Recent ridge-crest research suggests that further investigation of submarine spreading centers will provide multi-disciplinary insights in the Earth system sciences, elucidating mechanisms of past and present global change. Researchers working in the Northeast Pacific are developing the capability to observe moderate (100 km) to fine (1 m) scale ridge-crest phenomena; the suite of measurements from such endeavors will be crucial to understanding the complex, interdisciplinary nature of the ridge-crest systems. But even with relatively fine-scale observation and mapping improving, a recent scientific re-evaluation concludes that the dynamics and distribution of ridge-crest systems remain poorly constrained, especially on global scales.
Satellite data may provide a convenient way to extend local, high-resolution oceanographic understanding to a global assessment of submarine ridge-crest activity. If ridge-crest processes result in sea surface expressions, there is a constellation of satellites aloft with the potential to survey the global distribution, dynamics, and net impact of Earth's ridge-crest systems.
Several recent, direct observations and fortuitous encounters at sea have spurred renewed interest in ridge research. Remotely-operated vehicles, submersibles piloted by humans, and traditional oceanographic instruments have provided photographic and in situ observations of apparently recent -- and sometimes catastrophic -- volcanism along submarine ridges; sonar has generated bathymetric maps which reveal annual and decadal changes in geologic features on the seafloor; and data from the Navy's SOund SUrveillance System (SOSUS) have supplemented ship-board seismic detection of earthquake activity along some of the oceans' ridges.
Opportunities for the use of satellite remote sensing in ridge-crest research are already emerging from basic oceanographic observations. Mechanisms like "megaplumes" may translate ridge-crest activity into a detectable surface expression. In fact, significant information may already be contained in the archives of the TOPEX/POSEIDON radar altimeter, as well as a host of other satellite sensors.
Importantly, the workshop group (Kadko et al, 1994) considering the "Variations of Hydrothermal Activity in Space and Time" concluded that variability of hydrothermal systems on societal to orbital time scales is "possible, [and] potentially of great importance to questions of global change." Additionally, the group stated that on geologic time scales global variation is probable (in response to changing crustal generation rates) and regional variation is observed (in response to lithospheric rifting events).
Solid earth processes literally underlie the Earth's hydrothermal vent systems. Consequently, I propose to study the global impact of submarine ridge processes through participation in oceanographic observation and geological exploration. Simultaneously, I will consider the utility of satellite measurements in extending local and regional groundtruth to quantification of global impacts.
During the observational process, I will remain engaged in the search for mechanisms -- both theoretical and observed -- integrating ridge processes within the operation of the Earth system as a whole. I believe there are many connections to be made, for the scientific literature contains surprisingly few hypotheses linking submarine volcanic systems to other systems, and consequently, to global change. In search of a historical analog to contemporary greenhouse gas increases, Owen and Rea (1985) argue that past CO2-induced climate changes. . . were caused by pulsations in the intensity of sea-floor hydrothermal activity induced by tectonic rearrangements of sea-floor spreading centers, and that the most obvious example of this process occurred in the early Eocene. Subsequently, Shaw and Moore (1988) discover that mid-ocean ridge magma production was coincident with El Niño events, and Daniel Walker (1988) correlates seismicity on the East Pacific Rise with the Southern Oscillation Index. Most recently (1995), Walker reinforces his hypothesis with further data.
However, each of the correlations made thus far have suffered from a lack of observed, causative mechanisms. While conventional oceanographic research has revealed some mechanisms (see Table 1) and will inevitably reveal more, the development of sea-floor observatories and the capability to detect and quickly respond to transient ridge-crest phenomena will inevitably improve our ability to study a notoriously inaccessible site, thereby increasing the likelihood of observing mechanisms linking ridge systems with the rest of the Earth.
At the same time, the expense of ship-based oceanographic research provides an incentive to discern which small-scale, in situ observations might be extended globally through relatively cost-effective remote sensing technology. Hydrothermal vents occur on topographic highs along ridge-crests (Francheteau, 1983) and the cross-sectional area of the ridge reflects the underlying magmatic conditions (Macdonald, 1988; Scheirer, 1993). Since 99% of the variation in the Earth's mean sea surface is caused by the shape of the Earth's geoid (Koblinsky, 1993), which is predominantly influenced by density inhomogeneities in the mantle, a map of global hydrothermal activity could be inferred from steady state, mesoscale features in the geoid (and therefore the mean sea surface topography (Tsaoussi, 1994)). If the sea surface mimics (detectably) changes in the geoid due to bathymetric variations, areal extent of the submarine ridge network, and fluctuations in magmatic intensity (see Figure 1), then TOPEX/POSEIDON (and perhaps past or future satellite altimetry) data could be used to generate a global magmatic budget for the ridge systems.
