Sub-bottom temperature anomalies detected by long-term temperature monitoring at the TAG hydrothermal mound

During Aug. 1994 - Feb. 1995, geothermal measurements were carried out on the TAG hydrothermal mound (26 degrees 08'N, 44 degrees 49'W) of Mid-Atlantic Ridge. Two geothermal probes made reliable measurements. Six transient anomalies in subbottom temperatures were detected 20 m south of the central black-smoker complex (CBC), having durations 40 to 400 hours and magnitude 0.2 degrees C to > 30 degrees C. These anomalies are probably evidence for short-period fluctuations in the fluid circulation beneath the mound. The time variability in subbottom temperatures near CBC suggests that instantaneous heat flow measurements may not represent the time average of heat flow there. Although it is difficult to specify the origin of thermal anomalies during the drilling period by the Ocean Drilling Program, at least one of them seems to correlate with the drilling events because it was also accompanied by transient temperature anomalies at CBC and 50 m east of CBC observed by other instruments.