Fred T. Mackenzie

Atmospheric trace metals: global cycles and assessment of man's impact

Abstract Global data are presented for sources of atmospheric input for 20 trace metals, and the relative importance of natural and anthropogenic sources is assessed. Interference factors are calculated as (total anthropogenic emissions/total natural emissions) × 100. For lithophile metals such as Fe and Mn, interference factors are small. In contrast, the atmophile metals, such as As, Se and Hg, exhibit large interference factors. A significant degree of correlation exists between interference factors and enrichment factors, where enrichment factor is defined as the metal/Al ratio in atmospheric particulates divided by the metal/Al ratio in soils. For many of the trace metals, enrichment factors are of the same order of magnitude at high latitudes in both the Northern and Southern Hemispheres, and are larger at high latitude than at mid latitude. A simple mathematical model is used to calculate present-day enrichment factors in both hemispheres based on natural and anthropogenic influxes, effluxes, and transfer between hemispheres. The calculated enrichment factors are in good agreement with the observed enrichment factors for lithophile metals at both mid and high latitude, and for atmophile metals at mid latitude. However, calculated enrichment factors for atmophile metals are lower than observed enrichment factors at high latitude. To explain these results, we propose that for Hg, As and Se, and perhaps for other atmophile metals, there are significant fluxes from the sea surface to the atmosphere. If the estimated low-temperature fluxes of As, Se and Hg from the land and sea surfaces are included in the interference factor calculations for these metals, the factors are reduced to less than 100%.

Interactions of C, N, P and S Biogeochemical Cycles and Global Change

This is an up-to-date synthesis of the global biogeochemical cycles of C, N, P and S. Processes and models involving the flow of these elements in and between the ocean, atmosphere, biosphere and land are emphasized. Human-induced perturbations to the global cycles are discussed, and the role of these space scales of global change is considered, from the geological past to the present and the future. Feedback mechanisms that enhance or ameliorate global change are an important feature. The book is strongly interdisciplinary in scope.

Impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system

Fossil fuel combustion and agriculture result in atmospheric deposition of 0.8 Tmol/yr reactive sulfur and 2.7 Tmol/yr nitrogen to the coastal and open ocean near major source regions in North America, Europe, and South and East Asia. Atmospheric inputs of dissociation products of strong acids (HNO3 and H2SO4) and bases (NH3) alter surface seawater alkalinity, pH, and inorganic carbon storage. We quantify the biogeochemical impacts by using atmosphere and ocean models. The direct acid/base flux to the ocean is predominately acidic (reducing total alkalinity) in the temperate Northern Hemisphere and alkaline in the tropics because of ammonia inputs. However, because most of the excess ammonia is nitrified to nitrate (NO3−) in the upper ocean, the effective net atmospheric input is acidic almost everywhere. The decrease in surface alkalinity drives a net air–sea efflux of CO2, reducing surface dissolved inorganic carbon (DIC); the alkalinity and DIC changes mostly offset each other, and the decline in surface pH is small. Additional impacts arise from nitrogen fertilization, leading to elevated primary production and biological DIC drawdown that reverses in some places the sign of the surface pH and air–sea CO2 flux perturbations. On a global scale, the alterations in surface water chemistry from anthropogenic nitrogen and sulfur deposition are a few percent of the acidification and DIC increases due to the oceanic uptake of anthropogenic CO2. However, the impacts are more substantial in coastal waters, where the ecosystem responses to ocean acidification could have the most severe implications for mankind.

Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions—II. Applications

Abstract Equilibrium relations among common rock-forming minerals and aqueous solutions over a range of temperatures and pressures are known experimentally for a number of systems and can be calculated for others. This information permits prediction of the mass transfer involved in chemical reactions characteristic of geochemical processes. Calculations of this kind are used to examine various chemical and geologic implications of irreversibility in idealized models of weathering, evaporative concentration, diagenesis, hydrothermal rock alteration, and ore deposition.

