New research involving tiny fossil shells more than half a billion years old confirms that the Cambrian Explosion took place during a greenhouse climate.
The international study, led by researchers at the University of Leicester in the UK, has now been published in the journal Science Advances.
At the beginning of the Cambrian period around 540 million years ago, an evolutionary Big Bang took place. Where there had once been primarily single celled organisms, complex life swiftly emerged. The seas now teemed with diverse species, including early members of most major animal groups.
It has been long thought this “Cambrian Explosion” occurred during a greenhouse climate, but hard evidence for this was lacking.
“Because scientists cannot directly measure sea temperatures from half a billion years ago, they have to use proxy data — these are measurable quantities that respond in a predictable way to changing climate variables like temperature,” explains Thomas Hearing of the University of Leicester in the UK, first author on the new paper.
To go back as far as the Cambrian, tree rings and ice cores won’t do — trees were yet to appear, and the most ancient ice core ever recovered is only a few million years old. For extremely distant time scales, oxygen isotope ratios are used.
Oxygen-16 is the dominant oxygen isotope on Earth, but other naturally occurring ones also exist, such as the slightly heavier oxygen-18.
Depending on which isotope it contains, there are minute differences in the evaporation and condensation properties of water. Played out on a global scale, this can provide useful information about temperature patterns.
Marine animals inadvertently fix this information in place as they grow by incorporating oxygen isotopes from sea water into bio-minerals in bone or shells.
Fossils of ancient jawless vertebrates called conodonts are very useful paleo-thermometers. However, conodonts didn’t evolve until the late Cambrian, leaving temperatures early in the period something of a mystery.
In the new study, researchers were able to extract oxygen isotope data from minute fossil shells called brachiopods, isolated from limestone in the UK dated to between 510 and 515 million years old.
“Careful examination of these tiny fossils revealed that some of them have exceptionally well-preserved shell chemistry which has not changed since they grew on the Cambrian sea floor,” says Hearing.
Co-author Tom Harvey adds: “That chemical signature is often lost over geological time, so it’s remarkable that we can identify it in such ancient fossils.”
Analysis of the oxygen isotope ratios in the shells indicated that sea surface temperatures at high latitudes had been very warm, exceeding 20 degrees Celsius.
The researchers also ran climate model simulations for the early Cambrian, which produced consistent results. Taken together, the isotope data and the simulations suggest that the early Cambrian climate was a typical greenhouse climate, similar to more recent greenhouse periods in the late Mesozoic and early Cenozoic periods, around 140 to 60 million years ago, when dinosaurs roamed the Earth.
The research provides a more robust picture of the Earth’s climate during a critical time in evolution.
“We hope that this approach can be used by other researchers to build up a clearer picture of ancient climates where conventional climate proxy data are not available,” says Hearing.
