Average Thermal Structure Research Articles

Mafic High‐Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings

AbstractThe oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure‐temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 °C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction.

The Night Time Radiative Transport between the Earth's Surface, its Atmosphere, and Free Space

Estimates are made for the exchange of radiative fluxes between the Earth's surface, its atmosphere, and free space on the basis of their average thermal structure and estimated values for the emissivity and absorptivity of those individual entities. The radiative fluxes obtained reflect nighttime conditions but also indicate the components of those fluxes that must be subtracted from the input solar flux for daytime conditions. The estimates show that infrared radiant energy is lost to free space from both the Earth's surface and its atmosphere. Under the normal atmospheric temperature structure there is no net radiation from the atmosphere that reaches the Earth's surface to be absorbed by it, and thus the so-called “greenhouse effect”, as it is traditionally depicted, is devoid of physical reality. The only exception occurs during atmospheric inversion conditions, but even in the presence of such rare inversions, the radiant power lost to free space from the atmosphere as a whole overwhelms the radiant power absorbed by the Earth's surface from the warmer inversion layer above it. Inversion conditions are thus the only ones in which the so-called “greenhouse effect” can possibly have any form of physical reality. But such inversion conditions, however, are present over only a small fraction of the Earth's surface for limited periods of time. In addition, since the recent increases in atmospheric CO2 concentrations have a trivial effect on the atmosphere's total emissivity, the effect of those recent CO2 increases on the overall infrared radiative flux balance is insignificant. The analysis also suggests a cloud cooling mechanism for the Earth's surface and its atmosphere that is significantly different from the one traditionally cited. Cloud blockage such as that reflected in the Earth's albedo acts symmetrically with respect to both the input heating by solar radiation and the output cooling by infrared radiation to free space. On the other hand, it is shown that the increasing infrared emissivity of the atmosphere caused by the presence of clouds provides an effective cooling mechanism that is asymmetric.

Effect of metamorphic reactions on thermal evolution in collisional orogens

AbstractThe effects of metamorphic reactions on the thermal structure of a collisional overthrust setting are examined via forward numerical modelling. The 2D model is used to explore feedbacks between the thermal structure and exhumation history of a collisional terrane and the metamorphic reaction progress. The results for average values of crustal and mantle heat production in a model with metapelitic crust composition predict a 25–40 °C decrease in metamorphic peak temperatures due to dehydration reactions; the maximum difference between the P–T–t paths of reacting and non‐reacting rocks is 35–45 °C. The timing of the thermal peak is delayed by 2–4 Myr, whereas pressure at peak temperature conditions is decreased by more than 0.2 GPa. The changes in temperature and pressure caused by reaction may lead to considerable differences in prograde reaction pathways; the consumption of heat during dehydration may produce greenschist facies mineral assemblages in rocks that would have otherwise attained amphibolite facies conditions in the absence of reaction enthalpy. The above effects, although significant, are produced by relatively limited metamorphic reaction which liberates only half of the water available for dehydration over the lifetime of the prograde metamorphism. The limited reaction is due to the lack of heat in a model with the average thermal structure and relatively fast erosion, a common outcome in the numerical modelling of Barrovian metamorphism. This problem is typically resolved by invoking additional heat sources, such as high radiogenic heat production, elevated mantle heating or magmatism. Several models are tested that incorporate additional radiogenic heat sources; the elevated heating rates lead to stronger reaction and correspondingly larger thermal effects of metamorphism. The drop in peak temperatures may exceed 45 °C, the maximum temperature differences between the reacting and non‐reacting P–T–t paths may reach 60 °C, and pressure at peak temperature conditions is decreased by more than 0.2 GPa. Field observations suggest that devolatilization of metacarbonate rocks can also exert controls on metamorphic temperatures. Enthalpies were calculated for the reaction progress recorded by metacarbonate rocks in Vermont, and were used in models that include a layer of mixed metapelite–metacarbonate composition. A model with the average thermal structure and erosion rate of 1 mm year−1 can provide only half of the heat required to drive decarbonation reactions in a 10 km thick mid‐crustal layer containing 50 wt% of metacarbonate rock. Models with elevated heating rates, on the other hand, facilitated intensive devolatilization of the metacarbonate‐bearing layer. The reactions resulted in considerable changes in the model P–T–t paths and ∼60 °C drop in metamorphic peak temperatures. Our results suggest that metamorphic reactions can play an important role in the thermal evolution of collisional settings and are likely to noticeably affect metamorphic P–T–t paths, peak metamorphic conditions and crustal geotherms. Decarbonation reactions in metacarbonate rocks may lead to even larger effects than those observed for metapelitic rocks. Endothermic effects of prograde reactions may be especially important in collisional settings containing additional heat sources and thus may pose further challenges for the ‘missing heat’ problem of Barrovian metamorphism.

