View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Columbia University Academic Commons Late Neogene chronology: New perspectives in high-resolution stratigraphy W. A. Berggren Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 F. J. Hilgen Institute of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands C. G. Langereis } D. V. Kent Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 J. D. Obradovich Isotope Geology Branch, U.S. Geological Survey, Denver, Colorado 80225 Isabella Raffi Facolta di Scienze MM.FF.NN, Universita ‘‘G. D’Annunzio’’, ‘‘Chieti’’, Italy M. E. Raymo Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 N. J. Shackleton Godwin Laboratory of Quaternary Research, Free School Lane, Cambridge University, Cambridge CB2 3RS, United Kingdom ABSTRACT (Calabria, Italy), is located near the top of working group with the task of investigat- the Olduvai (C2n) Magnetic Polarity Sub- ing and resolving the age disagreements in We present an integrated geochronology chronozone with an estimated age of 1.81 the then-nascent late Neogene chronologic for late Neogene time (Pliocene, Pleisto- Ma. The 13 calcareous nannoplankton schemes being developed by means of as- cene, and Holocene Epochs) based on an and 48 planktonic foraminiferal datum tronomical/climatic proxies (Hilgen, 1987; analysis of data from stable isotopes, mag- events for the Pliocene, and 12 calcareous Hilgen and Langereis, 1988, 1989; Shackle- netostratigraphy, radiochronology, and cal- nannoplankton and 10 planktonic foram- ton et al., 1990) and the classical radiometric careous plankton biostratigraphy. Dis- iniferal datum events for the Pleistocene, age calibration of the GPTS (Mankinen and crepancies between recently formulated are calibrated to the newly revised late Dalrymple, 1979). Accordingly, a working astronomical chronologies and magneto- Neogene astronomical/geomagnetic polar- group was established (refer to authorship chronologies for the past 6 m.y. have been ity time scale. of this paper), assignments made, and the resolved on the basis of new, high-precision work initiated. The extended period re- Ar/Ar ages in the younger part of this in- INTRODUCTION quired to complete this work reflects the terval, the so-called Brunhes, Matuyama, intense activity in the field of Neogene and Gauss Epochs ( Chrons C1n–C2An; The past decade has witnessed a signifi- 5 geochronology during the past 5 yr and, in 0–3.58 Ma), and revised analysis of sea cant improvement in the precision and ac- particular, resolution of the apparent dis- floor anomalies in the Pacific Ocean in the curacy of late Neogene geochronology. This crepancies between the astronomical time older part, the so-called Gilbert Epoch has been achieved through the merging of scale and GPTS in 1993. This resolution (5 Chron C2Ar–C3r; 3.58–5.89 Ma). The improved radiochronologic dating tech- was achieved primarily by means of new, magneto- and astrochronologies are now niques and astronomical tuning methods high-precision Ar/Ar ages on the younger concordant back to the Chron C3r/C3An with the already familiar and classical boundary at 5.89 Ma. K-Ar method and geomagnetic polarity part of the stratigraphic interval treated The Neogene (Miocene, Pliocene, Pleisto- time scale (GPTS). The application of new here and by means of revised interpreta- cene, and Holocene) and Paleogene are techniques to old problems in geochronol- tions of sea floor anomaly patterns in the treated here as period/system subdivisions ogy, however, has resulted in a paradox; Pacific Ocean in the older part of the Plio- of the Cenozoic Era/Erathem, replacements namely, discrepancies between radiomet- cene–Pleistocene record. for the antiquated terms Tertiary and Qua- rically based and astronomically based In this paper we review the historical ternary. The boundary between the Miocene time scales for the later part of the Neo- framework and recent evolution of studies and Pliocene Series (Messinian/Zanclean gene. It is a consideration of these prob- in magnetostratigraphy, isotopic radiochro- Stages), whose global stratotype section and lems (and their resolution) that forms the nology, and astrochronology on late Neo- point (GSSP) is currently proposed to be in focus of this paper. gene geochronology. We then discuss the Sicily, is located within the reversed interval This study resulted from a request in 1991 chronostratigraphic framework of the late just below the Thvera (C3n.4n) Magnetic by M. B. Cita (University of Milan and chair Neogene and conclude with a tabulation of Polarity Subchronozone with an estimated of the International Union of Geological regional and global biostratigraphic datum age of 5.32 Ma. The Pliocene/Pleistocene Sciences Subcommission on Neogene Stra- events that have been calibrated to the newly boundary, whose GSSP is located at Vrica tigraphy) to one of us (Berggren) to chair a proposed astro-magneto-radiochronology. GSA Bulletin; November 1995; v. 107; no. 11; p. 1272–1287; 5 figures; 6 tables. 