24. BIOGENIC SEDIMENTOLOGY OF RADIOLARIAN ASSEMBLAGES IN A MIDDLE EOCENE DIATOM-RICH UNIT FROM THE EASTERN EQUATORIAL PACIFIC: ODP LEG 199, SITE 1219

Quantitative analysis of radiolarian assemblages at Ocean Drilling Program Leg 199 Site 1219 provides new information on ocean temperature variations during a significant productivity event. This event (which occurs within radiolarian Zone RP-15) is marked by the accumulation of diatoms in association with highly diverse radiolarian faunas. The data collected for this present work include (1) counts of radiolarian diversity and (2) estimates of the abundance ratio of the radiolarian groups Spumellaria and Nassellaria. The radiolarian assemblages within the diatom-rich unit do not differ greatly from one another in their composition. There are, however, some long-term trends in faunal abundances, as indicated by changes in the nassellarian:spumellarian ratio. Particular changes in this ratio that possibly are related to the diatom event involve (1) the increase of the abundance of actinommids above the diatom-rich sediments and (2) the occurrence of abundance maxima of “robust” artostrobids at the beginning and during the terminal phase of diatom production. These changes can tentatively be interpreted as a reaction of specific radiolarian groups to the increase of diatom productivity in shallow waters. Taken together with the earlier onset of calcareous nannoplankton deposition, the diatom-rich unit can be interpreted as marking a short-term gradual change in sea-surface temperature, together with a change in the equatorial current sys1Steiger, T.H. , 2006. Biogenic sedimentology of radiolarian assemblages in a middle Eocene diatom-rich unit from the eastern equatorial Pacific: ODP Leg 199, Site 1219. In Wilson, P.A. , Lyle, M. , and Firth, J.V. (Eds.), Proc. ODP, Sci. Results, 199, 1–19 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Silberbornstrasse 4, D-38889 Blankenburg Germany. T_Steiger@gmx.de Initial receipt: 26 April 2004 Acceptance: 29 September 2005 Web publication: 10 April 2006 Ms 199SR-217 T.H. STEIGER BIOGENIC SEDIMENTOLOGY OF RADIOLARIAN ASSEMBLAGES 2 tem that involved the introduction of cold waters by lateral transport rather than the vertical upwelling of water masses derived from the southeastern regions of the Pacific Ocean. INTRODUCTION Scientific ocean drilling in the central equatorial Pacific Ocean has concentrated on obtaining fundamental information about sedimentary successions and developing an overall biostratigraphic framework. As drilling techniques have improved, the research goals have changed somewhat—moving away from basic investigations and toward specialized objectives such as the reconstruction of changing climatic conditions and ocean-current patterns since the onset of sediment deposition. In addition to its general-interest objectives (those presented in the Ocean Drilling Program [ODP] Leg 199 Scientific Prospectus), Leg 199 focused on the sequence of calcareous and siliceous sediments in the area of the paleoequator during the early Tertiary (between 56 and 40 Ma) and on the evolution of biota there at that time. Of particular interest were (1) the effects of temperature on the position of the carbonate compensation depth and the stability of marine siliceous components and (2) the effects of the Late Paleocene Thermal Maximum on productivity. Previous drilling in this area was carried out during Deep Sea Drilling Project (DSDP) Leg 8 (Tracey, Sutton, et al. 1971), DSDP Leg 16 (van Andel, Heath, et al., 1973), and DSDP Leg 85 (Mayer, Theyer, Thomas, et al., 1985). Improved drilling techniques allowed the collection of high-recovery cores and almost undisturbed sediment sequences for most of Leg 199. In addition, multiple coring of several holes allowed continuous sedimentological analysis by letting gaps in the sediment record that were the result of drilling disturbances be filled with corresponding sediments from other holes at the same site. Leg 199 Sites 1215–1222 (a north–south oriented transect) were drilled on 56-Ma basaltic crust; an exception was Site 1218, which was drilled on 40-Ma crust. The sedimentary record obtained from Leg 199 sites showed predictable variations from north to south. Significant thicknesses of sediment were intersected at the southern sites for those sites that were at the equator between 40 and 25 Ma. Three sites (Sites 1219, 