Atmospheric response to deep-sea injections of fossil-fuel carbon dioxide

The possibility of controlling atmospheric carbon dioxide accumulation and attendant climatic effects from fossil-fuel burning by diverting a fraction of the combustion product and injecting it into the deep-ocean, as proposed by Marchetti, is analyzed using an atmosphere/mixed layer/diffusive deep-ocean model for the carbon cycle. The model includes the nonlinear buffering of CO₂ at the air/sea interface, and considers the long term trends associated with consuming an assumed fossil-fuel reserve equivalent to 7.09 × 10¹⁵ kg carbon as a logistic function of time as in the projections of Siegenthaler and Oeschger, except that atmospheric carbon dioxide levels are computed for five alternate strategies: (a) 100% injected into atmosphere, (b) 50% injected at oceanic depth of 1500 m and 50% into atmosphere, (c) 50% injected at sea floor (4000 m) and 50% into atmosphere, (d) 100% at 1500 m depth and (e) 100% at sea floor. Since no carbon leaves the system, all runs approached the same post-fossil fuel equilibrium after several thousand years, C _a - 1150 ppm, almost four times the pre-fossil fuel value (- 300 ppm). But the transient response of these cases showed a marked variation ranging from a peak overshoot value of 2800 ppm in the year 2130 for 100% atmospheric injection to a slight decrease to the pre-fossil fuel 300 ppm lasting till 2300 with a subsequent slow approach to equilibrium for the 100% deep-ocean injection. The implications of these results for an oceanic injection strategy to mitigate the climatic impact of fossil-fuel CO₂ is discussed, as are the ingredients of a second generation carbon cycle model for carrying out such forecasts on an engineering design basis.     

A geochemical study of magmatism associated with the initial stages of back-arc spreading

Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading.Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with Ce_N/Yb_N = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although Ce_N/Yb_N ratios and ⁸⁷Sr/⁸⁶Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher ⁸⁷Sr/⁸⁶Sr (0.7039) and Ce_N/ Yb_N (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present ⁸⁷Sr/⁸⁶Sr ratios.Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.     

Finite-element analysis of the transport of water and solutes in title-drained soils

The finite-element method based on a Galerkin technique was used to formulate the problem of simulating the two-dimensional (cross-sectional) transient movement of water and solute in saturated or partially saturated nonuniform porous media. The numerical model utilizes linear triangular elements. Nonreactive, as well as reactive solutes whose behaviour can be described by a distribution coefficient or first-order reaction term were considered. The flow portion of the model was tested by comparison of the model results with experimental and finite-difference results for transient flow in an unsaturated sand column and the solute transport portion of the model was tested by comparison with analytical solution results. The model was applied to a hypothetical case involving movement of water and solutes in tile-drained soils. The simulation results showed the development of distinct solute leaching patterns in the soil as drainage proceeded. Although applied to a tile drainage problem in this study, the model should be equally useful in the study of a wide range of two-dimensional water and solute migration problems.     

Epistemology and conceptions of people and nature in geography

The paper attempts to integrate two major issues of debate in geography which are conventionally discussed separately: epistemology and conceptions of the relationship between people and Nature. Whichever issue we start with, two concepts are essential: intersubjectivity and labour. Knowledge of natural objects is tied to labour, which both subjects ideas to a kind of test and changes our position within Nature. All knowledge depends upon intersubjective understanding between people, but this implies that in social science, and hence in human geography, the traditional subject-object relationship should be recast as a form of communication between ‘knowing-subjects’. These relationships are not limited to some separate sphere of epistemology but are constitutive of society in Nature itself. Labour, though traditionally overlooked in geography, is the most active and transformative process in changing Nature and society, and intersubjectivity is an essential element of the irreducibly social character of human life. In this attempt, the paper outlines and justifies a historical materialist perspective through critical engagement with positivist and humanist approaches. It is then argued that false conceptions of people and Nature in geography are grounded in the structure and selfunderstanding of our own society such that a critique of these ideas becomes a critique of our society.     

