87Sr/86Sr and Sr/Ca Investigation of Jurassic mollusks from Scotland: Implications for paleosalinities and the Sr/Ca ratio of seawater

Abstract

Measurements of 87 Sr/ 86 Sr, Sr/Ca, ! 18 O, and ! 13 C were performed on molluskan fos- sils from the Great Estuarine Group, Scot- land, to deduce paleosalinity patterns and hydrodynamic environments in this classic Jurassic marginal marine succession. Fos- sils are found as winnowed accumulations in shell beds that bear low-diversity assem- blages, consistent with deposition in a la- goonal paleoenvironment. Previous studies have concluded that estuarine mixing of seawater and freshwater took place based on correlated trends in ! 18 O and ! 13 C, most notably in the mussel Praemytilus strathair- densis from the Kildonnan Member. This interpretation is shown to be incorrect based on 87 Sr/ O, ! 13 C, 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 river- ine water transported over large distances. Lagoons containing the oyster Praeexogyra in the Duntulm Formation have similar in- ferred O and C isotope characteristics, but based on their 87 Sr/ 86 Sr 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 Es- tuarine Group were nearly hydrologically closed, at least over the interval represent- ing the molluskan invasion of these habi- tats, rendering O and C isotopes unreliable proxies of paleosalinity. Because strontium isotopes are not directly affected by evap- oration of the lagoons, knowledge of marine and freshwater 87 Sr/ 86 Sr ratios, along with estimates of Sr concentration, are used to calculate the proportion of seawater in the brackish water mixture. Unio is nearly al- ways the most radiogenic taxon compared to other genera in closely adjacent sedimen- tary strata, confirming its freshwater affin- ity. 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 Praeexo- gyra hebridica yields paleosalinities between 2‰ and 26‰, attesting to the wide salinity tolerance of this species. Thus far, only Neomiodon from the Dun- tulm and Staffin Bay formations show co- variant relationships between 87 Sr/ 86 Sr 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 Cal- lovian seawater composition of 0.7069. Us- ing values of DSr from modern aragonite se- creting 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 av- erage value of 2.3 for modern rivers but typical of Sr/Ca ratios in rivers draining arid climate regions. The semirestricted hy- drodynamic regime inferred from the high seawater-like ! 18 O values and high but uni- form Sr/Ca ratios in many of the Great Es- tuarine Group mollusks is consistent with a seasonally dry, Mediterranean-style climate for the Middle Jurassic of Britain.