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Journal of the Geological Society; 1997; v. 154; issue.2; p. 321-333;
DOI: 10.1144/gsjgs.154.2.0321
© 1997 Geological Society of London

Article

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

JAMES P. HENDRY1 & ROBERT M. KALIN2

1 School of Geosciences, The Queen's University of Belfast, Belfast BT7 1NN, UK (e-mail: j.hendry{at}qub.ac.uk)
2 Department of Civil Engineering, The Queen's University of Belfast, Belfast BT7 1NN, UK

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 ({delta}18O= –2.0 to +2.3{per thousand} PDB; {delta}13C= +0.9 to +3.7{per thousand} 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 {delta}18O-{delta}13C 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 ({delta}18O=+0.4{per thousand} SMOW). In contrast, isotopically heavy oyster calcites were produced in highly evaporated, nearshore lagoonal waters ({delta}18O up to +4.8{per thousand} 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 {delta}13C 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.


Keywords: England, Bathonian, Ostreidae, stable isotopes, salinity.




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