The major-ion composition of Carboniferous seawater

Abstract

The major-ion chemistry (Na+, Mg2+, Ca2+, K+, SO42−, 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 SO42−, which shows that the Carboniferous ocean was a “CaCl2 sea”, relatively enriched in Ca2+ and low in SO42− with equivalents Ca2+ >SO42−+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 SO42−/kg H2O (Mississippian), and ∼12mmol SO42−/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 SO42−. During this ∼200Myr interval, Ca2+ decreased and SO42− increased, but did not cross the Ca2+–SO42− chemical divide to become a MgSO4 sea (when SO42− 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.