This paper reports methodology and results of sedimentologic analyses performed on biogenic sediments collected during Ocean Drilling Program Leg 201 in the eastern equatorial Pacific (Sites 1225 and 1226). Particle size analyses were carried out with a laser particle sizer on both bulk and noncarbonate fractions to document changes in the relative proportions of the main biogenic, biocalcareous, and biosiliceous components of these sediments. The grain size distribution of these very fine grained sediments is generally polymodal and characterized by main modes at ~10, ~17, and ~40 µm and minor modes at ~100 µm and ~1 mm. The modes represent the common biogenic components of these pelagic sediments, including coccoliths (~2–10 µm), pennate (~10–20 µm) and centric diatoms (~20–50 µm), radiolarian tests (~40–100 µm), juvenile foraminifers (~40–50 µm), fecal pellets (>50 µm), and test and frustule fragments (~10–100 µm). Downcore variations of particle sizes correlate with both small-scale and main lithologic changes in the sediment column and show pronounced shifts at unit/subunit boundaries at both eastern equatorial Pacific sites. Larger particle sizes characterize the units dominated by biosiliceous sediments. This relationship is particularly pronounced for the deeply buried diatom oozes (deeper than ~200 meters below seafloor) deposited during the late Miocene "carbonate crash." Conversely, the samples having the smallest grain sizes are concentrated in the coccolith oozes deposited during the late Miocene to Pliocene "biogenic bloom." Our results also show that changes of some of the key dissolved chemicals involved in anaerobic microbial respiration and methanogenesis coincide with changes of sediment textures. In particular, the concentrations of the two main by-products of microbial respiration at these sites, reduced Fe and Mn, increase with the coarsening of the mean diameter and mode in the mixed diatom and radiolarian biosiliceous sediments of Pleistocene to Pliocene age and in the deeply buried late Miocene diatom oozes.
1Aiello, I.W., and Kellett, K., 2006. Sedimentology of open-ocean biogenic sediments from ODP Leg 201, eastern equatorial Pacific (Sites 1225 and 1226). In Jørgensen, B.B., D'Hondt, S.L., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 201 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/201_SR/112/112.htm>. [Cited YYYY-MM-DD]
2Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing CA 95039-9647, USA. Correspondence author: iaiello@mlml.calstate.edu
Initial receipt: 26 July 2004
Acceptance: 1 February 2006
Web publication: 14 July 2006
Ms 201SR-112