26. PLIOCENE DISCOASTER ABUNDANCE VARIATIONS, DEEP SEA DRILLING PROJECT SITE 606: BIOCHRONOLOGY AND PALEOENVIRONMENTAL IMPLICATIONS1 Jan Backman, Department of Geology, University of Stockholm and Pierre Pestiaux, Institut d'Astronomie et de Géophysique Georges Lemaitre, Université Catholique de Louvain2 ABSTRACT Abundance variations of six Pliocene species of the calcareous nannofossil genus Discoaster have been determined for the time interval between 1.9 and 3.6 Ma at DSDP Site 606, using a sample interval of 30 cm (<9 kyr.). These quan- titative data contain information which can be interpreted in terms of both biochronology and paleoclimatic develop- ment. The following datums have been examined: D. brouweri (extinction at 1.89 Ma), D. triradiatus (extinction at 1.89 Ma), D. triradiatus (peak abundance begins at 2.07 Ma), D. pentaradiatus (extinction between 2.33 and 2.43 Ma), D. surcu- lus (extinction between 2.42 and 2.46 Ma), D. asymmetricus (sharp decline in abundance at 2.65 Ma), and D. tamalis (extinction at 2.65 Ma). The age of the peak abundance of D. triradiatus is accurately determined for the first time. The remaining estimates agree with those previously obtained from the northern North Atlantic and Pacific oceans. Plots of changes in total Discoaster accumulation rate indicate a long-term trend of progressive cooling of sea-sur- face temperatures during preglacial times in the area of Site 606. This trend is overprinted by short-term abundance os- cillations. The orbital forcing of paleoclimatic change is strongly imprinted on these short-term variations, as indicated by spectral analysis. The 413-kyr. eccentricity period, the 41-kyr. obliquity period, and the 23- and 19-kyr. precessional periods (not separated in our record) are identified, and the eccentricity period is dominant. The eccentricity period co- incides with sharp changes in the Discoaster record at 2.3-2.4 Ma, 2.7-2.8 Ma, 3.1-3.2 Ma, and 3.5-3.6 Ma. Only the youngest of these cycles can be linked with major glacial buildup. INTRODUCTION Before DSDP Leg 94, DSDP Hole 552A was the only Neogene sequence successfully cored in the mid- and high- Drilling on DSDP Leg 81 provided the first accurate latitude North Atlantic region using the Hydraulic Piston age estimate of the initiation of large-scale Cenozoic gla- Corer. This is a poor record of deep-sea drilling for pa- ciation in the high-latitude North Atlantic region. The leoenvironmental purposes in the North Atlantic Ocean, age derived from Hole 552A is approximately 2.4 Ma, considering that this region was of key importance for and the evidence for the passing of this paleoclimatic global oceanographic and climatic development during threshold is based on the first occurrence of ice-rafted late Cenozoic times (see, e.g., Ruddiman and Mclntyre, material in the sediment record and oxygen-isotope analy- 1984). The Leg 94 sites were chosen to give good latitu- ses (Shackleton et al., 1984). The isotopic data of Shack- dinal coverage in the North Atlantic and to provide se- leton et al. (1984) indicated also that the preglacial Pli- quences of relatively high sediment accumulation rates. ocene was characterized by considerable climatic vari- The primary purpose of the present study is to evalu- ability, although no information was obtained on the ate further the usefulness of the nannofossil genus Dis- development of sea-surface temperature changes, since coaster as an indicator of sea-surface temperature change. benthic foraminifers were analyzed. Subsequently, Back- This evaluation includes spectral analysis to investigate man et al. (in press) addressed the question of Pliocene whether the paleoclimatic forcing of all main orbital el- sea-surface temperature development through analysis of ements (eccentricity, obliquity, precession) is imprinted abundance variations of the calcareous nannofossil ge- on the Pliocene Discoaster record. Such an evaluation nus Discoaster from the sample set used by Shackleton has to be based on quantitative data gathered from closely et al. (1984). These warm-water-preferring nannofossils spaced samples. In turn, this strategy promises to add occurred in distinctly low relative abundances at the lati- valuable biochronologic information regarding the Plio- tude of Hole 552A (56°N), but oscillated in abundance cene Discoaster extinction events. Of the Leg 94 sites at a periodicity corresponding to one of the main orbital drilled, we chose to study the southernmost (Site 606, elements (obliquity). Variation in the tilt axis of Earth is 37 °N), mainly because the data generated would repre- strongly related to insolation changes at higher latitudes, sent different paleoceanographic and paleoclimatic con- and is therefore associated with temperature change (see, ditions, as compared with Hole 552A (56°N). e.g., Berger and Pestiaux, 1984). Hence, the data of Back- Site 606 is situated in the subtropical