Discovery of the deep-sea NEOM Brine Pools in the Gulf of Aqaba, Red Sea

Deep-sea brine pools, formed by the stable accumulation of hypersaline solutions in seabed depressions, are relatively rare, occurring in the Gulf of Mexico, Mediterranean Sea, and Red Sea. Despite their rarity and small size (hundreds of square meters to a few square kilometers), they support significant macrofaunal and microbial biodiversity. Their anoxic, low-pH, and hypersaline conditions represent one of Earth's most extreme habitable environments, making them scientifically important for understanding early life and the search for extraterrestrial life. Red Sea brine pools are particularly noteworthy due to their extremophile microbes that produce bioactive molecules with potential therapeutic applications. Existing Red Sea brine pools are categorized into those found along the deep axial trough and those on the shallower coastal shelf. This study focuses on a newly discovered complex of brine pools in the Gulf of Aqaba, extending the known geographical range of Red Sea brine pools and offering a unique opportunity to study the impact of coastal processes on deep-sea environments.

Previous research has extensively documented the biodiversity and geochemistry of brine pools in the Red Sea and other locations. Studies have explored the microbial communities inhabiting these extreme environments, revealing diverse extremophiles with unique metabolic capabilities. The origin of Red Sea brine pools is often linked to the dissolution of subsurface evaporites deposited during the Miocene. Existing classifications categorize pools based on their location: those in the deep axial trough and those on the shallower coastal shelf. These studies highlight the significance of brine pools as unique ecosystems and potential sources of novel bioactive compounds. However, previous research lacked discoveries in the Gulf of Aqaba, and the importance of this location as a record of coastal processes has yet to be fully investigated. This study fills this gap by providing a detailed characterization of the newly discovered NEOM brine pools.

The NEOM Brine Pools were discovered during a 2020 research cruise aboard the R/V OceanXplorer using bathymetric and geophysical observations, including swath multibeam sonar and ROV surveys. The expedition employed a Sea-Bird 911+ CTD system with Niskin bottles to collect water samples for analysis of salinity, dissolved oxygen, temperature, and isotopes. Sediment cores were acquired using push cores with lengths ranging from 50 cm to 150 cm. Computed tomography (CT) scanning was used for high-resolution analysis of core lithology and grain size. Radiocarbon (¹⁴C) dating of bulk sediments from six horizons in the longest core established an age model. X-ray fluorescence (XRF) scanning provided elemental data, and X-ray diffraction (XRD) determined mineral composition. Stable isotope geochemistry was used to analyze carbon and nitrogen in organic matter to identify its source. 16S rRNA sequencing characterized microbial communities in the upper 20 mm of the cores. This approach combined various high-resolution techniques to comprehensively characterize the physical, chemical, and biological properties of the NEOM brine pools and their surrounding sediments.

The NEOM Brine Pool complex consists of one large (10,000 m²) pool and three smaller pools (<10 m²) located at 1770 m depth in the Gulf of Aqaba's Aragonese Deep. The main pool is elongated parallel to the coastline and lies near the toe-of-slope, close to the Arona Fault. The brine is hypersaline (160 PSU), anoxic (<10 µmol L⁻¹ O₂), and only slightly warmer than ambient seawater (<1 °C). Microbial staining indicates brine seepage from the toe-of-slope, likely originating from subsurface evaporites. Chemical analysis of the brine, including its SO₄²⁻/Cl⁻ and Na⁺/Cl⁻ ratios, and oxygen isotope measurements (δ¹⁸O), confirms a subsurface origin and rules out modern seawater evaporation as the primary source. The 150-cm long core from the main pool shows a 1200-year stratigraphy with three main depositional motifs: coarse siliciclastic intervals (30%, primarily quartz and feldspar), fine-grained fining-upward layers (55%, calcareous with high TOC), and a tan-colored clay interval (15%, high carbonate content). The coarse intervals are interpreted as tsunamites, deposited by high-energy events. The fine-grained layers are interpreted as turbidites from flash floods. Chemostratigraphic analysis (Rb/Zr, (Zr+Rb)/Sr, K/Fe ratios, quartz+feldspar abundances, TOC, δ¹³C) supports this interpretation. The NEOM Brine Pools support a diverse assemblage of megafauna, including eels, sharks, flatfish, and shrimp, possibly sustained by chemosynthetic microbial activity. 16S rRNA sequencing reveals microbial communities with aerobic and anaerobic metabolisms, stratified by depth and proximity to the brine-seawater interface.

The findings of this study address the research question by characterizing the newly discovered NEOM Brine Pools and their unique geological and biological properties. The discovery extends the known geographical distribution of brine pools and highlights their significance as archives of coastal processes. The preservation of detailed sedimentary layers, largely undisturbed by bioturbation, offers a valuable record of tsunamis, flash floods, and seismicity in the Gulf of Aqaba. This contrasts with heavily bioturbated sediments in other parts of the Gulf. The identification of tsunamigenic deposits, which recur approximately once a century, alongside more frequent flash flood deposits (four times per century), provides crucial insights into the region's geological history and hazard assessment. The close proximity of the pools to the coastline and the protective effect of the brine layer make them exceptional sediment traps, offering a high-resolution record of past events that would be difficult or impossible to obtain elsewhere. The study's findings contribute substantially to our understanding of brine pool formation, evolution, and their role as unique habitats and environmental archives.

The discovery and characterization of the NEOM Brine Pools represent a significant contribution to the understanding of deep-sea brine pools and their potential as sediment archives. Their unique location near the coastline and the exceptional preservation of sedimentary layers offer a high-resolution record of tsunamis, flash floods, and seismicity. Future research could focus on more detailed analysis of sediment layers to refine event frequency and intensity, exploring the potential of brine pools as indicators of regional paleoclimate variability, and assessing the long-term impacts of urbanization on this unique ecosystem.

The study's age model is based on radiocarbon dating which has uncertainties, particularly for younger samples. While the study provides strong evidence for a subsurface source of brine, the precise mechanisms of brine formation and migration require further investigation. The limited vertical resolution of some measurements, such as salinity near the brine-seawater interface, could be improved in future work. The study mainly focused on the main brine pool and could be further enhanced by detailed analyses of the smaller pools.