Abstract

Measurements of 87Sr/86Sr, Sr/Ca, δ18O, and δ13C were performed on molluskan fossils from the Great Estuarine Group, Scotland, to deduce paleosalinity patterns and hydrodynamic environments in this classic Jurassic marginal marine succession. Fossils are found as winnowed accumulations in shell beds that bear low-diversity assemblages, consistent with deposition in a lagoonal paleoenvironment. Previous studies have concluded that estuarine mixing of seawater and freshwater took place based on correlated trends in δ18O and δ13C, most notably in the mussel Praemytilus strathairdensis from the Kildonnan Member. This interpretation is shown to be incorrect based on 87Sr/86Sr ratios, which reveal negligible amounts of seawater-derived strontium in the shells of Praemytilus. Furthermore, the 87Sr/86Sr ratios are remarkably uniform between specimens of Praemytilus from the same shell bed despite large changes in δ18O, δ13C, and the Sr/Ca ratio. This is consistent with a lagoonal hydrology controlled more by seasonal changes in evaporation, precipitation, and flooding than by direct inputs of seawater or riverine water transported over large distances. Lagoons containing the oyster Praeexogyra in the Duntulm Formation have similar inferred O and C isotope characteristics, but based on their 87Sr/86Sr ratios, they have a higher proportion of seawater than the Praemytilus lagoons of the Kildonnan Member.

The data thus far suggest that many of the lagoonal environments in the Great Estuarine Group were nearly hydrologically closed, at least over the interval representing the molluskan invasion of these habitats, rendering O and C isotopes unreliable proxies of paleosalinity. Because strontium isotopes are not directly affected by evaporation of the lagoons, knowledge of marine and freshwater 87Sr/86Sr ratios, along with estimates of Sr concentration, are used to calculate the proportion of seawater in the brackish water mixture. Unio is nearly always the most radiogenic taxon compared to other genera in closely adjacent sedimentary strata, confirming its freshwater affinity. The purported marine-brackish bivalve Tancredia yields a low paleosalinity of 4‰, only slightly more saline than Unio and Praemytilus at 1–3‰. The oyster Praeexogyra hebridica yields paleosalinities between 2‰ and 26‰, attesting to the wide salinity tolerance of this species.

Thus far, only Neomiodon from the Duntulm and Staffin Bay formations show covariant relationships between 87Sr/86Sr and Ca/Sr ratio that can be reconciled with a dynamic response to seawater–freshwater mixing. The slopes of these trends converge at 0.7068, which is close to the early Callovian seawater composition of 0.7069. Using values of DSr from modern aragonite secreting mollusks, the Sr/Ca ratio of middle Jurassic seawater is constrained from the best correlated mixing line to be 5.4 ± 0.6 millimoles/mole. In contrast, Sr/Ca ratios in the Great Estuarine Group lagoons and continental watershed range from 5.4 to 19.0. These are much higher than the average value of 2.3 for modern rivers but typical of Sr/Ca ratios in rivers draining arid climate regions. The semirestricted hydrodynamic regime inferred from the high seawater-like δ18O values and high but uniform Sr/Ca ratios in many of the Great Estuarine Group mollusks is consistent with a seasonally dry, Mediterranean-style climate for the Middle Jurassic of Britain.

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