Henry N. Pollack

On the regional variation of heat flow, geotherms, and lithospheric thickness☆

Pollack, H.N. and Chapman, D.S., 1977. On the regional variation of heat flow, geotherms, and lithospheric thickness. Tectonophysics, 38: 279-296. Geotherm families in which surface heat flow is the principal independent variable have been constructed for continental and oceanic lithospheres. The continental model is characterized by geotherms in which surface heat flow is in equilibrium with heat flowing into the lithosphere at its base plus heat generated by radioactive decay within the lithosphere. The model accommodates the regional variation of the surface heat flow with proportional variations in the radioactivity of the surficial enriched zone and in the deeper heat flow. The proportionality is dictated by a new and general linear relationship between reduced heat flow and mean heat flow for a region (o* r 0.6 ifo), which enables both q* and the mean heat production of the enriched zone to be estimated from knowledge of the mean surface heat flow of a province. The oceanic model is characterized by the transient cooling of a semi-infinite medium with an initial temperature gradient and some near-surface radiogenic heat production. The model yields a heat flow in satisfactory agreement with observations in the oldest ocean basins. The depth at which both the oceanic and continental geotherms reach -0.85 of the melting temperature is shown to be a consistent estimator of the depth to the top of the low-velocity channel, or the thickness of the high-velocity lid overlying the channel. We identify the lid as synonymous with the lithosphere, and produce a global map of lithospheric thickness based on the regional variation of surface heat flow. The lithosphere is less than 100 km thick over most of the globe, but thickens appreciably and becomes more viscous beneath the Precambrian shields and platforms, regions of low heat flow. These characteristics of shields are consistent with recently reported models of the driving mechanisms of the plate system, which require greater retarding forces beneath plates with large continental areas.

Temperature trends over the past five centuries reconstructed from borehole temperatures

For an accurate assessment of the relative roles of natural variability and anthropogenic influence in the Earths climate, reconstructions of past temperatures from the pre-industrial as well as the industrial period are essential. But instrumental records are typically available for no more than the past 150 years. Therefore reconstructions of pre-industrial climate rely principally on traditional climate proxy records, each with particular strengths and limitations in representing climatic variability. Subsurface temperatures comprise an independent archive of past surface temperature changes that is complementary to both the instrumental record and the climate proxies. Here we use present-day temperatures in 616 boreholes from all continents except Antarctica to reconstruct century-long trends in temperatures over the past 500 years at global, hemispheric and continental scales. The results confirm the unusual warming of the twentieth century revealed by the instrumental record, but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies. The strength of temperature reconstructions from boreholes lies in the detection of long-term trends, complementary to conventional climate proxies, but to obtain a complete picture of past warming, the differences between the approaches need to be investigated in detail.

Cratonization and thermal evolution of the mantle

Abstract The stabilization of continental lithosphere to form cratons is accomplished by volatile loss from the upper mantle during magmatic events associated with the formation of continental crust. Volatile depletion elevates the solidus and increases the stiffness of the mantle residuum, thereby imparting a resistance to subsequent melting and deformation. Freeboard is maintained in part by the buoyancy associated with an increased Mg/(Mg + Fe) ratio in the mantle residuum following extraction of crustal material. Augmented subcratonic seismic velocities derive from the same shift in this ratio. The higher effective viscosity of the stabilized subcratonic upper mantle inhibits its entrainment in mantle convection, and locally thickens the conductive boundary layer. Heat approaching from greater depths is diverted away from the stiff craton to other areas that continue to transfer heat by convection, thus yielding a low surface heat flow within cratons. Cratonization by devolatilization and petrologic depletion was most effective in the Archean and has diminished in effectiveness over geologic time as the mantle temperature has fallen because of the declining store of internal heat. From the Archean to the present that ascending mantle material which has undergone partial melting has encountered the solidus at progressively shallower depth, has remained supersolidus over a smaller depth range, has temperatures which have exceeded the solidus by lesser amounts, has undergone diminishing degrees of partial melting, and has experienced less thorough devolatilization. At a given time the rate of production of continental crust is likely to be proportional to the depth extent and fraction of partial melting. Integration of the partial melt zone over time yields a growth curve that is similar to some continental crustal growth curves inferred from isotopic evolution.