Thermal convection heated both volumetrically and from below: Implications for predictions of planetary evolution

Solid-state thermal convection, as a model for the internal dynamics of planetary mantles, is generated both by volumetric heating and heating from below. A series of 2D and 3D numerical experiments is described where a bottom heat flux is prescribed as well as a constant fraction of volumetric heating with either free-slip or no-slip conditions for the two horizontal boundaries. The assumption that hot plumes rising in the interior of the layer act as a volumetric heat source leads to a first order scaling of thermal boundary layers. Cases with a no-slip boundary agree well with this scaling while cases with free-slip systematically deviate: a decrease of 20–30% is observed for the temperature difference across the boundary layer when the fraction of heating from below is increased from 0 to 1. These differences are attributable to a velocity structure near the boundary varying with the fraction of volumetric heating. The main planetary implications are that (i) the average thermal structure of the bulk mantle and lithosphere are not good indicators of the distribution of heat sources between the interior (e.g. radioactive heating, secular cooling) and the lower interface (e.g. core cooling); (ii) in contrast, the nature of volcanism (whether it is localized, hot spot like or widespread) reflect this distribution and should thus help to constrain the global thermal structure as well as the existence of a magnetic dynamo.

Thermochemical interpretation of one-dimensional seismic reference models for the upper mantle: evidence for bias due to heterogeneity

SUMMARY A 1-D reference model for the mantle that is physically meaningful would be invaluable both in geodynamic modelling and for an accurate interpretation of 3-D seismic tomography. However, previous studies have shown that it is difficult to reconcile the simplest possible 1-D physical model—1300°C adiabatic pyrolite—with seismic observations. We therefore generate a set of alternative 1-D thermal and chemical mantle models, down to 900 km depth, and compare their properties with seismic data. We use several different body and surface wave data sets that provide complementary constraints on mantle structure. To assess the agreement between our models and seismic data, we take into account the large uncertainties in both the elastic/anelastic parameters of the constituent minerals, and the thermodynamic procedures for calculating seismic velocities. These uncertainties translate into substantial differences in seismic structure. However, in spite of such differences, subtle trends remain. We find that models which attain (1) higher velocity gradients between 250 and 350 km; (2) higher velocity gradients in the lower transition zone; and (3) higher average velocities immediately beneath the 660-discontinuity, than 1300°C adiabatic pyrolite—either via a temporary shift to lower temperatures, and/or a change to a seismically faster chemical composition—provide a significantly better fit to the seismic data than adiabatic pyrolite. This is compatible with recent thermochemical dynamic models by Tackley et al. in which average thermal structure is smooth and monotonous, but average chemical structure deviates substantially from pyrolite above, in, and below the transition zone. Our results suggest that 1-D seismic reference models are being systematically biased by a complex 3-D chemical structure. This bias should be taken into account when attempting quantitative interpretation of seismic anomalies, since those very anomalies contribute to the 1-D average signal.

