Marcus G. Langseth

Geothermal convection through oceanic crust and sediments in the Indian ocean.

Closely spaced heat flow surveys at four sites on the flanks of the Central Indian Ridge and the Southeast Indian Ridge delineate a pattern of oscillatory heat flow which can only result from cellular convection of oceanic bottom water through the oceanic crust and overlying sediment. These cells have a wavelength of 5 to 10 kilometers and are presently active in sea floor 18 x 106, 25 x 106, and 45 x 106 years old of the Crozet Basin and in sea floor 55 x 106 years old of the Madagascar Basin. The precise measurement of nonlinear temperature profiles makes it possible to calculate the conductive and convective heat transfer components through the sea floor. Even in the oldest sites, geothermal convection is still a major component of heat transfer through both the crust and sedimentary layers. These observations coupled with the results of earlier oceanwide geothermal studies indicate that more than one-third of the entire surface area of the worlds ocean floor contains presently active geothermal convection that is cellular in plan form.

The Nicoya Convergent Margin—A region of exceptionally low heat flow

Over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the Nicoya Peninsula is characterized by remarkably low heat flow; values over the accretionary prism average 28 mW/m², and values in the trench and the ocean crust seaward of the trench average 14 mW/m². We attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation. Thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow. Disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits.

Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom-water potential temperature, turbidity, and sea-floor photography☆

Abstract Data on bottom-water potential temperature, turbidity and current indications show that in the Southern Ocean west of the Kerguelen Plateau, Antarctic Bottom Water (AABW) of Weddell Sea origin spreads northwards from the Atlantic—Indian Basin in two directions: (1) AABW enters the Agulhas Basin through relatively deep areas in the Mid-Indian Ridge at 20–25°E and possibly at 35°E, and flows northwards into the Mozambique Basin as far as its northern limits; (2) a more easterly spreading path extends from the Atlantic—Indian Basin through the Crozet into the Madagascar, Mascarene, Somali and Arabian Basins. The passage in the western branch of the Indian Ridge for the AABW spreading from the Crozet into the Madagascar Basin appears to be at 29-26°S and 60–64°E. East of the Kerguelen Plateau in the South Indian Basin, the bottom water formed mainly along the Adelie Coast and Ross Sea travels west towards the Kerguelen Plateau and then parallel to it. This water finally flows eastwards hugging the Southeast Indian Ridge. Significant deviations from this general circulation pattern occur due to local topographic effects. Some AABW in the South Indian Basin exits through a passage at 120–125°E in the region of the Australian—Antarctic discordance in the Southeast Indian Ridge and enters the South Australian Basin and subsequently the Wharton Basin. This passage is clearly indicated by the northward extension of a cold, bottom-water tongue as shown by the temperature distribution in the region; the bottom-water effects in the passage are reflected in the high turbidity and current lineations on the sea floor. In the Southern Ocean basins, bottom-water turbidity is generally high, reflecting in part the strong bottom-water activity. The effects of AABW circulation on the sea floor—in the form of well-developed small- or large-scale current ripples and erosional/depositional features, manganese-nodule formations, and unconformities and reworking of sediments observed in cores — are also marked in these basins. Even though the AABW in the Wharton Basin is cold, its spreading effects on the sea floor are minimal in this basin in contrast to the basins west of the Mid-Indian Ridge at comparable latitudes.

Sedimentation processes on the continental rise of northeastern South America

Abstract A section of the continental rise of northeastern South America northeast of the Orinoco delta contains physiographic features built by the interaction of southward-flowing North Atlantic Deep Water and turbidity currents generated in the Orinoco region during the last Pleistocene glacials. A sedimentary outer ridge of low relief (Demerara Outer Ridge) trends northeast along the rise and a field of westward-migrating sediment waves trending north-northwest is superimposed on the outer ridge. The sediment waves have a maximum amplitude and wavelength of 20 m and 4 km, respectively. Seismic profiler records indicate that the outer ridge was probably built during the Pleistocene. A major turbidity-current pathway adjacent to the outer ridge on the north supplied sediment to the southward-flowing North Atlantic Deep Water which then deposited this sediment down-stream on the outer ridge and formed the sediment waves. Piston cores from the outer ridge contain numerous silt—sand beds and appear to be contourites. The cores consist primarily of gray hemipelagic clay of a Late Wisconsin age and have high ( > 10 cm 1000 yrs ) sedimentation rates. In contrast, cores from the continental rise north of the turbidite channel are brown clays with relatively low sedimentation rates ( 3.0 cm 1000 yrs ) and do not contain silt—sand contourites.

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.

Apollo 15 measurement of lunar surface brightness temperatures thermal conductivity of the upper 1 1/2 meters of regolith

Abstract In situ measurements of lunar surface brightness temperatures made as a part of the Apollo Lunar Surface Experiments Package at the Apollo 15 Hadley Rille landing site are reported. Data derived from 5 thermocouples of the Heat Flow Experiment, which are lying on or just above the surface, are used to examine the thermal properties of the upper 15 cm of the lunar regolith using eclipse and nighttime cool-down temperatures. Application of finite-difference techniques in modeling the lunar soil shows the thermocouple data are best fit by a model consisting of a low-density and low-thermal conductivity surface layer approximately 2 cm thick overlying a region increasing in conductivity and density with depth. Conductivities on the order of 1 × 10−5 W/cm-°K are postulated for the upper layer, with conductivity increasing to the order of 1 × 10−4 W/cm-°K at depths exceeding 20 cm. An increase in mean temperature with depth indicates that the ratio of radiative to conductive transfer at 350°K is 2.7 for at least the upper few centimeters of lunar soil; this value is nearly twice that measured for returned lunar fines. The thermal properties model deduced from Apollo 15 surface temperatures is consistent with earth-based microwave observations if electrical properties measured on returned lunar fines are assumed.

