Bottom Water Temperature Variations Research Articles

A compositional kinetic model of hydrate crystallization and dissolution

Hydrates are crystallizing near and at the seafloor from gas vents on shelves where the sedimentation rate is high and hydrocarbons are being generated. When seafloor temperature, vent rate, or vent gas composition changes, these hydrates may become unstable and decompose. We have constructed a compositional kinetic model of hydrate crystallization and dissolution that can address these issues. The model crystallizes hydrate in compositional bins and allows each to dissolve at either a kinetically or compositionally controlled rate if vent gas composition or temperature causes it to become unstable. We empirically calibrate the model to venting at the Bush Hill hydrate mound in the offshore Louisiana Gulf of Mexico, show how variations in venting rate crystallize hydrate of diverse composition in the subsurface, and investigate how bottom water temperature variations similar to those measured could increase the rate of gas venting by destabilizing hydrates within a few meters of the seafloor. We show that increases in bottom water temperature can cause gas venting rates to increase ∼100%, as suggested by recent measurements, only if the dissolution kinetics are fast compared to the empirically calibrated crystallization kinetics and dissolution gases are removed rapidly enough that they do not thermodynamically inhibit the rate of dissolution. Model characteristics required to further investigate hydrate mound construction are identified.

Deep‐ocean temperature variations and implications for errors in seafloor heat flow determinations

The accuracy with which seafloor heat flow is determined depends on the temporal stability of bottom water temperature. Indirect tests for stability are provided most commonly by observing the uniformity of heat flow with depth. This criterion is met to a high degree of certainty at two sites in the eastern North Pacific Ocean, where colocated high‐quality probe and borehole heat flow data can be compared. A more direct test for stability is provided by long‐term observations of bottom water temperature. Previously published records and new data show temperature variations of only a few hundredths of a degree at sites in the central and eastern North Atlantic and the eastern North Pacific. Resultant gradient perturbations are geothermally insignificant (<5 mK m−1) at depths greater than 1–2 m below the seafloor, consistent with the uniformity of heat flow with depth observed in these areas. Geothermally problematic bottom water temperature variations are observed or inferred in the western North and South Atlantic and western South Pacific. Variations range up to ±0.15 K and are capable of producing gradient perturbations of up to 5 mK m−1 at depths as great as 5 m below the seafloor. While these data are instructive, their distribution is not adequate to provide general guidelines for estimating geothermal gradient perturbations. Data from shallower sites are needed in all oceans to define depth limits of acceptable bottom water temperature variability, and from other deep‐ocean locations where near‐source bottom water transients or vigorous deep‐water circulation dynamics are likely to be present.

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

Géli et al. [2001] (Eos, 17 July 2001, p. 317) recently presented three examples of particularly deep marine gravity‐corer temperature observations made to 18 m below the sea floor (mbsf). They noted large deviations of shallow temperatures from the trends of the deep thermal gradients and called into question the accuracy realized with short probes commonly used in detailed heat‐flow studies. We show this to be not a general concern, although attention must be given to perturbations from bottom water temperature variations in some deep ocean locations. We also show that the detail with which geographic and depth variability of heat flow must commonly be defined cannot be achieved with corer‐outrigger observations. Successful interpretation of widely spaced observations like those presented by Géli et al. [2001] requires complementary detailed data collected with multi‐penetration probes.

A constraint on the shear stress at the Pacific‐Australian plate boundary from heat flow and seismicity at the Kermadec forearc

New heat flow measurements and relocated hypocenters that constrain the subduction geometry of the Pacific plate at the Kermadec trench yield an estimate of shear stress on the thrust fault. With the exception of a few relatively high values (>60 mW m−2) on the upper forearc region near the active volcanic ridge, most of the 64 heat flow values along two profiles across the forearc range from 20 to 40 mW m−2. Corrections for bottom water temperature variations that caused nonuniform temperature gradients were required for most measurements. The means of the most reliable values for the northern and southern profiles are 28.9±5.8 mW m−2 (n = 33) and 28.9±7.2 mW m−2 (n = 19), respectively, and the measurements show no apparent systematic variation along the profiles over the range ∼50 to 150 km distance from the trench. The means of the 10 values at each of two sites on the Pacific plate seaward of the profiles are 57.2±6.3 and 60.2±6.6 mW m−2. Redeterminations of focal depths and fault plane solutions of earthquakes in the vicinity of the heat flow profiles indicate thrust faulting on a plane dipping 17°±2°. Calculated values of heat flow for a two‐dimensional analytical approximation to conduction through the upper plate, diffusion into the downgoing slab, and advection by that slab are consistent with either a uniform stress of ∼40±17 MPa along the thrust fault or stress increasing linearly at ∼0.5±0.2 MPa km−1 with distance from the trench axis. The comparable scatter in the heat flow measurements about those calculated from these simple stress distributions does not show one to be a better approximation than the other.

