University of British Columbia, Canada

Inoceramid bivalves bearing regular, concentric growth lines are present throughout laminated Cenomanian-Santonian "black shales" on Demerara Rise (tropical North Atlantic). Presence of inoceramids in deposits that are otherwise devoid of benthic macrofossils is a long standing paleontological anomaly, and what might control variations in growth rate suggested by growth lines at bathyal depths compounds the puzzle. To begin to address these questions we have analyzed sediment geochemistry relative to inoceramid distributions as well as shell isotopic variations. At ODP Sites 1260 and 1261, inoceramids are most abundant in the Turonian but occur sporadically in older and younger intervals. Sediment is laminated even where inoceramids are large, common, and bored. Similarly, trace metal contents of 30 bulk samples show no consistent differences between 15 samples with and 15 samples without inoceramids. All were enriched in Mo, V, Cr, Ni suggesting anoxic, sulfidic conditions within the sediment and possibly in the bottom water. One explanation for these contradictory data is that inoceramid colonization events were too brief to be systematically imprinted in the sedimentary record of benthic anoxia. For study of growth lines, three shell fragments with good preservation and well-expressed concentric growth lines were cleaned and 276 samples were milled across growth lines. These growth series exhibit cyclic delta (super 13) C and delta (super 18) O variations. Cathodoluminescence observations suggest some of the isotopic variation is diagenetic in origin, but the existence of lower delta (super 13) C values coincident with higher delta (super 18) O values in one shell is difficult to explain diagenetically. If primary, isotopic cycles in shells indicate cyclical variation in benthic conditions during the lifespan of an inoceramid and may suggest seasonal delta (super 18) O and/or delta (super 13) C signals reached the seafloor. Abstract 115468 modified by 128.206.45.133 on 7-11-2006