INOCERAMID OCCURRENCE, STABLE ISOTOPIC VARIATIONS AND BULK ROCK GEOCHEMISTRY FROM LATE CRETACEOUS BLACK SHALES OF DEMERARA RISE

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 δ¹³C and δ¹⁸O variations. Cathodoluminescence observations suggest some of the isotopic variation is diagenetic in origin, but the existence of lower δ¹³C values coincident with higher δ¹⁸O 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 δ¹⁸O and/or δ¹³C signals reached the seafloor.