Additionally, transient surface expressions (topographic or temperature anomalies) of megaplumes may be detectable from space. Preliminary investigation (Veirs, 1994 ) of TOPEX/POSEIDON data did not reveal (the only) event plumes observed in situ since the satellite's launch (Baker et al, 1993); however, an event of larger magnitude has been observed (Baker et al, 1986) and might be within the temporal coverage and resolving ability of past satellites. Additionally, in the high latitudes (above 60 degrees N or S latitude), the physical structure of the oceans may allow megaplumes to reach the surface (Lavelle, 1994), resulting in a thermal expression detectable by a host of heat-sensitive satellite instruments. Whether space-borne sensors have the sensitivity and spatio-temporal resolution to detect surface expressions of submarine volcanic activity from orbit awaits further exploration; the potential scientific benefit remains tremendous.
Part of the remote sensing scientists' predicament is the paucity of fine-scale (traditional oceanographic) observations of events which might be recorded by satellite sensors. In addition to assessment of the geological underpinnings of ridge systems, the lack of groundtruth motivates my fundamental interest in pursuing observational research via high-resolution multi-beam bathymetric and side-scan sonar surveys, and acquiring general in situ measurements (heat flux, biological, chemical, light attenuation, and salinity anomalies) and observations (from submersible, remotely operated vehicle, automated underwater vehicle, seismometers, and hydrophone arrays (SOund SUrveillance System).
With an enriched base of ridge observations in hand and an assessment of whether or not extant remote sensors have shed light on global ridge-crest studies, we will be much better equipped to specify the scientific objectives pertinent to the study of ridge-crest processes and mechanisms of global change. Simultaneously, the design of future EOS components, Earth Probes, Geostationary Earth Observation platforms, and other space-borne instruments will be better informed.
Baker, E.T., Massoth, G.J., Feeley, R.A., Embley, R.W., Thompson, R.E., and Burd, B.J. Hydrothermal Event Plumes from the CoAxial Seafloor Eruption Site, Juan de Fuca Ridge. June, 1994, Geophysical Research Letters.
Francheteau, J., and R.D. Ballard. The East Pacific Rise near 21oN, 13oN and 20oS: inferences for along-strike variability of axial processes of the Mid-Ocean Ridge. Earth and Planetary Science Letters 64:93-116, 1983.
Kadko, D., E. Baker, J. Alt, and J.Baross. RIDGE/VENTS Workshop: Global Impact of Submarine Hydrothermal Processes. Sept. 11-13, 1994.
Koblinsky, C. "Ocean Surface Topography and Circulation" reprinted from Atlas of Satellite Observations Related to Global Change (Gurney, R.J, Foster, J.L., and Parkinson, C.L., eds.), Cambridge University Press, 1993.
Lavelle, W. Personal communication, NOAA/PMEL, Sep.29, 1994.
Macdonald, K.C., and J.P. Fox. The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions. Earth and Planetary Science Letters, 88:119-131, 1988.
Owen, R.M., and D.K. Rea. Sea-Floor Hydrothermal Activity Links Climate to Tectonics: The Eocene Carbon Dioxide Greenhouse. Science, V. 227, 166-169, Jan. 11, 1985.
Reynolds, J. Aftermath of a sea-floor eruption. Nature, Vol. 367, p.115, 13 January, 1994.
Scheirer, D.S., and K.C. Macdonald. Variation in Cross-Sectional Area of the Axial Ridge Along the East Pacific Rise: Evidence for the Magmatic Budget of a Fast Spreading Center. Journal of Geophysical Research 98, 7871-7885, 1993.
Shaw, H.R., and J.G. Moore. Magmatic Heat and the El Nio Cycle. Eos, Vol. 69, Nov. 8, 1988.
Tsaoussi, L.S., and Koblinsky, C.J. An Error Covariance Model for Sea Surface Topography and Velocity Derived From TOPEX/POSEIDON Altimetry. Journal of Geophysical Research, June 29, 1994.
Veirs, S. Megaplume Meanderings: Searching for a Signal in the Sea. 1994.
Walker, D.A. Seismicity of the East Pacific Rise: Correlations With the Southern Oscillation Index? Eos, Vol. 69, Sep. 20, 1988.
Walker, D.A. More Evidence Indicates Link Between El Nios and Seismicity. Eos, Vol. 76, Jan. 24, 1995.