Fig. 6: Provenance of the NEOM brine.

a Compares the chemistry of other Red Sea pools using a cross plot of the ratio between sulfate and chloride (y-axis) vs. the ratio of sodium and chloride (x-axis). In this data space, the NEOM brine is plotted along with other Red Sea pools whose chemistry has been appropriately documented (data from Table S1 of Duarte et al.³² and Table 1 of Schmidt et al.⁶—sites mapped in Fig. 1). The color of the circles denotes the salinity of the brines; note bins are not all evenly spaced. With a salinity of 40 PSU, Gulf of Aqaba seawater plots in the top-left of this graph by virtue of its high SO₄²⁻/Cl⁻ but low Na⁺/Cl⁻ ratios. Except for Afifi, a pool reported to have an exceptionally high Na⁺/Cl⁻ ratio³², the brines varyingly occupy the lower portion of the graph because they present lower SO₄²⁻/Cl⁻ ratios than normal seawater and higher Na⁺/Cl⁻ ratios. The broken vertical line denotes the Na⁺/Cl⁻ ratio of halite, representing the maximum value that fluids dissolving salt can attain. The curve in (b) charts the modeled evaporation of the modern Gulf of Aqaba seawater under a humidity of 50% which would deliver a δ¹⁸O value of ∼16‰. The δ¹⁸O value of the NEOM brine, by contrast, is only 1‰, suggesting that it cannot have been sourced from evaporated seawater. A more plausible provenance for the brine is the dissolution of the sub-seabed Miocene evaporites (predominantly halite).