Paleogene calcareous nannofossil stratigraphy of ODP Leg 120 sites

Calcareous nannofossils are abundant in the Paleogene sediments recovered during Ocean Drilling Program Leg 120. Although no continuous Paleogene section was obtained, Sites 747 through 751 complemented each other so as to provide a virtually complete composite stratigraphic section. The calcareous nannofossil biostratigraphy at Sites 747, 748, and 749 is discussed. Correlation of calcareous nannofossil biozones and magnetozones at these sites suggests some diachrony with low-latitude areas, as well as on a regional basis. Changes in calcareous nannofossil diversity throughout the Paleogene are analyzed and interpreted as reflecting major paleoclimatic events.

Quantitative study of evolution in the Sphaeroidinella lineage in mid-Pleistocene sediment of ODP Hole 154-926A (Appendix A)

Morphological evolution in the late Neogene planktonic foraminifer Sphaeroidinella lineage involves a sudden increase of the percentage of specimens equipped with supplementary apertures (from <30% to >70%) in the mid-Pliocene (about 3.5 Ma). This evolutionary transition, marked by the first occurrence of specimens with large supplementary apertures in the lineage, is denoted the Sphaeroidinella event. Changes in the proportions of the supplementary apertures in the lineage were studied in 24 samples from ODP Hole 926A drilled in the equatorial Atlantic Ocean. In addition, detailed chronological models have been compiled for this section as well as for Pliocene sections from DSDP Holes 214, 502A, and 503B, where evolution in the lineage have been analyzed previously. Stratigraphic correlation of the studied sequences suggests that the Sphaeroidinella event took place at about 3.6 Ma in the eastern equatorial Pacific (Hole 503B) and at 3.5-3.6 Ma in the Caribbean (Hole 502A), while in the Atlantic Ocean (Hole 926A) and in the Indian Ocean (Hole 214) the event occurred after 3.5 Ma. The inferred diachrony of the mid-Pliocene Sphaeroidinella transition, which is considered to represent a prime example of punctuated anagenesis, suggests that this evolutionary modality may have an allopatric component. Its short duration (on average less than 50 kyr) and the detailed biochronology that could be established for this event qualifies it as a useful biostratigraphic tool in the low-latitude Pliocene oceans.

Integrated stratigraphy of the Messinian across the North Atlantic Ocean

The Messinian was a time of major climatic and paleoceanographic change during the late Cenozoic. It is well known around the Mediterranean region because of the giant anhydritelgypsum sequence and the suggested desiccation of the Mediterranean Sea. However, this interval is less constrained outside the Mediterranean region, where several paleoceanographic changes could have taken place because of the desiccation. Hence, we present an integrated stratigraphic framework for future Messinian paleoceanographic studies, determination of the effect of the Mediterranean desiccation on deep-water paleoceanography, and comparison of intra-Mediterranean paleoceanographic changes with those in the open oceans during the Messinian Stage. Four DSDP/ODP Holes (552A, 646B, 608, and 547A) from the North Atlantic Ocean and one land borehole from Morocco have been studied to integrate bio-, magneto-, and stable isotope Messinian stratigraphy with high resolution sampling. Our results produce the best assessment of the Tortonian/Messinian boundaries in all holes because they do not rely on any one signal. In paleomagnetic Subchronozone C3An1r in the Sale borehole and DSDP Site 609, a S/D coiling direction change of Neogloboquadrina pachyderma/acostaensis appears to indicate PMOW entering the northeastern Atlantic Ocean, at least reaching 50°N. Diachrony and synchrony of some important Messinian planktic foraminifera from these Atlantic DSDP/ODP holes and the Sale borehole, such as the LO of Gq. dehiscens, the LO of Gt. Eenguaensis, the FO and LO of Ct. conomiozea, the FO of Gt. margaritae s.s., the FO of Gt. puncticutata, and the FO of Gt. crassaformis are discussed for understanding some of the paleoceanographic changes. This integrated stratigraphic framework presented here allows much better North Atlantic correlations at this critical point in Messinian geologic history.