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Site U1306

At Site U1306, late Pliocene and Quaternary mean sedimentation rates were estimated to be ~18 cm/k.y., based on identification of seismic Reflector R1, which can be correlated to the mid-upper Pliocene at Site 646. The placement of the site was designed to yield a high-resolution (high sedimentation rate) Quaternary environmental record from a water depth within the main axis of the WBUC. Based on a nearby conventional piston core from a similar water depth (Core HU90-013-012; Hillaire-Marcel et al., 1994), we expect glacial intervals to be expanded relative to interglacial intervals.

Four holes were cored with the APC system at Site U1306, reaching a maximum depth of 309.3 mbsf. Hole U1306D was cored to 180.0 mbsf to provide necessary stratigraphic overlap for the upper portion of the sediment sequence. Five cores were obtained by drillover, and the average recovery was 102.5% for the cored interval.

The sediments at Site U1306 are designated as a single lithostratigraphic unit, composed of Holocene to uppermost Pliocene terrigenous and biogenic sediments that are gradationally interbedded at scales of a few meters or less. Calcium carbonate content is low, ranging from 0.3 to 12.3 wt% (mean = 3.2 wt%). The most common lithologies are silty clay, silty clay with diatoms, nannofossil silty clay, and silty clay nannofossil ooze. Dropstones are present throughout the cored interval, and large dropstones (~4 cm) are common to abundant. Centimeter- to decimeter-scale beds of olive-gray or greenish gray silty clay or clay with a high detrital carbonate content are present in all holes at Site U1306, but they are thinner and less common than at Site U1305.

Rich assemblages of calcareous, siliceous, and organic-walled microfossils are present at Site U1306, although benthic foraminifers are barren in many samples below 175 mcd. An oxygen isotope record based on N. pachyderma (sinistral) is presently in development (J.D. Wright, pers. comm., 2010). Large variations in abundance of microfossils occur downcore. Although preservation is moderate to good in the upper part of the succession, preservation generally decreases below ~170 mcd for calcareous and siliceous microfossils. All samples contain moderately well to well-preserved palynomorphs but variable numbers of dinocysts, which are abundant only in a few samples. Some redeposition is indicated by the presence of reworked nannofossils and palynomorphs of Cretaceous to Miocene age through the cored interval. The dominant components of each microfossil group reflect cold sea-surface temperatures (SSTs) for most of the time represented by the sedimentary sequence.

The sediments at Site U1306 carry well-defined magnetization components and document an apparently continuous sequence including the Brunhes Chron and much of the Matuyama Chron. The Jaramillo, Cobb Mountain, and Olduvai Subchrons are clearly identified. Within the Brunhes Chron, the Iceland Basin magnetic excursion (~188 ka) was observed in three of the holes.

A continuous stratigraphic composite section was constructed to ~337 mcd. Below 287 mcd, cores were recovered in two holes only, but the section is complete with only a single tenuous tie near the base of the record. The mean sedimentation rate, calculated using biostratigraphic and magnetostatigraphic datums, is 15.6 cm/k.y. for the entire section cored at Site U1306. Using only paleomagnetic datums, interval sedimentation rates vary between 12.4 and 19.3 cm/k.y.

Pore water chemical profiles at Site U1306 document very similar reactions to nearby Site U1305 (see Ennyu and Malone). Complete sulfate reduction is achieved at shallow depths at Site U1306 (85 mbsf) despite the low organic carbon content (mean = 0.3 wt%). Methane increases below 85 mbsf, reaching a maximum of 46,000 ppmv. Alkalinity reaches a maximum of 18.7 mM at the SMI, whereas calcium concentration attains a minimum value (3.7 mM), indicating carbonate mineral precipitation associated with methane oxidation. From 114 to 258 mbsf, sulfate increases slightly again (1.5 mM). This interval corresponds to pH and iron fluctuations, which are antithetic to each other and may indicate zones of anaerobic pyrite oxidation. Dissolved strontium remains at or below seawater values suggesting little or no carbonate dissolution or recrystallization.

Physical property records at Site U1306 are highly variable, recording lithologic and mineralogic changes. The higher carbonate content in the upper ~100 mcd results in lower NGR and magnetic susceptibility values than in the sediments below. Site U1306 sediments are characterized by an overall downcore increase in density (1.5 to ~1.8 g/cm3) and decreasing porosity (~70% to 50%).

Based on nearby piston cores, Site U1306 is expected to exhibit expanded glacial intervals, complementary to Site U1305 where interglacials are likely to be relatively expanded. The apparently complete Quaternary record recovered at Site U1306 provides a high-resolution (high sedimentation rate) record of detrital events derived from the instability of surrounding ice sheets. The site monitors the activity of the WBUC, which supplies a component of NADW to the Labrador Sea, and appears to have the attributes required for the generation of a well-constrained age model based on oxygen isotopes, micropaleontology, and RPI.