Detection of Human Influence on a New, Validated 1500-Year Temperature Reconstruction

Abstract Climate records over the last millennium place the twentieth-century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late-twentieth-century warming. A calibration method is suggested that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate. High variability in reconstructions does not hamper the detection of greenhouse gas–induced climate change, since a substantial fraction of the variance in these reconstructions from the ...

A global analysis of heat flow from Precambrian terrains: Implications for the thermal structure of Archean and Proterozoic lithosphere

Previous studies of heat flow from Precambrian terrains have yielded two different relationships: a global temporal relationship between heat flow and tectonic age, and a regional spatial relationship between heat flow and the proximity of Archean cratons. We analyze heat flow from tectonically stable Precambrian terrains worldwide to address questions associated with these two heat flow patterns: (1) Is the spatial relationship a global pattern? (2) Do the two heat flow relationships have a common underpinning? In answer to the first question, our data analysis reveals a widespread spatial pattern between heat flow and the proximity of Archean cratons, which is characterized by low heat flow in Archean cratons and Proterozoic terrains adjacent to cratonic margins and higher heat flow in Proterozoic terrains that are more than a few hundred kilometers from a craton. To address the second question, we examine three previous interpretative models of Precambrian heat flow: (1) simple cooling of a thermal boundary layer, (2) thicker lithosphere in Archean terrains than in Proterozoic, and (3) greater heat production in Proterozoic crust than in Archean. Model 1 predicts essentially no change in heat flow in terrains older than ∼1.5 Ga and therefore does not likely provide a common underpinning for the Precambrian heat flow patterns. Models 2 and 3, when combined with the special structural configuration of sutures, can independently yield both the spatial and temporal heat flow distributions and thus alone or together may be considered candidates to explain the Precambrian heat flow patterns. However, thermal models of the lithosphere in which the heat flow patterns are explained entirely by variations in crustal heat production cannot satisfy constraints on upper mantle temperatures beneath Archean cratons derived from xenolith thermobarometry. The thermobarometry constraints can be satisfied by models in which differences in lithospheric thickness are the principal factor controlling the surface heat flow distributions. If the heat flow patterns result primarily from variations in lithospheric thickness, then a different temporal heat flow relationship for Precambrian terrains can be proposed, one in which heat flow varies inversely with the age of stabilization of the lithosphere.

Regional geotherms and lithospheric thickness

Continental and oceanic geotherm families parametric in surface heat flow intersect the mantle solidus at a depth coincident with the top of the seismic low-velocity zone, thus allowing surface heat-flow variations to be used to map the thickness of the lithosphere on a global scale. Thermal models were developed that predict a lithospheric thickness of a few tens of kilometers in young oceans and continental orogenic provinces and more than 300 km in shield areas. The variable thermal structue of the mantle implies a greater viscosity beneath shields, which offers an explanation for the observed retarded motions of plates that bear shields.

Global Change and the Earth System

The Earth system in recent years has come to mean the complex interactions of the atmosphere, biosphere, lithosphere and hydrosphere, through an intricate network of feedback loops. This system has operated over geologic time, driven principally by processes with long timescales. Over the lifetime of the solar system, the Sun has slowly become more radiant, and the geography of continents and oceans basins has evolved via plate tectonics. This geography has placed a first-order constraint on the circulation of ocean waters, and thus has strongly influenced regional and global climate. At shorter timescales, the Earth system has been influenced by Milankovitch orbital factors and occasional exogenous events such as bolide impacts.

