Reconstructing past seawater pH from boron isotopes in carbonates

Abstract

Since the Industrial Revolution, the CO2 concentration in the atmosphere has increased substantially, enhancing the greenhouse effect, which is very much related to current global warming. Fortunately, the Earth has a series of reservoirs, especially its oceans, which trap a significant amount of the excess CO2. This marine absorption of CO2, however, results in progressive acidification of the oceans, which has detrimental and possibly devastating effects for marine organisms, particularly those that construct a skeleton of calcium carbonate (corals, mussels, clams, etc.). To understand the magnitude of this problem, it is crucial to know how seawater pH has oscillated in the past, particularly during those periods of time that are not covered by instrumental measurements, which are restricted to the last decade. For this purpose, the isotopic composition of boron in fossil biogenic carbonates provides a geochemical indicator, or proxy, which allows reconstruction of the past variations in seawater pH. In this review, we explain the theoretical grounds of this proxy, show the empirical calibrations carried out so far, briefly comment on some aspects of boron-isotope analysis, and list the results of paleo-pH reconstructions published so far.