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doi:10.2204/iodp.proc.301.106.2005

Thermal measurements

Determining conductive heat flow through sediments at Site U1301 was an important part of the Expedition 301 plan to assess hydrogeologic conditions in basement. It was shown with data collected during and after Leg 168 that temperatures in upper basement in Holes 1026B and 1027C are identical to within a few degrees. These two holes are separated by 2.2 km, and the top of basement in Hole 1027C is significantly deeper than that in Hole 1026B (see the "Expedition 301 summary" chapter). Upper basement temperatures would be considerably greater in Hole 1027C than in Hole 1026B if conditions were purely conductive. The upper basement temperature in Hole 1026C, located 1 km north of Hole 1026B, is somewhat cooler but still relatively consistent with other basement temperatures in the Second Ridge area. The measurement program in Hole U1301C, located 0.9 km south of Hole 1026B, was intended to determine whether nearly isothermal basement conditions extended this far south.

Temperature measurements

Two attempts to determine in situ temperatures in Hole U1301C were made with the APCT tool, and three attempts were made with the DVTP. Results are summarized in Table T21. The tools were held 10 m above mudline for 10 min during all deployments, to check bottom water temperature and intertool calibration, before measuring sediment temperatures. The first APCT tool measurement during Core 301-U1301C-3H yielded what appears to be a clean record, although close inspection reveals evidence of probe motion during the first 150 s following tool penetration (Fig. F67A). The data are well fit by the standard cooling model, giving an extrapolated equilibrium temperature of 10.4°C (Fig. F67B). The deployment with Core 301-U1301C-5H initially appeared to be less reliable. The record indicates that the tool moved repeatedly during the first 200 s following penetration (Fig. F67A). The fit to the analytical model is only fair, and the extrapolated equilibrium temperature is 11.5°C (Fig. F67B).

APC penetration was incomplete in deeper cores and pull-out tension reached 60,000 lb, so we ended the APCT tool measurement program and switched over to the DVTP. The first DVTP deployment followed collection of Core 301-U1301C-9H. The temperature record indicates that there was considerable tool motion during the first 200 s and additional motion throughout the rest of the record, but the data are still usable (Fig. F68A). Interestingly, fitting of extremely early time and late time data to the analytical model yields essentially the same equilibrium temperature, 21.0°C (Fig. F68B). The DVTP was deployed again following collection of Core 301-U1301C-14H. This core barrel did not stroke out completely, so prior to DVTP deployment the hole was advanced 3.0 m. Unfortunately, the DVTP failed to penetrate the undisturbed formation and the data collected are not usable (Fig. F68A). The DVTP was deployed a final time after collection of Core 301-U1301C-19H. The hole was advanced 1.0 m immediately prior to DVTP deployment to avoid pressing the tool into fill. The temperature record shows motion throughout the first 400 s after penetration, but the data stabilize near a maximum temperature of 58.2°C. Not surprisingly, fitting the late-time data to the equilibration model yields the same temperature.

Data interpretation

A plot of temperature versus depth in Hole U1301C shows that most of the data are consistent with a linear gradient of 0.228°C/m (Fig. F69A). This gradient was estimated from three subseafloor temperature values (Cores 301-U1301C-5H, 9H, and 17H) and a bottom water temperature of 1.88°C. This is the bottom water temperature determined during heat flow surveys prior to Expedition 301, and it is consistent with measurements made just above mudline with the APCT tool. The DVTP gave bottom water temperatures 0.05°C lower, and all DVTP values have been shifted to account for this difference. Uncertainties in estimated equilibrium temperatures are based on the fit of the data to the models used for extrapolation, using a reasonable range of sediment thermal conductivities (Table T21). Thermal conductivity is notoriously variable in turbidites, and it is difficult to determine the true value at the exact location of tool penetration for both the APCT tool and the DVTP. Uncertainties in equilibrium sediment temperatures are smaller than the symbols shown in the thermal gradient plot (Fig. F69A).

The equilibrium temperature from APCT tool deployment in Core 301-U1301C-3H is anomalously warm relative to the gradient suggested by the other data. There is a history of APCT tool equilibrium values being inconsistent with those determined with the DVTP (for example, see fig. 58A of Shipboard Scientific Party, 1997), but it is surprising that the measurement of Core 301-U1301C-5H is more consistent with DVTP data than of Core 301-U1301C-3H because the latter data appear to be less influenced by tool motion (Fig. F67A).

Because core recovery was limited and biased in Hole U1301C, we do not have a representative set of thermal conductivity data for the sediment section. Considerably more data were collected at nearby Sites 1026 and 1027, but even these data were likely biased by incomplete, selective recovery. Davis et al. (1999) adjusted thermal conductivity data collected during Leg 168 based on sediment thickness (determined during drilling) and seafloor heat flow (determined during predrilling site surveys with short probes). They used observed thermal conductivities to derive a thermal resistance (R_z) versus depth (z) function, a third-order polynomial, and then linearly shifted this function to force the equilibrium temperatures determined during drilling to yield the seafloor heat flow. The function derived for Sites 1026 and 1027 is

where R_z is in m²·K/W and depth is in meters. We use this same function for calculation of heat flow in Hole U1301C.

Heat flow in Hole U1301C is 280 mW/m² (Fig. F69B). This value is lower than that determined in Holes 1026A or 1026C, to the north, but is consistent with basement being roughly isothermal and sediment being thicker in Hole U1301C. The extrapolated temperature at the top of basement in Hole U1301C (estimated to be at ~265 mbsf) is 62°C, essentially the same value estimated for Hole 1026C and slightly lower than the value estimated in open Hole 1026B. Thus, the thermal data from Hole U1301C suggest that isothermality of uppermost basement temperatures extends along 2 km of the buried basement ridge, from Hole 1026C in the north to Hole U1301C in the south.

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