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doi:10.2204/iodp.proc.337.203.2017

Introduction

Integrated Ocean Drilling Program Expedition 337 explored the microbiology and hydrocarbon system associated with subseafloor coal beds in the Hidaka Trough offshore the Shimokita Peninsula, Japan (see the “Expedition 337 summary” chapter [Expedition 337 Scientists, 2013a]) (Inagaki et al., 2015, 2016). Hole C0020A (41°10.5983′N, 142°12.0328′E; 1180 m water depth) was drilled to 2466 meters below seafloor (mbsf) as an extension of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Site C9001, which was originally drilled to 647 mbsf (Aoike, 2007). Spot coring began at 1276.5 mbsf at an interval of approximately one core every 100 m, and near-continuous coring occurred between 1919.0 and 2002.4 mbsf across the coal-bearing target. Cuttings were collected over the entire hole at 10 m intervals but were not used in this study.

The recovered sediment sequence revealed a transition from offshore marine to nearshore/terrestrial environments that was described in four lithostratigraphic units (Figure F1) (see the “Site C0020” chapter [Expedition 337 Scientists, 2013b]) that span the late Oligocene to early Pleistocene (Phillips et al., 2016). Unit I (647–1256.5 mbsf; no core recovery) is composed of a hemipelagic, diatom-bearing silty clay, representing a continental slope environment. Unit II (1256.5–1826.5 mbsf; 7% core recovery) is composed of primarily silty shale with common intervals of siltstone and sandstone/unconsolidated sands. Unit II contains an increasing frequency with depth of coarse-grained sediments, terrestrial plant debris, bioturbation, and cross-lamination, along with a decrease in diatom abundance, suggesting a continental shelf to intertidal environment. Cruziana ichnofacies were observed in Unit II. Well logging results indicate a massive 60–70 m thick sandstone in Unit II and three coal layers (0.3–0.6 m thick) that were not sampled by coring. Unit III (1826.5–2046.5 mbsf; 35% recovery) contains multiple coal beds with intervals of sandstone/sand, siltstone, and shale. Flaser bedding, cross bedding, and authigenic carbonate nodules are present in Unit III. Sharp increases in magnetic susceptibility driven by authigenic magnetite precipitation are present in Unit III (Phillips et al., 2017). Unit III represents a nearshore environment with coals derived from brackish wetlands at the top of Unit III and freshwater wetlands at the base of Unit III (Gross et al., 2015). Unit IV (2046.5–2466 mbsf; 8% core recovery) consists of shale, sandstone, and siltstone, representing a tidal flat to fluvial environment, and is largely devoid of coal except for a 30 cm coal bed at the very bottom of the hole. Unit IV also contains numerous authigenic carbonate nodules.

Particle size distribution in marine and terrestrial sediments can provide insight into paleoenvironmental conditions driven by climatic and tectonic changes (e.g., Orton and Reading, 1993; Goman and Wells, 2000; Ding et al., 2002; Warner and Domack, 2002; Asikainen et al., 2007). In addition, grain size can influence microbial biomass and microbial activity and diversity in a range of marine and terrestrial environments (e.g., Dale, 1974; Bender and Conrad, 1994; Sessitsch et al., 2001; Noffke et al., 2002). Thus, grain size analysis of sediments at Site C0020 has the potential to provide insight on paleoenvironmental and microbial processes offshore Shimokita Peninsula since the late Oligocene.

A complete downcore sampling strategy for grain size analysis from each unit and throughout the entire hole was not possible because most cored samples from Site C0020 were consolidated or semiconsolidated, including all samples of siltstone and finer grained lithologies. These consolidated or semiconsolidated sediments cannot be accurately analyzed because intense disaggregation imparts error into the grain size distribution (Gealy, 1971; Thayer et al., 1974). However, multiple intervals of unconsolidated sands were recovered across Units II and III. No cores were collected in Unit I (only cuttings), and all sediments in Unit IV were lithified. Accordingly, we report the results of particle size analysis of 28 unconsolidated sand samples from Units II and III.

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