Natural Climate Change:A Geological Perspective

A presentation to the Seminar on Sustainable Development NBA 573, BEE 673 Sage Hall B-11If the 4.6 billion years the earth has been in existence is one day of geologic time, all of recorded earth history corresponds to the last one tenth of one second of that day- hardly enough time to form an opinion of what is going on in the room in which you are sitting, and hardly a good basis for forming a perspective on climate change. Geology provides the needed perspective. From direct evidence (glacial striations, ice-rafted drop stones, changes in pollen, ice cover, and sea level) and indirect evidence (changes in the isotopic composition of sediments and ice, and in the dust content of polar ice) geologists infer that the earth may have been alternatively completely frozen (-50C) and very warm (+50C) in Late Proterozoic time (800-600 million years ago). Thereafter the earth was warmer than present except for "ice ages" in Pennsylvanian and Permian time (~320-250 million years ago) and in the Quaternary (last 4 million years). Before the last decline in global temperature, England had a climate similar to that of Indonesia. Fossils of crocodiles and broad leaf tropical plants are preserved in the Eocene (40 million year-old) clays there. Continental glaciers developed in Antarctica ~34 million years ago and in Greenland ~8 million years ago. Starting about 2 million years ago continental glaciers periodically covered North America and Europe. Each glacial cycle was ~120,000 years in duration: 100,000 years of ice growth (ice-house conditions) and 20,000 years of warmer, interglacial, green-house conditions. The end of each interglacial was abrupt. The last glacial cycle started about 130,000 years ago. Ice reached Ithaca, New York about 45,000 years ago. At its maximum extent the ice reached to Long Island, and there was ~ 1 mile of ice over Ithaca. The ice age ended abruptly ~12,000 years ago and the last ice melted in Canada ~5,500 years ago. Our interglacial has probably been more uniform in temperature than previous interglacials, but non-the-less there have been significant and rapid climate changes within it. It was warmer than present during the Holocene Maximum (7000-4000 years ago) and during the Medieval warm period (1000-1400 AD), but colder that present during the Little Ice Age (1400-1860) when the Dutch skated on their canals and Washington and Jefferson commuted to Washington D.C. by sleigh. Recent geological studies suggest that recent minor climate changes (last 12,000 years) are caused by changes in solar energy output. These natural climate changes provide a context for discussing changes that might be caused by humans and for what can be meant by "sustainable". Of course climate change is real (40 million years ago England had the climate of Indonesia, but in the last few million years it has been subject to ice ages interspersed with interglacials). Human activity may be a factor, but other factors also operate. If we are to take out insurance (by controlling greenhouse gases, for example), we should consider what we wish most to insure against (another ice age or global warming) and how much we are willing to pay. My recommendation is to make commitments deliberately, wait for scientific clarification (which will come quickly), and avoid politicizing science. Objective science will be our best long-term protection

Types of Gas Hydrates in Marine Environments and Their Thermodynamic Characteristics

The hydrates in marine environment can be grouped into two categories,diffusion gas hydrates and vent gas hydrates. The diffusion gas hydrates occur widely in an area where bottom simulation reflector (BSR) was recorded in seismic profiles, and is a thermodynamic equilibrium system of hydrates and water with dissolved methane within gas hydrate stability zone (GHSZ). The hydrates are buried in a distance apart from the seafloor and are characterized by low concentrations. The vent gas hydrates occur in an area where gas vents out of the seafloor. It is a thermodynamic disequilibrium system of hydrate, water and free gas, occurs in a zone that extends from the base of GHSZ to the seafloor, and is characterized by high concentration. Reported evidences show that these two types of hydrates are possibly occurring in the South China Sea

Prediction of thermal conductivity in reservoir rocks using fabric theory

An accurate prediction of the thermal conductivity of reservoir rocks in the subsurface is extremely important for a quantitative analysis of basin thermal history and hydrocarbon maturation. A model for calculating the thermal conductivity of reservoir rocks as a function of mineral composition, porosity, fluid type, and temperature has been developed based on fabric theory and experimental data. The study indicates that thermal conductivities of reservoir rocks are dependent on the volume fraction of components (minerals, porosity, and fluids), the temperature, and the fraction of series elements (FSE) which represents the way that the mineral components aggregate. The sensitivity test of the fabric model shows that quartz is the most sensitive mineral for the thermal conductivity of clastic rocks. The study results indicate that the FSE value is very critical. Different lithologies have different optimum FSE values because of different textures and sedimentary structures. The optimum FSE values are defined as those which result in the least error in the model computation of the thermal conductivity of the rocks. These values are 0.444 for water-saturated clay rocks, 0.498 for water-saturated sandstones, and 0.337 for water-saturated carbonates. Compared with the geometric mean model, the fabric model yields better results for the thermal conductivity, largely because the model parameters can be adjusted to satisfy different lithologies and to minimize the mean errors. The fabric model provides a good approach for estimating paleothermal conductivity in complex rock systems based on the mineral composition and pore fluid saturation of the rocks. © 1994

The geochemical signatures of variable gas venting at gas hydrate sites

Diverse evidence suggests that gas-venting rates at sites of hydrate crystallization are variable in space and time, but the magnitude of these variations has been difficult to quantify. The hydrate crystallization model of Chen and Cathles [J. Geophys. Res. (Solid Earth) 108 (2003)] is used here to analyze 10 years of vent gas chemistry measurements at the Bush Hill hydrate mound and gas-venting site, Green Canyon 185, offshore Louisiana, Gulf of Mexico. The analysis suggests that, at any instant of time, gas vents at variable rates in different gas channels at the same site, and that the compositional differences in these vent gases are nearly as large as can be produced by hydrate crystallization. Almost two orders of magnitude differences in venting rate between individual gas channelways are suggested. Changes in the average vent gas composition over the last 10 years suggest the average venting rate varied by a factor of ∼2 or more over a few years. The average C + C composition of Bush Hill hydrates are leaner than could be crystallized from vent gases sampled over the last decade, indicating that the venting gas flux was slower in the past by a factor of ∼2. This is compatible with geologic generalizations that venting evolves from fast (mud volcano), to intermediate (hydrate crystallization), to slow (carbonate precipitation) if venting organized into more discrete vents with time. © 2004 Elsevier Ltd. All rights reserved. 3

Observational and critical state physics descriptions of long-range flow structures

Using Fracture Seismic methods to map fluid-conducting fracture zones makes it important to understand fracture connectivity over distances greater 10–20 m in the Earth’s upper crust. The principles required for this understanding are developed here from the observations that (1) the spatial variations in crustal porosity are commonly associated with spatial variations in the magnitude of the natural logarithm of crustal permeability, and (2) many parameters, including permeability. have a scale-invariant power law distribution in the crust. The first observation means that crustal permeability has a lognormal distribution that can be described asκ ≈ κ exp0 (α (ϕ −ϕ 0)), where α is the ratio of the standard deviation of ln permeability from its mean to the standard deviation of porosity from its mean. The scale invariance of permeability indicates that αφο = 3 to 4 and that the natural log of permeability has a 1/k pink noise spatial distribution. Combined, these conclusions mean that channelized flow in the upper crust is expected as the distance traversed by flow increases. Locating the most permeable channels using Seismic Fracture methods, while filling in the less permeable parts of the modeled volume with the correct pink noise spatial distribution of permeability, will produce much more realistic models of subsurface flow