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doi:10.2204/iodp.proc.336.107.2012

Results

A set of geophysical survey data was obtained during 6 working days containing 14 lines of multibeam data in combination with single-channel seismics and sediment echo-sounding, 5 heat flow surveys with 61 successful measurements, and physical properties measurements on cores such as thermal conductivity, electrical resistivity, and porosity.

Bathymetry

The EM120 was turned on during all our operations. Therefore, we were able to generate an improved bathymetric chart of North Pond compared to the existing one from 1989 (Atlantis II 123-2 cruise). The resulting map is shown in Figure F2, whereas Figure F5 shows a map which also contains seismic and Parasound lines together with the associated shotpoints.

The bathymetry shows North Pond as an ellipsoidal feature with a main direction of southwest–northeast. The water depth is, from the flanks to the innermost sections, between 4000 and 4500 m. A ridge is located in the northwestern part of the pond, which extends into the pond; ODP Site 1074 is located at its southernmost end. A separate small basin within the pond containing two mounds is located in the northeast part, and another mound is in the east of the pond. The surrounding steep flanks probably supply a large amount of the deposited sediment, which was also confirmed by the existence of turbidity flow structures in sediment cores from Site 395 (Melson, Rabinowitz, et al., 1979; Hyndman, Salisbury, et al., 1984; Beckert, 2009).

Seismic and sediment echo-sounding survey

The profiles running from southwest to northeast (Profiles 1–11) range between 18.2 and 19.6 km in length (except Parasound Profile 3, which is 10.3 km long). Perpendicular to these, Profiles 12–14 are shorter, with lengths between 11.3 and 11.9 km. An overview of the location of the seismic profiles is shown in Figure F5. All seismic and Parasound profiles, including navigation track charts, can be found in the “Appendix.”

The sediment echo-sounding (Parasound) system was able to penetrate the seafloor as deep as 80 m in the central basin (e.g., Fig. F6), whereas it was difficult to image the sediments close to the flanks of the pond because of strong diffractions and side echoes. These diffractions and echoes also made it impossible to locate the exact boundary between bare rock and sediments (Fig. F6) at the edges of the basin. Data within the basin show well-layered sediment packages, resolving layers with a thickness of several decimeters.

In general, the seismic measurements image the sediment/​basement boundary quite well; however, the clear image is masked in some locations and especially at the boundary of the pond by side echoes and hyperbolas because of rough basement topography. An example is given with Profile 10 in Figure F7. In the northeast, between Shotpoints 7614 and 7514 (Fig. F7), the basement seems to outcrop. This is in good agreement with the bathymetric map showing a shallower section and two mounds lying in the northeastern part of North Pond.

Heat flow

Overall, 61 heat flow measurements on 5 profiles were obtained during the cruise (Fig. F5). Two of the profiles (HF-1 and HF-5) run from southwest to northeast; the rest were obtained from southeast to northwest. All of them are located on multibeam and seismic profiles.

An example of a successful heat flow measurement is shown in Figure F8. Figure F8A shows the temperature increase with depth at a calculated temperature gradient of 16.5 K/km. Thermal conductivity increases slightly with depth with small-scale fluctuations around the mean value of 0.99 W/(m·K) (Fig. F8B; red dashed line). These can either be due to the accuracy of the needle probe or caused by turbidity layers originating from the flanks of the pond. As we do not have enough cores in the vicinity, these assumptions cannot be proven. Figure F8C shows the Bullard depth (Bullard, 1954), which is defined as the thermal resistivity versus temperature.

All heat flow values are summarized in Tables T1, T2, T3, T4, and T5. An overview of heat flow values at North Pond is shown in Figure F9, together with the results of Langseth et al. (1992). The measurements from Cruise MSM 11/1 confirm the results obtained two decades ago.

The heat flow survey was supposed to complete the coverage of North Pond, but due to the reduced number of working days during the cruise, measurements are still lacking in the southwest of North Pond.

Coring results and physical properties

During Cruise MSM11/1, 14 gravity cores (3–12 m long) were obtained from the sediments (GeoB13501–GeoB13514). Location and core recovery are shown in Table T6.

In situ thermal conductivity agrees very well with measurements on cores but also with the data published by Langseth et al. (1992) (Fig. F10). All other results of physical properties measurements can be found in the Pangaea database (www.pangaea.de).

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