Paleo-oceanography

When the Oceans Were Warm

Three and a half billion years ago, the oceans were very hot! “Temperatures could have been as high as 70°C,” say two French CNRS researchers in a study¹ that now confirms results obtained in the 1970s by American scientists who studied oxygen isotope concentrations in ancient cherts.² At that time, these results received a rather cool reception from the scientific community which questioned the validity of cherts' oxygen isotopic composition as reliable proxies to ascertain the environmental conditions of the Precambrian era (550 million years ago). Later, the discovery that the Earth might have experienced very cold times in the Precambrian was interpreted as another counter argument.

Cherts are quartz-based pieces of sedimentary rock, composed of one atom of silicon and two atoms of oxygen, that form in beds on the deep ocean floor. François Robert³ and Marc Chaussidon,⁴ decided to study both the silicon and the oxygen isotope ratios in nine Phanerozoic (550 million years ago) samples and 99 Precambrian ones from 23 geological formations. “The signature of a chert's silicon isotope should not be altered as is that of the oxygen,” explains Chaussidon, “since sediments might exchange oxygen with water, whereas silicon isotopes remain almost untouched.”

Although there are no chert samples as old as the Earth itself (4.5 billion years), they do date as far back as 3.5 billion years. The researchers used their “palaeothermometer” to gauge oceanic temperature over this period of time. How did it evolve? From a very warm state 3.5 billion years ago, it slowly cooled down about 800 million years ago to 30°C, a more stimulating temperature for ocean-dwelling creatures that needed oxygen to multiply, in contrast to warmer water where less oxygen can dissolve.

Some questions remain: How were the oceans' temperatures so high while the Earth underwent glacial phases? “These snowball periods did take place. Our results do not contradict this,” clarifies Chaussidon. “Over this very long period of time, our samples show a global trend but we do not have the resolution necessary to see shorter-term variations of the Earth's surface temperature.”

Samantha Maguire