7. Site 1146¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

The major objectives at Site 1146 (proposed site SCS-4) were to (1) recover a continuous sequence of hemipelagic sediments that would enable reconstruction of East Asian monsoon history from the upper Miocene (~10 Ma) to present; (2) establish the relationship between the orbital-scale variability of East Asian monsoon proxies with orbital forcing, glacial forcing, and internal feedbacks within the climate system; (3) test hypotheses of the relationship between Tibetan Plateau uplift, monsoon evolution, and global cooling by establishing whether monsoonal indices intensify or weaken during the late Miocene and whether the Miocene-Pliocene pattern of accumulation rates is consistent with models of Himalayan-Tibetan Plateau Complex uplift, monsoon intensification, and sea-level changes; and (4) compare the evolution of the East Asian monsoon in the South China Sea (SCS) with that of the Indian monsoon in the Arabian Sea (Ocean Drilling Program [ODP] Leg 117) to identify common causality.

Site 1146 is located at 19°27.40´N, 116°16.37´E, at a water depth of 2091 m (Figs. F9 in the "Leg 184 Summary" chapter; F5 in the "Seismic Stratigraphy" chapter). The site was initially located on seismic Line SO95-5 at common depth point 1049 (17:55) in an area of thick sediment fill within a down-dropped block of prerift basement (Fig. F12 in the "Seismic Stratigraphy" chapter). Based on the JOIDES Resolution (JR) seismic survey lines (see "Site 1146 [SCS-4]" in "Seismic Stratigraphy of Leg 184 South China Sea Sites" in the "Seismic Stratigraphy" chapter), Site 1146 was moved to shotpoint 3241 on Line JR184-3, ~0.5 nmi east of the originally proposed site (Fig. F11A, F11B, in the "Seismic Stratigraphy" chapter). Petroleum wells on the shelf and piston cores on the slope indicated that the sediments at Site 1146 would be hemipelagic muds and silts with moderate carbonate content. The total sediment thickness at the site location is >1.4 seconds below seafloor (sbsf), two-way traveltime, with a prominent double reflector at ~0.75 sbsf. Two regional reflectors have been identified in this area on the SO95-5 line: T₁, thought to be the Miocene/Pliocene boundary (~5.2 Ma), and T₄, thought to be lower-middle Miocene (15-16 Ma) (H.-K. Wong, pers. comm., 1998). On the JR184-3 line (Fig. F11 in the "Seismic Stratigraphy" chapter), Reflector T₁ lies at 0.2 sbsf and thus implies a sedimentation rate of ~38 m/m.y. In contrast, the available piston cores near Site 1146 have sedimentation rates of ~200 m/m.y. during the late Quaternary. Reflector T₄ lies at 0.54 sbsf, so that middle to upper Miocene sedimentation rates are estimated at 30 m/m.y. If the interpretation of Reflectors T₁ and T₄ is correct, Site 1146 should recover a middle Miocene to Pliocene sequence that underlies relatively thin Pleistocene deposits. Site 1146 was originally targeted for 520 m penetration; it was deepened to 600 meters below seafloor (mbsf) when the seismic stratigraphy showed that the deeper section at Site 1146 was likely missing at Site 1148 (proposed site SCS-5C). The 600 mbsf penetration at Site 1146 is equivalent to ~0.66 sbsf using the P-wave velocity data from physical properties measurements and logging.

Site 1146 lies above the current sill depth of the Bashi Strait (2600 m). Along with Sites 1147 and 1148 (3230 m), it offers both a water-depth transect and a full history of the margin during the Neogene and possibly the Oligocene. The sediments from this site should afford the opportunity to reconstruct a detailed, orbital-scale ( = 2 k.y.) record of how East Asian monsoon variability is related to orbital and glacial forcing as well as to internal feedbacks within the climate system. We expect that comparison of the East Asian and Indian monsoons will help to identify common sources of causality and that the long climate record from Site 1146 will enable us to test scenarios for the relationship between the Tibetan Plateau uplift, monsoon evolution, and global cooling. The biostratigraphy and logging data at Site 1146 should help us correlate our findings with the terrestrial record of China and thus make use of the extensive land-based records of climate and monsoon variability. As with its companion site in the southern SCS (Site 1143), the terrigenous accumulation rates at Site 1146 should yield another record of sediment erosion, weathering, and transport related to Himalayan-Tibetan uplift that can be compared with records from the Bengal Fan (ODP Leg 116) and Arabian Sea (Leg 117). The impact of both long-term and short-term sea-level changes will also need to be closely evaluated in interpreting the accumulation rates of these continental margin sediments. In summary, Site 1146 should give a detailed record of Neogene sedimentation and paleoceanographic changes related to the evolution of tectonics as well as global and regional climate.

The drilling program for Site 1146 was to triple core (using the advanced hydraulic piston corer [APC]) to refusal and to use the extended core barrel [XCB] to 520 m (at one or possibly two holes) with a standard logging program (triple combination tool, geological high-resolution magnetic tool [GHMT], and the Formation MicroScanner [FMS]-sonic tool) at one of the deep holes. Permission was received to deepen the hole to 600 m in an effort to recover a section suspected (on the basis of seismic stratigraphy) to be missing at Site 1148.