Richard P. Von Herzen

Submarine Thermal Springs on the Galápagos Rift

The submarine hydrothermal activity on and near the Gal�pagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Gal�pagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis.

Seafloor hydrothermal systems

The discovery of seafloor hydrothermal systems approximately two decades ago has led to a major reassessment of the Earths thermal and geochemical budgets and has revolutionized our understanding of biological processes. This review traces the development of the study of seafloor hydrothermal systems from the indirect evidence provided by conductive heat flow anomalies to the discovery of ≈ 350°C black smoker vents on the East Pacific Rise at 21°N. Although the review focuses on physical characteristics and processes, it outlines some key characteristics of vent fluid chemistry that provide constraints on physical models. Ridge crest systems have thermal power outputs ranging from 10 to 104 MW. They are transient systems, driven by magmatic heat sources, but episodic events such as megaplumes, the interplay between focused and diffuse venting, and other aspects related to their thermal, chemical, and biological evolution remain poorly understood. Advances will be made by continuing exploration and discovery to determine the full range of possible phenomena both on and off axis and in different tectonic settings. In order to understand the complete, integrated ridge system, however, future studies must include long-term monitoring of an active system, deep drilling into the reaction zone, and mathematical modeling that incorporates both physical and chemical constraints.

Heat Loss from the Earth: New Estimate

The average heat loss from the Earths interior is calculated from heat-flow values and tectonic models of sea-floor spreading. The value of 10.2 × 1012 cal/s (±1 5 percent) is about 32 percent larger than previous estimates, mainly due to the previously ignored contribution from the cooling lithosphere. The principal uncertainty derives from unknown parameters of the oceanic lithosphere.

Lake tanganyika: Water chemistry, sediments, geological structure

The water chemistry of the lake is uniform throughout its entire length and depth except for the nutrient minerals ammonia, nitrate, phosphate, and silica. Sediment fill in the lake is very massive. Because the sediments are almost entirely composed of biological debris, changes in the fossil inventory can be linked to the chemical and in turn biological evolution of the lake. Rates of depositions are about 30 to 50 cm per 1 000 years in the deep basins and 5 cm per 1 000 years in the sill area separating the southern and northern basins. Seismic profiles indicate graben-type structures. Magnetic surveys reveal no magnetic lineation typical of rifting. Free air and simple Bouguer anomalies suggest that lake and land structures are grossly similar. Low heat flow and similarity with surrounding values in Africa is consistent with a lack of active volcanicity and sea-floor spreading in Lake Tanganyika. A new concept on the evolution of a rift is proposed.

The Vema fracture zone and the tectonics of transverse shear zones in oceanic crustal plates

At 11°N latitude, the Mid-Atlantic ridge is offset 300 km by the Vema fracture zone. Between the ridge offset, the fracture consists of an elongate, parallelogram-shaped trough bordered on the north and south by narrow, high walls. The W-E trending valley floor is segmented by basement ridges and troughs which trend W10°N and are deeply buried by sediment. Uniform high heat flow characterizes the valley area. Seismically inactive valleys south of the Vema fracture, also trending W10°N, are interpreted as relict fracture zones. We explain the high heat flow and the shape of the Vema fracture as the results of secondary sea-floor spreading produced by a reorientation of the direction of sea-floor spreading from W10°N to west-east. This reorientation probably began approximately 10 million years ago. Rapid filling of the fracture valley by turbidites from the Demerara Abyssal plain took place during the last million years.The large amount of differential uplift in the Vema fracture is not explained by the reorientation model. Since the spreading rate across the valley is small compared to that across the ridge crest, we suggest that it takes place by intrusion of very thin dikes that cool rapidly and hence have high viscosity. Upwelling in the fracture valley will thus result in cosiderable loss of hydraulic head, according to models by Sleep and Biehler (1970), and recovery of the lost head could produce valley walls higher than the adjacent ridge crest. We further postulate that the spreading takes place along the edges of the fracture zone rather than in the center. This would account for the uniform distribution of heat flow along the fracture valley and for the lack of disturbance of the valley fill. As a consequence, a median ridge should form in the center, where head loss is compensated in the older crust; such a median ridge may be present. The width of the valley should be a function of the angle and time of reorientation, and of the spreading rate; the width so obtained for the Vema fracture is in accordance with the observed width. If this model is correct, the narrowness of the valley walls implies a thin lithosphere of very limited horizontal strength.