Global heat flow: A new look

A global heat flow map has been derived from existing observations supplemented in areas without data by an empirical predictor based on tectonic setting and age. In continental areas the predictor is based on the observed correlation of heat flow with age of last tectono-thermal event, and in oceanic regions on the observed relation of heat flow to age of ocean floor. The predictor was used to assign mean heat flow values to 5° × 5° grid areas on the globe, weighted according to the relative area of tectonic provinces represented. A spherical harmonic analysis to degree 12 of the heat flow field yields a mean value of 59 mW m−2, a rms residual of 13 mW m−2, and an amplitude spectrum which decreases gradually and almost monotonically fromn = 1. The spherical harmonic representation of the heat flow field is free of the unreal distortions which have characterized earlier analyses based on a geographically sparse data set. Areas with residuals greater than 15 mW m−2 comprise less than 19% of the area of the globe, thus indicating that most heat flow provinces have characteristic dimensions adequately represented in a 12-degree analysis.

Borehole climate reconstructions: Spatial structure and hemispheric averages

[1] Ground surface temperature (GST) reconstructions determined from temperature profiles measured in terrestrial boreholes, when averaged over the Northern Hemisphere, estimate a surface warming of � 1 K during the interval AD 1500– 2000. Other traditional proxy-based estimates suggest less warming during the same interval. Mann et al. [2003a] have raised two issues with regard to borehole-based reconstructions. The first focuses on the need for spatial gridding and area-weighting of the ensemble of borehole-based GST reconstructions to yield an average hemispheric reconstruction. The second asserts that application of optimal detection techniques show that the GST only weakly displays the spatial structure of the surface air temperature (SAT). We demonstrate the consistency of GST warming estimates by showing that over a wide range of grid element area and occupancy weighting schemes, the five-century GST change falls in the range of 0.89–1.05 K. We examine the subhemispheric spatial correlation of GST and SAT trends at various spatial scales. In the 5-degree grid employed for optimal detection, we find that the majority of grid element means are determined from three or fewer boreholes, a number that is insufficient to suppress site-specific noise via ensemble averaging. Significant spatial correlation between SAT and GST emerges in a 5-degree grid if low-occupancy grid elements are excluded, and also in a 30-degree grid in which grid element means are better determined through higher occupancy. Reconstructions assembled after excluding low-occupancy grid elements show a five-century GST change in the range of 1.02–1.06 K. INDEX TERMS: 1645 Global Change: Solid Earth; 1699 Global Change: General or miscellaneous; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; KEYWORDS: boreholes, paleoclimate, spatial analysis, surface temperature

Diversion of heat by Archean cratons: a model for southern Africa

Abstract The surface heat flow in the interior of Archean cratons is typically about 40 mW m −2 while that in Proterozoic and younger terrains surrounding them is generally considerably higher. The eighty-four heat flow observations from southern Africa provide an excellent example of this contrast in surface heat flow, showing a difference of some 25 mW m −2 between the Archean craton and younger peripheral units. We investigate two possible contributions to this contrast: (1) a shallow mechanism, essentially geochemical, comprising a difference in crustal heat production between the two terrains, and (2) a deeper mechanism, essentially geodynamical, arising from the existence of a lithospheric root beneath the Archean craton which diverts heat away from the craton into the thinner surrounding lithosphere. A finite element numerical model which explores the interplay between these two mechanisms suggests that a range of combinations of differences in crustal heat production and lithospheric thickness can lead to the contrast in surface heat flow observed in southern Africa. Additional constraints derived from seismological observations of cratonic roots, the correlation of surface heat flow and surface heat production, petrological estimates of the mean heat production in continental crust and constraints on upper mantle temperatures help narrow the range of acceptable models. Successful models suggest that a cratonic root beneath southern Africa extends to depths of 200–400 km. A root in this thickness range can divert enough heat to account for 50–100% of the observed contrast in surface heat flow, the remainder being due to a difference in crustal heat production between the craton and the surrounding mobile belts in the range of zero to 0.35 μW m −3 .