Heinrich Villinger

Iceberg scouring in Scoresby Sund and on the East Greenland continental shelf

Modern and relict scours produced by iceberg keels have been reported from a number of high latitude continental shelves in the Northern Hemisphere and Antarctica, but observations are largely absent from the East Greenland shelf. About 18 km3 yr−1 of icebergs are calved into the Scoresby Sund fjord system, East Greenland. These icebergs have submarine keels with drafts up to 550 m. Modern iceberg contacts with the sea floor are inferred from calculated keel depths and stationary icebergs identified on sequential Landsat imagery. Acoustic profiles (Parasound, 3.5 kHz) of the sea floor show that scouring is a significant process in water depths < 550 m. Iceberg scouring intensity varies inversely with water depth. The most intense scouring occurs at depths of < 300–400 m. Iceberg scours were observed on over 25,000 km2 of the East Greenland shelf, at 69–72°N and 75°N. Scours are being actively produced on the modern shelf, but relict scours probably dating back to the Late Weichselian are also present. The rate of iceberg production from Greenland Ice Sheet outlet glaciers, and iceberg drift tracks on the shelf, suggests that iceberg scouring is important over a significant proportion of the 500,000 km2 area above the shelf break around Greenland.

Heat flow through the Dead Sea rift

Abstract Geothermal measurements utilizing a special probe designed for the determination of heat flow in lakes have been made in the water-covered portions of the Dead Sea rift, i.e., Lake Kinneret, the Dead Sea and the northern part of the Gulf of Elat. Corrections have been applied for variations in bottom water temperature, sedimentation and topography. The preliminary results indicate a low to normal heat flow along the Dead Sea rift: the average of values in Lake Kinneret is 1.8 H.F.U., in the Dead Sea 0.7 H.F.U., and in the northern part of the Gulf of Elat 1.6 H.F.U. The values in the Dead Sea are comparable to other nearby continental values in Israel, and to those in the eastern Mediterranean.

Thermal regime of the Costa Rican convergent margin: 2. Thermal models of the shallow Middle America subduction zone offshore Costa Rica

At the Costa Rica margin along the Middle America Trench along‐strike variations in heat flow are well mapped. These variations can be understood in terms of either ventilated fluid flow, where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean, or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer. We model fluid flow within the subducting aquifer using Nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust. The sensitivity of these models to the initial thermal state of the plate and styles of fluid flow, either ventilated or insulated, is explored. Heat flow measurements on cool crust accreted at the East Pacific Rise are consistent with ventilated hydrothermal cooling that continues with subduction. These models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation. Heat flow transects on warm crust accreted at the Cocos‐Nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer. Near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench. Comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100°C and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust.

Hydrothermal heat flux through aged oceanic crust: where does the heat escape?

Abstract Recent publications suggest that most of the fluid flow in the upper oceanic crust is channelized through small volumes of rock and vented into the ocean. This implies that at flanks of generally thinly sedimented mid-ocean ridges, focused discharge at the seafloor should be concentrated most likely at outcrops, high-angle normal faults or seamounts. These vents should be associated with a significant heat flow signature. However, only few observations worldwide support this assumption up to now. On our quest for focused fluid exchange between young oceanic crust and the ocean we surveyed a 720 km long and 40–90 km wide off-axis portion of seafloor intersecting the East Pacific Rise near 14°14′S. A wealth of geophysical methods including high-resolution swath mapping bathymetry, single channel seismics, sediment echo sounding, magnetics and heat flow determinations were used. Heat flow data in the tectonic corridor cover crustal ages of 0.3–9.3 Ma. With respect to the conductive plate cooling model the data show the well-known pattern of low values close to the ridge, associated with vigorous hydrothermal circulation of cold seawater through the young upper crust, and a fast recovery to almost lithospheric conductive cooling values at a surprisingly young crustal age of 9.3 Ma. Although the sediment cover is fairly thin, measurements with a 3.6 m violin bow type heat probe were possible almost everywhere within the investigated area. A detailed survey between two large seamounts at 4.5 Ma revealed localized extremely high values of up to 618 mW/m 2 (275% of the expected heat flow) at the foot of the seamount. This is interpreted as a clear indication of focused discharge of hydrothermal fluid. If we, however, relate heat flow normalized by the expected conductive heat loss to the character of igneous basement, heat flow is highest in areas with an almost flat and sedimented basement, and lowest within ∼10–20 km of seamounts and other rough basement relief. We therefore hypothesize that the large number of seamounts covering the ocean floors governs a major amount of convective heat loss of aging oceanic lithosphere.

