Short-term environmental change in a Jurassic lagoon deduced from geochemical trends in aragonite bivalve shells

The Bathonian age Inverbrora Member of the Brora Coal Formation, northern S cotland, was deposited in a predominantly freshwater coastal lagoon setting . However, two prominent shell beds within the mostly argillaceous successi on have been interpreted as resulting from temporary inundation of marine w ater and establishment of brackish conditions. The ensuing lagoon hydrology and paleoenvironment were investigated by measuring stable isotopic (delta O-18 and delta C-13) and trace element (Mg, Sr, Mn, Ba) variations within five exceptionally well preserved aragonite shells of the bivalve Isognomon murchisoni, Transects were sampled across the visible growth banding with a resolution of 0.25-0.15 mm for stable isotopes, and 0.05 mm for trace ele ments. Oxygen and carbon isotope data define a broadly covariant trend (del ta O-18 between +0.4 parts per thousand and -4.8 parts per thousand Peedee belemnite [PDB]; delta C-13 between +2.2 parts per thousand and -0.7 parts per thousand PDB) that supports a variably brackish-water setting with slig ht seasonal evaporation. Absolute trace element values vary from shell to s hell but show consistent patterns of covariation, Profiles of Mg/Ca across shell layers display multiple sharp, episodic minima that are coincident wi th peaks in Sr/Ca and Ba/Ca, This is interpreted as documenting inundation of the restricted, brackish-water lagoon by relatively cooler seawater, lea ding to a temporary increase in metabolic activity of the bivalves. Less fr equent minima in Mg/Ca, Sr/Ca, delta O-18, and delta C-13 record fresh-wate r influx and dramatic salinity reduction in the Lagoon. Bivalve metabolic e fficiency was decreased, and in some cases breaks in shell growth occurred. This study shows that in hydrodynamically complex settings, trace element variation in bivalve shells can provide a detailed record of short-term env ironmental change.