Beyond evaporites: Assessing hypersalinity using boron-based salinity proxies

Boron-based paleosalinity proxies have been developed for both siliciclastic and carbonate sediments and sedimentary rocks, but to date modern analogs have been tested only over the salinity range from freshwater to normal-marine conditions (∼0–38 psu, or practical salinity units), with little attention given to hypersaline depositional systems (>38 psu). Here, we analyze boron and gallium concentrations in a known modern hypersaline facies (Abu Dhabi Lagoon) for comparison with an inferred ancient hypersaline facies (Devonian-Carboniferous Bakken Formation, Williston Basin, North America) to assess the applicability of the boron/gallium (B/Ga) and excess boron (Bxs) salinity proxies to hypersaline depositional systems. Our results show that B/Ga and Bxs effectively track salinity variation in the hypersalinity range for both siliciclastic and carbonate sediments. Although the precise thresholds between marine and hypersaline facies are tentative, the available data indicate ‘fuzzy thresholds’ of 10–12 for B/Ga and 70–100 ppm for Bxs in siliciclastic facies, with B/Ga > 12 and Bxs > 100 ppm unambiguously associated with hypersaline depositional conditions. In carbonate facies, B/Ga ratios can become artificially high as Ga concentrations approach zero, making the B/Ga proxy unreliable, although the Bxs proxy works well, with estimated proxy ranges of <0 ppm for brackish facies, ∼0–20 ppm for marine facies, 20–40 ppm for transitional marine-hypersaline conditions, and > 40 ppm for hypersaline facies. These findings robustly establish the validity of boron-based salinity proxies for recognition of ancient hypersaline facies.