Heat flow over the descending Nazca plate in central Chile, 32°S to 41°S: observations from ODP Leg 202 and the occurrence of natural gas hydrates

Bottom simulating reflectors (BSRs) were detected in multichannel seismic reflection data acquired in the vicinity of Isla Mocha across the southern Chile margin and near 33°S. Geothermal gradients were determined from the depth of the BSR that is interpreted to mark the thermally controlled base of a gas hydrate layer. Ground truth for the assessment and additional thermal constraints were provided by downhole measurements obtained during Ocean Drilling Program (ODP) Leg 202 in Site 1233 at 41°S and Sites 1234 and 1235 near 36°S. Both BSR-derived data and downhole temperatures were used to calculate heat flow anomalies and provide new constraints on the thermal regime of the continental slope and downgoing slab in Chile between 32°S and 41°S. Downhole chemical logs of Th, U, and K from Site 859 of ODP Leg 141 have been used to assess the radiogenic heat production in the margin wedge. Heat production is low (∼0.8 μW/m3). However, knowledge of this reduces the errors of estimating the contribution from frictional heating along the subduction thrust fault. With respect to the Eocene age of the incoming oceanic lithosphere, heat flow appears to decrease landward of the deformation front as expected due to the advective transport of heat into the subduction zone by the downgoing slab. Calculations of conductive fore-arc heat flow show that the modelled seafloor heat flow agrees with the measured heat flow only if there is negligible frictional heating. At 33°S, temperatures in the fault zone reach 100°C approximately 60 km landward of the deformation front and are coincident with the onset of earthquake activity and hence mark the up-dip limit of the seismogenic zone. The up-dip limit shifts seaward going to the south, reflecting the progressive southward decrease of lithospheric age of the subducting plate.

Hydrothermal activity and the evolution of the seismic properties of upper oceanic crust

In order to investigate the impact of off-axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust (layer 2A), we performed an extensive geophysical survey on the eastern flank of the East Pacific Rise at 14°S. Seismic refraction and heat flow data were obtained along a 720-km-long and 25 to 40-km wide corridor, covering thinly sedimented seafloor created since 8.5 Ma. The seismic data yield a seismic velocity of ∼2.9 km/s at the top of 0.5-m.y.-old basement rocks. Within about 8 m.y. the velocity increases gradually to a value of mature oceanic crust (∼4.3 km/s). Heat flow data, derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements, suggest that an open hydrothermal circulation system persists for at least 6–7 m.y. In crust older than 7 Ma, regional heat flow is close to values predicted by plate cooling models, suggesting that hydrothermal circulation is going to cease. Considering published dating of alteration minerals, it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10–15 m.y. This idea may explain why seismic velocities in the Pacific ocean have not changed significantly in igneous crust older than 8–10 Ma. In regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different; velocities increase rapidly and reach values of mature oceanic crust within 1–2 m.y. We therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust [Stephen and Harding, 1983; Rohr, 1994].

Miniaturized data loggers for deep sea sediment temperature gradient measurements

Abstract A new probe for autonomous high precision temperature measurements in deep sea sediments has been developed by Fa. Antares and the University of Bremen and extensively tested during several expeditions. The miniaturized temperature data loggers (MTLs) were constructed to be extremely robust, small (16 cm long) and easy to operate in waterdepths up to 6000 m. The temperature range extends from −5 to +60°C with an absolute accuracy of ±0.1 K. Higher absolute accuracy requires calibration with a high precision thermometer. The resolution of the A/D converter is 16 bit which leads to a temperature resolution of 0.001 K. The logging duration and sampling rate can be chosen independently. Up to 18 h of measurement are possible with the highest sample rate of 1 s. Data are stored in the non-volatile memory which holds up to 64 800 measurements. The probe is configured and data downloaded without opening the pressure case by using specialized software and interface hardware. The MTLs offer a wide variety of marine science applications wherever a high precision temperature measurement is of interest. Our main application is the investigation of temperature gradients in deep sea sediments where measurements of gradients over a large depth interval are required by fixing loggers on either different kinds of core barrels or on long lances. During four expeditions, we used the loggers to measure sediment temperature gradients in several geological settings. The design of the attachments to the different carriers was varied to optimize the results for different needs. These tests were highly successful and the data of high quality. Therefore the loggers can be recommended for regular use in marine applications.

