The grain sizes of the sedimentary components of Leg 201 eastern equatorial Pacific sites (e.g., biogenic tests, frustules, coccoliths, skeletal parts, and fragments) occur within the range of sizes detectable with the laser particle sizer (0.04 µm to 2 mm).
At Site 1225, the 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, minor modes at 17.18, 39.77, and 101.1 µm, and a very small mode at 948.32 µm (Fig. F3A; Table T1). The modes represent the common particles that compose 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 fragments of microfossil tests and frustules (~10–100 µm). Similar relationships are observed in the frequency curves from the bulk fraction of samples from Site 1226 (Fig. F4), which show two distinct peaks at 13.61 and 43.66 µm, although the peak at ~110 µm is only minor. Mean, mode, and median are smaller in samples from Site 1225 (Table T1) than in samples from Site 1226 (Table T2). Larger average grain sizes in samples from Site 1226 may in part reflect the greater number of diatom oozes sampled in the core sediments retrieved in this site, as also shown by generally lower weight percent TIC (see the "Appendix").
Total carbon data show that the noncarbonate fraction ranges between 10% and 80 wt% and is commonly <30% (see the "Appendix"). The particles of the noncarbonate fraction are the sedimentary components that survived dissolution by HCl and include biosiliceous tests such as diatom frustules, radiolarian tests, and other noncarbonate clasts. The frequency curves of the noncarbonate fraction at Sites 1225 and 1226 are very similar, characterized by a mode at ~15 µm that includes very small siliceous particles mostly composed of whole or dissolved/fragmented diatom frustules (Figs. F3B, F4B). The minor modes at 47.936 µm (43.667 µm) and 92.092 µm account for centric diatoms and radiolarian tests, respectively.
The particle size distributions of selected samples are shown in Figure F5, and examples of sediment microtextures are visualized by the smear slide microphotographs shown in Figure F6. Figures F7 and F8 show downcore variations of mean and mode of both the bulk and noncarbonate fractions after 30 s of sonication for Sites 1225 and 1226, respectively. In order to assess the relationships between sediment texture and lithology, the particle size data are plotted vs. depth and unit/subunit boundaries, luminosity (L*; generally carbonate-rich sediments have higher luminosity), and concentration of inorganic carbon (D'Hondt, Jørgensen, Miller, et al., 2003). The plot of Figure F8 shows that at Site 1226, downcore variations of sediment grain size mean and mode are generally in opposition of phase with the variations of sediment luminosity and inorganic carbon. Conversely, the sediment samples with the coarsest grain sizes were detected in intervals characterized by lower luminosity, lower inorganic carbon, and the highest concentrations of biosilica (both diatoms and radiolarians).
The Pleistocene and late Pliocene shallow subseafloor sediments at both eastern equatorial Pacific sites are characterized by meter-scale alternations between nannofossil-rich diatom and radiolarian oozes and between diatom- and radiolarian-rich nannofossil oozes. This small-scale lithologic variability is reflected by the variability of both mean and mode between the samples collected in this interval, reflecting small-scale changes in the relative abundance of the siliceous and calcareous biogenic components (Figs. F5A, F5B, F7, F8) (see the "Appendix"). An overall bimodal, rightly skewed shape characterizes the particle size distributions of the samples from the uppermost Subunit IA at both Sites 1226 and 1225. In Sample 201-1226E-1H-1, 1 cm (Figs. F5A, F6A, F6B), the mode is centered at ~40 µm and corresponds to the occurrence of whole and fragmented centric diatoms, although fragments of radiolarian and foraminifer tests (~30–60 µm) are also included in the mode. A secondary mode at ~17 µm reflects the occurrence of large numbers of pennate diatoms. Microscopic observation shows the typically highly asymmetric elongated shape of pennate diatom frustules (Fig. F6B). The width of these siliceous microfossils ranges between ~2 and ~5 µm, and their elongation is highly variable, commonly ranging between ~5 and >100 µm, although values around 20 µm are common. The samples from the Pleistocene sediments are also characterized by a third minor mode at ~120 µm that describes the occurrence of relatively large fecal pellets and whole radiolarian tests. Calcite coccoliths are the smallest particles of these sediments (less than ~10 µm). In Figure F5B, the grain size distribution of a sample of mixed nannofossil, radiolarian, and diatom ooze collected in Subunit IA at Site 1226 near the Pliocene/Pleistocene boundary (Sample 201-1226B-6H-3, 0 cm) shows three modes centered at ~14 (pennate diatoms) and ~40 and ~70 µm (both centric diatoms) (Fig. F6C). The minor modes at ~70 and ~130 µm include larger siliceous microfossils such as whole and fragmented radiolarian tests, silicoflagellates, sponge spicules, and juvenile foraminifers, as well as calcite fragments from foraminifer tests. The broad hump of the frequency plot between ~2 and 7 µm (Fig. F5B) accounts for the occurrence of coccoliths generally more abundant in samples from the Pliocene sediments.
