The rare earth element distribution in marine carbonates as a potential proxy for seawater pH on early earth

Understanding the marine environment of early Earth is crucial for
understanding the evolution of climate and early life. However, the master
variable of Archean and Proterozoic seawater, the pH, is poorly
constrained, and published ideas about the pH range encompass ~7 pH units
from mildly acidic to hyperalkaline. To better infer ancient seawater pH,
we examine the possibility of a seawater pH proxy using rare earth
elements (REEs) in marine carbonates. The principle is based on increasing
concentrations of heavy rare earth elements in solution relative to the
light REEs with decreasing pH due to REE complexation and scavenging. We
calibrated such an REE pH proxy using pH variability in modern seawater
and tested the proxy with ~100 REE measurements from 13 separate carbonate
formations. We compared our pH estimates derived from the REE proxy to
published pH estimates of Cenozoic and Neoproterozoic seawater that use
the established pH proxy of boron isotopes (δ11B). REE-pH estimates agree
with the Cenozoic and the Ediacaran δ11B-pH proxy based on the type of
carbonate and boron isotopic composition at corresponding times. The
uncertainty in our REE-pH proxy can probably be explained by model
assumptions, noise from freshwater influence, siliciclastic input, and
diagenesis. This proof-of-concept study demonstrates that the REE-pH
method provides pH estimates comparable to boron isotope pH estimates
within uncertainties, which potentially could constrain changes in
Precambrian seawater pH to better understand the coevolution of life and
early Earth’s environment.