Energy transport, overshoot, and mixing in the atmospheres of M-type main- and pre-main-sequence objects

We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence (PMS) objects. Despite the complex chemistry encountered in these cool atmospheres a reasonably accurate representation of the radiative transfer is possible, even in the context of time-dependent and three-dimensional models. The models provide detailed information about the morphology of M-type granulation and statistical properties of the convective surface flows. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency was expressed in terms of an equivalent mixing-length parameter alpha in the formulation of mixing-length theory (MLT) given by Mihalas (1978). Alpha amounts to values around 2 for matching the entropy of the deep, adiabatically stratified regions of the convective envelope, and lies between 2.5 and 3.0 for matching the thermal structure of the deep photosphere. For current spectral analysis of PMS objects this implies that MLT models based on alpha=2.0 overestimate the effective temperature by 100 K and surface gravities by 0.25 dex. The average thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e., stays close to radiative equilibrium conditions. Our models suggest that the rate of mixing by convective overshoot declines exponentially with geometrical distance to the Schwarzschild stability boundary. It increases at given effective temperature with decreasing gravitational acceleration.

Analytical representation of the plasmasphere electron temperature distribution based on Akebono data

A new approach is used to reveal the average thermal structure of plasmasphere at altitudes between 1000 and 10,000 km. This paper only considers the plasmasphere inside L = 3, and this region of space is sliced at L = 2 to one internal (denoted as equatorial) and one external (denoted as midlatitude) part. Each of these parts is divided into four altitude/local time zones: northern and southern daytime and northern and southern nighttime. Electron temperature (Te) distribution measured along every individual orbit within each zone is approximated by simple expressions that are functions of the geomagnetic latitude or L. The fitting is performed independently to each zone, and two coefficients are extracted from the individual fits: Te value at the equatorward border and the coefficient scaling the latitudinal shape. These coefficients are accumulated in each zone in order to obtain its average Te structure as a function of altitude. The fitting coefficients exhibit large scatter, which reflects the large day‐to‐day variability of plasmasphere Te. The fitting error is estimated at 6% and 12% for the equatorial and midlatitude zones, respectively. The average fit coefficients are used to check for seasonal variations in each zone and for a possible hemispheric asymmetry. It is determined that the winter sunlit plasmasphere is hotter than the summer one; the case for nighttime is reverse. The day/night amplitude is larger in the winter than in the summer, and the seasonal differences are more pronounced in the Southern Hemisphere. The latitude gradients of Te reveal a hemispheric asymmetry, which is largest during northern daytime winter. Solar activity is found to affect the plasmasphere Te. During the day at 3000 km and L = 2, Te increases by ∼2000°K when F10.7 changes from 70 to 300. The nighttime increase is ∼1000°K. At the equator, Te shows weaker dependence on solar activity. The average heat flux through 3000 km altitude is estimated at 3.7 × 109 (eV cm−2 s−1) during the day and 1.6 × 108 (eV cm−2 s−1) during the night.

Numerical simulations of surface convection in a late M-dwarf

Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD) simulations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of a prototypical late-type M-dwarf (Teff~2800K log(g)=5.0, solar chemical composition). The RHD models predict stellar granulation qualitatively similar to the familiar solar pattern. Quantitatively, the granular cells show a convective turn-over time scale of ~100s, and a horizontal scale of 80km; the relative intensity contrast of the granular pattern amounts to 1.1%, and root-mean-square vertical velocities reach 240m/s at maximum. Deviations from radiative equilibrium in the higher, formally convectively stable atmospheric layers are found to be insignificant allowing a reliable modeling of the atmosphere with 1D standard model atmospheres. A mixing-length parameter of alpha=2.1 provides the best representation of the average thermal structure of the RHD model atmosphere while alternative values are found when fitting the asymptotic entropy encountered in deeper layers of the stellar envelope alpha=1.5, or when matching the vertical velocity field alpha=3.5. The close correspondence between RHD and standard model atmospheres implies that presently existing discrepancies between observed and predicted stellar colors in the M-dwarf regime cannot be traced back to an inadequate treatment of convection in the 1D standard models. The RHD models predict a modest extension of the convectively mixed region beyond the formal Schwarzschild stability boundary which provides hints for the distribution of dust grains in cooler (brown dwarf) atmospheres.

