Physical oceanography during METEOR cruise M60/3 at the Logatchev hydrothermal field in the mid-Atlantic

The R/V METEOR cruise M60/3 took place from January 13 through February 14, 2004. Target area was the Logatchev hydrothermal field situated on the Mid-Atlantic Ridge (MAR) with main spots around 14°45'N and 44°59'W and 14°55'N and 44°55'W. The active Logatchev hydrothermal field lies on a small plateau on the eastern flank of the inner rift valley in 2900 m to 3060 m water depth. It is characterized by sites of active, high-T fluid emanation and sulfide precipitation as well as by inactive sites. CTD data for 17 stations located in the vicinity of the Logatchev hydrothermal field were recorded using a SEABIRD CTD Type 911, mostly for the entire water column. CTD sensors had been calibrated by SEABIRD directly before the cruise; additional calibrations of the data obtained, e.g. by salinometer measurements of selected samples were not accomplished. For most stations, no indication of hydrothermal plumes could be identified within the CTD-profiles. An exception is station M60/3-37-CTD-R for which the S/T plot evidences the intrusion of a component relatively depleted in salinity for the depth area from 2600m to 2700m water depth.

Hydrochemistry measured on water bottle samples during METEOR cruise M60/5

We compare alkalinity and total dissolved inorganic carbon (DIC) measurements made during the Transient Tracers in the Ocean, North Atlantic Study (TTO-NAS) in 1981 with modern measurements from a TTO reoccupation cruise in 2004 (M60/5). We find that the TTO-NAS alkalinity values are 3.6 ± 2.3 µmol/kg higher than modern alkalinity data tied to Certified Reference Materials. The TTO-NAS DIC values re-calculated from original alkalinity and discrete-pCO2 data using currently accepted constants are 3.8 µmol/kg higher than those reported in the revised TTO data set. This difference is reduced to 0.7 µmol/kg when our suggested correction to the TTO-NAS alkalinity is applied. These re-calculated DIC values are 2.4 µmol/kg too low relative to contemporaneous measurements made by the vacuum extraction/manometric Certified method. Application of this correction brings the TTO data into almost perfect agreement with modern measurements for slowly-ventilated deep water of the eastern Atlantic.