Boron geochemistry reveals the evolution of Dead Sea brines

Abstract

Well-known for their geological and natural singularity, the Dead Sea brines evolved from a marine ingression of the Mediterranean during the Pliocene. Dead Sea brines are currently almost ten times more concentrated than seawater and have a unique chemical composition with high boron isotope values (δ11Bbrine = ∼57‰). However, little is known on how these values were attained and their underlaying driving processes. Here we use boron isotopes (δ11B) combined with B/Ca and B/Li of lacustrine authigenic aragonites from the deep basin drill-core ICDP 5017-1, and Ein Gedi and Masada profiles to reconstruct past brine conditions. Comparing reconstructed δ11Bbrine from two key periods of contrasting hydro-climatic regimes we find that the brines of the late Holocene Dead Sea were enriched in 11B (δ11Bbrine = ∼60‰) relative to its glacial precursor Lake Lisan (∼57‰). With the aid of boron cycle modelling, we quantify the main boron fluxes in the basin. We show that the post-glacial δ11Bbrine enrichment is best explained by overall reduction of freshwater inflow to the lake and coeval increase in 10B sink through boron co-precipitation in evaporitic deposits and boron loss in atmospheric water vapour, consistent with the onset of warmer and drier climate in the Eastern Mediterranean during the Holocene. On geological time scales, adsorption of 10B on clastic sediments has acted as an important 10B sink and can explain the evolution of the high δ11Bbrine values.