. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography 20 CO > -*— o -Q o < E fl) 1- o c o o ^E4 2 3 E 03 Q 0 3 4 PROFILE NUMBER 6 7 8 9 10 246 246 246 246 249 248 2 56 2 56 2 60 262 " , " r-J r 243 243 243 244 246 11 12 13 14. 246 252 2 51 2 57 2 60 2 47 POTENTIAL TEMPERATURE (°C) 2 61 248 2 51 247 247 248 251 Pig. 2. A plot of the potential temperature vs. height above the lowest thermistor based on the data in Table 1 from Rona et al. [10] The hydrothermal manganese oxide occurs both
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography 20 CO > -*— o -Q o < E fl) 1- o c o o ^E4 2 3 E 03 Q 0 3 4 PROFILE NUMBER 6 7 8 9 10 246 246 246 246 249 248 2 56 2 56 2 60 262 " , " r-J r 243 243 243 244 246 11 12 13 14. 246 252 2 51 2 57 2 60 2 47 POTENTIAL TEMPERATURE (°C) 2 61 248 2 51 247 247 248 251 Pig. 2. A plot of the potential temperature vs. height above the lowest thermistor based on the data in Table 1 from Rona et al. [10] The hydrothermal manganese oxide occurs both as crusts on basalt talus and as veins filling fractures in the talus along the inner margins of steps on the southeast wall of the rift valley. Bottom photographs [15] and narrow-beam bathymetry [16] reveal that the steps range from tens to hundreds of meters in width, are tens of meters in height, and kilometers to tens of kilometers in length. The steps are interpreted as fault scarps. The manganese oxide is hypothesized to have been deposited by a sub-seafloor hydrothermal convection system involving the circulation of seawater through basalt [16], driven by intrusive heat sources beneath the rift valley. The discharge is thought to be focused by fractures in the rift valley wall which are overlain by a porous and permeable body of talus that may act to diffuse the fracture focused flow [16]. An abrupt temperature anomaly was measured in the water column over one of the steps on the southeast wall of the rift valley within the area of hydrothermal deposits, suggesting persistence of hydrothermal ac- tivity [9,10]. The temperature anomaly of +°C associated with an inverse gradient .of X 10"2 °C/m was measured within 20 m of the bottom, along a hor- izontal distance of 250 m, between water depths of 3000 and 2950 m using three thermistors mounted in a 4 m long towed vertical array (see Table 1 and Fig. 2). A second temperature profile made 5 km away on the s
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