How did life begin on Earth? What were those conditions like? These are just some questions that scientists studying the origins of life on early Earth have been grappling with for many years. It turns out, that answer lies in understanding Earth’s early oceans. A team of scientists led by Dr Meng Guo from the NTU Asian School of the Environment (ASE) has developed the most comprehensive Earth system model to date that is shedding new light on the critical role of ocean pH levels in the emergence of early life.
What were Earth’s early oceans like?
Current theories suggest that early life originated in Earth’s oceans billions of years ago. However, those ancient oceans were not initially conducive for life. Current estimates of pH levels in those early oceans ranged from being either very acidic or very alkaline, neither of which are ideal for the formation of life.
Why do the pH levels of the early oceans matter?
The conditions for the origins of life are very delicate, and any changes in pH levels will have a drastic influence on the formation of organic molecules. By determining the pH levels of Earth’s early oceans, scientists can figure out when the Earth evolved from an inhospitable environment to a habitable one and what those conditions were, and even possibly attempt to study and search for habitable planets – and therefore life – elsewhere in the universe.
Uncertainty around determining the pH of early oceans
This uncertainty in determining the pH of ancient oceans stems from several reasons. Firstly, there is a lack of ancient rock samples from the Precambrian, an era that encompasses the Hadean – the first geologic eon in Earth’s history, spanning 4.6 to 4 billion years ago – and the Archaen eon, which spans 4 to 2.5 billion years ago. Secondly, it is also difficult to accurately simulate how continental and seafloor weathering contribute to the global carbon cycle.
Another challenge arises from a debate between the traditional view that continental growth during the Hadean was slow and new evidence suggesting more rapid continental growth. The final challenge stems from assumptions about the mantle, a layer of mostly solid rock that lies underneath the Earth’s crust. It was previously assumed that the heat emanating from the mantle was rapidly and consistently increasing over time; however, evidence has shown that this is not the case.
These challenges contribute significantly to the uncertainty surrounding existing models used to determine the pH levels of the early oceans.
A comprehensive Earth systems model
Taking on the task of determining early ocean pH levels are Dr Meng Guo, from NTU ASE, and Professor Jun Korenaga from Yale University. Together, they have developed the most comprehensive Earth systems model to date.
The model combines tracking of the global carbon cycle and ocean pH levels, whilst considering the formation of continents and the Earth’s mantle cooling over time. It also accounts for changes in the amount of seawater throughout Earth’s history, a factor previous studies omitted.
In the absence of carbon-producing human activities, the amount of carbon dioxide in the atmosphere is affected by the natural exchange of gases between the atmosphere and the ocean as well as the release of gases from the mantle during volcanic activity.
The ocean’s carbon dioxide levels are in turn affected by seafloor and continental crust weathering, as well as aforementioned volcanic activity and atmospheric gas exchange. The amount of seawater also determines the concentration of carbon dioxide in the oceans.
The levels of carbon dioxide in the Earth’s crust are also affected by the exchange of elements between the crust and the mantle, which is influenced by the mantle’s cooling over time and mantle convection.
Essentially, the atmosphere, the crust and the mantle have been exchanging carbon and affecting one another’s levels throughout Earth’s history. However, the efficiency of these various processes in the storage and release of carbon has constantly changed over time.
This image shows how carbon, and other geochemicals like sodium and magnesium, are exchanged between the mantle, the crust and the atmosphere.
A habitable ancient ocean
By factoring these processes, Dr Guo’s model has shown that pH levels rose rapidly during the Hadean to the early Archaen, from a starting acidic pH of 5, to a neutral pH of 7. This period of rising pH levels took approximately 500 million years, which is rapid. This fast pH level rise is primarily due to the increased rates of seafloor and continental weathering during the Hadean. These increased weathering rates were driven by the Hadean’s unique conditions, which included quicker crust formation, distinct crust composition and characteristics, and faster tectonic plate movements.
This change in pH levels at the end of the Hadean created an ideal environment for the synthesis of organic molecules, and hence the beginnings of life on Earth. Dr Guo’s efforts have therefore given scientists a new perspective on when Earth likely became a habitable place, which is approximately 4 billion years ago. This lays the theoretical framework for understanding how early Earth’s environment evolved and subsequent theories surrounding the origins of life.
What next?
Dr Guo has shared that for her future studies, she will continue to investigate Earth’s evolving habitability by examining the history of plate tectonics and the chemical evolution of terrestrial reservoirs – parts of Earth that store water, carbon, nitrogen and other elements – including the atmosphere, crust, and mantle.
To read more about her research, click here.