Ca-SO4 crossover points in Phanerozoic seawater: evidence from fluid inclusions in halite
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Date
2021-12-15
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American Geophysical Union (AGU)
Abstract
The major ion (Mg2+, Ca2+, Na+, K+, SO42-, Cl-, HCO3-) chemistry of seawater has varied in the Phanerozoic and Neoproterozoic. Transitions between SO4-rich and Ca-rich seas, here called “Ca2+- SO42- crossover points” are of great biogeochemical interest because of their coincidence with shifts in seawater Mg2+/Ca2+ ratio and the mineralogies of potash evaporites. Crossover periods occurred four times over the past 550 Myr, when seawater chemistry shifted from Ca-rich to SO4-rich or vice versa. Here, chemical analyses of 2,466 fluid inclusions in marine halite define paleoseawater [Ca2+] and [SO42-] over the past 550 Myr, with emphasis on Ca2+- SO42- crossover periods. Three types of brine chemistries are recognized from [SO42-] and [Ca2+] concentrations: (1) SO4-rich (SO4>>Ca): inclusions with measurable SO42- (7-993 mmol/kg H2O, here termed mmolal) and low Ca2+ (<91 mmolal) (Late Neoproterozoic, Permian, Triassic, and Late Cenozoic); (2) Ca2+-rich (Ca >>SO4): inclusions with measurable Ca2+ (150-2000 mmolal) and very low SO42-, below detection (Cambrian, Silurian, Devonian, Mississippian, Pennsylvanian, Jurassic, and Cretaceous); (3) crossover periods (SO4≈Ca): inclusions have measurable but low concentrations of both Ca2+ (10-120 mmolal) and SO42- (10-210 mmolal) (Late Triassic to Early Jurassic, Eocene to Oligocene). The Late Pennsylvanian Paradox Formation represents a second type of crossover period. In that case, fluid inclusions vary from Ca2+-rich (3-670 mmolal) to SO4-rich (6-590 mmolal).The fourth transition from SO4-rich to Ca-rich seawater occurred during the Terminal Proterozoic to the Early Cambrian (ca. 544-515 Ma) (Brennan et al., 2004). Salts predicted to precipitate during evaporation of crossover periods, for example Eocene and Oligocene seawater are calcite, gypsum, halite, polyhalite, sylvite, carnallite, kieserite, and bischofite. The four crossover points parallel major transitions in: (1) Mg2+/ Ca2+ ratio of seawater; (2) aragonite and calcite seas; (3) MgSO4 and KCl potash evaporites; and (4) icehouse and greenhouse climates, suggesting a link between the composition of seawater, marine potash evaporites, carbonate mineralogy, and climate.
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Keywords
Calcium, Magnesium, Sodium, Chlorine, Seawater, Tertiary period, Mesozoic Era, Carbonates, Halite
Citation
Weldeghebriel, M. F., Lowenstein. T. K., García-Veigas, J., & Cendon, D. I. (2021). Ca-SO4 crossover points in Phanerozoic seawater: evidence from fluid inclusions in halite. Poster presented at the AGU Fall Meeting 2021, New Orleans, Louisiana and Online, 13-17 December 2021. In AGU Fall Meeting Abstracts (Vol. 2021, PP24B-02). Retrieved from: https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/879935