Tide-related variability of TAG hydrothermal activity observed by deep-sea monitoring system and OBSH

Hydrothermal activities were monitored by an ocean bottom seismometer with hydrophone (OBSH) and a composite measuring system (Manatee) including CTD, current meter, transmission meter and cameras at a small depression on the TAG hydrothermal mound in the Mid-Atlantic Ridge. Low-frequency pressure pulses detected by the hydrophone with semi-diurnal periodicity seem to correspond to cycles of hydrothermal upflow from a small and short-lived smoker vent close to the observing site. The peaks of pressure pulses are synchronous with the maximum gradient of areal strain decrease due to tidal load release. Microearthquakes with very near epicenters occur sporadically and do not appear to be directly correlatable to hydrothermal venting. Temporal variations in bottom water temperature also have semi-diurnal periodicity but are more complicated than the pressure events. Temperatures may be affected both by upwelling of hot water and by lateral flow of the bottom current changing its directions with ocean tide.

Estimation of thermal gradient and diffusivity by means of long- term measurements of subbottom temperatures at western Sagami Bay, Japan

Bottom water and subbottom temperatures were monitored for 1 year using three geothermal probes at a biological community area in western Sagami Bay, Japan. The subbottom temperatures are affected by bottom water temperature variations (BTV), most of which are negligible deeper than 50 cm below the sea floor. The steady-state geothermal gradient and subbottom depth of each sensor were determined from the average temperatures. The influence of BTV was been eliminated using the iterative, non-linear, least-squares method (Gauss-Newton method), so that the thermal diffusivities are determined with reasonable accuracy for each sensor of the probe. The result indicates that the long-term measurement of subbottom temperature can be a useful tool for determination of thermal gradient and diffusivity where BTV are significant. The average thermal diffusivity is 2.3 × 10 −7 m 2/s, which is consistent with typical values for marine sediments. The heat flow values are higher than 1 W/m 2 within 100 m of the community, and increase towards the community from 1 W/m 2 to 3.5 W/m 2. A transient temperature anomaly, starting on January 5th, 1992, appeared only at the lowermost sensor of the probe located 3 m to the east of the colony rim, with its amplitude up to 0.7°C. This anomaly may be attributable to a very localized heat source at a depth of 35 cm, possibly a warm fluid injection within a very thin linear fracture. A complicated flow regime may have occurred around the probe.

A Temperature Probe Survey on the Louisiana Shelf: Effects of Bottom-Water Temperature Variations: Geologic Note (1)

A survey of temperature gradients in the uppermost sediments of the Louisiana continental shelf was done in 1983 by Gulf Oil Corporation, using a 5-m-long temperature probe. The survey covered 450 sq mi, with an average grid spacing of less than 1 mi. The purpose was to detect heat flow anomalies due to subsurface fluid flow. However, thermal perturbations due to seasonal variation of bottom-water temperature mask the temperature gradients due to geological heat flow. Bottom-water temperature variations similar to those recorded near the survey area in 1963-1965 explain most of the observed spatial and temporal changes in shallow temperature-depth profiles. The data set from this detailed survey has been donated to a publicly accessible scientific database, to spur resear h into heat flow measurement on continental shelves.

