The Geochemistry of Qatar Coastal Waters and its Impact on Carbonate Sediment Chemistry and Early Marine Diagenesis

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 þ , Ca 2þ , Mg 2þ , K þ , Sr 2þ , Cl -, and SO 4 2-) and stable-isotope (d 18 O and dD) 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 H 2 S 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, d 18 O and dD continue to rise reaching values greater than 12 and 60%, respectively. The slope of the d 18 O-dD 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 d 18 O and d 13 C, as well as Sr 2þ and Ca 2þ concentration. The d 18 O of water and Pirenella from the same sample site exhibits a strong correlation (R 2 ! 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 d 18 O and d 13 C of the shells correlate positively, likely reflecting greater sequestration of 12 C into organic matter in more restrictive evaporative settings. The intercept of the d 18 O and d 13 C 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 (R 2 ! 0.85) between shell d 18 O and measured water salinity, reflecting their mutual control by degree of evaporation. The Sr 2þ content of the gastropods does not correlate well with any measured oceanographic parameter, or show evidence of systematic seasonal change.

Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

“…Measurements of Qatar coastal marine Mg levels and Mg/Ca ratios, however, show no evidence of ongoing dolomitization to even this degree. Indeed, evidence indicates that calcium carbonate precipitation and dissolution is fairly minor in waters concentrating from near‐normal marine to halite saturation (Rivers et al ., 2019a). This geochemical interpretation is consistent with the lack of sedimentary evidence for widespread dolomitization or even significant carbonate precipitation (i.e.…”

“…In the restricted Inner Lagoon of Khor Al Adaid (the Inland Sea, southern Qatar; Fig. 1), subtidal coastal marine waters range to 90 psu (Rivers et al ., 2019a), overlie a carpet of aragonite mud (Rivers et al ., 2020), where only scant dolomite is observed, and which is interpreted to be detrital in origin based on petrographic analysis (likely wind‐blown Eocene‐age material). Microbial mats ubiquitously form in the intertidal zone ringing the Inner Lagoon, and minor amounts of dolomite have been discovered forming as a primary precipitate (Diloreto et al ., 2019).…”

Dologrus: The impact of meteoric‐water‐controlled diagenesis following early‐marine dolomitization

“…The SEM images indicate that the transition from bedrock to neighbouring sediment involves dissolution of dolomite on the crystal-scale ( Figs 6 to 8), causing the separation of interlocking crystals that form the bedrock. Such dissolution would not be expected to occur in normal marine waters, because they are greater than ten times supersaturated with respect to stoichiometric dolomite (Morse & Mackenzie, 1990;Rivers et al, 2019b). Therefore, meteoric waters, or potentially mixed meteoric-marine waters, are implicated in the dissolution of the dolomite and the formation of the dolomite sediment.…”

Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

“…Rather, deviations from the theoretical +12& indicate fluxes of pyrite weathering, volcanic degassing, evaporite weathering, evaporite burial and sulphate reduction, among others (Markovic et al, 2016). Moreover, seawater evaporated to the point of gypsum saturation has d 18 O as high as +7 to +10& (Knauth & Beeunas, 1986;Rivers et al, 2019b), but modern gypsum has a mean d 18 O of ca +13& (Seal II et al, 2000). If dolomites formed in gypsum-saturated fluids with these seawater d 18 O values (+7 to +10&) between temperatures of 25°C and 35°C, they should have d 18 O dolomite between +6.5& and +11.8& (Horita, 2014).…”