U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
After 25 years of seafloor exploration, hydrothermal venting is known to discharge along divergent plate boundaries in every ocean, at all spreading rates, and in a diversity of geological settings. Ever since the discovery of submarine hydrothermal venting, and indeed even before that, prescient researchers have speculated on the global distribution of vent sites and the geologic conditions that control it. Eight years prior to the historic Galápagos discoveries [Corliss et al., 1979], Boström et al.  noted that iron- and manganese-rich sediments were preferentially found near "active" oceanic ridges. Moreover, both the extent and Fe+Mn content of these sediments increased with increasing spreading rate, suggesting that volcanism and "mantle outgassing" followed a similar pattern. This observation was the germ of a simple but challenging hypothesis: Hydrothermal activity increases linearly with the magmatic budget. (See Haymon  for a succinct history of the development of this hypothesis.) The "magmatic budget" hypothesis was perhaps first articulated by Francheteau and Ballard , who envisioned a simple geometry of increased magma supply and hydrothermal activity at segment centers bounded by transform faults.
Testing of this idea was limited for almost a decade by the lack of detailed segment-scale surveys of hydrothermal activity. Crane et al.  attempted the first multisegment plume survey, identifying several broad regions of hydrothermally warmed water along the Juan de Fuca Ridge (JDFR). Quantitative support of the link between magma supply and venting was first supplied by Haymon et al. , who visually surveyed most of a 2nd-order segment on the East Pacific Rise (EPR), and by Baker and Hammond , who used continuous plume mapping to ascertain the distribution of vent sites over an entire 1st-order segment, the JDFR. Accumulation of plume surveys, mostly on fast-spreading ridges, led to the proposal that, on a relative scale, hydrothermal activity increased linearly with spreading rate [Baker et al., 1995, 1996]. German and Parson  explored the extension of this relation to the slow-spreading Mid-Atlantic Ridge (MAR). They concluded that while the MAR results generally supported the model, the interplay of magmatic and tectonic processes could create dramatic departures, both positive and negative, from the over-arching linear trend.
In this paper, which includes a wealth of new hydrothermal data gathered since our previous review a decade ago [Baker et al., 1995], we present a comprehensive summary of the distribution of confirmed (from seafloor observations) or inferred (from water column measurements) active hydrothermal sites. We compare distributions on "fast" (rift valley absent) and "slow" (rift valley present) ridges, including a consideration of ultraslow and hotspot-affected ridges, to test the "magmatic budget hypothesis" described above.
While testing of this hypothesis seems straightforward, in fact several factors complicate the task. The most obvious difficulty is that quantitative knowledge of hydrothermal heat or chemical fluxes is rare and imprecise. Thus, we must use qualitative indices for hydrothermal activity, such as plume incidence ph (the fraction of ridge crest length overlain by a significant hydrothermal plume [Baker and Hammond, 1992]), or vent site frequency Fs (sites/100 km of ridge length). Large-scale distributions of plumes are most easily mapped using optical sensors, which are simple, inexpensive, and sensitive. The distributions described here are from light backscattering measurements (or light attenuation measurements transformed to light backscattering), given in terms of nephelometric turbidity units (NTUs) [APHA, 1985], determined from a laboratory calibration using formazine [Baker et al., 2001a]. NTU is the plume optical anomaly in excess of the NTU value of local ambient deep waters.
Another hindrance is the differing temporal scales of magma cycling and hydrothermal venting. Ideally, monitoring of hydrothermal activity at sites spanning the full range of the magmatic budget (or its proxy, spreading rate) at oceanic ridges would test this hypothesis effectively. Unfortunately, the temporal scale required to obtain statistically valid data, perhaps roughly the time required to accrete the full breadth of the neovolcanic zone at each site, is impossibly long. A reasonable alternative is to substitute spatial sampling for temporal sampling. In principle, a survey of multiple tectonic segments of the same spreading rate should provide a statistically significant measure of the hydrothermal activity characteristic of that spreading rate or magmatic budget. The actual ridge length necessary for a reliable survey remains to be established but almost certainly increases as spreading rate decreases. In practice, we consider ~200 km to be a minimum survey size for fast ridges, with longer surveys required for slow ridges.
A third complication is that hydrothermal circulation requires the confluence of a sufficient heat source and adequate permeability. While the availability of magmatic heat along the global ridge system is roughly proportional to the spreading rate, changes in the crustal permeability of the neovolcanic zone at either the local or regional scale are presently unconstrained even in relative terms [e.g., Fisher, 1998]. If bulk permeability is a strong function of spreading rate, for example, its effect on the development of hydrothermal circulation could blur the relationship between magma supply and hydrothermal activity [e.g., German and Parson, 1998].
The generation of hydrothermal circulation by heat sources other than magma cooling introduces more uncertainty. On slow- and ultraslow-spreading ridges where the basaltic magma flux is weakest, heat from gabbroic intrusions, from cooling of the lithospheric mantle [Cannat et al., 2004; Bach et al., 2002], and from exothermic serpentinization of ultramafic rocks [Kelley et al., 2001; Schroeder et al., 2002; Lowell and Rona, 2002] may increase the vent field population. Serpentinization, for example, may "contaminate" entire segments of slow-spreading ridge with plumes of dissolved products such as CH [Charlou and Donval, 1993], complicating the identification of magma-driven hydrothermal venting. Vent fields driven solely by serpentinization reactions will likely not be readily detected using our well established, optically based survey methods, however. Such fluid discharge is rich in dissolved gases but metal-poor, so that the precipitation of Fe-Mn oxyhydroxides following release into the water column is minimal [Kelley et al., 2001].
Finally, the hydrothermal data set to be considered must be representative of activity on ridges of all spreading rates. While it may be premature to characterize the available data as truly globally representative, we now have at least preliminary hydrothermal surveys from ridge sections across the entire spectrum of spreading rates.
Return to Abstract or go to next section
PMEL Outstanding Papers
PMEL Publications Search