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doi:10.2204/iodp.proc.303306.104.2006 Site U13041Expedition 303 Scientists2Background and objectivesThe objective at Integrated Ocean Drilling Program Site U1304 was to obtain a high-resolution (high sedimentation rate) Pliocene–Quaternary environmental record from a location within the central Atlantic ice-rafted debris (IRD) belt, at a water depth sufficient to sample North Atlantic Deep Water (NADW). A partially enclosed basin at the southern limit of the Gardar Drift just to the north of the Charlie Gibbs Fracture Zone provided a suitable location with water depth of 3024 m (Fig. F1). This basin lies to the east of the Reykjanes Ridge on oceanic crust associated with magnetic Anomaly 5 (~10 Ma). The site is located 217 km (117 nmi) west-northwest of Deep Sea Drilling Project Site 611 (Fig. F1), drilled in 1983 on the southern rim of the Gardar Drift. The mean sedimentation rate (in the Brunhes Chron) at Site 611 was found to be 2.7 cm/k.y. The sedimentation rates at Site U1304 are greater by a factor of about six, thereby achieving the objective of recovering a high-sedimentation-rate record in deep water at the southern limit of the Gardar Drift. Seismic data for positioning of Site U1304 were collected during the Knorr KN166-14 cruise in summer 2002. The local bathymetry was determined by SeaBeam survey (Fig. F2). Seismic reflection profiles were collected with the Lamont-Doherty Earth Observatory portable high-resolution acquisition system. A crossing ship track (Fig. F3) yielded high-quality multichannel seismic profiles (Fig. F4) that indicated that the site is optimally positioned in a thick (>900 ms two-way traveltime) well-stratified sediment pile. A piston core from this location (Core HU91-045-080P) collected during the 1991 cruise of the Hudson indicated moderate to high sedimentation rates (~10–15 cm/k.y.), good preservation of siliceous and calcareous microfossils, and the attributes for yielding isotopic and paleomagnetic (paleointensity) age control (Fig. F5). This has been confirmed by another piston core (KN166-14-13JPC) collected during the site survey cruise for Expeditions 303 and 306 conducted by the Knorr. Core KN166-14-13JPC, a 23.6 m core, extends into marine isotope Stage (MIS) 6 based on relative paleointensity and susceptibility correlations. Notwithstanding significant top-core stretching in Core KN166-14-13JPC, sedimentation rates appear to be ~10–20 cm/k.y., with elevated sedimentation rates during interglacial stages. Diatom mats are associated with MIS 5 in Core HU91-045-080P and in Core KN166-14-13JPC. Similar diatom mats, dominated by Thalassiothrix longissima, have been observed within MIS 5e in Core EW9303-17 (Bodén and Backman, 1996). Core EW9303-17 is located in 3233 mbsl water depth on the west side of the Reykjanes Ridge at 57°N, 37°W, ~500 km (270 nmi) north-northwest of Site U1304. The presence of diatom mats in the Quaternary of the North Atlantic has not been widely documented. Their occurrence is apparently localized to a narrow subarctic convergence zone between the cold, less saline surface water associated with the Labrador Sea current and the warmer North Atlantic current (see Bodén and Backman, 1996). This narrow convergence zone in the modern North Atlantic meanders northward, crisscrossing the mid-oceanic ridge (Ruddiman and Glover, 1975), and lies close to both Site U1304 and Core EW9303-17. Diatom mat deposition at Site U1304 was found to be episodic and discontinuous but present throughout the recovered sequence that spans the entire Quaternary. Although some thicker meter-scale mats are present, most are less than a few centimeters in thickness and are intercalated with clays and silts. If diatom deposition can be tied to the subarctic convergence zone, the 1.8 m.y. record recovered at Site U1304 will provide an unprecedented environmental record of movements of the convergence zone during the Quaternary. Good preservation of foraminifers, nannofossils, and diatoms and the potential for paleomagnetic and isotopic age control means that the environmental record comprising sea-surface and bottom-water characteristics, and detrital (Heinrich-type) stratigraphy, can be integrated into a paleointensity-assisted stratigraphy (PAC). |
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