How many vent fields? New estimates of vent field populations on ocean ridges from precise mapping of hydrothermal discharge locations
Introduction
Hydrothermal circulation is the dominant global agent for the transfer of heat, chemicals, and microbial life from the upper lithosphere to the ocean. The global fluxes of hydrothermal heat, hydrothermal fluids, and some (mostly conservative) hydrothermal chemical species from the neovolcanic zone of ocean spreading ridges (OSRs) are reasonably well estimated (e.g., Elderfield and Schultz, 1996). However, for many biological, chemical, and physical processes at the crust-ocean interface, a key variable in assessing the role of hydrothermal discharge is the number and spacing of distinct discharge sites (“vent fields”) along the 66,000 km of OSRs. The spacing of hydrothermal oases is critical to understanding the dispersal of chemosynthetic fauna (McGillicuddy et al., 2010, Vrijenhoek, 2010, Beaulieu et al., 2015) and recruitment success after ecosystem disruptions such as seabed mining (Hilário et al., 2015) and seafloor eruptions. Interest in the nature and number of low-temperature (<∼50 °C), diffuse venting sites, especially those isolated from sites of higher-temperature discrete venting, is increasing because Fe may be preferentially supplied to the ocean interior by diffuse discharge (German et al., 2015, Larson et al., 2015). Furthermore, the location of seafloor discharge provides our only direct information on the distribution of hydrothermal upflow sites, and thus on the pattern of hydrothermal circulation in the shallow crust (Hasenclever et al., 2014).
Thirty-five years of exploration has yielded an inventory of >500 active discharge sites on OSRs (Beaulieu et al., 2013, Beaulieu et al., 2015). These and other inventories rely on published descriptions of discharge sites, which commonly lump closely spaced sites (e.g., within 10 km; Hannington et al., 2011) into a single named site. Beaulieu et al. (2015) used these data to predict a total of sites on OSRs. This prediction implies a mean site spacing ranging from 25 km at ultrafast spreading rates (150 mm/yr) to 90 km at ultraslow rates (10 mm/yr). Similarly, Hannington et al. (2011) calculated that the spacing between 70 massive sulfide deposits on OSRs ranges from 10–330 km over the entire spreading rate range, roughly increasing with decreasing spreading rate.
However, direct evidence from crustal measurements and seafloor observations challenges the perception that vent fields are widely separated. Borehole measurements imply that the uppermost layer of very young crust is fractured and open, with bulk permeability as high as (Fisher and Becker, 2000). This permeability is seemingly inconsistent with spacings of tens of kilometers between discharge sites, especially along ridge sections underlain almost continuously with an axial melt lens. Visual surveys along ridge crests at length scales of 10–100 km are rare, but where available show that discharge sites are far more common than implied by the global statistics (Haymon et al., 1991, Auzende et al., 1996, O'Neill, 1998, Haymon and White, 2004).
We propose that the disagreement between these two views of hydrothermal site spacing is a product of sampling strategy and sensor capability. Mapping of discharge sites at the globally relevant 100–1000 km scale is based almost exclusively on the detection of dispersing, non-buoyant, hydrothermal discharge (e.g., German and Parson, 1998, Baker and German, 2004). Observations are occasionally continuous over several ridge segments (German and Parson, 1998, Baker et al., 2006) but more generally use discrete sampling at intervals of several to tens of kilometers (Son et al., 2014). In either case, mapping of discharge plumes predominantly relies on the detection of broadly dispersing, conservative (e.g., 3He, temperature) or quasi-conservative (e.g., optical, particulate Fe, dissolved Mn, CH4) tracers that can be detected many kilometers from their source. These tracers are highly sensitive to particle-rich, “black smoker” discharge, generally at temperatures >∼100 °C, but their broad dispersion makes it difficult to distinguish closely spaced sites with commingling plumes. Detecting isolated discharge with a negligible particle or dissolved metal signature (e.g., Lost City; Larson et al., 2015), or with disorganized flow that inhibits plume rise, is more problematical. Discrete sampling is apt to miss small sites, and temperature anomalies from such sites are commonly too weak to be reliably detected during a tow. In this paper, we use continuous tows of a sensor package sensitive to hydrothermal tracers both persistent and ephemeral, from sources both particle rich and particle poor, to show that the present vent field inventory along fast- to intermediate-rate spreading ridges may underestimate the true value by at least a factor of 3–6.
Section snippets
Sensor characteristics
Our surveys employed sensors that respond to both broadly dispersing (light backscattering, measured as Nephelometric Turbidity Units (NTU)) and ephemeral (oxidation–reduction potential (ORP)) tracers. NTU detects particle-rich discharge, generally at temperatures >∼100 °C, with copious “black smoker” minerals that create plumes extending tens of kilometers from their source. ORP detects hydrothermal discharge of all temperatures, including low-temperature diffuse venting as well as
Results
The combined distribution of ORP and NTU anomalies on each ridge section quantitatively locates sites that can then be compared to the distribution of vent sites listed in the authoritative InterRidge database. Because anomalies with a spatial scale as small as ∼1 km cannot be precisely illustrated on figures spanning hundreds of kilometers of ridge length, the location of every determined discharge site is tabulated in supplementary material Table S1, noting anomaly type, starting and ending
Discussion
To compare our results with published global vent distributions, we use values from 27 ridge sections (Beaulieu et al., 2015) totaling , or ∼20% of the global ridge system (Fig. 9). Vent field spacing is highly variable for slow- and intermediate-rate spreading ridges, decreasing to ∼20 km for fast-spreading ridges. For spreading rates >55 mm/yr (14 ridge sections), the reported spacings range from 12–220 km, far wider than our range of 3.3–19 km for the same rate interval. Improved
Conclusions
Decades of surveying along ocean spreading ridges, largely limited to discontinuous measurements of conservative and quasi-conservative tracers, has fostered a “common knowledge” that under-represents the true prevalence of hydrothermal discharge sites. Existing data lead to the conclusion that the mean spacing of active discharge sites on 14 sections of fast- and intermediate-rate spreading ridges ranges from 12–220 km. Our surveys tested this conclusion by towing optical and
Acknowledgments
We thank A. Fisher for comments on the manuscript. Three anonymous reviewers likewise provided valuable improvements. This research was supported by the NOAA/PMEL Earth-Ocean Interactions Program and the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement No. NA10OAR4320148. PMEL contribution 4379, JISAO contribution 2464.
