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The northern North Atlantic is the primary deep ventilator of the oceans, and it is now recognized that production of deep water in this area is intimately related to the global climate (Broecker et al., 1985; Dickson, 1997; Woods et al., 1999). Changes in the production of 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 the surface or near surface. Hydrographic time series from the North Atlantic, though sparse and sporadic, show natural variability on timescales of decades to centuries (Wunsch, 1992). The few observations that exist for the deep ocean show variability on similar timescales and at large spatial scales. Oceanographic observations indicate that the thermohaline structure of the North Atlantic has changed over the past 2030 y and the presence of significant variations in bottom water temperature (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. If this hypothesis is correct and because ocean bottom sediments continuously record changes in BWT, it is theoretically possible to reconstruct BWT histories anywhere in the ocean.
ODP Site 642 (Fig. F1) represents an ideal candidate to test this hypothesis for two reasons. It is located near Ocean Weather Ship Station (OWS) Mike, which has been in continuous operation over the last 50 y. Weekly temperature and salinity measurements at depths greater than 2000 m 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. Site 642 is located on the eastern margin of the Norwegian Sea (Fig. F1), 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.
To capture thermal transients associated with temporal variations in BWT, we will install a borehole observatory in a new 150 mbsf hole close to Site 642, consisting of a CORK (which will seal the borehole from the overlying ocean), thermistor string, data logger to make and record the temperature measurements, and a seal below the temperature string to ensure against formation fluids entering the borehole interval where the measurements are made. This configuration allows high-precision temperature measurements as a function of both depth and time. High-precision temperature measurements will be made at two timescales: in quick succession and over longer time intervals. Averaging a quick succession of temperature measurements is an effective way to reduce instrumental and environmental noise. Temperature measurements with an appropriate length of time between them can be used to directly monitor the propagation of transient temperatures (Chapman and Harris, 1992).
At 6712.7N, 255.8°E (water depth = 1289 m) near Hole 642E, operations will begin with a jet-in test followed by drilling in to 150 mbsf of 103/4 inch casing and reentry cone. The bottom of the cased hole will be sealed with a bridge plug, and then the CORK and thermistor string will be installed (Table T2).
The current operational plan for the new hole at Site 642 precludes a logging program. However, it is highly desirable to have a downhole record of temperature from Hole 642E to assess current background thermal conditions in the region. A high priority, if time and operations allow, will be to produce a vertical temperature profile, using a tool such as the Davis-Villinger Temperature Probe (DVTP) or the Lamont high-temperature tool.