--> Impact of Seawater Chemistry on Modern Subtidal Carbonate Sediments: Al-Wajh Lagoon, Northern Red Sea, Saudi Arabia

2018 AAPG International Conference and Exhibition

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Impact of Seawater Chemistry on Modern Subtidal Carbonate Sediments: Al-Wajh Lagoon, Northern Red Sea, Saudi Arabia

Abstract

Hydrochemical analyses of major ions (Na+, K+, Mg2+, Ca2+, Cl-, HCO3-, SO42-) and minor elements (B-, Ba2+, Br-, CO32-, Fe2+, I-, Li-, NO3-, PO43-, Si, Sr2+) were performed on 11 seawater samples collected from eight locations, inside and outside the Al-Wajh lagoon in the Northern Red Sea, to document and reconstruct hydrochemical and sedimentological heterogeneities between lagoonal and more distal sites. Seawater salinity, in fully open-marine (TDS = 42,900 – 43,810 mg/kg) and open-marine (TDS = 37,960 – 42,670 mg/kg) environments, is lower compared with salinities inside the lagoon (TDS = 44,300 – 47,410 mg/kg). Seawater alkalinity from the fully open-marine Red Sea and Al-Wajh Bank (open-marine and restricted-marine; HCO3 = 140 – 177 mg/kg,) and analog (Shark Bay, Australia; HCO3 = 135 – 141 mg/kg) sites, is elevated compared with global seawater (HCO3 = 105 mg/kg). Seawater in the Al-Wajh Bank region has restricted and smaller ranges compared with Shark Bay. Alkalinity is negatively correlated to salinity in Al-Wajh lagoon, in contrast to a positive trend in Shark Bay. Sediments from specific water depths were characterized by microscopic allochem counting, grain-size and neoichnology analysis (i.e., modern organismal behavior producing traces) to define environmental settings. Bioturbation intensity was measured by average apparent ichnofabric index (ii) and ranged from ii1.5 (0% bioturbation) to ii5.5 (100% bioturbation). Salinity heterogeneities are coupled with observed differences in carbonate sedimentation in the lagoonal and the Shark Bay analog versus more distal sites. Environmental stress related to elevated salinities is an important factor in limiting coral diversity, numbers, and growth within restricted-marine and hypersaline environments. Limitations on coral growth constrain coral fragment grain-sizes (i.e., to sand-size), resulting in lower abundance of gravel-sized fragments in restricted-marine and hypersaline environments. Sedimentological criteria, including grain-size distribution, ii, peloid types and content, and coral fragment abundance allow discrimination of environments on the basis of heterogeneities in salinity exposures (i.e., fully open-marine, open-marine, restricted-marine, and hypersaline environments). The best discriminators characterizing restricted-marine environments are: (1) the general lack of gravel-sized sediment with a slightly elevated mud content, and (2) a relatively low coral fragment abundance.