Carbonatization and mobility of Ti,Y, and Zr in Ascot Formation metabasalts,SE Quebec

The middle Ordovician Ascot Formation of southeastern Quebec consists of greenschist facies metamorphosed silicic to mafic pyroclastic rocks and lava flows and associated metasediments. Chemical analyses of lavas reveal a preponderance of metarhyolites and metabasalts, together with some porphyritic rocks with intermediate SiO₂ contents. The metabasalts exhibit wide ranges in concentrations of TiO₂ (0.25–2.0 wt.°), Y (9–46 ppm), and Zr (5–135 ppm). The extent of the ranges, and unusual interelement ratios, suggest that the concentrations of these normally immobile elements have been affected by secondary processes.There is a strong correlation between trace-element concentrations and the degree of carbonatization of the metabasalts. Low carbonate rocks are severely depleted in Ti, Y, and Zr whereas high carbonate rocks are depleted in Y and Zr and enriched in Ti. The differing movement of Ti can be explained in terms of variable chemical potential due to the various carbonatization reactions affecting titaniferous phases. Overall mobility of these generally immobile/rd elements is attributed to high CO₂ levels in the fluid phase during metamorphism.Extrapolation of the two alteration trends to a common origin enables one to infer primary concentrations of the trace-elements. Primary inter-element ratios arrived at in this way are compatible with an island-arc origin for the Ascot Formation although TiO₂ concentrations are a little high (1.5 wt/%).     

Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental,Chihuahua, Mexico

The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains 1% K₂O at 60% SiO₂. Trace element abundances of a typical ignimbrite with 73% SiO₂ are Sr 225 ppm, Rb 130 ppm, Y 32 ppm, Th 12 ppm, Zr 200 ppm, and Nb 15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.     

Nontidal circulation and mixing processes in the lower Potomac estuary

An analysis of the vertical structure of nontidal longitudinal currents and salinity in a reach of the lower Potomac River Estuary suggests that values for vertical eddy viscosity and eddy diffusivity scale with water depth H, tidal current amplitude U and bulk Richardson number according to conventional empirical formulas. However, the constant which relates the vertical eddy coefficients under conditions of neutral stability to UH is found to be an order of magnitude less than that expected for tidal conditions. Analyses also suggest that the degree of enhancement of longitudinal dispersion by the shear effect associated with the nontidal currents is a strong function of bulk Richardson number.     

Evaluation of cold-water carbonates as a possible paleoclimatic indicator

The common belief currently shared by many geoscientists concerning the climatic interpretation of limestones is that a warm-water environment is essential. This concept is not necessarily true because the rate and extent of terrigenous sediment dilution, rather than water temperature, is the primary factor determining whether or not a limestone forms at nearshore or continental shelf depths. Because carbonate productivity is lowest in cold climates, however, CaCO₃ abundance and the thickness of carbonate accumulations tend to be least at high latitudes. In this regard present-day continental shelves and beaches offer a poor model for comparing cold-water and warm-water carbonates because of the generally emergent continental tectonic framework, recent eustatic sea-level changes, and the presence of ice caps at the modern poles.Typically, the influence of climate on non-reef continental shelf and beach environments cannot be clearly distinguished by the presence or absence of major taxonomic groups. Faunal diversity and equitability are more sensitive in this regard. The absence of shelf-depth inorganic carbonate precipitates, micrite envelopes, and peloids may also point to the cold-water origin of a rock. Skeletal mineralogy and oxygen isotopes of certain unrecrystallized carbonates may be good paleoclimatic indicators; however, trace elements and physical-textural attributes of the carbonate fraction are probably temperature insensitive.Previous studies of high-latitude continental shelves have concentrated merely on the abundance of calcareous material and there is seemingly a disproportionate amount of information with respect to low-latitude carbonate studies. Further research on cold-water carbonates may open up new avenues for alternative paleoenvironmental and paleoclimatic interpretations.