Tectonic controls of Phanerozoic sedimentary rock cycling

The cyclic nature of the Phanerozoic sedimentary rock distribution, carbon-sulphur coupling, and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors. The distribution of preserved sedimentary mass in terms of rock mass remaining U. geologic age shows a minimum c. 300-350 Ma ago, which separates two subcycles of erosion and deposition of sedimentary rocks. The older subcycle was interrupted because of the major continental collisions of the Devonian and late Carboniferous. These collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses, leading to a decline in the probability of destruction and an increase in half-life of these older sediments. A strong correlation exists between the long-term cyclicity in the Phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs. This correlation is related to two principal tectonic modes of the Phanerozoic: oscillatory and submergent. It is postulated that the submergent mode of active plate convergence, obduction and subduction of sediments, large ridge volume, and high sea level gave rise to low erosion and sedimentation rates, less restricted environments of carbonate deposition, and relatively high atmospheric CO2 levels (high temperatures ?), resulting from an increased rate of production of CO2 from diagenetic and metamorphic reactions at subduction zones. As sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones, whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir. As sea level fell, reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors. These reservoir transfers are consistent with secular changes observed in the distribution of δ13C and δ34S in Sedimentary materials during the Phanerozoic. Petrographic examination of Phanerozoic oolite formations shows that öoids with preserved calcitic relict textures are characteristic of pre-Carboniferous carbonate rocks, whereas öoids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age. These changes in öoid mineralogy may be interpreted as reflecting changes in CO2 levels of the ocean-atmosphere system consistent with the above tectonic considerations. Atmospheric CO2 levels were higher prior to Carboniferous time, favouring formation of calcitic öoids and skeletal parts; after the Carboniferous, CO2 levels fell and aragonite and Mg-calcites of greater than 8 mol % Mg increased in abundance as precipitates.

Pleistocene History of Bermuda

The Pleistocene rocks of Bermuda consist of shallow-water, beach, and intertidal marine biocalcarenites; eolianites; and red soils, displaying complex facies relationships. Eolianites grade laterally into marine biocalcarenites, indicating deposition during times of high sea level, or interglacial episodes. During glacial times, red soils formed over the present area of Bermuda. Because no evidence for Pleistocene tectonism has been discerned, the Bermuda Islands may represent a good “tide gauge” for the assessment of Pleistocene eustasy. The determination of Bermudian stratigraphy requires recognition of environments of deposition and interpretation of the subsequent diagenetic history of the various units. Bermudian marine biocalcarenites can be distinguished from eolianites. The two red soils represent solutional unconformities, during which time fresh-water percolation induced diagenesis in the underlying rocks. Bermudian calcarenites can be divided into five diagenetic grades, representing increasing intensity of fresh-water alteration. Fossil land snails provide some criteria for the recognition of units. Radiochemical dates are available for most formations and allow a quantitative assessment of Pleistocene eustasy for the last 200,000 years. Bermudian stratigraphy records sea-level fluctuations and can only be interpreted rationally as the result of Pleistocene eustasy.

Experimental Study of Igneous and Sedimentary Apatite Dissolution: Control of pH, Distance from Equilibrium, and Temperature on Dissolution Rates

Apatite dissolution experiments were conducted using both a fluidized bed and stirred tank reactor over a range of pH, temperature, solution saturation state, and on non-carbonated and carbonated apatite compositions: igneous fluorapatite (FAP) and sedimentary carbonate fluorapatite (CFA), respectively. From 2 8.5 were not possible due to detection limits of the analytical techniques used in this study and the high insolubility of FAP. For the CFA compositions studied, the dissolution rate decreased with increasing pH from 4 < pH < 7. During early stages of the dissolution reaction for both FAP and CFA, mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P and Ca:F being greater than mineral stoichiometric ratios, suggesting that Ca was preferentially released compared to P and F from the mineral structure during the early stages of dissolution. An increase in reacted solution pH accompanies this early elevated release of Ca. As the dissolution reaction proceeded to steady state, dissolution became congruent. When normalized to BET measured surface area, FAP dissolved faster from 4 < pH < 7 compared to CFA. The apparent Arrhenius activation energy (Ea) of FAP dissolution over the temperature range of 25–55°C at pH = 3.0, I = 0.1, and pCO2 = 0 is 8.3 ± 0.2 kcal mol−1. Both the apparent exchange of solution H+ for solid-bound Ca at low pH in the early stage of dissolution and the Ea of dissolution suggest a surface and not a diffusion controlled dissolution reaction for FAP and CFA. The degree of undersaturation of the solution, ΔGR, with respect to FAP was important in determining the dissolution rate. At pH = 3.0, I = 0.1, and pCO2 = 0, the dissolution rate of FAP was ∼ 5× greater in the far-from-equilibrium region compared to the near-equilibrium slope region. A simple apatite weathering model incorporating the experimental results from this study was constructed, and numerical calculations suggest that during the Phanerozoic both the surface area of igneous rock available for weathering and the average global temperature were important factors in determining the P weathering flux from apatite dissolution. It is possible that elevated global temperatures coupled with relatively high surface area of igneous rock during the early- to mid-Paleozoic resulted in elevated P weathering fluxes, which along with climatic evolutionary pressures of the Neoproterozoic, facilitated the radiation of multicellular organisms, large-scale phosphorite deposition, and abundance of calcium phosphate shelled organisms during the early Cambrian.

