Stable isotopic, assemblage, and paleoenvironmental investigations of juvenile-ocean sediments recovered on Leg 122, Wombat Plateau, northwest Australia

A sedimentary sequence documenting the early history of the proto-Indian Ocean was drilled at Site 761. Directly above the post-rift unconformity, two lithologic units were recovered which reflect deposition in incipient oceanic environments. The lower unit, composed of sandstone, contains abundant belemnites and a few lenses composed of low-diversity coccolith assemblages. The second unit, composed of chalk, contains abundant calcispheres, thoracospheres, low-diversity coccolith assemblages, and a few radiolarians. Belemnites and organisms that produced calcispheres and thoracospheres are thought to be opportunistic. Their abundance, and the absence of a normal marine fauna and flora, reflects an unstable early ocean environment. Stable oxygen and carbon isotopic data for the two units fall into almost separate fields. Heavy δ¹⁸O values for the belemnites indicate that they have not been affected by recrystallization. Instead, these isotopic values are thought to indicate either the deep, cool habitat of the belemnites or strong vital effects. -from Author