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doi:10.2204/iodp.proc.318.107.2011 Site U13591Expedition 318 Scientists2Site summaryIntegrated Ocean Drilling Program Site U1359 (proposed Site WLRIS-04A) is located on the continental rise at 3009 meters below sea level (mbsl) (Fig. F1). The main objective at Site U1359 was to obtain an expanded record for the late Neogene to Quaternary to provide a history of climate and paleoceanographic variability and to investigate the stability of the East Antarctic Ice Sheet (EAIS) during the middle Miocene to Pleistocene extreme warm periods (e.g., Miocene climate optimum, early Pliocene, and Pleistocene marine isotope Stages 31 and 11). This record was to also provide the timing and nature of deposition of the upper seismic units (i.e., above unconformity WL-U6) defined on the Wilkes Land margin (De Santis et al., 2003; Donda et al., 2003). These units include a shift in sedimentary depocenters from the continental rise to the outer shelf, possibly corresponding to the transition from a dynamic wet-based EAIS to a more persistent cold-based EAIS (Escutia et al., 2002; De Santis et al., 2003) and inferred to occur during the late Miocene–Pliocene (Escutia et al., 2005; Rebesco et al., 2006). At Site U1359, unconformities WL-U6, WL-U7, and WL-U8 lie at approximately 4.61, 4.44, and 4.23 s two-way traveltime, respectively (approximately 520, 323, and 126 meters below seafloor [mbsf], respectively) (Fig. F2). Site U1359 is located on the eastern levee of the Jussieau submarine channel (Figs. F1, F2). The Jussieau channel is one of the intricate networks of slope canyons that develop downslope into channels and coalescing deep-sea fans (Escutia et al., 2000). Site U1359 is positioned in an upper fan environment where the levee relief (measured from the channel thalweg to the top of the levee) is ~400 m. Multichannel seismic profiles across the site show that widespread channels with high-relief levees occur on the Wilkes Land margin above unconformity WL-U5 (Escutia et al., 1997, 2000; Donda et al., 2003). The fine-grained components of the turbidity flows traveling through the channel and hemipelagic drape are inferred to be the dominant sedimentary processes building these large sedimentary levees (Escutia et al., 1997, 2000; Donda et al., 2003). Bottom currents can further influence sedimentation in this setting (Escutia et al., 2002; Donda et al., 2003). Similar depositional systems were drilled during Ocean Drilling Program (ODP) Leg 178 along the Antarctic Peninsula (Barker, Camerlenghi, Acton, et al., 1999) and ODP Leg 188 in Prydz Bay (O’Brien, Cooper, Richter, et al., 2001). At Site U1359, Holes U1359A–U1359D were drilled to total depths of 193.50, 252.00, 168.70, and 602.2 mbsf, respectively. In Holes U1359A and U1359B, the advanced piston corer (APC) system was used to refusal, followed by extended core barrel (XCB) drilling. Only the APC system was used in Hole U1359C. Hole U1359D was drilled using the rotary core barrel (RCB) system and core was only recovered below 152.2 mbsf. Silty clay with dispersed clasts is the dominant lithology observed throughout all holes at Site U1359. There are noticeable variations in the amount of biogenic components, bioturbation, and sedimentary structures, in particular the presence or absence of packages of silt–fine sand laminations and large variations in diatom abundance. Five distinct lithofacies are identified based on variations in the style of lamination, bioturbation, or the relative abundance of the biogenic component. Three lithostratigraphic units are defined on the basis of observed changes in facies associations (Figs. F3, F4). Lithostratigraphic Unit I (0–42.07 meters composite depth [mcd]) consists of decimeter-scale alternations of yellow-brown and olive-gray diatom-rich silty clays with dispersed clasts with occasional foraminifer-bearing clayey silt and sandy silt. Unit II (42.07–264.24 mcd) consists of bioturbated diatom-bearing silty clays interbedded with olive-gray diatom-bearing silty clays, which are mostly massive but contain decimeter-scale packages of olive-brown silty clay with silt laminations. Unit III extends from 264.24 mcd to the bottom of the cored section at 613.46 mcd and consists of bioturbated diatom-bearing silty clays interbedded with laminated silty clays. The laminated silty clays contain more subtle, but persistent, submillimeter- to millimeter-scale laminations compared to Unit