Thermal structure of Jupiter's atmosphere near the edge of a 5‐μm hot spot in the north equatorial belt

Thermal structure of the atmosphere of Jupiter was measured from 1029 km above to 133 km below the 1‐bar level during entry and descent of the Galileo probe. The data confirm the hot exosphere observed by Voyager (∼900 K at 1 nanobar). The deep atmosphere, which reached 429 K at 22 bars, was close to dry adiabatic from 6 to 16 bars within an uncertainty ∼0.1 K/km. The upper atmosphere was dominated by gravity waves from the tropopause to the exosphere. Shorter waves were fully absorbed below 300 km, while longer wave amplitudes first grew, then were damped at the higher altitudes. A remarkably deep isothermal layer was found in the stratosphere from 90 to 290 km with T ∼ 160 K. Just above the tropopause at 260 mbar, there was a second isothermal region ∼25 km deep with T ∼ 112 K. Between 10 and 1000 mbar, the data substantially agree with Voyager radio occultations. The Voyager 1 equatorial occultation was similar in detail to the present sounding through the tropopause region. The Voyager IRIS average thermal structure in the north equatorial belt (NEB) approximates a smoothed fit to the present data between 0.03 and 400 mbar. Differences are partly a result of large differences in vertical resolution but may also reflect differences between a hot spot and the average NEB. At 15 < p < 22 bars, where it was necessary to extrapolate the pressure calibration to sensor temperatures up to 118°C, the data indicate a stable layer in which stability increases with depth. Consistent with the indication of stability, regular fluctuations in probe vertical velocity imply gravity waves in this layer. At p > 4 bars, probe descent velocities derived from the data are consistently unsteady, suggesting the presence of large‐scale turbulence or gravity waves. However, there was no evidence of turbulent temperature fluctuations >0.12 K. A conspicuous pause in the rate of decrease of descent velocity between 1.1 and 1.35 bars, where a disturbance was also detected by the two radio Doppler experiments, implies strong vertical flow in the cloud seen by the probe nephelometer. At p < 0.6 bar, measured temperatures were ∼3 K warmer than the dry adiabat, possible evidence of radiative warming. This could be associated with a tenuous cloud detected by the probe nephelometer above the 0.51 bar level. For an ammonia cloud to form at this level, the required abundance is ∼0.20 × solar.

Temperature minima in the average thermal structure of the middle mesosphere (70–80 km) from analysis of 40‐ to 92‐km SME global temperature profiles

