© 2002, AAPG/DEG, 1075-9565/02/$15.00/0 Environmental Geosciences, Volume 9, Number 3, 2002 109–114 ZHENG ET AL.: COMPACTION OF MODERN SOFT SEDIMENTS 109 Compaction of Modern Soft Sediments during Core Sampling—An In Situ Investigation at an Estuary Site GUODONG ZHENG, BOKUICHIRO TAKANO, MOTOYUKI MATSUO, and YUTO TANAKA Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-Ku, Tokyo 153-8902, Japan ABSTRACT The vertical compaction of unconsolidated subsurface sediment was investigated at the Tama river estuary in Tokyo, Japan. The cores were taken by hand using polybutyrate tubes with differ- ent inner diameters, and core compaction ratios were measured in situ . Percent compaction varied with depth within the same core and with the inner diameter of a coring tube as well as its wall thickness. These results strongly suggest that vertical com- paction of soft sediments during coring should be well docu- mented in published articles relevant to the time-dependent analysis of sedimentary environments. Otherwise, comparison of one core with another could be misleading in discussions fo- cused on modern soft sediment. Key Words: soft sediment, vertical compaction, core sampling. INTRODUCTION After deposition, loose sediments typically exhibit poros- ity and volume reductions by releasing pore water and changing interstitial structure, and gradually are consoli- dated into denser material under compression by a loaded water column and sediments. The hydrostatic pressure from pore space water may be also a factor which influences compaction. Various physical, chemical, and mineralogical processes take place along with compression and compac- tion of sediments, which have been intensively studied from sedimentary petrology (Liu, 1979). Much attention has been paid to those variations derived from petroleum geology and groundwater engineering perspectives. Specifically, the petrological conversion after deposition and during deep burial has been well studied, as compaction behavior of loose sediments is critical for the understanding and model- ing of sedimentary basin evolution (Harrold et al., 1999) and petroleum reservoir properties (Issler, 1992; Toby et al., 1999). Another important concern about loose surface sedi- ments comes from civil engineering, because many tall build- ings have been constructed normally on recent estuarine sedi- ments. Thus, compaction of sediments during burial becomes • • important for urban development in estuaries such as the Shanghai Delta in China. In addition, volume contraction of modern soft sediments during field sampling, especially vertical contraction, is also critical, because core length or depth has been presented along with numerous measured data as an essential parame- ter for sedimentological studies in modern sediments. A chronological approach is one of the widely used techniques in related studies. By this method, a time scale on the core length or sediment depth can be developed. Even in recent years, vertical change in core length during sampling has re- mained controversial in environmental and global deep sea studies in which accurate sectional data of the cores are needed. Any kind of coring method has some advantages as well as disadvantages (e.g., Neale and Walker, 1996; Jahnke and Knight, 1997). Usually it is not easy to take cores with- out disturbing their properties (Skilleter, 1996). Such coring in lakes, seas, and oceans is extremely difficult because of thick water columns. In this connection, some estuary or tidal coastal areas could be suitable places for evaluating compac- tion of cores. In this study, one coring station in the estuary of the Tama river in Tokyo, Japan was selected to measure the compac- tion of soft sediment at the time of coring. At this site, tidal cycling supplies a suitable setting for us to investigate verti- cal variations of wet cores, because the surface water at this sampling station changes from time to time according to the tidal cycles of Tokyo Bay. Another reason for this selection is that the Tama river is a typical natural mass-transporting channel situated in the urban district, and presents an attrac- tive site for environmental monitoring (Takii, 1989), with several research projects having been carried out at this lo- cation by our laboratory (Kuno et al., 1998). This article will present information about how much compaction may be induced by coring soft sediments using a hand corer. SETTING AND INVESTIGATION Location and Sediments The Tama river is one of the major rivers flowing through the Tokyo metropolitan area into Tokyo Bay (Fig. 1). The