II. Clasts >2 mm in size occur throughout all lithostratigraphic units and are mostly dispersed in nature (i.e., trace to 1% in abundance). The sedimentology of Units I and II is consistent with levee deposition by low-density turbidity currents, whereas the facies associations in Unit III probably represent deposition in an environment influenced by periodic variations in contour current strength or saline density flows related to bottom water production, with turbidity currents having less influence than in the overlying units. The regular nature of the interbedding (i.e., beds 2–5 m thick) of the laminated and bioturbated facies within all three lithostratigraphic units suggests that the sedimentary record recovered from Site U1359 is cyclic in nature (Figs. F5, F6). The diatom-bearing and diatom-rich silty clays (Facies 1 and 2) were probably deposited by hemipelagic sedimentation in a higher productivity environment relative to the other facies. The clays and silty clays (Facies 3–5) indicate high terrigenous sedimentation rates and/or lower biogenic productivity, perhaps related to the duration of seasonal sea ice cover regulating light availability in surface water or wind-regulated control of the mixed layer depth, which in turn controls productivity. The opposite scenario may apply tor the diatom-bearing to diatom-rich silty clay facies (Facies 1 and 2). An increase in terrigenous input may result from ice advance across the shelf or increase in sedimentation from bottom currents. The passage of cold saline density flows related to bottom water production at the Wilkes Land margin (e.g., high-salinity shelf water flowing from the shelf into the deep ocean to form Antarctic Bottom Water [AABW]) should also be considered as a potentially important sediment transport mechanism. The depositional model for recovered sediments at Site U1359 may represent a continuum of all three processes, in addition to pelagic and ice-rafted components, as indicated by the presence of diatom remains and dispersed clasts throughout. Combined micropaleontology assigns the recovered successions at Site U1359 to the late middle Miocene to late Pleistocene (Fig. F7). Integrated diatom, radiolarian, foraminifer, and magnetostratigraphic data highlight a late Pliocene to early Pleistocene condensed interval (between ~2.5 and 1.5 Ma) and another one during the early late to mid-late Miocene (between ~9.8 and 7 Ma). Miocene diatom assemblages mainly include open-water taxa. In addition, a notable increase in the abundance of stephanopyxid specimens may be interpreted as either an indication of shallowing water depths or an increase in reworking of shallower water sediments. The lack of planktonic and benthic foraminifers suggests that bottom waters were corrosive during the late middle Miocene to calcareous foraminifers except for brief periods (e.g., around ~10 Ma, when calcareous benthic foraminifers were preserved). Also during the Pliocene, open-water taxa and variable abundances of benthic, neritic, and sea ice–associated taxa dominated diatom assemblages. The dinocyst assemblages predominantly comprise heterotrophic taxa, indicating that the biosiliceous-rich sediments were deposited in a high-productivity and sea ice–influenced setting. High abundances of sporomorphs reworked from Paleogene, Mesozoic, and Paleozoic strata suggest continuous strong erosion in the hinterland. The general lack of planktonic and calcareous benthic foraminifers suggests that Pliocene bottom waters were corrosive to the thin-shelled tests of planktonic foraminifers. Diatom and radiolarian Pleistocene assemblages at Site U1359 are dominated by typical Neogene Southern Ocean open-water taxa with variable abundances of benthic, neritic, and sea ice–associated diatom taxa. This indicates a pelagic, well-ventilated, nutrient-rich, sea ice–influenced setting, corroborated by the presence of heterotrophic-dominated dinocyst assemblages. The preservation of planktonic foraminifers in the Pleistocene indicates that bottom waters were favorable to the preservation of calcium carbonate. Further, pervasive reworked sporomorphs of Paleogene, Mesozoic, and Paleozoic age again point to continuing strong erosion in the hinterland. Paleomagnetic investigations at Site U1359 involved analysis of discrete samples from Holes U1359A, U1359B, and U1359D and measurement of archive halves from all four holes. A composite polarity log was correlated to the geomagnetic