Heat flux from black smokers on the Endeavour and Cleft segments, Juan de Fuca Ridge

We have estimated the heat flux from black smoker vents on the Juan de Fuca Ridge to evaluate their importance for heat transfer from young oceanic crust. The velocity and temperature of smoker effluent were measured from the manned submersible Alvin within a few centimeters of vent orifices, using a turbine flowmeter with an attached temperature probe. Exit velocity was calculated from a simple plume model, and vent orifices were measured in photographs and video records. The estimated power output from smokers alone is 49±13 MW for the Plume site, Vent 1 and Vent 3 on the southern Cleft segment near 45°N; 364±73 MW for the main vent field on the Endeavour Segment near 48°N; and 122±61 MW for the Tubeworm field 2 km north. The estimates for the Cleft and Tubeworm fields could be too low because of undiscovered vents. These values constitute only 4% to 14% of the total advective heat flux estimated for these vent fields from measurements in the nonbuoyant plume and of diffuse flow at the seafloor, indicating that most of the heat advected at these hydrothermal vent sites is carried by diffuse rather than focused flow. Values for individual smokers vary from 0.1 to 94 MW, with an average of 6.2 MW at the Endeavour field and 3.1 MW at the Cleft field. Our estimates agree well at all scales with those of Bemis et al. [1993] based on measurements made during the same dives, in some cases simultaneously, up to 50 m high in the buoyant plume. The good agreement between the two techniques implies that little diffuse flow at either high or low temperature is incorporated into the buoyant plumes generated by smokers at these sites. Velocity-temperature measurements at vents excavated by Alvin could not be modeled successfully, suggesting that vent structures may grow in equilibrium with the force of the exiting water such that orifice size is determined by volume flux. At the Endeavour field the heat flux is focused by faults.

Heat and fluid flux through sediment on the western flank of the Mid‐Atlantic Ridge: A hydrogeological study of North Pond

A comprehensive survey of heat flow, sediment thickness and bathymetry in a large intermontane sediment pond on the western flank of the Mid-Atlantic Ridge (North Pond) shows that only 20 to 25% of the heat escaping from the lithosphere flows through the sediments in the pond even though they completely cover a 70 km2 area of the seafloor. Three in situ pore-pressure gradient measurements gave values that were negative relative to hydrostatic, indicating that there is drawdown of water through the sediment at rates of 1 to 5 mm/yr. North Pond is a recharge zone for hydrothermal circulation that is probably driven by lateral pressure gradients produced by topographic relief.

Geothermal heat flux from hydrothermal plumes on the Juan de Fuca Ridge

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, and Woods Hole Oceanographic Institution, 1991.

Hydrothermal mounds and young ocean crust of the Galapagos: Preliminary Deep Sea Drilling results, Leg 70

A total of 32 holes at 5 sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November–December 1979) provide unique data on the origin of the hydrothermal mounds on the south flank of the Galapagos spreading center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (⩽100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with sea water and pelagic sediments above locally permeable zones of ocean crust. Mounds as much as 25 m in height form in less than a few × 10 5 yrs, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.

Near-axis heat flow measurements on the northern Juan De Fuca Ridge: Implications for fluid circulation in oceanic crust

Present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process. In axial regions, abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid. In older crust, where the insulating sediment cover is thick, previous studies have shown that basement topography is the dominant forcing factor for within-crust fluid circulation. In the intermediate region, where young crust is lightly sedimented, heat flow data are difficult to obtain with traditional techniques. To determine whether topography or permeability is the dominant process controlling fluid circulation in the near-axis region, we conducted a profile of heat flow measurements using the submersible ALVIN, on the Endeavour Segment of the Juan de Fuca Ridge. Our data indicate that topographic forcing is responsible for the long wavelength variations, with high heat flow at the ridge summits, and low values in the inter-ridge valleys. The locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge-valley topographic pair comprises a single circulation cell. This model predicts that the source area for the high temperature axial vents may be in the flanking inter-ridge valleys.