Heat flow in the Philippine Sea

Abstract The distribution of heat flow in the Philippine Sea is very complicated. Unlike other marginal seas behind island arcs and the Pacific Basin proper in front of arcs, the Philippine Sea has neither uniformly high nor low heat flow but shows numerous adjacent areas of high and low heat flow. Contouring is difficult but the pattern of heat flow distribution is fairly well established in the northwestern Philippine Sea, i. e. the area of the western Shikoku Basin and the Ryukyu Trench and Island Arc. The basin and trench southeast of the Ryukyu (Nansei-Shoto) Arc are characterized by low heat flow, and the area behind the arc by high heat flow. This distribution is consistent with the pattern over other trench and island arc systems. There is a well-defined area of high heat flow in the northern Shikoku Basin. Scattered measurements in the eastern part of the Philippine Sea behind the Izu-Bonin (Ogasawara) Arc are also predominantly high, suggesting, that the area of high heat flow in the Shikoku Basin may be part of a broad region of above average heat flow behind this arc. Several measurements west of the Mariana Arc indicate low heat flow in the trough between the outer and inner arcs and above average heat flow west of the inner ridge. Scattered measurements in the Philippine and Parece Vela basins south of latitude 22°N give highly variable results. High values predominate, however, more measurements are required before it can be established that these basins on the whole have significantly above normal heat flow.

Heat flow from the Mid-Ocean Ridges and sea-floor spreading☆

Abstract Observations of heat flow, topographic elevation and topographic slope are examined in the light of the hypothesis of ridge formation by ocean-floor spreading. By normalizing the distance of observations from the ridge axis in terms of the spreading rate, a large amount of data from all parts of the ridge system can be used to derive the pattern of heat flow with greater confidence than here-to-fore obtained. This analysis shows that the shape of the normalized anomaly of high heat flow over the ridge is independent of spreading rate, however, the amplitude of this anomaly over the fast-spreading Pacific ridges (3–6 cm/year) is generally 0.5 μcal/cm2sec larger than that over the Atlantic and Indian Ocean Ridge which are spreading at a slower rate (1–2 cm/year). More than 60% of the heat lost over the ridges in excess of the adjacent basin heat loss takes place in a narrow zone near the axis, corresponding to the part of the ridge that has been created in the last 6 million years. There is also a nearly linear relation between spreading rate and ridge topographic slope. The normalized slope, however, decreases slightly with increasing spreading rate. Simple mechanical models of ocean-floor spreading such as a thick plate cooling as it moves away from the axis or a thin plate moving over a shallow, near isothermal zone do not adequately explain the pattern of heat flow and topographic slopes derived in this paper. A complex model such as a thin crustal layer moving over a broad stagnation zone could fit the observed data. However, more definitive geophysical data are required to determine the details of this type of model.

Lunar microwave brightness temperature observations reevaluated in the light of Apollo program findings

Abstract Remote observations of the lunar radiowave emission are reexamined in the light of physical property data accumulated through the Apollo program. It is found that thermal and electrical properties determined for a number of different landing sites yield theoretical results in good agreement with remote observations for millimeter and short centimeter wavelengths. Theoretical models incorporating reflecting layers of rock and physical property data from the Apollo program are compared to the longer wavelength (5–500 cm) observational data to estimate a disk average steady state heat flow and a mean depth of the lunar regolith. It is found that a high heat flow, comparable to the heat flows measured at the Apollo 15 and 17 sites, is required to fit the available 5–20 cm wavelength remote data, and that a lunar surface layer relatively free of large boulders within the upper 10–30 m best fits the observations of a decreasing brightness temperature with wavelength for wavelengths greater than ∼ 50 cm.

Heat flow in the oceanic crust bounding Western Africa

Abstract Over four hundred widely distributed heat-flow measurements have been made in the eastern Atlantic Ocean between the Azores—Gibraltar Ridge and the Walvis Ridge, permitting a study of the correlation of heat flow with crustal age and tectonic province. The mean heat flow over crust less than 80 m.y. old is less than that predicted by cooling-lithosphere models. Near the ridge axis the standard deviation of the heat-flow values is as large as the mean, but it decreases rapidly near the 20 m.y. isochron to about half of the mean and then again near 70–80 m.y. to about one third of the mean. The heat-flow values observed over crust older than 80 m.y. are the same as predicted by a cooling-lithosphere model and show little scatter. In the younger ocean crust, water circulation between the crust and ocean bottom water is the dominant mechanism of heat transfer from the crust, but after 80 m.y., heat is transferred primarily by conduction. The age dependence of the mean heat flow and standard deviation north and south of the equator were examined independently and were found to be nearly identical. This implies that heat-transfer processes in the oceanic crust evolve in a similar manner in the two portions of the ocean. Over the older oceanic crust, regions delineated by their mean heat flow and the scatter in the heat-flow values correspond roughly with tectonic province. Volcanic regions have a higher and more variable heat flow than a normal oceanic basin. The scatter in the heat-flow values may be caused by water circulation between the crust and the bottom water. In the equatorial region, the Guinea and Sierra Leone basins have a uniform and relatively high heat flow, averaging 1.4 HFU (10−6 cal cm−2 sec−1). This may mean that the equatorial eastern Atlantic Ocean is underlain by an anomalously thin lithosphere, possibly caused by upwelling asthenosphere and higher shear-stress heating at the base of the lithosphere.