The Relative Impact of Regional Scale Land Cover Change and Increasing CO_2 over China

A series of 17-yr equilibrium simulations using the NCAR CCM3 (T42 resolution) were performed to investigate the regional scale impacts of land cover change and increasing CO2 over China. Simulations with natural and current land cover at CO2 levels of 280, 355, 430, and 505 ppmv were conducted. Results show statistically significant changes in major climate fields (e.g. temperature and surface wind speed) on a 15-yr average following     

A compilation of Australian heat‐flow measurements

A map is presented based on all known Australian heat‐flow estimates, including five new ones. A second map, based on the first, also is presented, but excludes any determinations judged for any reason to be unreliable. The data show that heat flow over large areas of the continent is effectively uniform to within 0.5 heat flow units (i.e. to within 20 mW m^‐2), with perhaps three major regional heat‐flow provinces being defined in western, central, and eastern Australia.     

DATA ANALYSIS WITH MINIMAL ASSUMPTIONS

The computer has made it possible to scrutinize data rapidly by means of graphics. This should be doneprior to the application of any model to the data, since the model must be validated before using it asa means of analyzing the data. The procedure is illustrated in terms of two examples of real experimentaldata.     

Enriched H2, CH4, and He concentrations in hydrothermal plumes associated with the 1996 Gorda Ridge eruptive event

The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of ³He, H₂, and CH₄ in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low ³He/heat ratio of ∼0.4×10^-12 cm³ STP gm^-1 °C^-1, while chronic plumes had ³He/heat ratios of 2.1–6.2×10^-12 cm³ STP gm^-1 °C^-1. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H₂ concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H₂. The data indicate, however, that maximum rates of H₂ production during event plume formation could have lasted a few days at most.     

The Escanaba Trough, Gorda Ridge hydrothermal system: Temporal stability and subseafloor complexity

The composition and temperature of vent fluids sampled from the active hydrothermal system in Escanaba Trough, Gorda Ridge in 2000 and 2002 remain unchanged from the only time this field was previously sampled, in 1988. ODP Leg 169 drilled nine bore holes at this site in 1996, some within meters of the vents, yet this disturbance has not impacted the measured compositions or temperatures of the fluids exiting at the seafloor. The fluids have maximum measured temperatures of 218°C and contain ∼20% more chloride than local ambient seawater. Our interpretation is that the fluid compositions are generated by supercritical phase separation of seawater, with much of the water-rock reaction occurring within the ∼400m thick sedimentary section that overlies the basalt at this site. The ODP drilling results provide information on the mineralogy and composition of materials below the seafloor, as well as direct constraints not typically available on the physical conditions occurring below the seafloor hydrothermal system. Calculations utilizing geochemical modeling software suggest the fluids are close to saturation with a suite of minerals found subsurface, suggesting equilibrium between the fluids and substrate. These results provide an explanation for why the fluids have remained chemically stable for 14 yrs. The pore water data from drilling suggest that the hydrology and chemistry of the hydrothermal system are much more complex within the sediment cover than would be expected from the surface manifestations of the hydrothermal system. While the pore waters have chloride contents both greater and less than the local seawater, only fluids with higher chloride contents vent at the seafloor. Our calculations suggest that at the current conditions the “brines” (fluids with chlorinity greater than seawater) are actually less dense than the “vapors” (fluids with chlorinity less than seawater). These density relationships may provide an explanation for why the “brines” are now venting preferentially to the “vapors,” a situation opposite to what is usually observed or inferred.     

Magnetometer array studies in India and the lithosphere

Two magnetometer array experiments were conducted in India during 1978–1980, under an Indo-Australian collaboration project, using 21 Australian three-component magnetometers of the Gough-Reitzel type. The first array study was made in the northwestern region covering the Aravallis, the Punjab, and the lesser Himalaya, while the second experiment was carried out in the southern peninsular shield area. Both these sets of geomagnetic deep sounding (GDS) observations yielded valuable results on the crustal and upper mantle structure in the two geologically and geophysically important regions of India.Geomagnetic induction patterns observed in northwest India have revealed a variety of electrical conductivity structures. The primary conductivity structure providing paths for induced currents is found to be striking at right-angles to the Himalayan Mountains. The conductivity structure is indicated to be a northward continuation of the Aravalli belt and, thus, suggesting the continuation of the Indian shield at depth into the base of the Himalayan foothills under the Ganga basin.The induction effects observed in the southern tip of peninsular India are by far the most complex geophysical phenomenon due to the simultaneous occurrence of the sea coast, the crustal and upper mantle conductivity anomalies between India and Sri Lanka under the sea, and the day-time equatorial electrojet as part of the external heterogeneous inducing field. It is further complicated by the existence of a conductive step, structure along the coastline at the Moho boundary and a “graben” structure in the Palk Strait, as revealed by the array observations.     

Electrical conductivity from Australian magnetometer arrays using spatial gradient data

Quiet daily magnetic variations recorded by magnetometer arrays in Australia are analysed to obtain electromagnetic response parameters for two parts of the Australian continent remote from electrical conductivity anomalies. The parameters are based on measurements of vertical-field and horizontal-field spatial gradient, and three different methods are followed in their computation. The response parameters are checked for consistency with a compilation of globally-determined Earth apparent resistivities, and are then interpreted for one-dimensional conductivity structure in the two different parts of the continent. There is evidence that the rise in electrical conductivity from 10^?1 S m^?1 to 10⁰ S m^?1 which occurs at a depth of order 500 km beneath central Australia may occur at a substantially shallower depth of order 230 km beneath southeast Australia.