Abstract
Four tropical ODP/IODP sites were quantitatively investigated to enhance the Oligocene calcareous nannofossil biostratigraphy between 24-30 Ma. Despite the low diversity in the Oligocene, our data show additional bioevents and a new taxon-range zone in the tropical region. Sphenolithus avis appeared in upper Zone NP23 in the Indian Ocean, providing an additional bioevent between the base events of S. distentus and S. ciperoensis. Sphenolithus ciperoensis appeared to be rare and sporadic but an increase in abundance was synchronously found in the tropics at 28.73 Ma which is correlated with middle Chron C10 r in Hole 1237B. Sharp extinction levels include the top events of Helicosphaera compacta and S. predistentus in which the latter is more distinguishable than the top of S. distentus. The base of Triquetrorhabdulus carinatus differs within our tropical sites by showing inconsistent ages. Slightly prior to the NP24/25 boundary, S. ciperoensis reached its highest abundance during the decline of S. distentus and S. predistentus in the tropics. Confined to the early part of that abundance shift, an acme of gigantic S. ciperoensis, with a spine ratio (T/B) larger than 1.23, was observed together with S. ciperoensis s.s only in the Indian and Pacific Oceans which was probably related to fertility changes in the photic zone. The acme of gigantism lasted longer in the SE Pacific Ocean for about similar to 500 kyr, which is correlated with the lower-middle part of Chron C9 n (27.41-26.86 Ma), while in the Indian Ocean it was only found with low abundance at 26.86 Ma. Chiasmolithus altus, which was only observed in the western Pacific Ocean, abruptly terminated at 26.60 Ma. We introduce here the new Crassidiscus backmanii taxon-range zone, observed in our four tropical and mid latitude sites with a significant short duration. The base of C. backmanii is correlated with upper Chron C8 r in Hole 1237B at 26.19 Ma, while it is correlated with lower Chron C8 n in Hole U1438B at 25.78 Ma; hence the reliability of this biozone needs further examination. An event of a maximum abundance, however, shows similar ages in our tropical sites yielding an age of 25.75 Ma correlated to middle Chron C8n.2 n in Hole 1237B and of 25.71 Ma correlated with lower Chron C8 n in Hole U1438B. The top of S. avis occurs slightly below the top of S. ciperoensis, thereby, providing an additional bioevent in the Chattian.