The sedimentologic analyses presented in this paper were performed on sediment samples collected during Ocean Drilling Program (ODP) Leg 201 in the eastern equatorial Pacific, Sites 1225 and 1226 (D'Hondt, Jørgensen, Miller, et al., 2003) (Fig. F1). The main purpose of this cruise was to document the extent of subseafloor life and compare the rates of microbial activities in two contrasting depositional settings: the open-ocean pelagic sediments of the oligotrophic eastern equatorial Pacific basin and the hemipelagic ocean-margin deposits of the eutrophic Peru margin basin. The lowest microbial rates were found in the biogenic sediments of the eastern equatorial Pacific, which also contained some of the lowest cell concentrations ever observed in deep-sea sediments. However, all of the microbial processes observed in the ocean-margin sediments were also observed in the open-ocean sediments (D'Hondt, Jørgensen, Miller, et al., 2003). In particular, onboard and postcruise geochemical, microbiologic, and lithologic analyses indicated that the highest subseafloor rates of microbial activity in the eastern equatorial Pacific occur in deeply buried biosiliceous sediments of Pliocene and Miocene age (D'Hondt et al., 2004; Parkes et al., 2005). Leg 201 data document large-scale relationships between lithology (and, conversely, paleoceanography), distribution, and rates of microbial activities (D'Hondt, Jørgensen, Miller, et al., 2003). However, postcruise research suggests that these relationships can also be verified at smaller lithologic boundaries, including subunit boundaries and meter-scale bedding alternations (D'Hondt et al., 2004; Parkes et al., 2005). The dominant electron-accepting pathways of the open-ocean eastern equatorial Pacific sediments are sulfate, manganese, and iron reduction. The comparison between interstitial water dissolved inorganic carbon (DIC) and ammonium profiles from the two sites also indicates that mineralization of organic material at the relatively organic carbon (OC) rich Site 1226 is several-fold higher than at Site 1225. The concentrations of methane, ammonium, DIC, and alkalinity peak in the middle of the sediment columns, whereas sulfate concentrations are lowest in the middle and lower parts of the sediment column. Nitrate and oxygen are only present at the ocean and basement interfaces. In the lower portion of the sediment columns, the vertical sequence of successive reduction zones is reversed as a result of water flow through the underlying basaltic basement (D'Hondt, Jørgensen, Miller, et al., 2003).
Modern deposition at the eastern equatorial Pacific sites occurs beneath a region of relatively high productivity and enhanced accumulation of biogenic sediments (Chavez and Barber, 1987). Site 1225 is located near the present boundary between the South Equatorial Current and the North Equatorial Countercurrent at 3670 m water depth. Site 1226 is located ~300 km south of the Galapagos Islands at 3297 m water depth beneath the present-day boundary between the South Equatorial Current and the nutrient-rich Peru Current (Fig. F1). Lithology, stratigraphy, and age model for Sites 1225 and 1226 (and nearby Sites 851 and 846) are described by Mayer et al. (1992), Pisias et al. (1995), and D'Hondt, Jørgensen, Miller, et al. (2003).
The biogenic sediments of Sites 1225 and 1226 consist mainly of Miocene to Pleistocene nannofossil (coccolith) ooze with varying amounts of biosiliceous (diatoms, radiolarians, and sponge spicules) and biocalcareous (foraminifers) microfossils and also accessory minerals (e.g., pyrite, plagioclase, dolomite, and hematite). The main mineralogic phases present in these sediments are calcite and silica (opal-CT and quartz) and reflect the composition of the two main biogenic components. Amorphous silica (opal-A) composes the frustules of living diatoms and is the major silica phase of the biosiliceous sedimentary component.
The stratigraphy of the eastern equatorial Pacific sites offers a continuous record of marine biogenic sedimentation from the early Miocene at Site 1226 (16.5 Ma) and the middle Miocene (11 Ma) at Site 1225 over cooling and subsiding basaltic crust (Mayer et al., 1992). Stratigraphic (vertical) changes of both lithology and physical properties account for the subdivision of the sedimentary column into units and subunits (D'Hondt, Jørgensen, Miller, et al., 2003). These lithologic changes reflect the evolution of biogenic sedimentation during the Neogene in the eastern equatorial Pacific, which earlier studies divided into three main paleoceanographic phases: the middle to late Miocene "carbonate crash," the late Miocene to early Pliocene "biogenic bloom," and late Pliocene to modern sedimentation (Pisias et al., 1995).
Particle size analyses, in conjunction with other sedimentologic and lithologic data, can improve the characterization of very fine grained biogenic sediments. Particle size variations can also be used to evaluate lithologic/textural controls and effects of microbial activity on biogenic sediments. Grain size analysis is a classic sedimentologic tool commonly used for the study of siliciclastic deposits, where particle sizes reflect the processes that generated the clasts, including weathering, erosion, transport, and sedimentation. There are relatively few examples of grain size studies of fine-grained biogenic sediments (e.g., Paull et al., 1988). However, in the last few years the number of detailed sedimentologic studies of pelagic and hemipelagic sediments has increased because analytical instruments that perform relatively rapid and automated analyses of very small particles have become available (McCave et al., 1995; Stuut et al., 2002; Warner and Domak, 2002). Grain size analyses of marine sediments have been successfully used in conjunction with other paleoceanographic proxies to document past changes in intensity of bottom currents and upwelling. For example, grain size analyses of carbonate oozes in sediment cores from the Walvis Ridge, southeast Atlantic Ocean, indicate that the last glacial episodes are marked by increasing fragmentation of foraminifer oozes (Stuut et al., 2002). Particle size analyses of Antarctic glacial marine sediments were successfully used by Warner and Domak (2002) as a paleoenvironmental proxy and correlated to downcore variations of magnetic susceptibility.
The analysis of fine-grained sediments has been commonly based on settling velocities and density differential between particles and a settling fluid (Stokes law, which applies only to perfectly spherical particles). However, most particles, including biogenic tests and test fragments, are not spherical, and the use of Stokes law yields grain sizes that tend to be finer than the actual particle sizes (Murray, 2002). Modern tools for particle size analysis are automated instruments that estimate grain size distribution based on different properties, including changes in intensity of a light beam that interacts with particles dispersed in a fluid (photohydrometer), X-ray attenuation (sedigraph), and electroresistance (Coulter counter). The laser particle sizer is a recently developed laser diffraction–based instrument that offers the most effective way to perform rapid analyses of very fine grained sediments on very small samples (<1 g). Particle size analyses were carried out with a Beckman-Coulter LS 13 320 laser particle size analyzer attached to an aqueous module equipped with a pump and a built-in ultrasound unit. The measured size distributions were analyzed from 0.04 µm to 2 mm. Measurements of such a wide particle size range are possible because the particle sizer is composed of two units: a laser beam for conventional (Fraunhofer) diffraction (from 0.4 µm to 2 mm) and a polarized intensity differential scatter (PIDS) unit, which measures particles based on the Mie theory of light scattering (0.04 µm; Beckman Coulter Inc., 2003).
The acquisition of biogenic sediment grain size data for samples from the eastern equatorial Pacific with a Beckman Coulter laser particle sizer first required the development of strategies for pretreatment and analysis of the samples. In particular, the "Materials and Methods" section, below, illustrates the experimental designs tested to establish disaggregation procedure and pretreatment of the indurated core sediment samples for grain size analysis with the particle sizer. The results of the tests and of the analyses are discussed in the following "Discussion" section, and the grain size data are reported in the "Appendix."