Terrestrial heat flow from project CESAR, Alpha Ridge, Arctic Ocean

During two months in spring, 1983, a multidisciplinary study, project CESAR, was undertaken from the sea ice across the eastern Alpha Ridge, Arctic Ocean. In the geothermal program, 10 gradiometer profiles were obtained; 63 determinations of in situ sediment thermal conductivity were obtained with the same probe, and 714 measurements of conductivity using the needle probe method were obtained on nearby core. Weighted means of the thermal conductivity of the sediment are 1.26 W/mK ( in situ) and 1.34 W/mK (core), consistent with the compacted sediment encountered across the ridge and with the lithology. Calculated terrestrial heat flow values, corrected for the regional topography, range from 37 to 72 mWm −2; the average is 56+/−8 mWm −2. Some temperature and heat flow versus depth profiles exhibit non-linearities that can be explained by physically reasonable (but otherwise unsubstantiated) variations in bottom water temperatures preceding the measurements; models are hypothesized that reduce the curvatures. Two heat flow values considerably higher than others in the area may be explained by higher bottom water temperature over several years, while the low value is consistent with a recent deposition from a slump. This hypothetical modelling reduces the scatter of heat flows and reduces the average to 53+/−6 mWm −2. The CESAR heat flow is somewhat greater than expected for a purely continental fragment but is consistent with crust of oceanic origin. The heat flow is similar to values obtained in Cretaceous back-arc basins. Based on the oceanic heat flow-age relationship, the heat flow constrains the age of the ridge to 60–120 million years. The heat flow observed on other aseismic features in the world's oceans suggests that the Alpha Ridge has experienced no significant tectono-thermal event in the last 100 million years.

Towards determining the thermal state of old ocean lithosphere: heat-flow measurements from the Blake--Bahama outer ridge, north-western Atlantic

Summary. Detailed suites of heat-flow measurements have been completed at four sites on and to the east of the southern end of the Blake-Bahama outer ridge in the north-west Atlantic on ocean crust which ranges in age from roughly 115 to 155 Ma. The principal purpose of these measurements was to compare instrumental techniques and to examine the decay of heat flow with age on old ocean floor. Multi-penetration gradient measurements were made at the youngest site with a 2.5 m outrigger probe, and multipenetration heat flow measurements at the other three sites with a new 3.5 m violin-bow type instrument. In addition, at all four sites the gradient was measured using outriggers on a 9 m piston or gravity corer, and conductivities were measured on the sediment recovered. The average of thermal conductivities measured on cored sediments agreed to better than 2 per cent with the average of conductivities measured in situ at the oldest three sites. Such good agreement is surprising given the problems associated with making measurements on sediments recovered in cores. The greatest measurement errors in determining heat flow were found to be caused by temperature disturbances generated by the piston coring process. Gravity coring reduced these disturbances but provided a less representative section for measuring thermal conductivity. The violin-bow probe, which measures average interval conductivities in situ, provided the most reliable heat flow determinations. The accuracy of these determinations was limited primarily by the presence of thermal disturbances due to bottom water temperature variations. The heat flow averages at all four sites are virtually identical: they range from 47mW m-2 (1.12 μcal cm-2s-1) to 49mW m-2(1.17μcal cm-2s-1) with standard errors of less than 2 mW m-2 (0.05 μcal cm-2s-1). The constancy and the value of heat flow over lithosphere of this age, and the local shallowing of depth with increasing age in the region containing the heat flow sites cannot be explained by either a simple plate or a boundary layer lithosphere cooling model. The unexpectedly high heat flow and shallow depths may result from lithospheric thinning and perhaps dynamic forces associated with a thermal perturbation beneath the plate.

Heat transfer and intraplate deformation in the central Indian Ocean

Nineteen new heat flow measurements made across deformed oceanic lithosphere in the Central Indian Ocean, and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region. Many of the temperature‐depth profiles are nonlinear. Upward convection of water is the most likely explanation for the curvature of the temperature profiles, since other possible causes, including variations in bottom water temperatures, conductivity changes with depth in the sediments, and experimental error, can be eliminated. This interpretation reguires water velocities of the order of 7×10−8 m/s, which is unusual because the lithosphere is relatively old (72–82 m.y.) and a thick sedimentary cover (1−2.5 km) is present. These observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment‐water interface. We infer that extra heat is being generated at shallow depths (perhaps less than 35 km) in the plate, although the specific mechanism by which deformational energy is converted into heat is difficult to determine.

