Global Change and the Earth System

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94868/1/eost14816.pd

Mantle devolatilization and convection: Implications for the thermal history of the Earth

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95357/1/grl3652.pd

Thermal stresses at the oceanic-continental margin

Dissimilar temperature profiles beneath oceans and continents give rise to thermoelastic stresses at and adjacent to the oceanic-continental margin. Computations of the magnitude and orientation of the maximum shear stress field reveal a zone of shear dipping beneath the continent from the margin.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33354/1/0000752.pd

On the use of the volumetric thermal expansion coefficient in models of ocean floor topography

The use of the volumetric thermal expansion coefficient, instead of the linear coefficient, in successful models of ocean floor topography implies that the elastic rigidity of the lithosphere relaxes, enabling isostasy to be achieved. However, the presence of a thin elastic lid in the lithosphere, inferred from gravity investigations, implies some rigidity at the top of the lithospheric column and suggests that the volumetric thermal expansion coefficient derived from rheologically uniform models of the topography is about 15% too small.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23262/1/0000196.pd

Climate change inferred from borehole temperatures

Temperature changes at the Earth's surface propagate downward into the subsurface and impart a thermal signature to the rocks that can be analyzed to yield a surface temperature history over the past few centuries. Thus subsurface temperatures have the potential to extend the 20th century meteorologic temperature record back well into the pre-industrial era and therefore to provide information relevant to an assessment of the role of greenhouse gases in atmospheric warming. Short period variations in surface temperature are attenuated at shallow depths, whereas longer period excursions propagate deeper. The ability to resolve details of the surface temperature history diminishes with time. Care must be taken to identify and evaluate local anthropogenic temperature perturbations such as urbanization, deforestation and wetland destruction and microclimatic effects associated with topography and vegetation patterns, in order to isolate true regional climate change. Investigations in North America indicate significant regional variability in the surface temperature history inferred from borehole profiles, similar to that observed in the meteorologic record of the 20th century.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30810/1/0000468.pd

Cratonization and thermal evolution of the mantle

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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26023/1/0000095.pd

Temperature trends ever the past five centuries reconstructed from borehole temperatures

For an accurate assessment of the relative roles of natural variability and anthropogenic influence in the Earth's 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(1-5), 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(6), but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies(2-5). 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62610/1/403756a0.pd

Variable seasonal coupling between air and ground temperatures: A simple representation in terms of subsurface thermal diffusivity

The utility of subsurface temperatures as indicators of temperature changes at Earth's surface rests upon an assumption of strong coupling between surface air temperature (SAT) and ground surface temperature (GST). Here we describe a simple representation of this coupling in terms of a variable thermal diffusivity in the upper meter of the subsurface. The variability is tied to daily SAT, precipitation, and snow cover, but does not incorporate the physical details of these and the many other factors that influence the air-ground interface in many high-fidelity land-surface models. Our simple model reduces the difference between observed and modeled temperatures by a factor of 3 to 4 over a model with uniform diffusivity driven only by SAT. This simple representation of air-ground coupling offers a means of simulating subsurface temperatures using only archived meteorological records and creates the potential for examining the long term character of air-ground temperature coupling

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

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 [mu]W m-3.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26589/1/0000130.pd

Modern and ancient geotherms beneath southern Africa

Estimates of subsurface temperatures in the Archean craton of southern Africa during the Archean derived from diamond thermobarometry studies are remarkably similar to temperatures estimated for the same depths today, even though heat production in the earth and the mean global heat flow were probably substantially higher in the Archean. We present multi-dimensional numerical models for the thermal environment of the Archean craton in southern Africa during the Archean in which deep mantle heat is diverted away from the craton toward the surrounding oceanic lithosphere by a lithospheric root beneath the craton. Extrapolation of present-day models to thermal conditions appropriate for the Archean is inadequate to explain the similarity of present-day and Archean temperatures in the cratonic root. Reconciliation of the modern and ancient temperature estimates requires either relaxation of the constraints that the cratonic crustal heat production and/or the earth's mean mantle temperature were higher in the Archean than they are today, or that substantial "erosion" of the lithosphere comprising the cratonic root has occurred since the Archean. The latter possibility could perhaps result from revolatilization of the cratonic root in association with thermal perturbations in the mantle, for which there is evidence in southern Africa in the form of post-Archean tectonic and igneous activity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27346/1/0000371.pd