The major-ion composition of Carboniferous seawater

The major-ion chemistry (Na⁺, Mg²⁺, Ca²⁺, K⁺, SO₄^2-, and Cl^-) of Carboniferous seawater was determined from chemical analyses of fluid inclusions in marine halites, using the cryo scanning electron microscopy (Cryo-SEM) X-ray energy-dispersive spectrometry (EDS) technique. Fluid inclusions in halite from the Mississippian Windsor and Mabou Groups, Shubenacadie Basin, Nova Scotia, Canada (Asbian and Pendleian Substages, 335.5-330Ma), and from the Pennsylvanian Paradox Formation, Utah, USA, (Desmoinesian Stage 309-305Ma) contain Na⁺-Mg²⁺-K⁺-Ca²⁺-Cl^- brines, with no measurable SO₄^2-, which shows that the Carboniferous ocean was a "CaCl₂ sea", relatively enriched in Ca²⁺ and low in SO₄^2- with equivalents Ca²⁺ >SO₄^2-+HCO₃^-.δ³⁴S values from anhydrite in the Mississippian Shubenacadie Basin (13.2-14.0 ‰) and the Pennsylvanian Paradox Formation (11.2-12.6 ‰) support seawater sources. Br in halite from the Shubenacadie Basin (53-111ppm) and the Paradox Basin (68-147ppm) also indicate seawater parentages.Carboniferous seawater, modeled from fluid inclusions, contained ~22mmol Ca²⁺/kg H₂O (Mississippian) and ~24mmol Ca²⁺/kg H₂O (Pennsylvanian). Estimated sulfate concentrations are ~14mmol SO₄^2-/kg H₂O (Mississippian), and ~12mmol SO₄^2-/kg H₂O (Pennsylvanian). Calculated Mg²⁺/Ca²⁺ ratios are 2.5 (Mississippian) and 2.3 (Pennsylvanian), with an estimated range of 2.0-3.2.The fluid inclusion record of seawater chemistry shows a long period of CaCl₂ seas in the Paleozoic, from the Early Cambrian through the Carboniferous, when seawater was enriched in Ca²⁺ and relatively depleted in SO₄^2-. During this ~200Myr interval, Ca²⁺ decreased and SO₄^2- increased, but did not cross the Ca²⁺-SO₄^2- chemical divide to become a MgSO₄ sea (when SO₄^2- in seawater became greater than Ca²⁺) until the latest Pennsylvanian or earliest Permian (~309-295Ma). Seawater remained a MgSO₄ sea during the Permian and Triassic, for ~100Myr.Fluid inclusions also record a long interval, from the Early Cambrian to the Middle Devonian, when seawater had low Mg²⁺/Ca²⁺ ratios (<2) that coincide with calcite seas. The Mg²⁺/Ca²⁺ ratio of seawater rose from 0.9 in the Middle Devonian, to 2.5 in the Middle/Late Mississippian, 2.3 in the Middle Pennsylvanian, and 3.5 in the Early Permian. The transition from calcite seas to aragonite seas, established from the mineralogy of oölites and early marine cements, occurred in the Late Mississippian. Fluid inclusions show that seawater Mg²⁺/Ca²⁺ ratios rose above 2 by the Middle to Late Mississippian coinciding exactly with the shift to aragonite seas. Aragonite seas existed for ~100Myr, from the Late Mississippian until the Late Triassic/Early Jurassic.