Diverse Carbonates in Exoplanet Oceans Promote the Carbon Cycle

TL;DR

This paper explores how diverse carbonate chemistry in exoplanet oceans influences the carbon cycle, emphasizing the importance of chemical diversity for maintaining temperate climates and guiding the search for extraterrestrial life.

Contribution

It demonstrates that carbonate diversity extends the conditions under which the carbon cycle operates in exoplanet oceans, highlighting implications for planetary habitability.

Findings

Ocean pH decreases with increased atmospheric CO2

Carbonate diversity broadens the parameter space for the carbon cycle

Deep carbonate compensation depths occur at high temperatures and CO2 levels

Abstract

Carbonate precipitation in oceans is essential for the carbonate-silicate cycle (inorganic carbon cycle) to maintain temperate climates. By considering the thermodynamics of carbonate chemistry, we demonstrate that the ocean pH decreases by approximately 0.5 for a factor of 10 increase in the atmospheric carbon dioxide content. The upper and lower limits of ocean pH are within 1-4 of each other, where the upper limit is buffered by carbonate precipitation and defines the ocean pH when the carbon cycle operates. If the carbonate compensation depth (CCD) resides above the ocean floor, then carbonate precipitation and the carbon cycle cease to operate. The CCD is deep (>40 km) for high ocean temperature and high atmospheric carbon dioxide content. Key divalent carbonates of magnesium, calcium and iron produce an increasingly wider parameter space of deep CCDs, suggesting that chemical diversity promotes the carbon cycle. The search for life from exoplanets will benefit by including chemically more diverse targets than Earth twins.