1220, and 1221) contained signals of both southern and northern hemisphere positions (Shipboard Scientific Party, 2002). The sediments studied from Leg 199 sites range in age from the late middle Eocene to the Eocene/Oligocene boundary. During that time, the ecological and productivity conditions changed from those characteristic of an oceanic, deepwater milieu to those that marked the change from siliceous to carbonaceous lithologies at the Eocene/Oligocene boundary. The drilled sediments spanned a 20-m.y. time period that covers the passage of the equator over the Inter-Tropical Convergence Zone (ITCZ). The sediments that were deposited contain accumulations of radiolarians, diatoms, and nannofossils that are characteristic of specific productivity conditions within the Eocene ITCZ. This paper is the result of a pilot study of 30 samples from Hole 1219A, from a diatom-rich unit that is both underlain and overlain by radiolarian-rich sediments. T.H. STEIGER BIOGENIC SEDIMENTOLOGY OF RADIOLARIAN ASSEMBLAGES 3 GEOGRAPHIC SETTING Site 1219 is located at 7°48.019′N, 142°00.940′W, 3° north of the Clipperton Fracture Zone (Fig. F1). The drill site represents the southernmost location of a north–south oriented transect drilled on 56-Ma crust. The distance from Site 1219 northward to Site 1220 is ~400 km, and the distance to Leg 8 Sites 70 and 71 is ~300–400 km (Tracey, Sutton, et al., 1971). Site 1219 was evidently situated in the southern hemisphere until 29 Ma (late Oligocene equator crossing); equatorial and tropical conditions existed there between the middle Eocene and the early Miocene (Shipboard Scientific Party, 2002). These conclusions are based on reconstruction of equator passage and paleogeographic position using a fixed hotspot model (Gripp and Gordon, 1990; Engebretson et al., 1985). Site 1219 sediments contain numerous levels of diatom-bearing sediments that are related to the high-productivity events at 40 Ma and during the equator passage (Fig. F2). Neighboring sites are not included in this pilot study, either because of the scarcity of diatoms in the smear slides from Site 1220 or because of the poor sediment recovery at neighboring sites. Floral and faunal changes in the corresponding diatomrich unit at Site 1218 will be compared with the present results in a separate study. SCIENTIFIC PURPOSE Previous investigations on siliceous organisms in the central equatorial Pacific Ocean have concentrated on radiolarianand diatom-based biostratigraphic zonation, using as their basis sediments cored during DSDP and ODP legs and others obtained from box cores and piston cores (Moore, 1971; Dinkelman, 1973; Schrader, 1976; Fenner, 1984; Nigrini, 1985). Oceanographic research related to the occurrence of radiolarians and diatoms has focused on their lateral distribution relative to the present current system, sea-surface temperatures, temperature gradients, thermocline influences, salinity, and nutrient supply (comp. Blueford et al., 1990; Goll, 1976). Radiolarian depth zonations (Kling, 1979) have been obtained from different water masses and for low and high latitudes, and radiolarian associations have been recognized that represent specific temperature and bathymetric regimes (Petrushevskaya, 1971; Casey, 1993). Recent paleoceanographic research has investigated depositional patterns of siliceous and calcareous microorganisms and nannoorganisms with the purpose of delineating the spatial distribution of productivity zones and (in combination with ocean-climate models) of investigating the recycling of silica (Leinen, 1985; Ragueneau et al., 2000; Moore et al. 2001; Huber, 2002). The primary scientific goal of the present study is to describe and interpret the changes in radiolarian associations from the Zone RP-15 diatom-rich unit at Site 1219. The “radiolarites” underlying this unit are composed either of hard radiolarian chert that could not be penetrated, drilled, or recovered without problems or of soft brownish to pink radiolarian accumulations that are identified as radiolarites. These underlying radiolarian deposits range from middle to late Eocene in age and represent the overall signal of unique oceanic conditions described and modeled by Huber (2002). From an ecological standpoint, the radiolarian mass accumulations are a result of the evolutionary potential of radiolarians, the supply of silica into the ocean system, and the 160°W 150° 140° 130°