Century-Scale Variation of Seafloor Temperatures Inferred from Offshore Borehole Geothermal Data
Seiichi Nagihara, Kelin Wang, 2001. "Century-Scale Variation of Seafloor Temperatures Inferred from Offshore Borehole Geothermal Data", Geological Perspectives of Global Climate Change, Lee C. Gerhard, William E. Harrison, Bernold M. Hanson
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A large amount of hydrographic data obtained in the last three to four decades indicates that temperatures in the deep ocean have been changing globally However, because of the scarcity of older data, it is difficult to trace the ocean thermal history farther back in time. In this study, we examine the possibility of using subseafloor borehole temperature data to estimate the history of the bottom-water temperature (BWT) in the last two to three centuries. The thermal signal associated with BWT fluctuation slowly propagates into the subseafloor rock formation, perturbing the otherwise steady-state temperature field. It is possible to extract this signal and reconstruct the BWT history by inverting the borehole temperature measurements. We make such an attempt using data obtained from a borehole drilled on 669-m-deep seafloor at Ocean Drilling Program Site 1006 in the Straits of Florida. The observed temperature-depth profile in the depth range of 26 through 349 m below seafloor shows significant curvature in the upper 100 m. The BWT history reconstructed from this profile indicates that the long-term average BWT in the early eighteenth century was about 1°C lower than the present value. It decreased to a minimum at about the turn of the century, and then gradually increased to the present value. The pattern of the inferred BWT variation is similar to that of the surface air temperature at Key West, Florida, and the global surface air temperature average.