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Site U1304

The objective at Site U1304 was to obtain a deep water record from the southern edge of the Gardar Drift (Fig. F1; Table T1) to compare with the intermediate depth site on the northern part of the Gardar Drift sampled during ODP Leg 162 at Site 983. Site U1304 is north of the Charlie Gibbs Fracture Zone, 217 km west-northwest of DSDP Site 611. The mean sedimentation rate at Site U1304 (15 cm/k.y.) is about six times that in the same stratigraphic interval at DSDP Site 611.

Four holes were cored with the APC coring system to a maximum depth of 243.8 mbsf at Site U1304. Correlation of cores among holes at Site U1304, utilizing mainly magnetic susceptibility and natural gamma radiation, provides a continuous stratigraphic sequence to ~258 mcd with a single potential break within an 8 m thick diatom mat at ~199 mcd. The spliced composite section relies on sections from Holes U1304A and U1304B because good weather conditions during the early occupation of Site U1304 led to excellent recovery and good core quality.

The sediments at Site U1304 are predominantly interbedded diatom oozes and nannofossil oozes with less common intervals of clay and silty clay that also contain abundant nannofossils and/or diatoms. Calcium carbonate content ranges from 5 to 70 wt% and organic carbon content is low (generally <0.5 wt%). This sedimentary succession has been designated as a single unit because the various lithologies are generally interbedded on a scale of only centimeters to decimeters. Most contacts between nannofossil ooze and clay intervals are gradational, although sharp contacts are also observed. The contacts between diatom ooze beds and the other lithologies are generally sharp. Redeposited beds of silt and sand-sized particles are rare, as are disturbed units related to mass-transport processes (e.g., slumps and debris flows). Thus, the section cored at Site U1304 apparently represents a relatively continuous pelagic section, where the sediments record changes in productivity in response to oceanographic and climatic conditions.

Recurring laminated diatom sequences are the most prominent feature at Site U1304 (Shimada et al., 2008). The thicker diatom mats are clearly distinguished by very low magnetic susceptibility values. Diatom assemblages are dominated by needle-shaped species of the Thalassiothrix/​Lioloma complex. All other fossil groups investigated, coccoliths, planktonic and benthic foraminifers, radiolarians, and palynomorphs, are present in high to moderate abundance and are well preserved (see Liu; Yamasaki et al., 2008). Biostratigraphic datums were mainly derived from coccoliths and are consistent with datums provided by diatoms, planktonic foraminifers, dinoflagellate cysts, and magnetostratigraphy. The composite sequence covers the uppermost Pliocene and the entire Quaternary. The microfossil assemblage indicates only minor redeposition. Excellent preservation of benthic and planktonic microfossils enhances the potential for a high-resolution environmental record. The site monitors North Atlantic Deep Water (NADW) and sea-surface temperatures and may provide a record of central Atlantic detrital-layer stratigraphy. Oxygen isotope stratigraphies based on both planktonic and benthic foraminifers are presently in development for this site. Published stable isotope data at Site U1304 has demonstrated that the site provides a particularly high resolution record of the last interglacial, where sedimentation rates were ~40 cm/k.y., where frequent thin diatom-rich layers inhibit bioturbation (Hodell et al., 2009). Planktonic oxygen isotope data from a number of planktonic foraminifer species are compared with benthic oxygen and carbon isotope data, yielding a unique picture of surface and deep water evolution during marine isotope stage (MIS) 5 (Hodell et al., 2009). The results from this study of MIS 5 at Site U1304 illustrate the potential of the site for detailed studies of Quaternary paleoceanography.

Preliminary paleoceanographic interpretation of the microflora and microfauna reveals large-amplitude changes in surface water temperature and trophic conditions. Diatom layers were formed during both cold and warm phases, according to interpretation of diatom and planktonic foraminifer assemblages (Shimada et al., 2008). The presence of the benthic foraminifer Epistominella exigua documents recurring flux pulses of fresh organic matter to the seafloor. A shift from dominance of autotrophic to heterotrophic dinocyst assemblages is recorded after 1.2 Ma, which may suggest a general change in trophic conditions of the surface ocean.

Site U1304 sediments document a magnetostratigraphic sequence that includes the Brunhes Chron and part of the Matuyama Chron, the Jaramillo and Cobb Mountain Subchrons, and the top of the Olduvai Subchron. Mean sedimentation rates of 17.8 cm/k.y. are estimated for the last 0.78 m.y. and 12.2 cm/k.y. for the interval from 0.78 to 1.77 Ma, with an overall mean sedimentation rate of 14.9 cm/k.y. The shore-based magnetic stratigraphy has resolved two magnetic excursions at this site and has refined the position of the major reversals (C. Xuan and J.E.T. Channell, unpubl. data). The RPI record at this site is in the process of being developed and awaits correlation to the oxygen isotope records.

Site U1304 pore water profiles indicate active sulfate reduction (minimum value of 2.8 mM reached at 214 mbsf) with corresponding increases in alkalinity and ammonium, although alkalinity values do not reach concentrations expected for the degree of sulfate reduction. Calcium concentrations decrease downcore to 2.7 mM, a ~75% reduction from standard seawater values.

The Quaternary sequence recovered at Site U1304 provides a high-resolution, high-sedimentation rate (average ~15 cm/k.y.) record of environmental change at a sensitive location close to the sub-Arctic convergence between the surface Labrador Current and the North Atlantic Current (see Bodén and Backman, 1996). Good preservation of both calcareous and siliceous microfossils, abundant benthic foraminifers, and a high-fidelity magnetostratigraphic record indicate that the environmental record, including monitoring of NADW, can be placed in a tight chronological framework based on oxygen isotope and RPI data.