The coccolith oozes of the late Miocene to Pliocene "biogenic bloom" at both sites (Subunit IB at Site 1225 and Subunit IC at Site 1226) are characterized by very small grain sizes and generally show a single prominent mode and closer to log-normal particle size distribution (Figs. F5C, F6D). Downcore variations of both grain size mean and mode are only minimal and reflect the relatively homogeneous, calcareous lithology of the sediments from these stratigraphic intervals (Figs. F7, F8; see the "Appendix"). The frequency plot of Figure F5C (Sample 201-1225A-16H-3, 0 cm) shows the typical grain size distribution of a diatom-rich coccolith ooze from Site 1225 (Subunit IB), including a dominant mode at ~6 µm, with the most common size for coccoliths ranging between ~2 and ~10 µm (Fig. F6D). The hump between ~12 and 20 µm and the secondary mode at ~48 µm reflect the occurrence of pennate and centric diatoms, respectively. Fecal pellets and fragments of foraminifers account for the minor mode at ~140 µm (Fig. F5C).
The coarser grain sizes of the deeply buried sediments deposited during the late Miocene "carbonate crash" reflect the abundance of the biosiliceous (mostly diatoms) microfossils over the calcareous coccoliths (Figs. F7, F8). The texture of the samples collected from these sediments are characterized by a polymodal grain size distribution similar to the texture of the biosiliceous Pliocene ooze (compare the frequency plots of Fig. F5A and F5D). In the frequency plot of Figure F5D (Sample 201-1225A-24H-3, 0 cm), the mode centered at ~23 µm accounts for the large amount of pennate diatoms (Fig. F6E, F6F). The occurrence of centric diatoms is reflected by the secondary mode at ~40 µm, whereas fecal pellets and whole and fragmented radiolarian tests account for a third minor mode at ~110 µm. Finally, the broad hump below ~10 µm, which also persists in the noncarbonate fraction, accounts for coccoliths and for fragmented/dissolved diatom frustules (Fig. F6G). These very small biosiliceous particles may be the result of fragmentation of diatom frustules by bottom currents winnowing the seafloor. Alternatively, the small size of the frustules can be the product of silica dissolution due to the opal-A to opal-CT silica phase change detected in these Miocene sediments by onboard mineralogic analyses (Shipboard Scientific Party, 2003).
At both Sites 1225 and 1226, the sediments directly overlying the basaltic oceanic crust have high concentrations of Fe and Mn oxides, which typically form reddish micronodules, mainly around radiolarian tests and fecal pellets (Figs. F5E, F6H, F6I). Sample 201-1226B-47X-2, 13 cm, from a radiolarian-rich nannofossil ooze shows that the oxides occur clustered in two main sizes: ~45 µm (mode) and ~110 µm (minor mode). Coccoliths in this sample are very abundant, and microscopic observations indicate that their size can be as large as ~12 µm, although they usually range between ~7 and ~9 µm (Figs. F5E, F6H, F6I).
In Figures F7 and F8, the downcore variations of sediment mean and mode are compared to pore water concentrations of the two main by-products of subseafloor microbial activity in Leg 201 eastern equatorial Pacific Sites 1225 and 1226: reduced Fe and reduced Mn. These two chemical species show peak concentrations in the mixed, siliceous, and calcareous late Pliocene to Pleistocene sediments of Subunits IA at Site 1225 and IA and IB at Site 1226. As previously discussed, these relatively young and shallow subseafloor sediments are characterized by relatively coarse and variable grain sizes (e.g., the mean diameter ranges between ~10 and ~18 µm) (Figs. F7, F8; see the "Appendix"). At Site 1225, coarser grain sizes of the bulk and, in particular, noncarbonate fractions match peak concentrations of dissolved Mn at 3.6 meters below seafloor (mbsf) and a broad zone of relatively highly dissolved Fe centered at ~25 mbsf. Similar relationships are also verified for Site 1226, where dissolved Mn peaks just at the sediment/water interface and again at 9 mbsf. Pore water data also show active Mn reduction in the coarser biosiliceous oozes of the uppermost Subunit IA at both sites.
A correlation between coarsening of sediment texture and increased levels of microbial activity is also observed in the deeply buried late Miocene diatom oozes in Subunit IC at Site 1225 and, in particular, in Subunit ID at Site 1226. At the latter site, Mn reduction is the dominating electron-accepting pathway in the biosilica-rich opal-A and opal-CT sediments below 260 mbsf, with peaks at 300 mbsf and near the basement (Fig. F8). Conversely, at Site 1225 the dominant electron-accepting pathway is Fe reduction. Dissolved iron at this site shows a large peak centered at ~230 mbsf, which overlaps with minor Mn reduction, as demonstrated by a broad zone of the dissolved element centered at ~250 mbsf.