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 short-term BWT perturbations, 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.
Site 642 (Fig. F3) represents an ideal candidate to test this hypothesis for two reasons. First, it is located near Ocean Weather Ship Station (OWS) Mike, which has been in continuous operation during the last 50 y. Weekly temperature and salinity measurements at depths >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. Second, 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.
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