polarity timescale (GPTS) of Gradstein et al. (2004), documenting a complete Pliocene section from the top of Chron C2An to the bottom of Chron C3An (Fig. F8). A gap including Chron Cn2 and a period of extremely slow (and probably discontinuous) sediment accumulation from Chron C3Ar to the top of Chron C5n aligns with the biostratigraphic assessments. Routine headspace gas analyses were carried out on samples from Holes U1359A–U1359D, and 71 samples were taken for analyses of weight percent carbonate, carbon, nitrogen, and sulfur content, as well as major and trace element analyses. Furthermore, 51 interstitial water samples were taken close to the microbiology samples from the top ~20 m (0.1–20.1 mbsf) of the holes. CaCO3 contents for most samples vary between <1 and 3.2 wt%. A distinct carbonate-rich layer with a CaCO3 content of 39.7 wt% was found at 372.45 mbsf and corresponds to a minor lithology of diatom-bearing nannofossil ooze. On the basis of the distribution patterns of the major and trace elements, four broad intervals can be distinguished between 0 and ~200, ~210 and ~310, ~310 and 536, and 547.39 and 594.79 mbsf. The interstitial water measurements reveal chemical gradients that are consistent with active diagenesis of buried organic matter within the sulfate reduction zone (SRZ). Significant levels of sulfate at the bottom of the observed profile (~23 mM at 20.1 mbsf) imply that the sampled interval did not reach the carbon dioxide (methanic) reduction zone (see the “Site U1357” chapter for contrasting behavior). Microbiological sampling was conducted in Hole U1359B and was supported with pore water sampling (Fig. F9). A total of 52 ten-centimeter whole rounds were taken from the top 20 m and frozen at –80°C for onshore phospholipid analyses and molecular 16S rRNA sequencing. Between 20 and 200 mbsf, seventeen 5 cm3 samples were taken and preserved for onshore molecular 16S rRNA sequencing. The physical property program for Site U1359 includes routine runs on the Whole-Round Multisensor Logger (WRMSL), which includes the gamma ray attenuation (GRA) bulk density, magnetic susceptibility, and P-wave velocity logger (PWL) sensors, as well as natural gamma ray (NGR) measurements. P-wave velocity was also analyzed, and samples were taken for moisture, density, and porosity measurements from Holes U1359A, U1359B, and U1359D. Thermal conductivity measurements were taken in cores from all holes. Cyclicities at several scales are observed in the intervals where the magnetic susceptibility ranges between 40 and ~100 instrument units. Furthermore, the NGR data together with the magnetic susceptibility and GRA density data were used to correlate the four holes drilled at Site U1359 and to define a composite record (see “Stratigraphic correlation and composite section”). In addition, pronounced lower density values between 50 and 65 mbsf (50 and 65 m core composite depth below seafloor, method A [CCSF-A]), below the lithostratigraphic Unit I–II transition, suddenly drop at ~99.5 mbsf (~101 m CCSF-A), which coincides with the lithologic change from diatom-bearing to diatom-rich silty clays (lithostratigraphic Subunit IIa–IIb transition), as well as a shift to slightly lower values at ~248 mbsf (~264 m CCSF-A; lithostratigraphic Unit II/III boundary). Downhole logging measurements in Hole U1359D were made after completion of RCB coring to a total depth of 602.2 mbsf (drilling depth below seafloor [DSF]). Three tool strings were deployed in Hole U1359D, the triple combination (triple combo), Formation MicroScanner (FMS)-sonic, and Versatile Sonic Imager (VSI). Hole U1359D was divided into two logging units (100–260 and 260–606 mbsf) on the basis of the logs (Fig. F10). The upper logging unit is characterized by high-amplitude swings in bulk density, NGR, and resistivity values. The transition to the unit below is gradual. Logging Unit 2 is characterized by generally lower amplitude bulk density and resistivity variations than the unit above, but the 2–5 m scale alternations are still clearly defined. NGR continues to show high variability, and several large drops in NGR values are observed between 350 and 450 mbsf. Near the base of the hole at 574–580 mbsf, a 6 m interval of higher bulk density and resistivity indicates a cemented bed or series of cemented beds. Heat flow at Site U1359 was estimated at 62.4 mW/m2, a typical value for the ocean floor. |
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