This study demonstrates that standard petrographic microscopy in combination with high-resolution particle size analysis performed with a laser particle sizer equipped with a PIDS module (resolution 0.04 µm to 4 mm) is a powerful tool to detect and quantify compositional and textural changes of very fine grained pelagic sediments such as those of the eastern equatorial Pacific (ODP Leg 201, Sites 1225 and 1226). The small (<1 g) amount of sample required for the analysis makes this method particularly useful for describing small-scale lithologic and sedimentologic variations. The methodology that has been proposed for pretreatment and grain size analyses includes 30 s of sonication while the samples are sitting in the aqueous module of the laser particle sizer (Fig. F2). The method proposed also includes the separate grain size analysis of the bulk and noncarbonate fractions (the latter achieved by reacting the sediment with HCl; see the "Appendix").
Our analyses indicate that in samples from Site 1225, the overall shape of the average grain size distribution of the bulk fraction at 30 s of sonication is polymodal and characterized by a main mode at 9.87 µm (Site 1226 = 13.61 µm), minor modes at 17.18, 39.77 (Site 1226 = 43.66 µm), and 101.1 µm (Site 1226 = 110 µm), and a very small mode at 948.32 µm (Figs. F3, F4; Tables T1, T2). The modes represent the common biogenic components of these pelagic sediments, including coccoliths (~2–10 µm), pennate (~10–20 µm) and centric (~20–50 µm) diatoms, radiolarian tests (~40–100 µm), juvenile foraminifers (~40–50 µm), fecal pellets (>50 µm), and test and frustule fragments (~10–100 µm).
At both sites, downcore variations of the textural characteristics of the sediments reflect lithologic variations and, consequently, variations in the proportions of coccoliths, pennate diatoms, centric diatoms, radiolarian and foraminifer fragments, fecal pellets, and trace amounts of silt-sized clasts and volcanic glass shards (Figs. F5, F6). Particle size statistics show that grain size changes correlate with main lithologic changes of the sediment column and show pronounced shifts at unit/subunit boundaries (Figs. F7, F8). At Site 1226, downcore variations of the mean diameter and mode are in opposition of phase with the coccolith-dominated oozes characterized by higher luminosity and inorganic carbon content. Conversely, they show a strong positive correlation with the darker, biosilica-rich sediments (both diatoms and radiolarians), showing the lowest concentrations of inorganic carbon. Our analyses indicate that the samples characterized by the smallest mean diameter and mode are concentrated in late Miocene to Pliocene coccolith-dominated calcareous sediments deposited during the "biogenic bloom." Conversely, coarser particle sizes were detected in the biosilica-rich radiolarian and diatom oozes of Pliocene and Pleistocene age and, in particular, in the deeply buried diatom oozes (deeper than ~200 mbsf) deposited during the late Miocene "carbonate crash."
Our results also show that at both Sites 1225 and 1226, downcore textural changes correlate with changes of some of the key dissolved chemicals involved in anaerobic microbial respiration and methanogenesis. Pore water concentrations of reduced Fe and reduced Mn are higher in the sediment of two stratigraphic intervals characterized by high biosilica content and larger mean diameter and mode: the shallow subseafloor mixed, coccolith, radiolarian, and diatom oozes of Pleistocene to Pliocene age and the deeply buried late Miocene, opal-A, and opal-CT diatom oozes (Figs. F7, F8). This relationship is particularly pronounced in the late Miocene "carbonate crash" diatom oozes of Site 1226, which host unusually high levels of subseafloor microbial life, as indicated by an increased number of total and dividing cells (D'Hondt, Jørgensen, Miller, et al., 2003). In conclusion, our sedimentologic analyses suggest that in the sediments of the eastern equatorial Pacific, the depth distributions of modern microbial processes are related to the sediment's textural properties, which in turn were largely determined by past oceanographic conditions.