Composition of the Primordial Ocean Just after Its Formation: Constraints from the Reactions between the Primitive Crust and a Strongly Acidic, CO2-Rich Fluid at Elevated Temperatures and Pressures

Hisahiro Ueda,Takazo Shibuya

doi:10.3390/min11040389

Abstract

The Hadean was an enigmatic period in the Earth’s history when ocean formation and the emergence of life may have occurred. However, minimal geological evidence is left from this period. To understand the primordial ocean’s composition, we focused on the ocean’s formation processes from CO2- and HCl-bearing water vapor in the high-temperature atmosphere. When the temperature of the lower atmosphere fell below the critical point, high-temperature rain reached the ground surface. Then, hydrothermal reactions between the subcritical fluid and primordial crust started. Eventually, a liquid ocean emerged on the completely altered crust as the temperature decreased to approximately 25 °C. Here, we conducted two experiments and modeling to simulate the reactions of hypothetical primordial crustal rock (basalt or komatiite). The results indicate that the primordial ocean was mildly acidic and rich in CO2, Mg, and Ca relative to Na, irrespective of the rock type, which is different from the modern equivalents. Therefore, unlike the present seawater, the primordial seawater could have been carbonic, bitter, and harsh rather than salty.

Highlights

The Hadean was an enigmatic period in the Earth’s history when ocean formation and the emergence of life occurred [1,2,3]
The specific compositions of the ocean have not been reported from experiments or thermodynamic calculations simulating fluid
ΣCO2 concentration had been already sufficiently decreased at the timing of the first fluid sampling (350 ◦ C); the formation of the Mg-rich core of the type I carbonate was already terminated at that time (Figures 3b and 4b,c,e)

Summary

Introduction

The Hadean was an enigmatic period in the Earth’s history when ocean formation and the emergence of life occurred [1,2,3]. The oldest zircon grains with ages extending to ~4.4 Ga [4] were found from the clastic rocks in Australia. Their oxygen isotopic evidence indicates the incorporation of low-temperature weathering products under liquid water at ~4.3 Ga (e.g., [5]), indicating that the Earth’s surface temperature had cooled down enough to stabilize the liquid ocean at this age. The specific compositions of the ocean have not been reported from experiments or thermodynamic calculations simulating fluid–

Methods

Results

Conclusion