Independent geological and micropaleontological lines of evidence suggest a ~200 kyr, period of intense warming covering the Paleocene/Eocene boundary interval (PETM). It has been suggested that this warming was initiated by a massive release of methane from the continental slopes. Among other groups of organisms, calcareous nannofossils are characterized by a remarkable increase of warm water taxa during this interval. Here we report the tropical response of calcareous nannofossils to the Paleocene/Eocene interval in ODP Site 1260B (283.15-276.35 mbsf). The PETM is about 1.38 m thick (279.88-278.5 mbsf) as defined by the onset and termination of the delta13C excursion and is marked by a ~12 cm thick laminated clay layer at the start of the excursion. A 30 cm thick interval (279.75-279.45 mbsf) directly above the clay layer, is characterized by common Discoaster spp. and Coccolithus subpertusus (syn. Ericsonia subpertusus) and the first occurrences of Discoaster araneus, Rhomboaster cuspis and Tribrachiatus bramlettei. All five taxa are interpreted as proxies for warm surface waters. The increase of Discoaster spp. during the PETM is solely caused by the onset of the new species D. araneus, which is here seen as a malformed Discoaster related to specific PETM conditions. These possibly include an acidification of the surface waters and/or higher salinity. The Rhomboaster/Tribrachiatus group, which first occurs in the PETM, is also thought to be a proxy for increased salinity. The abundance of the genus Toweius, indicative for mesotrophic conditions, declines simultaneously. Discoaster abundance decreases in the later stages of the PETM, being substituted by Chiasmolithus and Campylosphaera indicating a change to more eutrophic conditions. The genus Fasciculithus, very common in the tropical Paleocene, suffered a dramatic decline in the clay layer (basal PETM), without recovering later in the Eocene. This decline and the subsequent extinction of Fasciculithus, a solid robust nannolith, are here thought to be related to a calcification crises, perhaps caused by a high CO2 concentration and an acidification of the oceans. The observed changes in the composition of the calcareous nannofossils were relatively short lived, following the course of the PETM, which was characterized by warm surface waters with possibly low pH conditions
