Discovery of Afifi, the shallowest and southernmost brine pool reported in the Red Sea

Abstract

The previously uncharted Afifi brine pool was discovered in the eastern shelf of the southern Red Sea. It is the shallowest brine basin yet reported in the Red Sea (depth range: 353.0 to 400.5 m). It presents a highly saline (228 g/L), thalassohaline, cold (23.3 °C), anoxic brine, inhabited by the bacterial classes KB1, Bacteroidia and Clostridia and the archaeal classes Methanobacteria and Deep Sea Euryarcheota Group. Functional assignments deduced from the taxonomy indicate methanogenesis and sulfur respiration to be important metabolic processes in this environment. The Afifi brine was remarkably enriched in dissolved inorganic carbon due to microbial respiration and in dissolved nitrogen, derived from anammox processes and denitrification, according to high δ¹⁵N values (+6.88‰, AIR). The Afifi brine show a linear increase in δ¹⁸O and δD relative to seawater that differs from the others Red Sea brine pools, indicating a non-hydrothermal origin, compatible with enrichment in evaporitic environments. Afifi brine was probably formed by venting of fossil connate waters from the evaporitic sediments beneath the seafloor, with a possible contribution from the dehydration of gypsum to anhydrite. Such origin is unique among the known Red Sea brine pools.

Figure 1

(A) Map showing the location of the Afifi brine pole relative to other brine pools in the Red Sea. Hot and cold brine pools are shown is white squares and green dots, respectively. (B) Bathymetry of the Afifi brine pool based on multibeam echosounder surveys and (C) cross section of the brine as recorded on the echosounder, where the brine surface, seen as a thin line, has a strong sound reflection.

Figure 2

Temperature and depth profiles from the Afifi brine pool along the brine seawater interface (A) and all the water column (B).

Figure 3

Prokaryote communities of the Afifi brine body and the oxic sea water above the brine. (A) Abundance of total prokaryote cells measured by flow cytometry (left histograms) and of bacterial and archaeal 16S rRNA gene copies measured by quantitative Real Time PCR (central and right histograms). (B) Bacterial taxonomic classes. (C) Archaeal taxonomic classes. (D) Taxonomic classes (Bacteria and Archaea) with significant differences (q ≤ 0.05) in mean proportions between the oxic deep seawater and the brine body. (E) Bacterial and archaeal predicted functional categories with significant differences (q ≤ 0.05) in mean proportions between the oxic deep sea water and the brine body.

Figure 4

(A) Vertical profiles of ⁴⁰Ar/³⁶Ar ratios of a Red Sea water column at a location nearby the Afifi brine pool, with no underlying brine (grey symbols) and the Afifi brine pool, sampled on March 2, 2017 and March 3, 2017. The ⁴⁰Ar/³⁶Ar ratios in both the surface and the brine are consistent with those observed in a location where there is no brine pool. (B) Vertical profile of oxygen and nitrogen to argon ratios in the Afifi brine pool and overlaying Red Sea water. The icon, representing an echosounder profile of the Afifi brine pool, shows the depth where the interface between overlaying Red Sea waters and the brine pool waters is located.

Figure 5

Concentration and carbon isotopic composition of Total Inorganic Carbon (TIC) in the Afifi brine pool waters and the overlaying Red Sea water. The icon, representing an echosounder profile of the Afifi brine pool, shows the depth where the interface between overlaying Red Sea waters and the brine pool waters is located.

Figure 6

Vertical profiles of (A) Percentage of oxygen, relative to the sum of all major dissolved gases (N₂ + O₂ + Ar), excluding CO₂. Isotope composition of δ¹⁸O (B) and δ¹⁷O (C) of the dissolved oxygen in the Afifi brine pool waters and the overlaying Red Sea water. The icon, representing an echosounder profile of the Afifi brine pool, shows the depth where the interface between overlaying Red Sea waters and the brine pool waters is located. Note that that no isotopic oxygen data is reported inside the brine, since the concentration was too low to resolve this.

Figure 7

Vertical profiles of (A) nitrate concentration, (B) the isotopic composition o nitrogen in nitrate, (C) the isotopic composition of oxygen in nitrate in the Afifi brine pool waters and the overlaying Red Sea water, and (D) the relationship between the isotopic composition of oxygen and nitrogen in nitrate in the water column of the Afifi brine pool. The icon, representing an echosounder profile of the Afifi brine pool, shows the depth where the interface between overlaying Red Sea waters and the brine pool waters is located.

Figure 8

Vertical profiles of (A) nitrogen to argon ratios, and (B) the isotopic composition of nitrogen (N₂) in the gasses dissolved in the waters of the Afifi brine pool. The icon, representing an echosounder profile of the Afifi brine pool, shows the depth where the interface between overlaying Red Sea waters and the brine pool waters is located.

Figure 9

The relationship between oxygen and deuterium isotopes in waters of the Afifi brine pool (red symbols) and overlaying Red Sea waters (open symbols), and the corresponding vertical profiles of these isotopes in the waters of the Afifi brine pool. The icon, representing an echosounder profile of the Afifi brine pool, shows the depth where the interface between overlaying Red Sea waters and the brine pool waters is located. The solid line shows the relationship expected under evaporation (MWL) and the dotted line shows the fitted regression line, corresponding to the equation shown.