Siliceous microfossils in middle Eocene sediments of ODP Holes 120-748B and 120-749B

The Middle Eocene Climatic Optimum (MECO) is a major transient warming event that occurred at ~ 40 Ma and reversed a long-term cooling trend through the early and middle Eocene. We report the results of a high-resolution, quantitative study of siliceous microfossils at Ocean Drilling Program Sites 748 and 749 (Southern Kerguelen Plateau, Southern Ocean, ~ 58°S) across a ~ 1.4 myr interval spanning the MECO event. At both sites, a significant increase in biosiliceous sedimentation is associated with the MECO event. Rich siliceous planktonic microfossil assemblages in this interval are unusual in that they are dominated by ebridians, with radiolarians as a secondary major component. Silicoflagellates and diatoms comprise only a minor fraction of the assemblage, in contrast to siliceous microfossil assemblages that characterize modern Southern Ocean sediments. Based on our new siliceous microfossil records, we interpret two ~ 300 kyr periods of elevated nutrient availability in Southern Ocean surface waters which span the peak warming interval of the MECO and the post-MECO cooling interval. A diverse assemblage of large silicoflagellates belonging to the Dictyocha grandis plexus is linked to the rapid rise in sea-surface temperatures immediately prior to peak warmth, and a pronounced turnover is observed in both ebridian and silicoflagellate assemblages at the onset of peak warming. The interval of peak warmth is also characterized by high abundance of cosmopolitan ebridians (e.g., Ammodochium spp.) and silicoflagellates (e.g., Naviculopsis spp.), and increased abundance of tropical and subtropical diatom genera (e.g., Asterolampra and Azpeitia). These observations confirm the relative pattern of temperature change interpreted from geochemical proxy data at multiple Southern Ocean sites. Furthermore, rapid assemblage changes in both autotrophic and heterotrophic siliceous microfossil groups indicate a reorganization of Southern Ocean plankton communities in response to greenhouse warming during the MECO event.

Radiolarian biostratigraphy and chronology of radiolarian events of ODP Leg 199 sites and EW9709 sediment cores

A generally rich radiolarian fauna ranging in age from Quaternary to early Eocene (Zone RP7) was found at five of the eight sites drilled during Ocean Drilling Program (ODP) Leg 199. Of particular interest are the stratigraphically complete assemblages that range in age from middle Miocene (Zone RN5) to early Eocene (Zone RP7), composites of Sites 1218, 1219, and 1220. At the same sites, multisensor track (MST) data show consistent cycles in gamma ray attenuation density, color, and carbonate content that can be correlated on a submeter scale from the early Miocene to early Eocene. In addition, the magnetic reversal records from these three sites allow construction of an absolute timescale. A series of 305 radiolarian morphologic first and last occurrences and evolutionary transitions for radiolarians were determined and correlated directly with the accompanying MST and paleomagnetic data, resulting in a detailed and accurate dating of events. Since many of the bioevents are found at more than one site, it was also possible to test their reliability within the study area. Twelve new species are described: Calocycletta (Calocycletta) anekathen, Dorcadospyris anastasis, Dorcadospyris copelata, Dorcadospyris cyclacantha, Dorcadospyris ombros, Dorcadospyris scambos, Eucyrtidium mitodes, Theocyrtis careotuberosa, Theocyrtis perpumila, Theocyrtis perysinos, Theocyrtis setanios, and Thyrsocyrtis (Pentalacorys) orthotenes.

Range charts of stratigraphically important radiolarian species in sediments of ODP Leg 165 sites

The Paleogene sequences from three sites in the Caribbean were examined for radiolarians. In general, samples are highly lithified, requiring lengthy and repetitive cleaning procedures, and the assemblages are usually fragmented and/or partially dissolved. Both abundances and preservation of the assemblages vary considerably from site to site and within a single site; even within a single sample more than one degree of preservation was observed. It was possible, however, to construct at least partial stratigraphies for each of the three sites. Because the abundance of radiolarians is high even in extremely poorly preserved assemblages, we conclude that the differences in biogenic silica preservation are the result of postdepositional processes and not productivity. In both Sites 999 and 1001, near the Paleocene/Eocene boundary (Bekoma bidartensis Zone [RP7]), there is a short interval in which the abundance and preservation state of the radiolarians improves relative to overlying and underlying assemblages. In each case the intervals corresponds to the level, identified by calcareous microfossils, as representing changes in paleoceanographic conditions associated with the late Paleocene thermal maximum.