Highly diverse sediment types in both open-ocean and ocean-margin provinces of the eastern tropical Pacific Ocean were studied during Leg 201. The lithostratigraphy and physical properties of each of these sites are described in the Leg 201 Initial Reports volume (D'Hondt, Jørgensen, Miller, et al., 2003). Further descriptions are provided in the reports from previous DSDP and ODP cruises to the same sites: Peru Basin DSDP Leg 34 (Yeats, Hart, et al., 1976), Peru margin ODP Leg 112 (Suess, von Huene, et al., 1988), and equatorial Pacific ODP Leg 138 (Mayer, Pisias, Janecek, et al., 1992). Only a few new data obtained following Leg 201 and pertinent to the above discussions of the biogeochemistry and microbiology will be presented in the following.
At the eastern equatorial Pacific sites (1225 and 1226) total inorganic carbon (TIC) originates mostly from calcareous nannoplankton, and concentrations vary in major part between 6 and 10 wt%. The late Miocene carbonate crisis is, however, apparent from low TIC contents in the diatom-rich zone between ~200 and 300 mbsf (Meister et al., this volume b; Mayer, Pisias, Janecek, et al., 1992). The Peru margin sediments generally have TIC contents, between <1 and 2.5 wt%. These low inorganic carbon concentrations are related to the predominant sedimentation of siliceous and siliciclastic material, with lowest values where upwelling, and thus diatom productivity, are strongest.
The cored sediments vary 100-fold in modern sedimentation rates (Skilbeck and Fink, this volume). The deep-sea sediments (Sites 1225, 1226, 1230, and 1231) have sedimentation rates of 1.5–2.4 cm/k.y., in accordance with earlier determinations in the eastern equatorial Pacific. Of the Peru margin sediments, Site 1227 has only a thin layer of upper Holocene sediment below which 15,700 yr of sediment is missing. Sites 1228 and 1229 have maintained sedimentation rates of 32–98 cm/k.y. over the past few thousand years. The temperatures of the cored sediments at the open-ocean sites vary from 2°C at the sediment surface to a maximum of 26°C at 400 mbsf. The shallow Peru margin sediments have temperatures of 8°ñ–13°C at the sediment surface and 16°–20°C at the bottom of the drilled interval. Thus, all sediments are well within the temperature range of psychrophilic to mesophilic microorganisms.
Grain size of the sediments is a biologically important property for several reasons. It affects permeability and thus the possibility of fluid flow through the sediment. This in turn determines the diffusive or advective transport of dissolved species in the pore water and thereby affects biogeochemical zonations and substrate fluxes to the deep biosphere. It also determines the pore space available for microbial cells in the deep subsurface and thereby their ability to multiply and to actively move. Particle studies using a laser particle size analyzer show mean diameters of bulk particles in Site 1225 and 1226 sediments of 20–30 µm, which is larger than the normal size range of the dominant coccoliths (4–14 µm), suggesting some degree of aggregation (Aiello and Kellett, this volume). In the nannofossil ooze deposited during the late Miocene–Pliocene "biogenic bloom," more than 50% of the sediment is composed of particles of <12 µm size. Unless the pore space between the coccoliths is clogged by fine particles, the pore space in unlithified oozes should certainly be sufficiently large for prokaryotic cells to move freely in the deep subseafloor sediments.
Sediment permeability was analyzed experimentally in core samples from most of the sites drilled in order to evaluate the possibility of advective flow (Gamage et al., this volume). These samples represent depths from 11 to 400 mbsf and include both open-ocean and ocean-margin subsurface environments. Permeabilities vary over two orders of magnitude, from 8 x 10–19 m2 to 1 x 10–16 m2, the lower representing lithified oozes and clay-rich sediments, the higher representing unlithified oozes and silt-rich sediments.
