Site 1146

Site 1146 (proposed site SCS-4) is located 19°27.40'N, 116°16.37'E, at a water depth of 2092 m (Table 1), within a small rift basin on the mid-continental slope of the northern South China Sea (Fig. 16). Site 1146 was intended to take advantage of the relatively shallow slope basin with moderate sedimentation rates to recover a continuous sequence of hemipelagic sediments that would enable reconstruction of East Asian monsoon history from the middle to upper Miocene (~10 Ma). Such a long-term record at orbital-scale resolution (2 k.y.) would allow comparisons of East Asian monsoon variability with orbital forcing, glacial forcing, and internal feedbacks within the climate system and provide a new set of constraints on the possible relationship between Tibetan Plateau uplift, monsoon evolution, and global cooling. Specifically, we seek to determine 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 uplift, monsoon intensification, and sea-level changes. Lastly, we envisioned that the Site 1146 record of the East Asian monsoon would provide an appropriate counterpart to Site 1143 in the southern SCS and Site 722 in the Arabian Sea and that their comparison could serve to identify potential sources of common causality.

We cored three APC/XCB holes at Site 1146. Hole 1146A reached the target depth of 607 mbsf with an average recovery of 100%, Hole 1146B reached 245 mbsf with an average recovery of 99%, and Hole 1146C reached 599 mbsf with an average recovery of 101% (Table 1). We requested approval to deepen Site 1146 beyond the originally approved 520 m penetration to extend the paleoenvironmental record in time, after the sediment age turned out to be younger than expected and after we had confirmed that no significant hydrocarbon concentrations occurred in the entire interval recovered. Following completion of Hole 1146A, three successful downhole logs were acquired with the triple combo (85-600 mbsf), FMS-sonic (239-600 mbsf),

and GHMT (239-600 mbsf) tool strings. Downhole and bottom-water temperature measurements yielded a thermal gradient of 59°C/km at Site 1146.

Drilling at Site 1146 recovered a lower Miocene through Pleistocene section of relatively carbonate-rich, hemipelagic nannofossil clays with a basal age of ~19 Ma (Fig. 17). The core-logging data enabled construction of a continuous mcd scale and a continuous spliced record from 0 to 266.7 mcd. A discontinuous ("floating") mcd scale and splice were also developed for the interval spanning 266.7-640.9 mcd, which is the bottom of the cored sequence. We expect that postcruise correlation with downhole logging data will allow construction of a complete, continuous section to 640 mcd.

The Pleistocene-age sediments are composed of greenish gray nannofossil clay that is relatively enriched with quartz, plagioclase, chlorite, and illite relative to lower sections. This unit grades downhole into clayey nannofossil ooze within the Pliocene section. The Pleistocene sediments average ~21% carbonate and are characterized by slightly lighter intervals with higher carbonate content. Planktonic foraminifers are abundant and have good preservation for the site's entire interval. The upper part of the section (0-110 mcd) shows constant or slightly increasing values in CR (L*), MS, and NGR, a high scatter in grain densities, and normal compaction-related increase in bulk density and P-wave velocity. This interval is characterized by high-amplitude variations representing orbital-scale cyclicity. Minor sediment components include large pyrite nodules that preserve organic structures (Xenophyophorians) and thin, light-gray ash layers, often strongly dispersed by bioturbation, as well as isolated pumice clasts. Large (>1 cm in diameter) pteropods, diatoms, silicoflagellates, radiolarians, and sponge spicules are common.

Total organic carbon decreases systematically from the Pleistocene sediments (1% at the top of the section) to trace abundance (<0.2%) below the mid-Pliocene (~225 mcd). Interstitial water profiles at Site 1146 were primarily driven by the removal and release of elements in the process of organic matter reduction, with sulfate reduction the dominant process above 68 mcd and methanogenesis dominant below. Depletion of methanogenesis products such as alkalinity, phosphate, and ammonium with depth suggests that methanogenesis is active only in the upper sediments. This agrees with the interpretation of higher methane levels and higher order hydrocarbons in headspace gas samples below 200 mcd as gases that have migrated into this site, either laterally or from depth. Dissolved silica and strontium correlate with the amount of biogenic silica and carbonate in the sediments, respectively. A major decrease in the dissolved silica concentration is observed between 109 and 140 mcd (~0.7-1.1 Ma) and accompanies a decrease in the amount of biogenic silica to near-zero values. A similar decrease was observed at Sites 1144 and 1145 at an age of ~0.8 Ma.

Paleomagnetic measurements reveal the Brunhes/Matuyama transition at 115 mcd, the upper Jaramillo transition at 133 mcd, and the lower Jaramillo transition at 138 mcd. Between 99.7 and 101.3 mcd, a swing in declination with correlative very low inclinations could document the Big Lost (geomagnetic) Event (dated at 510 to 650 ka). A transition from reverse declinations to normal occurs between 160.5 and 165.8 mcd, possibly marking the Olduvai Event. The biostratigraphy places the Pleistocene/Pliocene boundary between 185.5 and 195.1 mcd. At Site 1146, the sedimentation rates decrease downcore; the Pleistocene section has the highest rates, with an average linear sedimentation rate of 150 m/m.y. and total and carbonate MARs of 11.5 and 2.3 g/cm2/k.y., respectively.

