Christopher R. German

Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean

Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earths crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2u2009cmu2009yr-1—which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6–1.3u2009cmu2009yr-1) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100u2009km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.

Diverse styles of submarine venting on the ultraslow spreading Mid-Cayman Rise

Thirty years after the first discovery of high-temperature submarine venting, the vast majority of the global mid-ocean ridge remains unexplored for hydrothermal activity. Of particular interest are the world’s ultraslow spreading ridges that were the last to be demonstrated to host high-temperature venting but may host systems particularly relevant to prebiotic chemistry and the origins of life. Here we report evidence for previously unknown, diverse, and very deep hydrothermal vents along the ∼110 km long, ultraslow spreading Mid-Cayman Rise (MCR). Our data indicate that the MCR hosts at least three discrete hydrothermal sites, each representing a different type of water-rock interaction, including both mafic and ultramafic systems and, at ∼5,000 m, the deepest known hydrothermal vent. Although submarine hydrothermal circulation, in which seawater percolates through and reacts with host lithologies, occurs on all mid-ocean ridges, the diversity of vent types identified here and their relative geographic isolation make the MCR unique in the oceans. These new sites offer prospects for an expanded range of vent-fluid compositions, varieties of abiotic organic chemical synthesis and extremophile microorganisms, and unparalleled faunal biodiversity—all in close proximity.

Hydrothermal plumes above the East Scotia Ridge: an isolated high-latitude back-arc spreading centre

We have identified first evidence for the presence of submarine hydrothermal activity along the East Scotia Ridge an isolated back-arc spreading centre located at 55–60°S in the Atlantic sector of the Southern Ocean. Using a combination of in situ optical light-scattering sensor data, and total dissolvable Mn concentrations, we demonstrate the existence of hydrothermal plumes overlying two segments of this ∼500 km ridge-crest; both segments exhibit anomalous topography and at least one segment is also underlain by an axial magma chamber seismic reflector. Future investigation of the fauna that inhabit these remote hydrothermal environments may provide an important ‘missing link’ between the distinct biogeographical provinces delimited from previous investigation of northern Atlantic versus eastern Pacific vent-sites.

Sources and fluxes of hydrothermal heat, chemicals and biology within a segment of the Mid-Atlantic Ridge

We have conducted a segment-wide study of the chemical, biological and thermal effects of hydrothermal activity, in a bathymetrically enclosed, 50-km-long segment of the Mid-Atlantic Ridge, at 29°N. Our study involved long-term monitoring of bottom current speeds and temperatures coupled with ‘snap-shot surveys of the concentrations of hydrothermal shrimp, manganese and particulate plumes occupying the water column enclosed by the axial valley. These data are combined with the calculated volume transport rates for the segment and the results compared with the heat and manganese flux from the Broken Spur vent field, located in the center of the segment. Our current meter data show that the basin-like bathymetry acts to restrict and control flow of water. Warm and shallow water, exported from the segment, is replaced by colder and deeper water. Water column profiles showed an increase in temperature of the water within the ‘Broken Spur segment, compared with background stations located outside the ridge axis. The average refresh rate for the water column enclosed by the segment was found to be about 262 days. From these data, we calculate that the segment-wide flux of juvenile shrimp is 20×106 individuals per year. We show that the flux of manganese (exported at a rate of 5–6 mol/s) is matched by input from high-temperature effluent discharge at the Broken Spur vent field. We also calculate that the minimum average heat flux exported from the segment is about 275 MW. Of this, more than 90% is pervasively dissipated throughout the segment floor by conductively heated sea water, or simple thermal conduction, that carries little or no manganese to the water column. The remaining 10% or less of the heat flux results from localized, manganese-rich, high-temperature hydrothermal venting.