Site 1145

Site 1145 (proposed site SCS-2) is located 19°35.04'N, 117°37.86'E, at a water depth of 3175 m (Table 1), near the base of the northern continental margin of the South China Sea (Fig. 14). The location of Site 1145 was selected to provide a deep-water contrast to Site 1144 (i.e., below the depth of the Bashi Strait, 2500 m) so that the water-mass depth gradients and ventilation history of the South China Sea could be reconstructed for the Quaternary. Site 1145 was also envisioned as a mid-Pliocene through Pleistocene record of the intensification of the winter monsoon, coincident with the development of loess in China.

We cored three APC/XCB holes at Site 1145. Hole 1145A reached the target depth of 200 mbsf with average recovery of 93%, Hole 1145B reached 200 mbsf with an average recovery of 90%, and Hole 1145C reached 198 mbsf with an average recovery of 96%. The total core recovered at this site was 555 m, representing 93% of the cored interval (Table 1). Downhole and bottom-water temperature measurements revealed a thermal gradient of 90°C/km, which is consistent with the water depth of the site.

Site 1145 recovered a continuous sequence of hemipelagic clays of late Pliocene to Holocene age with a basal age of ~3 Ma (Fig. 15). A complete composite (spliced) section was constructed over the entire depth of 213 mcd. The site is noteworthy for its excellent paleomagnetic stratigraphy and distinct patterns of cyclicity.

The Pleistocene-age sediments at Site 1145 mainly consist of intensely bioturbated clay with distinct light carbonate-rich clay layers of ~0.5 to 4 m thickness. Although average carbonate content is low during the Pleistocene (averaging ~10%), the light carbonate-rich layers were clearly recorded in the CR data, especially the lightness parameter (L*). Other core-logging data (GRA bulk density, NGR, and MS) are also characterized by high-amplitude cyclical fluctuations over the entire section. These cycles are best defined in the NGR data from the APC interval (above ~133 mcd), where the dominant wavelength changes at ~80 mcd from ~10-15 m in the upper interval to ~2-5 m in the lower interval. The first five major CR and NGR intervals are easily correlated with the marine oxygen isotope interglacial Stages 5, 7, 9, 11, and 13. Identification of light layers became more problematic with depth as the average sediment composition became more carbonate rich and the contrast between dark and light layers decreased. In the lower half of the Pleistocene, below ~80 mcd, the average values of GRA, MS, and NGR increase to roughly double the value. The abrupt increase in these values is accompanied by a sudden decrease in porosity from ~70% to 55%. Porosity clearly defines an interval of excursion from a linear trend between 80 and 170 mcd. The porosity decrease alone cannot account for the changes observed in MS and NGR over the interval from 80 to 170 mcd; we therefore infer a significant change in lithology that affects porosity and other physical properties, including average grain density. At ~133 mcd, GRA bulk density shows a sharp offset caused by the change from APC to XCB coring. XCB cores are moderately disturbed by partial remolding and incorporation of drilling slurry.

The upper part of the section (0-86 mcd) is characterized by small amounts (<10%) of biogenic silica, mainly in the form of radiolarians, diatoms, sponge spicules, and silicoflagellates. Green layers (typically 1-3 cm thick) and less distinct green mottles, as well as slightly yellowish gray patches that probably represent traces of bioturbation, appear frequently throughout the Pleistocene section. The occurrence of complete pteropod tests in the upper section (0-~70 mcd) along with abundant calcareous nannofossils and planktonic foraminifers reflects the good carbonate preservation in the upper Pleistocene. Evidence of downslope transport at Site 1145 includes several complete echinoderms, echinoderm fragments remineralized by pyrite, and several wood fragments. Although a few cores contain fresh angular volcanic glass shards within burrows and dispersed ash in low concentrations, volcanic ash represents a very minor component of the total sequence.

