New insights on the role of reverse weathering in determining seawater geochemistry: a lithium isotope study

The lithium isotope composition (δ7Li) of ancient seawater is a fundamental proxy for the reconstruction of past conditions on Earth. At any given time, seawater δ7Li depends on the magnitude and isotope composition of the major input and output fluxes, which themselves result from interactions between climate, Earth surface processes and tectonics. Characterizing lithium fluxes during modern times and past eras can thus help determining Earth surface response to environmental changes.

Despite a general consensus on the characterization of the major fluxes of lithium to seawater, questions remain regarding the fate of lithium during early-diagenetic reactions taking place at the ocean floor. In particular, the amount of lithium incorporated into new secondary aluminosilicates formed at the seafloor (i.e., reverse weathering), and the Li isotope fractionation associated with this, are poorly constrained.

Here, we tackle these questions with new lithium isotope data and lithium concentrations measured on the clay-sized fraction of terrestrial and marine sediments from the Chilean active margin. The dataset is composed of riverbed sediments from major rivers along the Chilean climatic gradient (26°S to 41°S), hemipelagic sediments from marine core-tops, and three gravity cores located at 36°S, 33°S and 30°S offshore. It allows us to a) detect a systematic overprint of terrestrial signals in the marine realm, explainable only by the formation of authigenic aluminosilicate phases at the ocean floor, b) determine the Li isotopic signature of reverse weathering products, and c) discuss the global modern fluxes of lithium into and out of seawater.