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doi:10.2204/iodp.proc.303306.214.2010

Site U1307

The location of Site U1307 was chosen for access to Pliocene sediments below the Quaternary sequence drilled at Site U1306. A thinner Quaternary sedimentary sequence at Site U1307 allows the Pliocene sequence to be sampled using the APC system.

Two holes were cored at Site U1307, reaching a maximum depth of 162.6 mbsf. Two partial strokes of the APC required RCB drilling of two intervals in Hole U1307A that were difficult to penetrate using APC (50.5–52.5 and 73.7–77.7 mbsf). Average recovery was 102% for the cored intervals at Site U1307. Coring was terminated because of excessive heave when a passing storm system affected drilling operations.

The lower Pliocene to Pleistocene sedimentary succession at Site U1307, which is divided into three units, records variations in the input of terrigenous and biogenic components (mostly quartz, detrital carbonate, nannofossils, and foraminifers). Unit I (0–49.55 mcd) is composed of Quaternary mixtures of foraminifers, silty clay, and nannofossils (silty clay with foraminifers, foraminifer silty clay, and nannofossil silty clay). Minor lithologies include eight discrete foraminifer silty sand and sandy foraminifer ooze beds. Unit II (49.55–133.86 mcd) is composed mainly of Pleistocene to upper Pliocene silty clay with a minor biogenic component. Unit III (133.86–173.6 mcd) comprises upper to lower Pliocene silty clay, silty clay with nannofossils, and nannofossil silty clay. With the exception of the foraminifer sand beds, calcium carbonate content is low (mean = 3.8 wt%).

Calcareous, siliceous, and organic-walled microfossils are common to rare with moderate to poor preservation. A possible hiatus (~0.25 m.y. in duration) or condensed interval (~1.21–1.45 Ma) is indicated by nannofossil biostratigraphy at ~56–61 mcd. The dominant components of each microfossil group reflect subpolar to polar conditions during the Pleistocene. In the lower upper Pliocene (before 2.74 Ma), the nannofossil assemblage suggests warmer surface water conditions. Sarnthein et al. (2009) measured oxygen isotope data and SST proxies in the 2.7–3.5 Ma interval at Site U1307 and interpreted the results in terms of a cooling by ~6°C and freshening by ~2 psu from 3.2 to 3.0 Ma, attributable to changes in the East Greenland Current, that they associate with a phase of closure of the Central American Seaway, thereby implicating this tectonic event in the onset of Northern Hemisphere Glaciation.

Paleomagnetic directional data yield an almost continuous sequence and permit unambiguous identification of the Brunhes, Matuyama, and Gauss Chrons. Within the Matuyama Chron, the Jaramillo, Olduvai, and Reunion Subchrons are clearly recognized. Within the Gauss, the Kaena and Mammoth Subchrons are also recognized, with the base of the section corresponding to the top of the Gilbert Chron.

With only two holes drilled, it was impossible to construct a complete spliced record for Site U1307. However, several long intervals of overlap between holes allowed segments to be correlated between holes (0–56.5, 76.4–104.7, and 104.7–146.2 mcd), which were then appended in the record. The mean sedimentation rate calculated using biostratigraphic and magnetostatigraphic datums is 4.8 cm/k.y. Using only magnetostratigraphic datums, interval sedimentation rates vary between 2.7 and 7.6 cm/k.y.

As for the other Eirik Drift sites, pore water geochemical profiles reflect the influence of organic matter remineralization reactions (see Ennyu and Malone). Pore water sulfate decreases linearly from seawater values to ~1 mM at 79 mbsf. The methane profile is atypical, decreasing from 200 ppmv near the sediment/water interface to a low of ~30 ppmv at 54 mbsf and increasing again below the sulfate reduction zone to a high of 26,000 ppmv. Calcium and strontium attain minimum values (5.5 mM and 76 µM, respectively) at the base of the sulfate reduction zone where alkalinity reaches a maximum (10 mM), suggesting carbonate mineral precipitation.

Physical property records at Site U1307 document high-frequency changes in sediment composition. The variability in sediment composition recorded in magnetic susceptibility, NGR, and density likely reflect changes in paleoceanographic conditions in the overlying and surrounding water masses and ice sheets at a range of timescales. Site U1307 sediments are also characterized by an overall downcore increase in density (from 1.55 to ~1.76 g/cm³) and variable but generally decreasing porosity (~70% to 40%).

Site U1307 demonstrates that the Pliocene sediments of the Eirik Drift are located at penetration depths achievable with the APC. Apart from one possible hiatus (at 1.2–1.4 Ma) the sedimentary record at Site U1307 is apparently continuous with a mean sedimentation rate of ~5 cm/k.y. The base of the recovered section correlates to the uppermost Gilbert Chron, indicating that the record extends to ~3.6 Ma. The sediments from Site U1307 will provide information on the history of bottom and surface currents and of the Laurentide and Greenland ice sheets. Age control for seismic reflectors, provided by Site U1307 stratigraphy, will provide important constraints on the sedimentary architecture of the Eirik Drift.

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