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Site U13151

Expedition 306 Scientists2

Background and objectives

Ocean Drilling Program (ODP) Site 642 (Integrated Ocean Drilling Program Site U1315), located on the Vøring Plateau in a water depth of ~1280 m, was visited during ODP Leg 104 (Fig. F1). The main objectives of Leg 104 were to investigate the paleoceanographic and tectonic history of the Norwegian-Greenland Sea (Eldholm, Thiede, Taylor, et al., 1987). A total of five holes were drilled at Site 642. In Hole 642E, a 1229 m deep sequence was drilled that is composed of upper Eocene–Quaternary biogenic and terrigenous sediments with volcaniclastics in the lower part (Units I–IV; 0–315 meters below seafloor [mbsf]) and Eocene tholeiitic (upper series) and andesitic (lower series) basalt flows with interbedded volcaniclastic sediments (from 315 to ~1229 mbsf) (Fig. F2). The location of Site 642 was revisited during Expedition 306 to install a borehole observatory in a new hole close to this site (Hole U1315A) to record the bottom water temperature (BWT) variability over the last ~100 y.

The northern North Atlantic is the primary deep ventilator of the oceans, and it is now recognized that production of deep water in the northern North Atlantic is intimately related to global climate (Broecker, 1987; Dickson, 1997; Woods et al., 1999). Changes in the production of North Atlantic Deep Water (NADW) may be the result of, or lead to, regional or global climatic changes. Unfortunately, there is a lack of long-term observations and those that do extend back in time are concentrated at or near the surface. Oceanographic observations indicate that the thermohaline structure of the North Atlantic has changed over the past 20–30 y suggesting the presence of significant variations in BWT (Roemmich and Wunsch, 1984; Antonov, 1993).

It is hypothesized that subbottom temperature-depth profiles can be used to construct BWT histories at timescales on the order of decades to a century. The conductive thermal regime of oceanic crust comprises the superposition of two processes: the outward flow of heat from the Earth's deep interior and perturbations to the deep regime by changes of BWT at the seafloor. The latter effects operate on a relatively short timescale (decades, centuries, and millennia), whereas the former process operates on a geologic timescale, with secular changes taking place over millions of years. In the context of the short-term BWT perturbations, the outward flow of heat from the interior is seen as a quasi-steady-state process. Because oceanic sediments have a low thermal diffusivity, changes in BWT diffuse slowly downward by conduction, perturbing the background thermal regime. These measurable anomalies are a direct thermophysical consequence of BWT variations and as such are a straightforward measure of temperature, not a proxy. Resolution analysis indicates that 100 y of temperature change is potentially recoverable from high-precision temperature-depth logs in boreholes 200 m deep.

To capture thermal transients associated with temporal variations in BWT, we established a borehole observatory in a new 170 m deep hole (Hole U1315A) close to Site 642, consisting of a circulation obviation retrofit kit (CORK) to seal the borehole from the overlying ocean and a thermistor string and data logger to make and record the temperature measurements. This configuration allows high-precision temperature measurements as a function of both depth and time.

Site 642 represents an ideal candidate to test this hypothesis for two reasons:

  1. It is located near Ocean Weather Ship Station (OWS) Mike (Fig. F3) which has been in continuous operation over the last 50 y. Weekly temperature and salinity measurements at >2000 m depths have been made since 1948 (Gammelsrød et al., 1992). These measurements represent the longest homogeneous time series from the deep ocean. They will be used to check the efficacy of our measurements and analysis as well as to provide a direct test of our hypothesis.

  2. It is located on the eastern margin of the Norwegian Sea (Fig. F3), a climatically sensitive area that records the changing hydrographic character and horizontal exchange of deep water from the Greenland Sea, Arctic Ocean, and Norwegian Sea. As such, BWT histories will yield insight into the complex interplay between these important water masses.

The operational plan for the new hole near Site 642 precluded a logging program in that hole. To assess current background thermal conditions in the region, however, a downhole logging of temperature was carried out in Hole 642E using the Lamont-Doherty Earth Observatory (LDEO) high-temperature tool. In addition to the temperature tool, the triple combination (triple combo) and Formation MicroScanner (FMS) logging tool strings were run.

1 Expedition 306 Scientists, 2006. Site U1315. In Channell, J.E.T., Kanamatsu, T., Sato, T., Stein, R., Alvarez Zarikian, C.A., Malone, M.J., and the Expedition 303/306 Scientists. Proc. IODP, 303/306: College Station TX (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.303306.114.2006

2Expedition 306 Scientists’ addresses.

Publication: 9 September 2006
MS 306-114