Although the volume of volcanic ash found in Leg 184 sediments is not large, it does provide some constraints on the activity of the volcanic arcs of the region. All recovered ashes are thin, generally <5 cm, and are light colored in the Pleistocene, reflecting a dominant dacitic-rhyolitic composition of the arc's explosive fraction. Most of the ashes were deposited since 1 Ma on the northern margin and since 2 Ma in the south. This trend is similar to the global pattern noted by Kennett and Thunell (1975), although it has been disputed by several authors (e.g., Ninkovich and Donn, 1976). The uphole increase in volcanic ash may reflect either more volcanic eruptions during the Pleistocene or the diagenetic alteration of chemically unstable volcanic glass during burial. This latter explanation may account for much of the pattern on the northern margin, since older ashes in this area tend to be devoid of glass and are simply composed of angular quartz, mica, and other accessory mineral grains. In contrast, at Site 1143, fresh glass is found in Miocene-aged ash beds. Certainly the Philippine Arc is not a recent feature; unless wind directions have radically changed, ashes from this arc were probably deposited over the entire cored interval.
Green Layers and Iron Sulfides
Green clay layers are a common, yet volumetrically small, part of the sequence at most of the Leg 184 drill sites. They occur as discrete layers as thick as 3 cm and even more commonly as disrupted layers, patches, or mottles. XRD analyses show that they are not glauconite, and no clear relationship is observed between green clay layers and depth of burial. Most of the layers are confined to the Pliocene-Pleistocene (Fig. 21) except for a lower Miocene set recovered at Site 1148. Their common association with burrows and patches caused by burrowing suggests that they may be linked to the presence of organic matter. Certainly their green color is suggestive of reducing conditions, which are linked to organic matter alteration. They do not seem to be equivalent to green layers found by Lind et al. (1993) on the Ontong Java Plateau and by Gardner et al. (1986) from the Lord Howe Rise, which were interpreted as altered volcanic ash. In the case of the South China Sea, the green clay layers are interbedded with clear tephra-bearing unaltered volcanic glass. No appreciable change in the background sediment is noted over these intervals; thus, the diagenetic environment seems uniform between beds. Other diagenetic minerals noted in the Leg 184 sediments are iron sulfide minerals (well-crystallized golden-colored pyrite often present as nodules, concretions, and replacement burrows) and fine-grained, black sulfide dispersed in the sediment. The latter style is described as FeS in the cores but as this is chemically unstable, this material must also be pyrite in mineralogy. The lack of a clear regional pattern either in depth or age in the distribution of pyrite or FeS suggests that the minerals' development reflects only local variations in sediment composition and burial.
Synthesis-Environmental History of the South China Sea
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