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Nature © Macmillan Publishers Ltd 1998 8 letters to nature NATURE | VOL 392 | 2 APRIL 1998 485 Shallow continental shelf and slope waters may also act as lowsalinity conduits of younger terrestrial organic matter (J.E.B., unpublished data, and ref. 18), where margins are affected significantly by rivers and estuaries. However, most of this material must also be degraded in nearshore waters or sequestered in sediments as it does not appear to comprise a significant component of open ocean DOC 29 and POC susp seaward of the shelf-slope front. The isotope signatures of DOC and POC susp at the coastal-open ocean boundaries (that is, slope and rise waters) here indicate that this carbon has mainly a non-recent marine origin and is older than organic carbon from the North Atlantic and Pacific central gyres. If this material propagates seaward, possibly along isopycnal surfaces, it may represent a source of old DOC and POC to intermediate and deep waters of the interior ocean 4 .Ⅺ

Post-stack reprocessing of the PROBE seismic data set has resulted in a refined tec-

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Hole 857C, drilled in Middle Valley, northern Juan de Fuca Ridge, during Leg 139 of the Ocean Drilling Program, penetrated about 470 m of turbidite sediments. Below the sediments the hole intersected a series of basaltic sills interbedded with sediments. Hole 857C is located south of an active hydrothermal vent field in an area where seafloor heat flow measurements show values exceeding 0.8 W/m 2 . This hole was successfully logged with the seismic and lithodensity tool string. Open-hole temperatures at a depth of 480 mbsf are as high as 222°C. The porosity profile of Hole 857C, based on log-derived wet-bulk densities, forms the basis for the calculation of a thermal conductivity profile using a binary mixture of sediment grains and seawater and a geometric mean model. The thermal conductivity of the grains is assumed to be 2.6 W/m-K (Davis and Seeman, this volume). High temperatures encountered in Hole 857C require that temperature effects on the thermal conductivity must be included. The variation of the thermal conductivity of seawater is well known. The decrease of the thermal conductivity of the grains with increasing temperature is calculated after Sass et al. (1992) and Chapman et al. (1984). A purely conductive vertical heat flow is assumed to allow the iterative modeling procedure caused by the nonlinear nature of the problem. The model calculations show that the results of a simple model with constant thermal parameters are similar to those of a model that includes a temperature-dependent seawater thermal conductivity and temperature coefficient of the grain thermal conductivity after Sass et al. (1992). This surprising result is a consequence of the competing effects of decreasing porosity with depth, decreasing thermal conductivity of the grains with depth (and temperature), and the variation of the thermal conductivity of seawater with temperature. Results based on temperature corrections of Chapman et al. (1984) give unrealistically high temperatures at the base of the sediments. Varying the grain thermal conductivity between 2.6 and 3.2 W/m-K results in a range of thermal conductivity profiles that predict temperatures that are in agreement with observed vent fluid temperatures.