Primitive life emerged in a cool ocean: Dr Itay Halevy, Exclusive Interview

Was the Ocean boiling? Was it cool? Did first life orginate in a boiling ocean water or normal tempereture? The scientists are divided on these questions. Some believe it was extremely hot at least 70˚ Celsius while others believe it was closer to the 15˚C what we enjoy today.

Recently, a team of scientists led by Dr Itay Halevy, Associate Professor of Geochemistry, Department of Earth and Planetary Sciences, Weizmann Institute of Science, Israel made some breakthrough on this issue. In an exclusive interview with Siddheshwar Shukla, Associate Editor, Devdiscourse, Dr. Itay shares the findings of his study and it's relevance.

What are the major findings of your research on first ocean temperature?

To start with, I would like to mention that the Geoscientists studying the conditions prevailing in Earth's earliest oceans rely on very few datasets. One of the most important, and also the most controversial, is a trend in the oxygen isotope composition of minerals precipitated out of sea water. Different interpretations of the same datasets have led to two divergent explanations for the trend. In one, the oceans were much warmer than today, and early sea water had an oxygen isotope composition much the same as today. In the other, the oceans' temperature was similar to today's and the oxygen isotope composition of ancient seawater must have been different—more depleted in the heavy isotope of oxygen, O(18).

Our results strongly favor the second explanation, that Earth's oceans have become gradually enriched in the rare and heavier O(18) over time, by about one or two percent. We are talking about a relatively small difference between the isotopic composition of sea water today and in the deep past, but this small difference has given rise to isotopic records in sedimentary rocks that parts of our scientific community have interpreted to reflect early oceans that were close to boiling. We now understand that this was not the case, and that over the past two billion years or so, ocean temperatures have varied around an average of about 15 degrees Celsius, much like today's average temperature.

What kind of samples and scientistific tests were used in this study?

To investigate the theory further we choose tiny ovoidal grains – ooids – composed of fine concentric layers of iron oxides that precipitated from ancient seawater. The ooids had been collected from dozens of sites around the world and they represented different geological periods – the oldest being around two billion years old.

To measure the oxygen isotope ratios, we partnered with Prof. Aldo Shemesh, also from the Department of Earth and Planetary Sciences at the Weizmann Institute of Science. Prof. Shemesh is an expert in high-precision measurement of oxygen isotopes in miniscule samples, and together with his group, we developed and tested methods for accurately measuring isotope ratios in the tiny iron oxide samples and this gave us a new tool that we did not previously have.

Two types of iron oxides were collected and analyzed: goethite and hematite. But to understand what the results were telling us about the early oceans, we needed a way to link the numbers obtained from the oxides to ancient seawater. To do so, we synthesized both oxides in the lab, over a range of temperatures, and measured the isotope ratios in these. This calibration then helped take the data from the ancient rusty samples and reconstruct the oxygen isotope composition of seawater from two billion years ago to the present day.

The geoscientists who consider the first ocean temperature to be very high also argue that it was due to the very high temperature of the Earth at that time. It's because of sustained rain, the earth cooled gradually. What are your findings?

The sequence of events described above happened much, much earlier than the times that we are considering. The oldest sedimentary rocks are almost 4 billion years old, and this is well after the atmosphere cooled enough for H2O to condense and form the oceans. These rocks display the most O(18)-depleted values, and progressively younger rocks are progressively more enriched in O(18), defining the trend that we set out to understand. The most O(18) depleted values have been interpreted to reflect hot oceans, but again, this was hundreds of millions of years after the oceans first condensed.

If the first ocean temperature is considered as cool as it is today, how will you explain the phenomena which were associated with very hot temperature of the primitive Earth?

The earliest Earth was very hot, but it cooled to form the oceans only a few hundreds of millions of years after its formation. The sedimentary rock record spans the ages of about 3.8 billion years ago to the present day, and during this time span the Earth was already cool enough to host an ocean. The question is, how cool? Close to boiling, or like today? We think that it was like today.

How will your findings be helpful in understanding the mystery of the earliest form of ocean, earth and the origin of life on this planet?

Our findings suggest that the ocean that existed almost 4 billion years ago, was already characterized by temperatures not much different than today's ocean. We cannot constrain the history of the surface environment earlier than the oldest sedimentary rocks. However, we do know that after its accretion from multiple smaller bodies, the Earth was very hot and all of the volatiles were in gaseous form in the primordial atmosphere. As the surface cooled, the oceans condensed, and this set in motion the geochemical cycles and put in place climate regulation mechanisms. It seems as though these climate regulation mechanisms were efficient enough to sustain habitable conditions on Earth's surface for the past 3.5-4.0 billion years. Primitive life emerged, I would argue, in a cool ocean, which was very different in its chemical composition (and isotopic composition) from today, but similar in temperature.

Thank you!