Abstract

The southern margin of the Arabian Gulf is a “classic” shallow-water, evaporative, carbonate-producing setting. The sediments and early diagenetic products creating the “Great Pearl Bank” of the United Arab Emirates to the east and accumulating in the coastal regions of Qatar to the west have long been studied as modern analogs for ancient evaporitic carbonate deposits of the rock record. An integrated study measuring the chemistry of Qatar subtidal coastal waters, evaporating tidal-pond waters (to halite saturation), and meteoric pond waters was undertaken encompassing both the dry (fall) and wet (winter/spring) seasons of 2016–2017. Measured parameters included temperature, pH, dissolved oxygen, and alkalinity, as well as major-ion (Na⁺, Ca²⁺, Mg²⁺, K⁺, Sr²⁺, Cl^–, and SO₄^2–) and stable-isotope (δ¹⁸O and δD) composition. Initial concentration by evaporation (to ∼ 90 practical salinity units (psu)) is interpreted to drive minor diagenetic aragonite precipitation. Further evaporation initially causes minor aragonite dissolution followed by gypsum and halite precipitation. One pond showed evidence of ongoing replacive dolomitization interpreted to be driven by H₂S formation and oxidation in association with microbial breakdown of organic matter. The stable-isotope composition of water in restricted ponds is a function of the degree of evaporation and dilution by meteoric waters during the wet season. Unexpectedly, beyond 350 psu, δ¹⁸O and δD continue to rise reaching values greater than 12 and 60‰, respectively. The slope of the δ¹⁸O–δD regression line exhibits no differences between dry and wet seasons.

During collection of coastal waters (up to ∼ 90 psu), live Pirenella cingulata (previously Cerithidea cingulata) gastropods were collected and their shells analyzed for δ¹⁸O and δ¹³C, as well as Sr²⁺ and Ca²⁺ concentration. The δ¹⁸O of water and Pirenella from the same sample site exhibits a strong correlation (R² ≥ 0.85) with a slope of ∼ 1, suggesting that the shells may be a useful chemical archive for the isotopic composition of past oceans. The δ¹⁸O and δ¹³C of the shells correlate positively, likely reflecting greater sequestration of ¹²C into organic matter in more restrictive evaporative settings. The intercept of the δ¹⁸O and δ¹³C correlation shifts between dry and wet seasons, and is interpreted to reflect average seawater temperature differences during recent growth. There is also a strong correlation (R² ≥ 0.85) between shell δ¹⁸O and measured water salinity, reflecting their mutual control by degree of evaporation. The Sr²⁺ content of the gastropods does not correlate well with any measured oceanographic parameter, or show evidence of systematic seasonal change.