gyre at latitude man et al. (in press) indicate that short- and long-term 37°N and longitude 35°W; the water depth is 3007 m. paleoclimatic change may be delineated by determining The moderate water depth and the fact that Discoaster Discoaster abundance oscillations. spp. are among the least dissolution-susceptible of all Cenozoic nannofossils (Lohmann and Carlson, 1981) sug- gest that the influence of dissolution on our abundance Ruddiman, W. F., Kidd, R. B., Thomas, E., et al., Init. Repts. DSDP, 94: Washington (U.S. Govt. Printing Office) data is negligible. A sample interval of 30 cm has been 2 Addresses: (Backman) Department of Geology, University of Stockholm, S-106 91 used, representing roughly 8.9 kyr. above the base of the Stockholm, Sweden; (Pestiaux) Institut d'Astronomie et de Géophysique Georges Lemaitre, Université Catholique de Louvain, Louvain-la-Neuve, Belgium. Mammoth and about 4.5 kyr. below that boundary. The 903 J. BACKMAN, P. PESTIAUX taxonomy of the species involved, and the counting meth- mean value in the pre-2.65-Ma record. The virtual ab- od adopted, are discussed by Backman and Shackleton sence of D. tamalis in the post-2.65-Ma record both at (1983). The chronology adopted results from combined Site 606 and in Core V28-179 suggests that the presence of magnetostratigraphic data (Clement and Robinson, this D. asymmetricus above a level corresponding to 2.65 Ma volume) and biostratigraphic data (this study). We have is indigenous and not caused by sediment mixing. For not used material from Hole 606A. Because our princi- this reason it is not possible to assign a precise chrono- pal interest was to compare the results from Site 606 logical estimate for the true last occurrence of D. asym- with those obtained from Hole 552A, we have investi- metricus. Nevertheless, the abundance decline at 2.65 gated the same time interval, between approximately 1.9 Ma appears to provide a consistent datum in the Atlan- and 3.6 Ma. The time-control for the spectral analysis is tic and Pacific oceans for both species, which can be based on linear interpolation between the magnetostrati- easily determined using a quantitative method of inves- graphic control points. tigation. BIOCHRONOLOGY D. surculus and D. pentaradiatus The succession of late Pliocene Discoaster extinction Comparison of abundance patterns of D. surculus and events was established more than a decade ago (e.g., D. pentaradiatus in different cores and ocean basins (Fig- Bukry, 1973), and the abundance patterns from Site 606, ure 1; Backman and Shackleton, 1983; Backman et al., shown in Figure 1, do not deviate from this order (from in press) suggests that the last occurrences of these two oldest to youngest): D. tamalis, D. asymmetricus, D. species provide only approximative biochronological in- surculus, D. pentaradiatus, and D. brouweri. However, formation. All available data suggest that these extinc- the precise chronology of Pliocene Discoaster extinction tions occurred in the earliest part of the Matuyama, be- events has been much debated. In the sediment record tween approximately 2.35 and 2.45 Ma, and that D. sur- of Site 606, several factors provide a good basis for pre- culus usually disappeared slightly earlier than D. pentara- cise chronological estimates of the Discoaster datums: diatus. In the record of DSDP Hole 552A, for example, the comparatively high sediment accumulation rate, the D. surculus disappeared in conjunction with the entry recognizable presence of all relevant magnetostratigraphic of the first ice-rafted pulse and D. pentaradiatus disap- reversal boundaries, the fact that these boundaries oc- peared with the entry of the second ice-rafted cycle (Back- cur over short stratigraphic intervals, and the short sam- man, 1984). At Site 606, it is virtually impossible to de- ple intervals used. cide from the abundance pattern (Fig. 1) whether the The investigated Discoaster datums all occurred in presence of D. pentaradiatus between 74.0 and 77.5 m the late Gauss and early Matuyama. The age-depth re- sub-bottom results from sediment mixing or corresponds lationships of the base of the Olduvai, top of the Gauss, to a local low abundance caused by the onset of glacial and top of the Kaena suggest a uniform sediment accu- conditions in the North Atlantic. This gives an uncer- mulation rate of approximately 3.4 cm/kyr. tainty of as much as 0.1 m.y. (2.33-2.43 Ma) for the last occurrence datum of D. pentaradiatus at Site 606. Simi- D. tamalis and D. asymmetricus larly, the disappearance of D. surculus probably occurs Backman and Shackleton (1983) demonstrated that between 77.0 and 78.5 m sub-bottom (2.42-2.46 Ma). D. tamalis and D. asymmetricus exhibited pronounced covariation in abundance patterns during mid-Pliocene D. brouweri and D. triradiatus times in the equatorial Pacific (V28-179) and the North Takayama (1970) observed that D. triradiatus became Pacific (V32-127).