Date of this Version
Calcareous nannofossils occur in lagoonal sediments recovered from Sites 873, 874, and 877 drilled on Wodejebato Guyot in the central Pacific Ocean. Nannofloras are present only in a few layers, from which several smear slides were prepared. All the specimens were counted, and the relative abundance of taxa was computed as percentages. Assemblages are characterized by excellent preservation, very high diversity (= number of taxa), but extremely low abundance. Both diversity and abundance decrease from the inner perimeter ridge along the northern edge of the atoll (Sites 877 and 874) to the inner lagoon (Site 873), but the assemblage composition is very similar at the three sites. The dominant species are Prediscosphaera spp., Cribrosphaerella ehrenbergii, Cretarhabdus spp., Biscutum constans, and Zygodiscus erectus. Watznaueria barnesae is common but not dominant. Holococcoliths were observed at all sites. All taxa are represented by dwarf specimens.
At Site 877, the CC22 Zone of late Campanian age was determined; at Sites 873 and 874, nannofloral assemblages indicate a Campanian age. The Cenomanian-restricted species Corollithion kennedyi was found at Site 877, suggesting the presence of older sediments on Wodejebato Guyot. Evidence of reworking is also given by the occurrence of recrystallized, broken, and often phosphatized specimens of long-ranging taxa. Reworking is more abundant in the lagoon than at the inner ridge.
The investigation was extended to the upper Campanian interval recovered at Site 869, which was drilled during Leg 143 on the oceanic floor south of Wodejebato Guyot. Here, nannofloras are represented by normal-sized specimens, and the assemblage composition is typically oceanic, suggesting that the nannoplankton at Sites 873,874, and 877 adjusted to restricted environmental conditions. Calcareous nannoplankton occur in extant lagoons, where coccoliths are found in the sediments and in the water samples. Lagoonal nannofloras are also preserved in the fossil record of various ages: Neogene, Oligocene, Paleocene, Early Cretaceous, and Middle to Late Jurassic. There are similarities and differences between the nannofloras from Wodejebato and assemblages from other ancient lagoons. The small size of the coccoliths seems to be a peculiarity of lagoonal assemblages and may be related to the unusual environmental conditions. The genus Braarudosphaera—inferred to be a low salinity form—is usually absent.
The relatively high abundance of fertility indicators B. constans and Z. erectus at Sites 873, 874, and 877 is possibly controlled by the proximity of Wodejebato Guyot to the paleoequatorial upwelling belt during the Campanian. In fact, similar nannofloras were previously reported from mid-Cretaceous sites at comparable paleolatitudes in the central-western Pacific. This interpretation is substantiated by the record of Site 869, where a few layers are relatively enriched in Prediscosphaera spp., C. ehrenbergii, B. constans, and Z. erectus, although nannofloral assemblages for the most part are affected by diagenesis and dominated by such dissolution-resistant taxa as Watznaueria barnesae, Cretarhabdus spp., and Micula decussata. The present study suggests that C. ehrenbergii and Prediscosphaera also possibly bloomed under relatively fertile surface-water conditions in the Late Cretaceous.