The Pliocene-age sediments at Site 1146 (~190-310 mbsf) are distinguished by significantly higher carbonate content (an average of 47% compared to 21% in the Pleistocene). The transition to higher carbonate occurs in the upper Pliocene and is accompanied by an increase in the CR L* values and a decrease in the NGR and MS. This transition interval at ~235 mcd is also characterized by a very pronounced decrease in porosity and an increase in bulk density. This corresponds to a general downhole decrease in accumulation rate at that depth. The sediments grade from the overlying greenish gray nannofossil clay to homogeneous to rarely mottled, light brownish gray foraminifer and nannofossil clay mixed sediment. A small number of thin (<1-2 cm ) dark ash layers, containing large volcanic glass shards as long as 1 cm, occur in the lower part of the interval. A major increase in dissolved strontium in the Pliocene/Pleistocene section between 109 and 350 mcd corresponds to the increase in the percent carbonate in that interval. In both cases, the availability of dissolvable biogenic sediments appears to be a strong control on dissolved concentrations of silica and strontium at Site 1146. The Pliocene/Miocene boundary at Site 1146 is between 308.42 and 317.99 mcd. The Pliocene section has an average LSR of 39 m/m.y. and total and carbonate MARs of 4.3 and 1.9 g/cm2/k.y., respectively.

The Miocene-age sediments at Site 1146 (310-642 mbsf) grade from light brownish gray foraminifers and nannofossil clay mixed sediment of the late Miocene to the green nannofossil clay of the middle to early Miocene. This transition is marked by a progressive change in the sediment color from brownish gray to a distinct greenish gray. The late Miocene to early Pliocene-age interval is slightly more carbonate rich than the middle-Miocene sediments, in which kaolinite and quartz become significant contributors to the mineral composition. In this lower interval, characteristic bluish green nannofossil clay appears, which contains large amounts of pyrite as nodular irregular layers or as finely disseminated particles. The carbonate content declines throughout the Miocene: 53% in the upper Miocene, 35% in the middle Miocene, and 30% in the lower Miocene. The green nannofossil clay of the early to middle Miocene is characterized by relatively high concentrations of smectite and illite. Over much of the Miocene-Pliocene interval, the CR L* signal correlates well with the carbonate pattern but NGR and MS signals are depressed, which implies a carbonate dilution effect. Negative excursions in the chromaticity ratio a*/b* at 325 mcd, 355 mcd, and 418 mcd correspond to distinct green intervals observed in the cores. Interval 420-550 is marked at 420 mcd by a pronounced downhole increase in NGR and MS, which is most likely the result of the lower carbonate content and associated L* reflectance below that depth. The top of interval 550-642 is defined by a sharp decrease in the chromaticity ratio a*/b*, corresponding to a visual color change in the cores. The MS values also drop at this depth, affirming that the drop in chromaticity is accompanied by a change in mineralogy. Overall, the sediment character is typical of deposition at bathyal depths on a continental slope, with oxygenated bottom water implying water depths exceeding the oxygen minimum zone (~600 m). However, the bulk mineralogy suggests that either a change in the source of the terrigenous material or a change in the weathering regime of the source region took place over time.

The concentration of methane (headspace analysis) increased downhole from <10 ppm at the top to a maximum of 85,000 ppm at 599 mcd. Ethane (C2H6) and propane (C3H8) initially appeared at 536 mcd and peaked at 608 mcd with concentrations of 155 and 7.3 ppm, respectively. The C1/C2 ratio reached a minimum of 345 at the bottom of the hole. A major decrease in the interstitial water salinity and chloride content occurs below 500 mcd, corresponding to the top of the interval of highest methane values, the appearance of higher order hydrocarbons, changes in lithologic color, and changes in physical properties. All of these changes appear to correlate with seismic reflector T4, which can be traced back to a possible fault ~1 nmi from the site. This suggests that hydrocarbon and freshwater signals may have migrated laterally and that the other sedimentary changes are a diagenetic response to this change in environment.

At Site 1146 calcareous nannofossils are abundant and well preserved, although nannofossil preservation deteriorates below ~530 mcd. Benthic foraminifers are generally few but become more abundant in the lower part of the section. However, we found no clear evidence for reworked benthic foraminifers from the shelf and upper slope. Planktonic foraminifers are abundant and well preserved. The age at the bottom of the section is ~19 Ma. The Miocene section has relatively constant rates. The upper Miocene has an average LSR of 28 m/m.y. and total and carbonate MARs of 3.7 and 2.0 g/cm2/k.y., respectively; the middle Miocene has an average LSR of 28 m/m.y. and total and carbonate MARs of 4.2 and 1.4 g/cm2/k.y., respectively; and the lower Miocene has an average LSR of 31 m/m.y. and total and carbonate MARs of 4.9 and 1.4 g/cm2/k.y., respectively.

Overall, Site 1146 provides one of the most continuous Neogene sections ever recovered by the Ocean Drilling Program. The sediments are relatively high in carbonate and have rates of 30-150 m/m.y., which will enable construction of an orbital-scale stratigraphy back to the middle Miocene. This site will enable the reconstruction of monsoon proxies that can be used to test hypotheses about the late-Miocene intensification of the Asian summer monsoon and its relationship to tectonic events.

Site Summaries - Sites 1147 and 1148

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