TEMPORAL AND SPATIAL VARIABILITY OF HYDROTHERMAL MANGANESE AND IRON AT CLEFT SEGMENT, JUAN-DE-FUCA RIDGE

A unique data set for hydrothermal Mn and Fe was collected at Cleft se gment on the Juan de Fuca Ridge between 1983 and 1991. The data set in cludes observations of focused and diffuse venting fluids and neutrall y buoyant plumes formed by chronic and episodic venting. Manganese/hea t and iron/heat ratios for plumes from the north end of the Cleft segm ent were combined with independently determined estimates of plume hea t flux to yield annually averaged chronic venting fluxes for Mn of 0.3 6 +/- 0.17 mol s-1 and for Fe of 0.61 +/- 0.34 mol s-1. Over 6 years o f plume measurements at North Cleft segment, observed episodic hydroth ermal discharge accounted for approximately 15% of the total vented Mn and approximately 35% of vented Fe. The chronic fluxes for Mn and Fe at a second venting center located at the south end of the Cleft segme nt were estimated to be approximately equal to the fluxes at North Cle ft segment. Chronic plumes at North Cleft segment are mixtures of focu sed and diffuse discharge that contribute heat, Mn, and Fe in variable proportions. Similar examination of South Cleft segment data strongly suggests the presence of an as yet unobserved venting source relative ly depleted in Mn and Fe but contributing substantially to the overall heat. Temporal and spatial variations in the concentrations of Mn and Fe and in Mn/heat and Fe/heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes. Manganese/heat and iron/heat ratios of megaplumes suggest they may have derived from res ervoirs of diffuse fluids while smaller event plumes may have formed b y different processes and have properties similar to chronic plumes. T he accurate assessment of segment-scale hydrothermal fluxes of Mn and Fe requires coordinated measurements of representative seafloor source s and the neutrally buoyant plume that integrates all seafloor dischar ge.