Heat transfer in the oceanic crust of the Brazil Basin

A detailed geothermal survey has been made on the western flank of the Mid‐Atlantic Ridge in the Brazil Basin where the crust is 20 m.y. old. The area is generally covered by a 150‐ to 200‐m thick layer of foraminiferal ooze, but basement outcrops are numerous. Sedimentary temperature profiles indicate anomalously low temperatures at the base of the sedimentary layer, which results from cold bottom water penetrating laterally into the upper igneous crust and absorbing much of the geothermal heat. Net influx rates of 10−8 m s−1 are implied by a simple aquifer model. A negative pressure drop across the sedimentary layer is required by this interpretation. Refraction studies indicate low seismic velocities (3.3 to 4.5 km s−1) near the top of layer 2, and a large velocity gradient with depth. The velocity structure in the upper kilometer of the crust is related to the porosity, from which it is inferred that permeabilities there are high. Many of the 52 temperature profiles show a significant decrease in gradient with depth. The curvature can be attributed to one or a combination of the following causes: (1) upward advection of pore water at rates of 10−7 s−1, (2) bottom water temperature variations, (3) variation of conductivity with depth, and (4) experimental errors. Interpretation of curvature in terms of an advective model leads to results hard to reconcile with present knowledge of pressure variations and permeabilities of the oceanic crust.

The effects of the geothermal gradient on amino acid racemization

The “second region” of the kinetic curve for the racemization (epimerization) of isoleucine in foraminifera is defined from previously published data corrected for the thermal history experienced by the samples. These kinetic parameters are applied to racemization data from Deep Sea Drilling Project Sites 332A and 333 from the Deep Drill Valley, Mid-Atlantic Ridge, and Site 148 from the Aves Ridge, in order to determine whether there are any observable effects due to the geothermal gradient. The data for Site 148 clearly show an increase of temperature with depth. The data for the two sites of the Mid-Atlantic Ridge do not show this clear relationship. When bottom water temperature variations are taken into consideration, the effects of the geothermal gradient become apparent. Since the degree of racemization is dependent upon both age and temperature, a knowledge of the age of a sample places constraints upon its thermal history, and hence on the heat flow at the location since deposition of the sample. The crude heat flow models thus developed are compatible with present geologic and geophysical information. It appears probable that detailed heat flow models may be developed by improving analytical precision.

Heat flow through the Dead Sea rift

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.

Geothermal gradients in British lake sediments

Temperatures have been measured in the water and sediments of Loch Ness and Windermere to investigate the possibility of determining the geothermal heat flow through the bottom of deep British lakes. The accuracy is limited by corrections for annual and long term variations in bottom water temperatures.In Loch Ness (max depth 232 m) the annual temperature variation of the deep water is normally <0.5°C. There is evidence of a benthic water layer overlying the sediments in the deepest parts of the loch in which the annual variations are probably not more than 0.2°C. When corrections are made, the mean of 13 measurements of heat flow in Loch Ness is 1.45 µcal cm−2 s−1. Values increase steadily from 1.03 µcal cm−2 s−1 in the northeast to 1.74 in the southwest, with local high values near the Foyers granite intrusion.Annual variations in bottom water temperature in Windermere (max depth 65 m) are >2.5°C, though variations in the benthic layer are less than this. Reliable geothermal heat flow measurements were only possible because an extended series of measurements was available. The mean of three acceptable results was 1.69 µcal cm−2 s−1.

Geothermal measurements in five small lakes of northwest Ontario

The heat flow through the floors of five small lakes of known thermal history on the Canadian Shield was measured with a modified Bullard probe. A small correction for seasonal bottom water temperature variations was applied to temperature gradient measurements, and the heat flows are corrected for glaciation, lateral temperature gradients, sedimentation rates, and lateral thermal conductivity changes. Four lakes have an average heat flow of 49 ± 4 mW/m2 (1.2 ± 0.1 μcal/cm2 s). A high heat flow in the fifth lake is thought due to unusual refraction effects. The heat generation–heat flux combination yields a point that falls near accepted lines for the Canadian Shield.