1272 LATE NEOGENE CHRONOLOGY ASTROCHRONOLOGY When the first deep-sea sediment d18O records were presented by Emiliani (1955), he proposed that the quasi-cyclic variations in d18O were due to astronomical forcing at the 41 000 yr tilt frequency. Although this has proven to be the case for the interval prior to 1 m.y. B.P. (Pisias and Moore, 1981; Ruddiman et al., 1986), better time control (Shackleton and Opdyke, 1973; Hays et al., 1976) demonstrated that climate records since then have been dominated by a 100 000 yr cycle, although the shorter orbital cycles of tilt and precession are also pre- served. The first step toward astrochronol- ogy was taken by Hays et al. (1976), who proposed that a modest adjustment to their initial time scale, well within geological con- straints, would bring the peak variance of the obliquity cycles to exactly the same fre- quency calculated for obliquity by astro- physicists (i.e., 41 000 yr). Moreover, after making this adjustment they found im- proved agreement between the shorter cy- cles and the orbital precessional frequencies at 19 000 and 23 000 yr. Thus, the method of ‘‘tuning’’ time scales was established. By the middle 1980s, the large number of oxygen isotope records that had been gen- erated were being used to further refine the ‘‘orbital’’ time scale (Prell et al., 1986; Pisias et al., 1984; Imbrie et al. 1984; Martinson et al., 1987), which, for the past 300 000 yr, was proposed to be accurate to within 5000 yr (Martinson et al., 1987). The astronomically calibrated time scale was extended to ca. 800 000 yr B.P., still without generating any conflict with published geological con- straints (Imbrie et al., 1984). These time scales proved extremely popular because they proposed ages for many characteristic features of the late Pleistocene d18O record that are easily recognizable in deep-sea Figure 1. Oxygen isotope records from Deep Sea Drilling Project Sites 552A and 607, and records (e.g., isotopic stages; Fig. 1). Ocean Drilling Program Site 677, for the interval spanning isotope stages 24–35. Shaded Initial attempts at extending the orbital intervals indicate depth range within which the Jaramillo magnetic reversals (top and chronology back to the Pliocene (Pisias and bottom) fall in Sites 552 and 607 (after Shackleton et al., 1990). Moore, 1981; Williams et al., 1988) were hampered by the relatively poor quality of the available core material. The develop- lated the ages of magnetic reversals back to reversals (Shackleton et al., 1990; hereafter ment of the hydraulic piston corer by the the top of the Gauss Chron. Once again, the SBP90). In favor of the SBP90 time scale Deep Sea Drilling Project, which allowed study posed no serious conflicts with prior was the significant improvement in the co- the recovery of continuous, high-sedimenta- radioisotopic estimates other than a signif- herency of the planktonic isotopic records tion-rate sections, led to the generation of icantly shorter Olduvai event (170 000 yr as with the highly modulated record of orbital continuous, high-resolution isotopic records compared with the published duration of precession. Hilgen (1991a), simultaneously back to the middle Pliocene (and by now 220 000 yr). examining Pliocene sedimentary sections in back to the Miocene). Using a continuous Shortly after this, a ‘‘radical’’ orbital time southern Italy, also proposed an astronom- and orbitally tuned d18O record from Deep scale was proposed, which required a signif- ical chronology with the age of the Matu- Sea Drilling Project Site 607, Ruddiman et icant ('6%) adjustment to the published yama/Gauss boundary 6% older than the ac- al. (1989) and Raymo et al. (1989) recalcu- ages of the Pliocene–Pleistocene magnetic cepted value. Geological Society of America Bulletin, November 1995 1273 BERGGREN ET AL. The orbitally tuned time scale of SBP90 duced variations in climate (e.g., Ryan, was based on the premise that one can cor- 1972; Cita et al., 1978; Vergnaud-Grazzini et relate specific cycles in d18O records to spe- al., 1977; Calvert, 1983; Rossignol-Strick, cific well-dated orbital variations. The great- 1983; Thunell et al., 1984; Rohling and est potential for error in such an exercise Gieskes, 1989). arises from the possibility that one might not In southern Italy, sapropel-bearing se- recognize all the orbitally forced ‘‘cycles’’ in quences are found in the upper Pliocene– the sedimentary record, either because of lower Pleistocene Narbone Formation natural sedimentation rate perturbations (Fig. 2). As in upper Pleistocene piston or because of a hiatus due to natural causes cores, sapropels in the Narbone Formation or the coring process. The possibility of are not distributed evenly in the stratigraph- overlooking a cycle increases in the older ic record but occur in clusters on various sections where fewer high-resolution records scales: large-scale clusters commonly com- exist for intercomparison.