ISOTOPIC EQUILIBRIUM, VITAL, AND DIAGENETIC EFFECTS AMONG COMPONENTS OF THE LATE PERMIAN MAGNESIAN REEF, FORD FORMATION, SUNDERLAND, ENGLAND

The Magnesian reef formed around the margins of the Late Permian Zechstein Sea in modern day northern England. The reef is founded on a basal coquina consisting of brachiopods, bryazoa, bivalves, gastropods, crinoids, and voluminous early diagenetic cements. Though most of the reef is pervasively dolomitized, in the Tunstall Hills near Sunderland, one 5 m section preserves texturally pristine basal coquina limestones that have escaped dolomitization. We have initiated a pilot study to assess isotopic equilibrium, vital effects, and diagenesis among cements and skeletal components microdrilled from thin sections of the basal coquina.

Preliminary results suggest that the δ¹³C from shells of some brachiopod species match those of cement coatings, indicating that both likely precipitated in equilibrium with seawater. However, δ¹³C and δ¹⁸O values are inconsistent in different brachiopod species. Continuing work will elucidate whether such differences result from vital or diagenetic effects. Diagenetic effects likely explain discrepancies in the punctate terebratulide Dielasma. In δ¹³C—δ¹⁸O space, values from shell regions containing more versus fewer punctae define a linear trend with values from neomorphosed encrusting aragonite cements (adjusted for calcite-aragonite equilibrium). CL images indicate that punctae are filled with dully-luminescent cement, similar to the aragonite crusts. An end-member mixing model suggests that ~30% of the punctae-rich portion of shell has been in-filled or altered to values of the cement crust. Because such values probably exceed punctal density, it is likely that after mantle degradation punctae acted as conduits for diagenetic fluids and were loci for less obvious shell alteration as well as observable cement infilling.

These preliminary results imply that vital effects may exist for some brachiopod species, though not all, and that punctate brachiopods are particularly susceptible to diagenesis, so should be cautiously incorporated in isotopic studies.