Early to Middle Miocene Planktonic Foraminiferal Datum Levels of the Equatorial and Subtropical Pacific

Quantitative planktonic foraminiferal analysis of the equatorial and subtropical Pacific DSDP Sites 77B, 319 and 296 revealed more than 35 datum levels for the Early and Middle Miocene. Relative ages for these datum levels were derived from the estimated sediment accumulation rate of Site 77B, as calibrated by certain foraminiferal and radiolarian datum levels previously tied to the paleomagnetic record. This indirect approach of dating foraminiferal datum levels resulted in age estimates that are in relatively close agreement with ages of Middle Miocene foraminiferal datum levels previously tied to the paleomagnetic record by Ikebe et al. (1972), Berggren and Van Couvering (1974), Ryan et al. (1974), and Tsuchi et al. (1979). However, considerable age differences are apparent in the Early Miocene datum levels. Several intervals marked by severe calcium carbonate dissolution, often associated with a hiatus, are identified in the Early Miocene sequences. These dissolution intervals occur at about the same stratigraphic intervals in many deep-sea sections in the Pacific Ocean and form an important marker horizon for regional biostratigraphic correlations; they appear to have been caused by sea level changes accompanied by major paleoclimatic fluctuations. Early to Middle Miocene planktonic foraminiferal datum levels of the equatorial and subtropical Pacific INTRODUCTION The availability of Neogene deep-sea sections provided by the DSDP (Deep Sea Drilling Project) has refined micropaleontological biostratigraphy through the use of datum levels [first (FA) and last (LA) appearances of species] in conjunction with paleomagnetic stratigraphy (Berggren, 1 972; Ryan et al., 1 974; Theyer and Hammond, 1974; Van Couvering et al., 1976; Saito, 1977). In recent years paleomagnetic calibration of diatom and radiolarian datum levels by Theyer and Hammond (1 974), Burckle (1 977, 1 978), Burckle and Opdyke (1 977), and Theyer, Mato and Hammond (1978) have provided increasing dating accuracy and biostratigraphic resolution. There has not been equal progress in datum-level calibration in the calcareous faunas of the early Neogene due to a general lack of calcareous faunas in piston cores suitable for paleomagnetic stratigraphy. This shortcoming makes it difficult to establish a precise biochronology for foraminiferal events. However, age estimates can be established indirectly through multiple microfossil correlations in deep-sea sections containing both calcareous and siliceous faunas. Such an effort has been made for the Late Miocene interval of the temperate and subtropical Pacific (Keller, 1980). The present study extends this effort to the Early and Middle Miocene and in addition has newly identified several apparently synchronous datum events that provide an unusually detailed biostratigraphic control. DATUM LEVELS AND BIOSTRATIGRAPHY A synchronous datum level may be defined as a horizon in which a species seems to appear or disappear approximately at the same time over a given climatic/oceanographic region usually characterized by a specific water mass. The most objective way to determine synchroneity of a datum level is by determining time-equivalence in many paleomagnetically analyzed stratigraphic sequences. Such an approach is usually impractical both from the standpoint of the time involved in such a study and the availability of deep-sea sections with well-preserved faunal records. There are presently few continuous early Neogene deep-sea sections with good calcareous faunas in the Pacific, and no paleomagnetic stratigraphies are available from these cores. Hence, this study was based on 2 early Neogene sections with relatively well-preserved calcareous faunas and (where available) siliceous faunas, in which timeequivalence was assumed from paleontological criteria. One excellent calcareous and siliceous biostratigraphic record is available for Site 77B. The radiolarian record at this site was initially studied by Goll (1972). Later, Westberg and Riedel (1978) restudied the Middle to Late Miocene sequence in an attempt to determine the accuracy of correlations using radiolarian events in the Pacific. Recently, a paleomagmicropaleontology, vol. 26, no. 4, pp. 372-391, 1980 372 This content downloaded from 157.55.39.195 on Tue, 26 Apr 2016 05:26:09 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol 26, no. 4, 1980 TABLE 1 DSDP Site locations and water depths. Water depth Site Latitude Longitude (in meters) Location 77 00'28.90'N 133"13.70'W 4291 Equatorial Pacific 319 13'01.04'S 101 31.46'W 4296 Bauer Deep 296 29'20.41'N 133'31.52'E 2920 Kyushu Ridge netic calibration of radiolarian events in the equatorial Pacific was provided by Theyer and others (1978). These studies, together