References (61)
- et al.
Hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N: plume distribution and relationship to the apparent magmatic budget
Earth Planet. Sci. Lett.
(1994) - et al.
Where are the undiscovered vents on oceanic spreading ridges?
Deep-Sea Res., Part II
(2015) - et al.
Mining of deep-sea seafloor massive sulfides: a review of the deposits, their benthic communities, impacts from mining, regulatory frameworks and management strategies
Ocean Coast. Manag.
(2013) - et al.
Crustal construction and magma chamber properties along the Eastern Lau Spreading Center
Earth Planet. Sci. Lett.
(2013) - et al.
Distributions of hydrothermal activity along the Mid-Atlantic Ridge: interplay of magmatic and tectonic controls
Earth Planet. Sci. Lett.
(1998) - et al.
Hydrothermal exploration with the autonomous benthic explorer
Deep-Sea Res., Part I
(2008) - et al.
Hydrothermal Fe cycling and deep ocean organic carbon scavenging: model-based evidence for significant POC supply to seafloor sediments
Earth Planet. Sci. Lett.
(2015) - et al.
Hydrothermal exploration of mid-ocean ridges: where might the largest sulfide deposits be forming?
Chem. Geol.
(2016) - et al.
Diking, young volcanism and diffuse hydrothermal activity on the southern Mid-Atlantic Ridge: the Lilliput field at
Mar. Geol.
(2009) - et al.
Fine-scale segmentation of volcanic/hydrothermal systems along fast-spreading ridge crests
Earth Planet. Sci. Lett.
(2004)
Hydrothermal vent distribution along the East Pacific Rise crest (9°09′–9°54′N) and its relationship to magmatic and tectonic processes on fast-spreading mid-ocean ridges
Earth Planet. Sci. Lett.
Axial crustal structure of the Lau back-arc basin from velocity modeling of multichannel seismic data
Earth Planet. Sci. Lett.
Fluxes of fluid and heat from the oceanic crustal reservoir
Earth Planet. Sci. Lett.
Dispersal connectivity and reserve selection for marine conservation
Ecol. Model.
Stealth export of hydrogen and methane from a low temperature serpentinization system
Deep-Sea Res., Part II
Post-eruption succession of macrofaunal communities at diffuse flow hydrothermal vents on Axial Volcano, Juan de Fuca Ridge, Northeast Pacific
Deep-Sea Res., Part II
Opposing trends in crustal thickness and spreading rate along the back-arc Eastern Lau Spreading Center: implications for controls on ridge morphology, faulting, and hydrothermal activity
Earth Planet. Sci. Lett.
Larval dispersion along an axially symmetric mid-ocean ridge
Deep-Sea Res., Part I
The first measurements of hydrothermal heat output at , East Pacific Rise
Earth Planet. Sci. Lett.
Designating networks of chemosynthetic ecosystem reserves in the deep sea
Mar. Policy
Biogeography and ecology of the rare and abundant microbial lineages in deep-sea hydrothermal vents
FEMS Microbiol. Ecol.
Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: submersible observations
J. Geophys. Res.
On the global distribution of hydrothermal vent fields
Abundant hydrothermal venting along melt-rich and melt-free ridge segments in the Lau back-arc basin
Geophys. Res. Lett.
High-resolution surveys along the hot spot-affected Galápagos Spreading Center: 1. Distribution of hydrothermal activity
Geochem. Geophys. Geosyst.
Hydrothermal cooling along the Eastern Lau Spreading Center: no evidence for discharge beyond the neovolcanic zone
Geochem. Geophys. Geosyst.
Structure, temporal evolution, and heat flux estimates from the Lucky Strike deep-sea hydrothermal field derived from seafloor image mosaics
Geochem. Geophys. Geosyst.
An authoritative global database for active submarine hydrothermal vent fields
Geochem. Geophys. Geosyst.
Diffuse flow on and around hydrothermal vents at mid-ocean ridges
Oceanography
Biogeographic, climatic and spatial drivers differentially affect α-, β- and γ-diversities on oceanic archipelagos
Proc. R. Soc. B, Bio. Sci.
Cited by (82)
A review of zooplankton and deep carbon fixation contributions to carbon cycling in the dark ocean
2022, Journal of Marine SystemsCitation Excerpt :By 2009, 521 hydrothermal vent sites had been identified (Le Bris et al., 2017). This number increased to 630 by 2016 (Baker, 2017), with the general finding that hydrothermal vents occur every 2–20 km along the global ridge system (Baker et al., 2016). Based on this estimated distribution globally, Beaulieu et al. (2015) estimate that there are still about 900 active venting sites remaining to be discovered.
Near-bottom survey of hydrothermal sulfide on an extremely magmatic accretion segment along an ultraslow-spreading ridge
2022, Deep-Sea Research Part I: Oceanographic Research PapersUpper crustal Vp/Vs ratios along the northern East Pacific Rise derived from downward-continued streamer data
2023, Geophysical Journal International