The Characteristics And Provenance Of The Sediment In The Southwestern Okinawa Trough Since The Mid-Holocene And Their Environmental Implications

Owing to its location, geomorphology and hydrodynamic conditions, the southernmost part of the Southern Okinawa Trough (SOT) acts like an efficient receptacle for sediments from Taiwan and the East China Sea shelf. The region is also known for the Kuroshio Current, which turns northwestards and brances off from its mainstream before continuing to flow to the North Pacific. Between 29 May and 29 May 2005, one core was drilled at Site MD05-2908 (24o48.04′N, 122 o29.35′E) off northeast Taiwan during IMAGES XII-MD147 Marco Polo1. The site is located on a gentle topographic high on the southern slope of the southern Okinawa trough at 1275 m water depth. Site MD05-2908 was designed to obtain a high resolution record of paleoceanographic changes of the Kuroshio Current during the latest Quaternary. The core is 34.17m long with high water content; the sediments in core MD05-2908 consist of homogenous dark gray hemipelagic clay and fine silt-sized sediments with three insignificant thin layers of very fine sand and silt. The depth-age relationship of core MD05-2908 is well constrained by the seventeen AMS14C dating points and the 34.17m core has a continuous record of the past 6,800 years. The so high-resolution record is a good material for research the millennial to decade scales of the historic climate change.Grain size analysis shows that, the sediment of core MD05-2908 is mainly made of clayey silt, generally uniform but small changes in different depths.Subsection comparision indicates that the grain size coursing downwards gradually, especially in the bottom where sand in unignored. It probably reveals the enhancement of terrigenous clastic material. XRD diagrams display different minerals including 4 clay minerals and few quartz and feldspars in <2μm fraction of sediment. In descending order of abundance, the clay mineral assemblage consists of illite (～68%), chlorite (～17%), smectites (～10%) and kaolinite (～5%). In eastern Taiwan, illite and chlorite are abundant in widespread slates and schists as well as in soils, whereas kaolinite and smectites only occur locally. The dominance of illite, chlorite, and unaltered feldspars as inferred from geochemical data suggests that the immature nature of sediments and sedimentary rocks is probably a result of low residence times in the source region or river basin and quick removal of materials from the soil profile by steep, mountainous rivers (physical weathering dominates). The well-known earthquake and storm triggered extremely high physical weathering rate in Taiwan is consistent with these results.We selected 41 layers for heavy minerals analysis, and the heavy minerals consist mainly of Chlorite(29%),Hornblende(22%),Dolomite(10%),Biotite(8%),Muscovite(7%),Epidote(7%). The sediments in the core are weakly weathered, lightly abraded and bad sorted, which is derived from near source and did not experience seriously transformation by ocean. The content of stable mineral is small and mineral maturity index is low. Based on studies of detrital minerals it is unraveled that the coarse-grained sediments are of single and stable source and have the same trait as intermediate-acidic granite and metamorphites.Major elements analysis shows that the main components on the sediment core are SiO₂, Al₂O₃ and Fe₂O₃, which account for as much as 82%. There is no obverse change in the core, which is quite similar with the value of sediments from the East China Sea shelf. According to the distribution of elements and the results of the R-model factors analyses, the sediments of the South western Okinawa Trough are mainly composed of terrigenous composition, such as SiO₂,Al₂O₃,Fe₂O₃,MgO,K₂O,Cr,Cu,Ni,Zn,Pb,Rb and Mn, and biogenic compositions, such as CaO,Sr and Ba. The elements are devided into dirrerent groups, combined with the minerals distributions, every elements group are given a different source indication.The sediments span across the mid-Holocene (6.8 ka B.P.) and have remarkablely high sedimentation rates between 1.8 and 21.2 m/ka, which is well consistent with the modern observations from sediment traps. We identified five 70―200 a periods of abnormally rapid sedimentation events at 6790―6600 a B.P., 5690―5600 a B.P., 4820―4720 a B.P., 1090―880 a B.P., and 260―190 a B.P., during which the highest sedimentation rate is up to 21.2 m/ka. As the most significant sediment source, the Lanyang River in northeastern Taiwan annually deliver about 10Mt materials to the coastal and offshore region of northeast Taiwan, a portion of which could be carried northward by currents toward the study area. Therefore, we concluded that the 5 abnormally rapid sedimentation events may be related to intensified rainfall in Taiwan and thus increased materials to our study area at that time. However, a few extreme-rapid sedimentation events cannot be explained by normal river runoff alone. The large earthquakes or typhoons induced hyperpycnal discharge of fluvial sediment to the ocean may also act as a potential source supply to the Okinawa Trough. Mass wasting processes, seafloor morphology and grain sizes play important roles in transporting sediment from near shore to deep water. Frequent earthquakes, typhoon-induced floods and ocean currents add to the complexities of sedimentary processes on the southwestern Okinawa Trough.