Are oxygen and carbon isotopes of mollusc shells reliable palaeosalinity indicators in marginal marine environments? A case study from the Middle Jurassic of England

Abstract

Stable isotope analyses of non-luminescent, texturally-pristine oyster calcites (Praexogyra hebridica) are presented from Bathonian inner-ramp limestones in central-eastern England. A relatively large range of values (δ 18 O= –2.0 to +2.3‰ PDB; δ 13 C= +0.9 to +3.7‰ PDB) define a crude positive covariance, which can be regionally correlated with an offshore tonearshore increase in faunal restriction. This palaeoenvironmental stress gradient has previously been interpreted as a consequence of fluctuating nearshore palaeo-salinities, due to episodic or seasonal runoff from surrounding hinterlands. Covariant δ 18 O-δ 13 C trends from euryhaline bivalves are commonly ascribed to regional salinity gradients, the most isotopically depleted examples representing the greatest influence of fresh water. However, Bathonian palaeogeography indictates that the most isotopically-depleted oysters inhabited relatively offshore settings. Assuming an average growth temperature of 25°C, these shells were secreted in near-normal sea water (δ 18 O=+0.4‰ SMOW). In contrast, isotopically heavy oyster calcites were produced in highly evaporated, nearshore lagoonal waters (δ 18 O up to +4.8‰ SMOW). Modelling the oxygen isotope and chemical evolution of residual waters during evaporation suggests that up to 65% of the nearshore water mass was lost, probably in a complex of hydrodynamically choked lagoons. Evaporites were not precipitated in the sediment, and geochemical modelling suggests that the lagoon waters may have been brackish rather than marine in origin. The regional δ 13 C trend is contrary to that predicted on the basis of palaeosalinity and may have resulted from variations in organic carbon production, decay and fixation in early diagenesis.