hand, is “all over the geologic record,” Goldberg said. Carbonate mud from the seafloor contains calcium carbonates from tiny, shelled microbes as well as direct precipitation from seawater. (Over time, carbonate mud becomes limestone.)

Carbonate mud is abundant and contains the same chemical compound as shells, but geologists haven’t relied on it for climate reconstructions because of the assumption that muds are more susceptible to chemical changes than shells are. As the sediment is buried and eventually converts to sedimentary rock, it can be exposed to water and heat that throw off the oxygen isotope ratio from the time of the calcium carbonate’s formation.

Goldberg and his coauthors challenged that assumption by testing carbonate mud samples from Svalbard (Norway) and Newfoundland (Canada) that appeared to be well preserved. After collecting rock samples from both sites, they ran a clumped-isotope analysis, which looks for pairs of isotopes that indicate whether water exposure significantly altered the oxygen isotope ratio of the original formation conditions. They found that the oxygen isotope composition of the samples was relatively unchanged, making them a viable option for climate reconstruction.

Because shelled animals evolved during the time period the researchers were analyzing, they could check their mud-based temperature reconstruction against a lower-resolution temperature reconstruction based on tried-and-true fossil records. It was a match.

“The paper is a welcome addition to previous clumped isotope paleothermometry efforts…and…a significant step forward in improving the fidelity of the earliest parts of the Phanerozoic record,” Gregory Henkes, an assistant professor in the Department of Geosciences at Stonybrook University who was not involved with the study, wrote in an email.

The study’s findings also match up with existing models that link the early Paleozoic’s leaps and bounds in animal evolution to cooling of previously warm oceans, though Henkes noted that the description of very high ocean temperatures in particular “warrants further testing against paleoclimate models.”

Goldberg said that although it’s difficult to draw direct comparisons to today’s climate, one thing is clear: “Animal life doesn’t like it when it gets too hot.”

Now that the researchers know their methods for analyzing carbonate mud work, they hope to use the same methods to go even further back in geologic and evolutionary time—something Bergmann’s lab is working on.

It turns out “clear as mud” can actually be a good thing—when it comes to paleoclimatology, at least.

—Clara Chaisson (@clarachaisson), Science Writer

Citation:

Chaisson, C. (2021), Geologists have a new tool for reconstructing the ancient climate, Eos, 102, https://doi.org/10.1029/2021EO155909. Published on 15 March 2021.

Text © 2021. The authors. CC BY-NC-ND 3.0
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