STABLE-ISOTOPE SCLEROCHRONOLOGY OF RUDIST BIVALVES - GROWTH-RATES ANDLATE CRETACEOUS SEASONALITY

The almost complete translation of shell accretion to vertical growth was a successful morphogenetic adaptation of rudist bivalves, which re placed coral-algal-hydrozoan communities in most Tethyan shallow-marin e environments during the Cretaceous. Growth rates of rudist shells ar e, therefore, important in reconstructions of Cretaceous shallow-marin e benthic communities. Carbon and oxygen isotopic compositions were an alyzed in sclerochronological profiles through the shells of two Late Cretaceous rudist bivalves from Greece and Turkey. Values of delta(13) C vary between +0.8 parts per thousand and +2.2 parts per thousand in the hippuritid Vaccinites ultimus and between +0.8 parts per thousand and +2.6 parts per thousand in the radiolitid Gorjanovicia cf, costata , Ranges in delta(18)O are -5.3 parts per thousand to -2.9 parts per t housand and -4.6 parts per thousand to -2.1 parts per thousand, respec tively, A distinct cyclicity of delta(13)C and delta(18)O in both shel ls reflects Late Cretaceous seasonality and indicates annual vertical- growth increments of 45 mm in V. ultimus and 54 mm in Gorjanovicia, Th e concentrations of minor elements are similar to those of extant biva lves with low-Mg calcite shells.