[Ca2+] and [SO42-] in Phanerozoic and terminal Proterozoic seawater from fluid inclusions in halite: The significance of Ca-SO4 crossover points

Chemical analyses of 2,618 (1,640 new and 978 published) fluid inclusions in marine halite were used to define paleoseawater [Ca2+] and [SO42-] over the past 550 million years (Myr). Three types of fluid inclusion brine chemistries were recognized based on measured [Ca2+] and [SO42-]: (1) SO4-rich with [SO42-] >> [Ca2+]; (2) Ca-rich with [Ca2+] >> [SO42-]; and (3) Ca-SO(4)crossover points with [Ca2+] approximate to [SO42-]. The SO4-rich and Ca-rich fluid inclusion chemistries oscillated twice in the terminal Proterozoic and Phanerozoic. Transitions between SO4-rich and Ca-rich seas, here called Ca2+- SO42- crossover points occurred four times: terminal Proterozoic-Early Cambrian (544-515 Ma), Late Pennsylvanian (309-305 Ma), Triassic-Jurassic boundary (similar to 200 Ma), and Eocene-Oligocene (36-34 Ma). New fluid inclusion analyses using laser ablation-inductively coupled plasma-mass spectrometry better defined the [Ca2+] and [SO42-] in seawater at the Late Pennsylvanian and Eocene-Oligocene crossover points and the timing of the Triassic-Jurassic crossover point. Crossover points coincide with shifts in seawater Mg2+/Ca2+ ratios, the mineralogies of marine non-skeletal carbonates and shell building organisms (aragonite vs. calcite) and potash evaporites (MgSO4 vs. KCl types). Phanerozoic and terminal Proterozoic trends in seawater [Ca2+] and [SO42-] also coincide with supercontinent breakup, dispersal, and assembly cycles, greenhouse-icehouse climates, and modeled atmospheric pCO(2). Paleoseawater [Ca2+] and [SO42-] were calculated from the fluid inclusion data using the assumption that the [Ca2+] x[ SO42-] ranged from 150 to 450 mmolal(2), which is 0.5-1.5 times the [Ca2+] = 11 x[SO42-] = 29 product in modern seawater (319 mmolal2). Two additional end-member scenarios, independent of the [Ca2+] x[SO42-] = 150-450 mmolal(2) assumption, were tested using constraints from fluid inclusion [Ca] and [SO4]: (1) constant [SO42-] = 29 mmolal as in modern seawater, and variable [Ca2+], and (2) constant [Ca2+] = 11 mmolal as in modern seawater and variable [SO42-]. Mg2+/Ca2+ ratios calculated from the three scenarios were compared to independent data on the Mg2+/Ca2+ ratios from skeletal carbonates (echinoderms and corals) and mid-ocean ridge flank calcite veins. Constant [Ca2+] of 11 mmolal is unlikely because this relatively low concentration generated unreasonably low seawater [SO42-] during most of the past 550 Myr and high Mg2+/Ca2+ ratios compared to independent data. Constant [SO42-] of 29 mmolal produced unreasonably high seawater [Ca2+] and lower Mg2+/Ca2+ ratios than those derived from fluid inclusions, echinoderms, corals, and calcite veins. Variable [Ca2+] and [SO42-] showed the best agreement with the Mg2+/Ca2+ ratios derived from fluid inclusions, echinoderms, corals, and calcite veins. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Chemical analyses of fluid inclusions in marine halite were used to determine the concentrations of calcium ions and sulfate ions in paleoseawater over the past 550 million years. The study found oscillations in the concentrations of calcium ions and sulfate ions in ancient seawater, which were related to tectonic events, climate changes, and continental drift. The use of a new analysis method produced more accurate results that were consistent with other studies. The findings of this research are important for understanding the evolution of ancient marine environments and Earth's dynamic processes.