MANGANESE DISTRIBUTION IN THE WATER COLUMN NEAR THE AZORES TRIPLE JUNCTION ALONG THE MID-ATLANTIC RIDGE AND IN THE AZORES DOMAIN

As part of a multidisciplinary research effort aimed at quantifying mi d-ocean ridge processes near the Azores, we conducted a survey of the water column above the mid-Atlantic Ridge (MAR) in the vicinity of the Azores Triple Junction. Manganese is a tracer of hydrothermal activit y intimately related to mid-ocean ridge processes. This paper reports on 23 vertical depth profiles that were analyzed for total dissolvable manganese (TDM). TDM inputs attributable to hydrothermal circulation could be observed along the MAR in all of the southern Amar (36 degree s 15'N), Famous (36 degrees 45'N), and Lucky Strike (37 degrees 03'N a nd 37 degrees 17'N) segments and south of the Kurchatov fracture zone (40 degrees 10'N). To date, seafloor observations of hydrothermal acti vity on the seabed have been confirmed at Lucky Strike (37 degrees 17' N) and at the Rainbow site (36 degrees 14'N). Large-scale TDM distribu tion features along the axial valley of the MAR include a decrease in TDM concentrations from south to north (36 degrees N to 38 degrees 30' N), followed by an increase to 40 degrees N. In the basins within the Azores archipelago, we found the lowest TDM background levels of this study (0.4-0.6 nmol l(-1)) and, based on our data no firm evidence for hydrothermal inputs of TDM. In the MAR axial valley, we observe both a more elevated TDM background (0.5-1.0 nmol l(-1)) and evidence for p robable hydrothermal TDM inputs. This suggests that hydrothermal input s contribute to a low-level chronic TDM plume throughout the axial val ley of the MAR between 36 degrees and 40 degrees N. (C) 1998 Elsevier Science Ltd. All rights reserved.