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HYDROTHERMAL vents along mid-ocean ridges are an important source of elements such as lithium, silicon, manganese and iron to the world's oceans1. The venting produces both episodic and steady-state hydrothermal plumes with unique thermochemical signatures in the mid-water column. The particulate phases in these plumes (predominantly iron oxides and hydroxides) also scavenge phosphorus, vanadium, arsenic, lead, polonium and several rare-earth elements from sea water2–5. Thus, on a global scale, hydrothermal plumes are both a source for some elements and a sink for others. Ultimately, the particulate metals precipitated from plumes form extensive regions of metalliferous sediments over the crests and flanks of mid-ocean ridges6, 7. Although the metalliferous sediment coverage is vast and well documented, only a tiny fraction of the vents responsible for these sediments have been located (Fig. la). To date, both the number and location of hydrothermal vents and the detailed distribution of chemical constituents within the resultant plumes are poorly understood because of under-sampling of the mid-ocean ridges and the overlying waters. Here we present the results of high-resolution mapping of the chemical and thermal characteristics of hydrothermal plumes in near real time using a novel submersible chemical analyser (Scanner)8, 9 and a conductivity/temperature/depth/trans-missometer instrument package (CTDT)10. We show that the kinetics of iron oxidation in the plume can be used to constrain estimates of the plume's age, and that variation in the ratio of manganese content to excess heat can be explained by the mixing of several different vent fluids.

SINCE the discovery of high-temperature venting on the East Pacific Rise in 19791, it has been expected2,3, because of the physical properties of sea water at pressures and temperatures encountered during submarine hydrothermal circulation4,5, that phase-separated fluids would discharge from ridgecrest vents. Although this notion is supported by the reported large deviations in vent-fluid chlorinity relative to that of sea water (–40%−+200%)6,7, by observations of venting at P–T conditions clearly within the two-phase region (220 bar and 420°C)8 and by fluid-inclusion data9,10, unequivocal identification11 of phase-separated venting fluids has remained elusive. Here we report observations of chloride- and metal-depleted, gas-enriched fluids from a shallow vent field on the Juan de Fuca Ridge which confirm the expectation that phase-separated effluents are delivered to the deep ocean from some sea-floor venting systems.

The first vent fluid samples recovered from submarine hydrother-mal systems on the Galapagos Rift1 and at 21° N on the East Pacific Rise (EPR)2 had a nearly identical ratio of 3He/heat of ~0.5 x l0−12cm3STPcar−1, even though the two hydrothermal systems were separated geographically and had widely differing fluid exit temperatures (~20 and ~350 °C, respectively)3–5. Jenkins et al.3 combined this ratio with independent estimates of the flux of mantle 3He through the oceans6, to calculate a global oceanic hydrothermal heat flux of 4.9 x 1019cal yr−1, which is in excellent agreement with geophysical estimates for this flux7,8. Other investigators then combined this 3He flux with measured ratios of various chemicals in vent fluids to 3He (such as Mn/3He and Si/3He) to estimate global hydrothermal fluxes for these species1,9,10. Here we show that 3He/heat ratios vary by over an order of magnitude between submarine hydrothermal systems, suggesting that early measurements of the 3He/heat relation are not representative of all hydrothermal systems, and that flux calculations based on the oceanic 3He flux must be undertaken with caution.

Hydrothermal particles originate in buoyant plumes emanating from seafloor thermal vents1–4 and accumulate as metalliferous sediments found along oceanic spreading centres5–7. Observational evidence of the transport pathway of hydrothermal particles is scarce, however, and the structure, extent and particle concentrations of hydrothermal plumes remain largely conjectural. To evaluate the potential of hydrothermal plumes as particle transport agents, we mapped their distribution along and across the 80–100-m-deep axial valley of the southern Juan de Fuca Ridge in the north-east Pacific Ocean (∼44°30′ N, 130° W), an area of known hydrothermal activity8–11. Continuous light-scattering measurements, reported here, defined numerous particle plumes centred 30–120 m above bottom (m.a.b.); elemental analysis of the plume particles and the theta (potential temperature)–S (salinity) signatures of the plume water verified their hydrothermal origin. Concentrations of hydrothermal particles remained elevated at ridge-crest depths at least 100 km from the ridge, indicating distant off-axis dispersal of hydrothermal emissions.

The study of hydrothermal vents at oceanic spreading centres has led to a re-evaluation of some basic tenets concerning energy flux through oceanic ecosystems and the sources and sinks for dissolved ions in sea water. Hydrothermal vents vary considerably, from relatively low-temperature (

The horizontally advected plumes which originate from buoyant hydrothermal fluid discharges and subsequently mix with entrained ambient sea water have been detected hundreds of kilometres from their vent sources by distinctive chemical hydrothermal tracers such as 3He, Mn and Fe (refs 1–6). Non-conservative plume tracers, such as Mn and Fe, undergo dynamic oxidation–precipitation reactions as the plume is advected away from the vent sources1,4,7,8. Although some of these transformations may be mediated by microbial activity9,10, hydrothermal plumes have largely escaped microbiological examination beyond tens of metres from their vent origins11,12. Microbe–metal interactions in a hydrothermal plume were specifically addressed during our recent expedition (Vent–Plume '85) to the southern Juan de Fuca Ridge (SJFR). The early results, reported here, provide strong evidence of major microbiological influence over Mn scavenging and particulate-Mn distributions.