Global temperatures have been derived for the upper stratosphere and mesosphere from analysis of Solar Mesosphere Explorer (SME) limb radiance profiles. The analysis of additional SME wavelength radiances at 304, 313, and 442 nm provides for extension of the original SME 60‐ to 90‐km temperature climatology (Clancy and Rusch, 1989a) to a much expanded altitude coverage of 40–92 km. The SME temperatures represent fixed local time observations at 1400–1500 LT, with partial zonal coverage of 3–5 longitudes per day over the 1982–1986 period. These new SME temperatures are compared to the CIRA 86 climatology (Fleming et al., 1990) as well as stratospheric and mesospheric sounder (SAMS); (Barnett and Corney, 1984), National Meteorological Center (NMC); (Gelman et al., 1986), and individual lidar and rocket observations. Significant areas of disagreement between the SME and CIRA 86 mesospheric temperatures are 10 K warmer SME temperatures in the 55‐ to 65‐km altitude region and 10–20 K warmer SME temperatures at altitudes above 80 km. The 1981–1982 SAMS temperatures are in much closer agreement with the SME temperatures between 40 and 75 km. Although much of the SME‐CIRA 86 disagreement probably stems from the poor vertical resolution of the observations comprising the CIRA 86 model, some portion of the differences may reflect 5‐ to 10‐year temporal variations in mesospheric temperatures. The CIRA 86 climatology is based on 1973–1978 measurements. Relatively large (1 K/yr) 5‐ to 10‐year trends in temperatures as functions of longitude, latitude, and altitude have been observed for both the upper stratosphere (Clancy and Rusch, 1989a) and mesosphere (Clancy and Rusch, 1989b; Hauchecorne et al. 1991). The SME temperatures also exhibit enhanced amplitudes for the semiannual oscillation (SAO) of upper mesospheric temperatures at low latitudes, which are not evident in the CIRA 86 climatology. The so‐called mesospheric “temperature inversions” at wintertime midlatitudes, which have been observed by ground‐based lidar (Hauchecorne et al., 1987) and rocket in situ measurements (Schmidlin, 1976), are shown to be a climatological aspect of the mesosphere, based on the SME observations. The winter midlatitude “temperature inversion” is evident in both winter hemispheres and appears more distinctly as a low‐altitude (75 km) temperature minimum at southern midlatitudes. Furthermore, the SME temperatures indicate the related presence of low‐latitude temperature minima at 80‐km altitude during the equinoxes. Both the low‐latitude semiannual minima and the midlatitude annual minima in middle mesospheric temperatures appear to result from the strong SAO in mesospheric temperatures.

Wind‐induced internal seiches in Lake Zurich observed and modeled1

During August and September 1978, 31 current meters and 120 temperature sensors were deployed to record every 10 or 20 min at various depths at 12 moorings (with wind speed and direction at three moorings) in Lake Zurich. We explore here the baroclinic (internal seiche) response to wind impulses, observed as fluctuations in isotherm depth and current speeds. Those fluctuations and their energy spectra are compared with the predictions of two models fitted to basin topography and to the observed average thermal structure: a two‐layered variable‐depth (TVD) model developed by D. J. Schwab, fitted to basin topography and incorporating a two‐dimensional horizontal grid, and C. H. Mortimer’s two‐layered modification of a simpler procedure originally developed for surface seiche calculations by A. Defant.The dominant responses to wind impulses were internal seiches of the first longitudinal mode (average period 44 h). Weaker signals from the second (24 h) and third (17 h) modes were also seen in spectra of temperature and current fluctuations. The models displayed patterns of thermocline displacement and current which, in periodicity and structure, were closely similar to those observed. Predictions of the Defant model were less precise, particularly for current.Founded on linear theory and neglecting the effects of rotation, the models were unable to reproduce two features occasionally seen in the lake motions: clockwise or anticlockwise rotation of current direction; and internal surges arising when storms induced large‐amplitude downstrokes of the thermocline at one basin end or the other. The lake’s internal response was principally dependent on the timing, strength, and duration of the wind impulse, relative to and interacting with internal seiche motions already in progress.

The Role of Vertical Motion in the Heat Budget of the Upper Northeastern Pacific Ocean

Abstract A study of the heat budget of the upper northeastern Pacific Ocean demonstrates the importance of heat changes due to vertical motion. Bathythermograph observations are used to form time series of monthly average thermal structure at ocean weather station November (30°N, 140°W) and various 5° and 2° quadrangles between California and Hawaii. These time series are first used to explore correlations between heat content in a 250 m thick layer and the depth of the 14°C isotherm. High negative correlations between these quantities suggest that vertical motion at 250 m, indicated by the fluctuations of the 14°C isotherm, must play a significant role in altering the heat content of the upper layer. A heat budget equation is derived that includes the heat changes due to vertical motion. At all test locations, correlations between terms show that a stronger relationship exists between surface heat exchange and heat content change when the heat changes due to vertical motion are included. The mean square ...