Total organic carbon (TOC) declined steadily from more than 1% at the top of the section to much lower amounts (~0.2%) at the base, consisting of mostly marine organic matter. Only trace amounts of methane (<16 ppmv) and no other hydrocarbon gases were detected in sediments at Site 1145. Interstitial water profiles are characterized by relatively constant chloride values close to those of the seawater value. The sulfate, alkalinity, ammonium, and phosphate profiles show significant changes in the upper part of the sedimentary column (0-~100 mcd), which appear to be caused by the diagenesis of organic matter via sulfate reduction. Sulfate reduction is incomplete, and methanogenesis is a minor process in these sediments. Dissolved magnesium and potassium concentrations decrease linearly with depth. Dissolved calcium concentrations decrease downhole to ~44 mcd, indicating that sulfate reduction and alkalinity production are promoting inorganic calcite precipitation. Concentrations then remain low and relatively constant between 50 and 100 mcd. Below this interval, calcium increases to a maximum near the bottom of the hole. Below 80 mcd, dissolved strontium concentration increases continuously, which most likely reflects dissolution of biogenic silica and/or reactions involving alteration of volcanic glass. Low lithium concentrations at the top of the hole probably result from an uptake of Li+ during authigenic calcite precipitation. Dissolved silica increases slightly at the top of the hole at 24.9 mbsf and then is high and constant between 24.9 and 72.4 mbsf. Below this interval, H4SiO4 concentrations decrease abruptly and then vary within a lower range of values. This shift was also observed at the higher accumulation rate of Site 1144 at 420 mcd and is synchronous (~1 Ma) at both sites.

The paleomagnetic records at Site 1145 clearly established the Brunhes/Matuyama transition at 93 mcd, the upper Jaramillo transition at 110 mcd, and the lower Jaramillo transition at ~116 mcd. Farther downcore the overprint resulting from the coring process increases, but long core measurements identify the Gauss/Matuyama boundary at 190 mcd as a jump of the inclination from ~25° to 45° (no declination is obtained from XCB cores). Demagnetization of the discrete samples proved to be more efficient in removing the overprint, and the Olduvai Event was clearly revealed from 155 to 165 mcd. The inclinations around the Gauss/Matuyama boundary are closer to their expected values. From 190 mcd to the bottom of Hole 1145A, two samples yield unequivocal reverse inclinations. This may indicate that the upper Kaena reversal (3.04 Ma) had been reached in this hole, although this conclusion conflicts with the biostratigraphic age for the bottom of the hole and must be confirmed with shore-based analysis. Based on paleomagnetic and biostratigraphic control, the Pleistocene/Pliocene boundary can be placed at ~160 mcd. At Site 1145, the upper Pleistocene interval (0-70 mcd) had a linear sedimentation rate of 210 m/m.y. and total and carbonate MARs of 14 and 1.4 g/cm2/k.y., respectively. The lower Pleistocene interval had lower rates, with an average LSR of 60 m/m.y. and total and carbonate MARs of 6.2 and 0.7 g/cm2/k.y., respectively.

In the Pliocene-age section at Site 1145, average carbonate contents (17%) and variability (10%-30%) increase. Although planktonic foraminifers and calcareous nannofossils remain common to abundant, planktonic foraminifer preservation degraded from good to poor downsection, and the nannofossils were occasionally to commonly reworked. Benthic foraminifers are generally few. The CR and bulk data both increase in the Pliocene section and reflect the increased carbonate content. Other properties display cyclic variability but show no longer term trends. The biostratigraphic age of the oldest sediments recovered at Site 1145 is estimated at 3.12-3.35 Ma. The Pliocene section has the lowest rates observed at Site 1145, with LSRs of 38 m/m.y. and total and carbonate MARs of 4.5 and 0.8 g/cm2/k.y., respectively.

In general, both carbonate and noncarbonate accumulations decrease downhole at Site 1145, with a small increase in the lower Pleistocene. Overall, the variations in the noncarbonate components dominate the accumulation rate pattern as in Site 1144. Site 1145 provides a high-quality section with excellent paleomagnetic stratigraphy for the resolution of orbital-scale variations of climate changes that may be related to the onset of extensive loess deposition in China at ~2.4 Ma.

Site Summaries - Site 1146

Table of Contents