Heat flow measurements in the inlets of southwestern British Columbia

The geothermal heat flux has been measured at 15 sites in the inlets of southwestern British Columbia by using the ocean probe technique. Corrections have been applied for variations in bottom water temperature, sedimentation, thermal refraction by the sediment prism, topography, warm-rim effect, Pleistocene thermal history, uplift, and erosion. The accuracy of the values is about ±0.3 μcal cm−2 s−1 (±13mW m−2). The relative accuracy between the values is about half of this range. The data indicate a pattern of low heat flow from the coast inland about 200 km to the heads of the inlets (13 values average 0.9 μcal cm−2 s−1 or 37 mW m−2). Stations at the heads of two inlets (1.5 μcal cm−2 s−1 or 63 mW m−2) mark the transition to high heat flow further inland (about 2.0 μcal cm−2 s−1 or 84 mW m−2). From west to east the heat flow transition coincides with the first occurrence of recent volcanic centers and hot springs, the transition from high to low Bouguer gravity, and the transition to low deep crustal and upper mantle electrical resistivity and to recent regional uplift. The coast low heat flow is explained partly by low crustal radioactive heat production and in part by the heat sink effect of the cold oceanic Juan de Fuca plate being subducted along the continental margin. The high inland heat flow probably is produced by the upward convective transport of heat by magma and thinning of the lithosphere that occurs when the sinking plate reaches a depth where temperatures are sufficient for partial melting. Low mantle temperature and thus high density in both the continental lithosphere and the underlying subducted oceanic lithosphere under the coast zone explain the high Bouguer gravity and thick crust in this region compared with the low gravity and thin crust inland. To the north of about 51°N where at present there is transform fault motion along the continental margin, there probably was subduction prior to about 10 m.y. ago. The gravity and crustal thickness data suggest that the high density in the subducted oceanic lithosphere has disappeared but that upwelling and a hot thin continental lithosphere persist inland of 200 km from the continental edge.

Heat flow through the Southern California Borderland

Fifty three new heat flow measurements together with 16 published data indicate that the heat flow (average of best values equals 1.74 μcal/cm2 s) through basins of the southern California borderland is intermediate between heat flows through the Sierra Nevada and the Basin and Range provinces. In general, geothermal gradients in the borderland decrease with depth in the sediments, corresponding to an increase in thermal conductivity of 2–3% per meter. Owing to rapid sedimentation, surface heat fluxes may be locally masked from their steady state values by as much as 30%. Topographic corrections are usually less than 5%. Seasonal bottom water temperature variations are insignificant, but long-term climatic change in the last 37,000 yr may have reduced the gradient by 2–3%. Anomalous values were found in close proximity to turbidity current channels. The heat flow distribution shows two significant geographic trends. First, heat flows increase systematically southeastward in response to a late Cenozoic southeastward-migrating triple junction. Second, heat flows increase slightly toward the continent, suggesting that the offshore basins were formed progressively landward. In specific basins the measured values are uniform (standard deviation ≦ 0.1 HFU, where 1 HFU = 1 μcal cm−2 s−1), but in other basins, only the corrected values are uniform. In the Santa Cruz Basin the corrected values are dominated by a north-south linear trend.

Geothermal measurements in deep-sea drill holes

A method of measuring the in situ sediment temperatures in deep bore holes drilled to depths of several hundred meters or more beneath the sea floor has been developed. The technique, as presently used aboard the Deep-Sea Drilling Project (DSDP) drilling vessel Glomar Challenger, involves the emplacement of a temperature sensor, located below a self-contained digital temperature recorder package, a short distance into the undrilled, thermally undisturbed sediment at the bottom of the drill hole. By measuring the in situ temperature at various depths in a single drill hole it is possible to calculate the thermal gradient for various intervals in the hole. This information, in conjunction with thermal conductivity data measured aboard ship on the sediment cores recovered from the drill hole, permits computation of the heat flow through the oceanic crust. Heat flow values measured in deep drill holes in the Indian and Pacific oceans and in the Bering and Red seas are in generally good agreement with the regional geothermal flux as determined by conventional near-surface heat flow measurements, suggesting that the thousands of existent shallow heat flow values are representative of the earth's heat flux. Where multiple downhole temperature measurements made at one site permit calculation of interval heat flow values, there is no consistent indication of a significant vertical increase or decrease in heat flux, such as might be caused by long-term changes in bottom water temperature or the upward migration of interstitial fluids. We note, however, that a more detailed set of temperature measurements in a single hole is required to verify this conclusion. Downhole heat flow values made within a specific physiographic region, such as the Red Sea or the Ninety East ridge, appear to be less variable than, but equal to, the heat flow values calculated using thermal gradient measurements made at shallower depths beneath the sea floor. This observation is in accordance with theoretical considerations which indicate that temperature measurements in deep drill holes are less susceptible than conventional heat flow measurements to the disturbing thermal effects of small-scale surface topography, short-term variations in bottom water temperatures, and local sedimentary processes (slumping, erosion).