12/05/2026
Life likely started in the Archean Ocean. What were the temperatures of these
early oceans? Oxygen isotope compositions of different sediments have been
used extensively to answer this question. The d18O values of Archean sediments
are strikingly lower than their younger equivalents, but interpretations of these
data are extremely varied. High ocean temperatures, low d18O values of the early
oceans, or simply massive diagenesis of the sediments over the billions of ensuing
years have all been considered. There is no consensus between the different
camps. Here we use a completely different approach, using the triple oxygen
isotope composition of mantle eclogites to define the d18O value of the Archean
ocean. Mantle eclogites are subducted and metamorphosed equivalents of oceanic
crust. Unlike sediments, which are subject to surficial alteration over billions of
years, subducted ocean crust becomes metamorphosed and then isolated
isotopically ‘frozen’– for billions of years. We find that the range of triple
oxygen isotope values of fresh altered oceanic crust and those of Archean
eclogites are nearly identical. Given that the temperatures of hydrothermal
alteration have not changed through time, and the isotopic composition of the
mantle has not changed through time, then the similar triple isotope range of
modern and Archean metamorphosed oceanic crust indicates an unchanging
ocean isotope composition over 3 Ga. Eliminating the oxygen isotope
composition of the Archan ocean as a variable allows for better estimates of
Earth’s early surficial conditions.