During Leg 184 in the South China Sea (SCS) characteristic lithologic features, called green clay layers (GCLs) (Wang, Prell, Blum, et al., 2000), were observed in the Pliocene-Holocene intervals at five of the six drilled sites (Fig. F1; Table T1). It is not the first time that such features have been found in marine sediments. Green and purple bands were noted in sediments from Leg 130 in the Ontong Java Plateau (Lind et al., 1993), and similar layers were observed in deposits from the Rio Grande Rise (Supko, Perch-Nielsen, et al., 1977) and the Lord Howe Rise (Gardner et al., 1986). Inside the SCS, green clay granules and foraminifer tests infilled with green clays were reported from Holocene and Pleistocene sediments in the Vietnam inner and outer shelf (Markov et al., 1996).
Several hypotheses were offered to explain the nature and the origin of these layers: (1) the presence of glauconite, (2) the alteration of volcanic material, or (3) alteration of the original clay, but the question remained unanswered. A total of 22 samples (11 green clay and 11 background sediment) (Fig. F2) were collected from Ocean Drilling Program (ODP) Sites 1147 and 1148 in the SCS. Here, we present the results of a multiproxy investigation that comprises bulk and clay mineralogy, grain size, organic matter, and phosphorus sedimentary phase characterization, bulk inorganic geochemistry by X-ray fluorescence (XRF), and statistical analysis of the temporal distribution of the GCLs. The main goals of this study are to determine the nature of the GCLs, understand their genesis, and establish any possible correlation with external factors, such as sea level changes, climate variations, and tectonism.