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doi:10.2204/iodp.proc.303306.214.2010

Site U1305

Site U1305 is located close to the southwest extremity of the Eirik Drift, 82.2 km south of ODP Site 646 (Fig. F1; Table T1). The thickness of the sediments above the mid-upper Pliocene seismic Reflector R1 (~540 m) is almost twice that at Site 646. The water depth (3518 m) means that the seafloor at Site U1305 lies below the main axis of the Western Boundary Undercurrent (WBUC) and hence preserves expanded interglacial intervals and relatively condensed glacial intervals. The mean Quaternary sedimentation rate (>17 cm/k.y.) provides a high-resolution record of ice sheet instability, and changes in surface and deep water masses.

Three holes were cored at Site U1305 with the APC system to a maximum depth of 287 mbsf. The sediments at Site U1305 are designated as a single unit dominated by varying mixtures of terrigenous components and biogenic material, primarily clay minerals, quartz, detrital carbonate, and nannofossils. Calcium carbonate content ranges from 1 to 49 wt%. The most common lithologies are dark gray to very dark gray silty clay and nannofossil ooze with silty clay. In addition, olive-gray sandy silt laminae and centimeter- to decimeter-scale intervals of silty clay with detrital carbonate are present. The sediments are gradationally interbedded at scales of a few meters or less.

Calcareous, siliceous, and organic-walled microfossils show generally good preservation and abundance in the upper ~200 mcd. However, the abundance of microfossil assemblages is variable below this depth, with generally poorer preservation. All microfossil groups investigated are dominated by subpolar to polar assemblages. Planktonic foraminifers show a bimodal size distribution. The small test-sized planktonic foraminifers, which are cold-water species, coexist with increased abundance of benthic foraminifers, possibly indicating transport by bottom currents. An oxygen isotope stratigraphy based on N. pachyderma (sinistral) is presently in development (C. Hillaire-Marcel, pers. comm., 2009).

Sediments at Site U1305 carry well-defined magnetization components and appear to provide useful records of polarity transitions. Natural remanent magnetization (NRM) intensities decrease by about one-half below 166 mcd. Directional magnetization data allow identification of the Brunhes Chron and part of the Matuyama Chron, including the Jaramillo, Cobb, and Olduvai Subchrons. The Cobb Mountain Subchron and the top of the Olduvai Subchron are less clearly identified because of the incomplete removal of the normal polarity drill string magnetic overprint. A relative paleointensity record is presently being developed for this site (A. Mazaud, pers. comm., 2009).

A continuous stratigraphic composite section was constructed to ~295 mcd with a single problematic interval between 197.2 and 206 mcd. The mean sedimentation rate calculated using biostratigraphic and magnetostatigraphic datums is 17.5 cm/k.y. for the entire section. Using only paleomagnetic datums, sedimentation rates are relatively uniform with the exception of a higher mean sedimentation rate between 1.07 and 1.19 Ma (from the base of the Jaramillo to the top of the Cobb Mountain Subchrons) that averages 29.3 cm/k.y.

Despite the low organic carbon content (mean <0.4 wt%), organic matter diagenesis dominates the pore water chemistry (see Ennyu and Malone). Sulfate decreases linearly downcore and is completely reduced by 58 mbsf. Methane increases immediately below the sulfate reduction zone, reaching a maximum of 46,000 ppmv at 228 mbsf. Ethane fluctuates between 2 and 14 ppmv within the methanogenic zone, but no higher hydrocarbons were detected. Alkalinity increases downcore, reaching a maximum of 18.9 mM at the sulfate/methane interface (SMI). Calcium reaches a minimum of 2.58 mM at the same depth, suggesting carbonate precipitation associated with anaerobic methane oxidation at the SMI. Similar to other Expedition 303 sites, dissolved strontium at Site U1305 is at or below seawater values, indicating little or no carbonate dissolution or recrystallization.

Physical property records at Site U1305, in particular magnetic susceptibility and density, are highly variable, recording lithologic and mineralogic changes. Low magnetic susceptibility and density values usually coincide with the presence of silt-sized detrital carbonate. The detrital carbonate (Heinrich-type) layers are characterized by a very specific biomarker association, pointing to an Ordovician oil shale close to Hudson Strait as the source of these layers (Hefter et al., 2007). Natural gamma radiation (NGR) increases at the transition between detrital carbonate- and siliciclastic-dominated background sediments, suggesting a relative increase in the clay component. Site U1305 sediments are also characterized by an overall downcore increase in density (from 1.3 to ~1.9 g/cm³), decreasing porosity (from 80% to 62%), and low P-wave velocities (1500–1600 m/s).

Data from wireline logs in Hole U1305C span the interval from 95.3 to 265.9 mbsf. The triple combination (triple combo) tool string was successfully deployed, yielding downhole records of density, porosity, NGR, electrical resistivity, and photoelectric factor. Density and porosity are generally inversely related, with density increasing and porosity decreasing downhole. Density and gamma ray logging data show similar downhole trends to those observed in core data.

The analysis of MST, biostratigraphic, and paleomagnetic data indicates that a complete and continuous high-resolution record (mean sedimentation rate = 17.3 cm/k.y.), covering the uppermost Pliocene and Quaternary, has been recovered at Site U1305. The record represents a rich archive of environmental change that documents episodes of instability in the surrounding (Laurentide, Greenland, and Inuitian) ice sheets, the history of surface currents and deep water currents, and hence the strength of the WBUC that contributes to NADW. Good preservation of both planktonic and benthic foraminifers for isotopic analysis and a high-fidelity paleomagnetic record indicate that the environmental record can be integrated into a high-resolution stratigraphy.

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