Norbert E Kaul

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.

Aging of oceanic crust at the Southern East Pacific Rise

The oceanic crust covers almost 57% of the Earths surface and is created by seafloor spreading at mid-ocean ridges. Although crustal structure is similar everywhere, seismic experiments near spreading ridges indicate that seismic velocities in the top of the igneous crust are typically much lower than those in mature oceanic crust. While profound differences between juvenile and mature crust have long been recognized, little is known about the relationship between crustal aging and the properties of oceanic crust. German researchers from the Universities of Hamburg and Bremen explored seafloor created over the last 8 million years at the “super-fast” spreading East Pacific Rise south of the Garrett Fracture Zone (14–16°S) during a 52-day marine geophysical survey aboard the R/V Sonne. The seafloor in that area spreads at a rate of 150 mm/yr. The researchers studied age-dependent trends in the structure and properties of upper oceanic crust; this was the first study in nearly two decades to use an integrated approach to study variations and heat transfer in the upper crustal structure.

Temperature measurements and thermal gradient estimates on the slope and shelf edge region of the Beaufort Sea, Canada

In situ temperature measurements were conducted at 63 gravity-core stations during the 2013 expedition with the CCGS Sir Wilfrid Laurier in the Canadian Beaufort Sea. Outriggers attached to the outside of the gravity core-barrel were used to mount portable miniature temperature loggers (MTL) for down-core in situ temperature measurements. Several sub-regions were investigated during the expedition including two shelf-slope crossings, three mud volcano-type expulsion features, as well as two canyon sites. The last site visited was at the Gary Knolls, just east of the Mackenzie Trough at water depths of less than 100 m. Overall, temperature data obtained from the MTLs were of high quality at most stations and the data acquisition technique was proven to be robust and easy to adapt in the Arctic. However, depth determination for each logger position remains the largest challenge as no additional pressure sensor was used with the MTLs. Instead, depths were estimated based on the apparent core penetration and the geometry of the outriggers. The most significant result from this work is the discovery of the very large apparent geothermal gradients associated with the two expulsion features (EF) Coke Cap and the mud volcano at 420 m water depth. Temperatures measured within the top 2.5 meter below seafloor suggest geothermal gradients of up to 2.94oC/m (Station 96, 420m EF) and 1.37 oC/m (Station 58, Coke Cap EF). Away from the centre of the EFs, thermal gradients decrease to values of 0.5oC/m for Station 99 at the 420 m EF, and 0.92oC/m at Station 21 at the Coke Cap EF. Temperature data across the slope-shelf transect and the two transects across the canyon heads did not reveal considerable geothermal gradients, but show a water-depth dependent trend in temperature. From deep to shallow water, temperature appear to decrease until the most negative temperature values are found on the shelf itself at water depths of ~100 m (-1.2 to -1.4oC). Overall, data from the top 1.0 to 1.5 meter below seafloor are likely affected by seasonal variations in the water column temperature and may not be used to define geothermal gradients. With an optimal full penetration of the core barrel, the deepest temperature data are from ~2.3 mbsf, which limits the accuracy of the estimated geothermal gradients as only few data points (2 – 4) can be used in the calculations.

A Fluid Pulse on the Hikurangi Subduction Margin: Evidence From a Heat Flux Transect Across the Upper Limit of Gas Hydrate Stability: Fluid Pulse on the Hikurangi Margin

A transect of seafloor heat probe measurements on the Hikurangi Margin shows a significant increase of thermal gradients upslope of the updip limit of gas hydrate stability at the seafloor. We interpret these anomalously high thermal gradients as evidence for a fluid pulse leading to advective heat flux, while endothermic cooling from gas hydrate dissociation depresses temperatures in the hydrate stability field. Previous studies predict a seamount on the subducting Pacific Plate to cause significant overpressure beneath our study area, which may be the source of the fluid pulse. Double-bottom simulating reflections are present in our study area and likely caused by uplift based on gas hydrate phase boundary considerations, although we cannot exclude a thermogenic origin. We suggest that uplift may be associated with the leading edge of the subducting seamount. Our results provide further evidence for the transient nature of fluid expulsion in subduction zones.

LIRmeter: A new tool for rapid assessment of sea floor parameters. Bridging the gap between free-fall instruments and frame-based CPT

Physical and geotechnical parameters of marine sediments are of vital interest to fields like foundation-planning of offshore structures, surveying of cable routes, sediment dynamics, sediment manipulation (dredging, plowing), ground-truthing of acoustical surveys, risk assessment and mine burial predictions. Therefore, characterization of geotechnical in-situ parameters with dynamic penetrometers is of interest for research, consulting and the offshore industry, because in-situ methods, and especially dynamic tests, are generally less time consuming than static tests or measurements on samples. In addition, recovery of sediment samples from the seafloor may alter the sediment characteristics (pressure decrease, temperature change) and limits the information value. To date, the determination of fundamental parameters like shear strength, bearing capacity or grain size is predominantly done ex-situ. However, in-situ assessment of these parameters leads to a better characterization of marine sediments due to direct measurement under field conditions.

Comment [on “Deep-penetration heat flow probes raise questions about interpretations from shorter probes” by Géli et al.]

Geli et al. [2001] (Eos, 17 July 2001, p.317) present sub-sea floor thermal profiles collected with temperature sensors and data loggers attached to 18-m-long sediment cores. Some of these thermal profiles include significant non-linearities, particularly within the shallowest 5 m below sea floor (mbsf). Geli et al. [2001] assert that thermal data collected with data loggers attached to long core barrels provide a more precise means of interpreting marine heat-flow data than do thermal data collected with shorter, conventional heat flow probes. This assertion is not supported by the data presented. Geli et al. [2001] also suggest that the observed non-linearities are indicative of perturbations to steady-state, conductive conditions within sea-floor sediments resulting from recent changes in bottom water temperature (BWT) or pore fluid flow. We contend that the data shown by Geli et al. [2001] are more likely explained by sampling artifacts or errors in interpretations described below. Each of these possibilities must be confidently eliminated before one can interpret the data to indicate bottom-water temperature changes or fluid flow. We also urge taking a more balanced view of the strengths and weaknesses of sea-floor, heat-flow data collected with different tools, and caution against dismissing data collected with conventional probes as being generally less reliable than those collected with long piston cores.

MATLAB scripts to calculate the water budget in the Kumano Basin, offshore Japan

Two matlab scripts which simulate water emission from the subducting plate and in the inner accretionary prism.