Site 1144

Site 1144 (proposed site SCS-1) is located 20°3.18'N, 117°25.14'E, at a water depth of 2037 m (Table1), on a sediment drift on the northern margin of the South China Sea (Fig. 12). Site 1144 was chosen to take advantage of the extremely high sedimentation rates to recover a continuous sequence of hemipelagic sediments that will enable reconstruction of the paleomonsoon history on a millennial, centennial, or higher resolution time scale for the mid- to late Pleistocene (~1 Ma). Site 1144 data will allow comparison of SCS records with orbital-scale and higher frequency records from ice cores, marginal seas, and terrestrial deposits. Situated along the northern margin of the SCS, Site 1144 is ideally located to reconstruct the sedimentologic, isotopic, and faunal/SST changes associated with development of the winter monsoon. This site also provided the northern constraints for reconstruction of SST gradients in the South China Sea.

We cored three APC/XCB holes at Site 1144. Hole 1144A reached the target depth (453 mbsf) with an average recovery of 104%, Hole 1144B reached 452 mbsf with 99% recovery, and Hole 1144C was APC cored to the target depth of 199 mbsf with an average recovery of 100%. The total recovery for Site 1144 was 1113 m, representing 101% of the cored interval (Table 1). Following completion of Hole 1144A, wireline logs were successfully acquired from 87 to 452 mbsf for all three tool suites (triple combo, FMS-sonic, and GHMT). The temperature gradient within the sediment column at Site 1144 is anomalously low (24°C/km), about one-third of the expected value. Reasons for this may include the rapid sedimentation rate or subsurface flow.

Drilling at Site 1144 recovered a mid- to upper Pleistocene and Holocene sequence of rapidly accumulating, hemipelagic clays with a basal age of ~1.1 Ma (Fig. 13). The recovered sequence spans 452 mbsf (519 mcd). The continuous spliced record (mcd scale) extends from 0.0 to 235.41 mcd. Splice construction below this interval was precluded by incomplete core recovery and alignment of coring gaps. However, a discontinuous ("floating") mcd depth scale was constructed from 235 to 519 mcd based on correlations among cores from the three holes.

The Pleistocene-age sediments at Site 1144 are notable for their high sedimentation rates and organic carbon content, the cyclicity of their physical properties, and the variations of "iron sulfide" and pyrite. The sediments represent a rapidly accumulating drift deposit with hemipelagic sedimentation of clay with quartz silt and nannofossils, completely homogenized by bioturbation. Variations in the lithology reflect the abundance of iron sulfides (dominant in uppermost interval, 0-283 mcd), siliceous biota (highest proportion in middle interval, 283-404 mcd), and pyrite (dominant in lowermost interval, 404-518 mcd). Minor lithologies (5%-10% of the sediment) include clay with quartz silt and sponge spicules, clay with quartz silt and diatoms, clay with silt, silty clay, and clay. Ash layers and green clay layers are rare throughout, but a number of rather indistinct green clay layers and patches occur over the same intervals that contain more discrete layers at Site 1143. Downslope redeposition is suggested by thin layers of foraminifer ooze with small amounts of pyrite and a variety of macrofossils, including gastropods, scaphopods, pteropods, fragments of echinoderms, and poorly defined shell debris. Wood debris as long as 4 cm was also observed. Bulk X-ray diffraction (XRD) analysis revealed that the mineralogy follows the overall visual homogeneity of the sediment.

Carbonate content of this Pleistocene section is low and ranges from 10% to 20%. Unlike at other sites, CR data did not correlate well with shipboard carbonate measurements taken at much lower sampling resolutions. However, they do show detailed, high-resolution variations with patterns similar to recognized glacial-interglacial scale variations. Once properly calibrated, these data will provide useful proxies for paleoceanographic studies. Over the upper section of Site 1144 (0-100 mcd), increases in bulk density and NGR and decreases in porosity reflect rapid compaction, with superimposed fluctuations of glacial-interglacial and higher frequencies. Both core data and downhole logging show a markedly decreased rate of compaction, and the superimposed signal variability is of larger amplitude over the next 320-m interval (100-420 mcd). Magnetic susceptibility is low and rather featureless through these two upper intervals, apart from some spikes representing ash layers. However, an abrupt, threefold increase in MS takes place at ~420 mcd and is associated with a decrease in porosity from 55% to 50% as well as increased downhole sonic velocities. These changes correspond to the depth of a prominent seismic reflector. Because of the scarcity of biostratigraphic markers in this section of high sedimentation rates, we have not yet determined if a hiatus exists at that depth and/or if the section may reflect past mass wasting events in the sediment drift.

Organic carbon is relatively high at Site 1144 and decreases from ~1.5% near the core top to 0.3% at 519 mcd. Methane ranged from 0.3% to > 6% and was biogenic in origin. No heavier hydrocarbons were observed. The interstitial waters at Site 1144 reflected sulfate reduction in the upper few meters and ammonia production throughout the remaining core. Clay alteration accounted for a number of the interstitial water profiles. A few profiles indicated a change at ~420 mcd, where physical properties and seismic records also detect a boundary. However, the chemistry gives no special insight into the nature of this boundary or layer.

The chronostratigraphy of Site 1144 is primarily derived from the calcareous nannofossil and planktonic foraminiferal zones and events and is aided by the abundance patterns of several planktonic foraminifers, siliceous microfossils, benthic foraminifers, and pteropods. Because of the extremely high sedimentation rates at this location, only eight of the standard biostratigraphic zones and markers could be identified over the past 1.1 m.y. Unfortunately, the magnetopolarity stratigraphy was also limited by the high sedimentation rates and poor magnetization. Only the Laschamp Event (0.04 Ma) is tentatively identified at 23.5-25.5 mcd. The temporal pattern of magnetic intensity apparently correlates well with other intensity records and is consistent with the biostratigraphic age model. Sedimentation rates varied within the section but tend to decrease downhole, with the exception of the lowermost 100 m. Over the upper half of the site (0-250 mcd, 0-0.31 Ma), the linear sedimentation rates average 870 m/m.y., and the total and carbonate MARs (g/cm2/k.y.) average 85 and 11, respectively. In the lower half of Site 1144 (250-519 mcd, 0.31-1.03 Ma), the LSR averages 50 m/m.y., and the total and carbonate MAR (g/cm2/k.y.) average 64 and 9.6, respectively.

In summary, Site 1144 offers an exceptionally high sedimentation-rate section with well-defined variability of many properties for the study of climate-ocean response on the orbital, millennial, centennial, and higher resolution time scales for the mid- to late Pleistocene (~1 Ma). The data from Site 1144 will be among the highest resolution marine records and should enable direct comparison of the South China Sea climates with records from ice cores, laminated sequences, and terrestrial deposits.

Site Summaries - Site 1145

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