The low permeability appears to prevent vertical transport of pore fluid in Peru margin sediments to the extent that the upward flux of dissolved seawater ions from the underlying Miocene brine takes place primarily by molecular diffusion over hundreds of meters. The low permeability of the 120- to 420-m-thick sediments overlying the basaltic crust of the eastern tropical Pacific sites also appears to prevent vertical advective transport of pore fluid. Accordingly, none of the Leg 201 sites show chemical evidence of vertical flow. The crust itself, however, appears to be fractured and permeable to an extent that allows slow lateral flow of seawater. The existence of such fluid flow had already been indicated by earlier studies of geothermal gradients and pore water chemistry. Yet, it was a striking observation during Leg 201 that the most readily consumed oxidants, oxygen and nitrate, penetrated into the sediment also from below at Sites 1226 and 1231. It is an important conclusion from this observation that the basalt is not a source of energy-rich compounds such as hydrogen that may fuel the deep biosphere from below. Such compounds would obviously be excellent substrates for microorganisms utilizing the available oxygen or nitrate in seawater in the crust. On the other hand, the availability of electron acceptors with high free-energy yields may support bacterial utilization of products such as methane and ammonium, derived from previous microbial activity in the sediment column above. Furthermore, the flow of oxic seawater through the crust may accelerate basalt weathering and ultimately enhance chemical flux to the ocean.
Bacterial processes in the subsurface sediment drive authigenic formation and dissolution of minerals such as pyrite, barite, dolomite, and apatite. The dynamic formation of such minerals is most clear in ocean-margin sites with highest microbial activity. Pyrite formation is distinctly due to high iron content of mainly terrestrial origin and enhanced sulfide production resulting from high organic carbon burial. Barite formation is controlled by penetration of seawater sulfate from above and upward migration of Ba2+ ions from deeply buried barite of planktonic origin. The barite horizons are therefore found just below the sulfate–methane transitions and reflect a dynamic cycle by which barite, upon further burial below the sulfate zone, dissolves, diffuses upward, and reprecipitates in the lower sulfate zone.
Intermediate minima in dissolved Ca2+ and Mg2+ within the upper 30 m of Peru margin sediments suggest precipitation of dolomite or other carbonate phases (Donohue et al., this volume). Dolomite was indeed found at all sites drilled on the Peru margin, from the shelf to the Peru Trench (Meister et al., this volume a). Dolomite occurs in distinct layers that probably formed relatively early after burial, based on the Sr isotopic composition of the diagenetic carbonate (Meister et al., submitted [N3]). Thus, on the Peru shelf the youngest dolomite layers often appear between 10 and 30 mbsf, where the diatomaceous sediment is still uncompacted. It is likely that formation of dolomite was not a continuous process but was coupled to geochemical interfaces, which may have migrated upward or downward as a result of varying sediment composition and organic carbon content (Meister et al., submitted [N3]). In particular, depletion of sulfate, which may inhibit dolomite formation, and the concurrent increase of alkalinity are controlling factors for dolomite precipitation (Baker and Kastner, 1981; Meister et al., this volume a). Such favorable chemical conditions are found at the sulfate–methane transitions, where microbial density and activity are particularly high (D'Hondt, Jørgensen, Miller, et al., 2003). Although formation of dolomite has been observed to take place in bacterial culture experiments and in surface sediments of a tropical lagoon (Vasconcelos et al., 1995; Warthmann et al., 2000), crystal morphologies found in ODP cores were distinctly different and did not provide direct evidence of a bacterial origin.
Biogenic magnetic minerals, magnetosomes, are formed as inclusions in the cells of magnetotactic bacteria that generally live under microoxic conditions but may also be found in anoxic zones. The magnetosomes consist primarily of magnetite but may also be greigite or pyrite and may be preserved in sediments over many millions of years. In spite of the persistence of magnetite toward iron-reducing bacteria, the magnetic minerals are chemically attacked by free sulfide in organic-rich sediments. The presence of magnetosomes is therefore indicative not only of suitable growth conditions for magnetotactic bacteria at the time of sediment deposition but also of the changing geochemical conditions that the buried sediment has undergone since then (e.g., Hesse and Stolz, 1999). In the sediments drilled during Leg 201, magnetosomes were recovered from all investigated core samples, both from open Pacific Site 1225 and Peru margin Site 1227, and even in layers with low magnetic susceptibility and significant sulfide concentrations (Ford et al., this volume). The presence of magnetosomes enabled detailed studies of magnetostratigraphy, which, together with biostratigraphy, provided an accurate age model for Site 1225 (Niitsuma et al., this volume).