with the nannofossil biostratigraphy of Site 77B (Bukry, 1972), provide an independent check on the ordered succession of foraminiferal datum levels and permit age estimates to be extrapolated from the sediment accumulation curve based on the dating of these microfossil events. The foraminiferal biostratigraphy of Site 77B was initially studied by Jenkins and Orr, who also illustrated many species (Jenkins and Orr, 1972, pls. 1-41). These authors found that the effects of selective dissolution on Blow's (1969) zonal index species created major problems in the application of his zonal scheme and therefore developed a new zonation for faunas that have undergone selective dissolution (Jenkins and Orr, 1972; Orr and Jenkins, 1977). The major differences between this new zonal scheme and Blow's are in the interval equivalent to Blow's Zones N7-N9 (Orr and Jenkins, 1977, fig. 3), in which the calcareous assemblages are severely affected by dissolution. I have also applied Blow's zonation in this study and have encountered similar problems in zoning the Early Miocene; Zones N5 to N6 contain a hiatus, and the fauna of Zone N7 is severely dissolved. However, there were no problems in zoning the Middle Miocene. The fact that Site 77B is one of the rare sites with a nearly complete Neogene calcareous and siliceous faunal record made this an ideal stratigraphic sequence to 1) obtain a quantitative record of equatorial foraminifera; 2) identify faunal events (FA and LA) that appear to be synchronous over a give region; and 3) compare these faunal events with the radiolarian, diatom, and nannofossil datum levels and thereby obtain an indirect method of dating foraminiferal events. Any attempt to identify datum events in a section necessitates at least one reference site that provides a continuous faunal sequence. It has been extremely difficult to find a continuous Early to Middle Miocene reference section for Site 77B ;instead, 2 sites, one with an Early Miocene sequence (Site 296) and one with a Middle Miocene sequence (Site 319), were chosen (table 1). The Southeast Pacific Site 319 has a continuous Middle Miocene sequence resting on oceanic crust. The Southwest Pacific Site 296 has an Early Miocene sequence; the Middle Miocene section has been removed by a hiatus. While the foraminiferal records at these 2 sites form an excellent control for the Site 77B faunas, siliceous faunas are absent in Site 319 and are not well preserved in Site 296 (Ling, 1975). Preliminary foraminiferal and nannofossil biostratigraphies for Site 296 were prepared by Ujiie (1975) and Ellis (1975), and for Site 319 by Quilty (1976), Bukry (1976) and Blechschmidt (1976). METHODS Methods of sample preparation are described in detail by Keller (1980). Samples of approximately 20 cc of sediments were washed through a 62-,Lm screen. Quantitative counts were obtained from splits (using a modified Otto microsplitter) of approximately 300-500 individuals, and these data are provided in tables 4 to 6. Intervals affected by high solution rates were identified by samples containing less than 500 specimens per 20 cc of sediments, as compared to several thousand specimens ordinarily found in tropical to subtropical faunas. DISSOLUTION INTERVALS Carbonate dissolution is a factor in virtually all deepsea sections, with the effects of dissolution on foraminifera ranging from "sugary" tests to tests with thin walls and holes to fragmented tests. One result of selective dissolution on foraminifera may be the differential removal of species of such genera as Orbulina and Globigerinoides (Berger, 1970). Therefore, biostratigraphic resolution may be impaired where zonal indexes are based on solution-sensitive species. Besides these general dissolution effects on faunas, certain stratigraphic intervals show relatively increased dissolution. Samples in these highly affected intervals may contain a few to a few hundred specimens of dissolution-resistant species in the genera Globorotalia, Globoquadrina and Sphaeroidinella. These highlyaffected intervals are often associated with a hiatus, appear to occur at specific geologic times, and can be recognized in deep-sea sections throughout the Pacific (Keller, 1 980). It appears that these synchronous dissolution events are triggered by large-scale climatic/oceanographic events, and that they provide stratigraphic markers for regional correlations as well as being evidence for major paleoclimatic and paleoceanographic events. In the deep-sea sections studied here, Middle Miocene faunas of Sites 77B and 319 show moderate selective dissolution that mostly affected species of Orbulina. Early Miocene faunas of Sites 77B and 296 are more severely dissolved as is indicated by the near absence of species of Globigerinoides and Globigerina. Dissolution intervals (intervals containing less than 500