Pore pressures and permeabilities measured in marine sediments with a tethered probe

A new probe for measuring pore fluid pressures in marine sediments has been constructed and tested in two hydrologic environments on the Juan de Fuca Ridge. The instrument utilizes a highly incompliant high-sensitivity differential pressure transducer, and a small (4mm) diameter sensing probe that telescopes within a larger diameter 1.5-m-long (currently 2.5 m) strength member. The small probe diameter and the low compliancy serve to minimize the time required for pressures to approach equilibrium after penetration. Observed decay times are short enough to permit the instrument to be used in a tethered mode. The mechanical configuration enables the small diameter probe to penetrate the sediments without buckling and to be pulled out without bending. This allows multiple penetrations to be completed during a single instrument lowering. Ten measurements were made during the development and testing of the instrument in fine-grained turbidite sediments on the Juan de Fuca Ridge. Four penetrations were uninterrupted by mechanical disturbances or instrumentational problems, and although equilibrium conditions were approached during only one penetration (2.7 hours maximum undisturbed decay period), all provided useful constraints on equilibrium pore pressures. Values at two sites where sediment cover is thick and heat flow relatively low were unresolvably different from hydrostatic. Values estimated from two penetrations located within a few hundred meters of a hydrothermal vent field within the sedimented axial valley of the northern Juan de Fuca Ridge, Middle Valley, exceeded hydrostatic and yielded pressure gradients of roughly 0.5 kPa m−1. Permeabilities were estimated from the rates of decay of the penetration transients. Values ranged from 5×10−16 to 1×10−14 m2 and agreed well with values measured on core samples collected near the probe penetrations which ranged from 8×10−16 to 1×10−14 m2. Using the values determined from the probe measurements from Middle Valley, the implied rate of fluid flow upward through the sediments in the vicinity of the vent field is roughly 5×10−10 m s−1, or 15 mm yr−1. With the current practical limit of resolution of roughly 10−10 m s−1 (3 mm yr−1) this direct measurement technique provides a means of determining pore fluid flow rates that is roughly 1 order of magnitude more sensitive than the method of estimating the rate of flow from the perturbation of the conductive thermal regime measured by typical marine heat flow instruments.

Inversion of marine heat flow measurements by expansion of the temperature decay function

SUMMARY Marine heat flow data, obtained with a Lister-type probe, consists of two temperature decay curves, frictional and heat pulse decay. Both follow the same physical model of a cooling cylinder. The mathematical model describing the decays is non-linear as to the thermal sediment parameters thus a direct inversion is not possible. To overcome this difficulty, the model equations are expanded using a first-order Taylor series. The linearised model equations are used in an iterative scheme to invert the temperature decay for undisturbed temperature and thermal conductivity of the sediment. The inversion scheme is tested first for its theoretical limitations using synthetic data. Inversion of heat flow measurements obtained during a cruise of R/V SONNE in 1996 and needle probe measurements in material of known thermal conductivity show that the algorithm is robust and gives reliable results. The programme can be obtained from the authors.

Thermal regime of the Costa Rican convergent margin: 1. Along-strike variations in heat flow from probe measurements and estimated from bottom-simulating reflectors

The thermal structure of convergent margins provides information related to the tectonics, geodynamics, metamorphism, and fluid flow of active plate boundaries. We report 176 heat flow measurements made with a violin bow style probe across the Costa Rican margin at the Middle America Trench. The probe measurements are collocated with seismic reflection lines. These seismic reflection lines show widespread distribution of bottom‐simulating reflectors (BSRs). To extend the spatial coverage of heat flow measurements we estimate heat flow from the depth of BSRs. Comparisons between probe measurements and BSR‐derived estimates of heat flow are generally within 10% and improve with distance landward of the deformation front. Together, these determinations provide new information on the thermal regime of this margin. Consistent with previous studies, the margin associated with the northern Nicoya Peninsula is remarkably cool. We define better the southern boundary of the cool region. The northern extent of the cool region remains poorly determined. A regional trend of decreasing heat flow landward of the deformation front is apparent, consistent with the downward advection of heat by the subducting Cocos Plate. High wave number variability at a scale of 5–10 km is significantly greater than the measurement uncertainty and is greater south of the northern Nicoya Peninsula. These heat flow anomalies vary between approximately 20 and 60 mW m−2 and are most likely due to localized fluid flow through mounds and faults on the margin. Simple one‐dimensional models show that these anomalies are consistent with flow rates of 7–15 mm yr−1. Across the margin toe variability is significant and likely due to fluid flow through deformation structures associated with the frontal sedimentary prism.