Stabilities of synthetic magnesian calcites in aqueous solution: Comparison with biogenic materials

Abstract Free-drift dissolution data and inverse time plots were used to evaluate the stabilities of synthetic and biogenic magnesian calcites in aqueous solutions at 25°C and 1 atm total pressure. Synthetic phases with MgCO 3 concentrations below 6 mole percent have stoichiometric ion activity products that are less than the value for calcite, whereas the values for phases with higher concentrations are greater than that of calcite. For synthetic phases, stability is a smooth function of composition and all phases (up to 15 mole percent MgCO 3 ) have values of ion activity products less than that for aragonite. These results agree with those of Mucci and Morse (1984) derived from precipitation of magnesian calcites in aqueous solutions. “Average” seawater at 25° and 1 atm total pressure is supersaturated with respect to all synthetic phases in the compositional range studied. Biogenic samples are less stable than synthetic phases of similar Mg concentrations and stability is not a smooth function of composition. Biogenic materials with compositions greater than 11–13 mole percent MgCO 3 have ion activity products greater than that for aragonite. The difference in stability between biogenic materials and synthetic phases is due to greater variation in chemical and physical heterogeneities found for the biogenic samples. If it is assumed that the results of the dissolution experiments reflect only differences in Gibbs free energies of formation between synthetic phases and biogenic materials of similar Mg concentration, the biogenic materials are 200–850 j/mol less stable than the synthetic phases. Only the results of synthetic dissolution experiments should be used to model the thermodynamic behavior of the magnesian calcite solid solution. The results for the synthetic phases, however, may not be appropriate to use for interpreting diagenetic reaction pathways for magnesian calcites in modern sediments, except as a basis of comparison with the behavior of natural materials.

Influence of the human perturbation on carbon, nitrogen, and oxygen biogeochemical cycles in the global coastal ocean

Abstract The responses to human perturbations of the biogeochemical cycles of carbon (C), nitrogen (N), and oxygen (O) in the global coastal ocean were evaluated using a process-based model. In this model, the global coastal ocean is represented by two distinct zones: the proximal zone which includes large bays, the open water part of estuaries, deltas, inland seas, and salt marshes; and the distal zone which includes the open continental shelves down to a depth of 200 m. The biogeochemical model of the elemental cycles in the coastal ocean describes the dynamics of transfer processes and their interactions (primary production, mineralization, sediment deposition, and burial). The coastal sediment submodel describes element recycling in the sediments and allows the assessment of the evolution of denitrification under various environmental forcings. Initial values for the biogeochemical fluxes of carbon, nitrogen, and oxygen between the various reservoirs of the coastal ocean and the exchange of materials at the boundaries with the rivers and the open ocean are estimated based on current literature data. Before anthropogenic activities, the global coastal ocean was a net autotrophic system with a net export flux to sediments and open ocean of 20 Tmol organic C/yr. The magnitude and direction of this net flux is strongly dependent on the flux of organic carbon from the coastal ocean to adjacent reservoirs. The proximal coastal region was slightly heterotrophic at 8.4 Tmol C/yr, consistent with estimates by other authors; the distal coastal ocean was autotrophic at the rate of 28.4 Tmol C/yr. To simulate the evolution of the coastal ocean under human influence during the past 50 yr, the global coastal ocean model was perturbed by increasing the global riverine fluxes of dissolved inorganic nitrogen (DIN) and total organic matter (TOM) and the depositional flux of atmospheric nitrogen relative to the preanthropogenic condition. Model results show that over the past 50 yr, primary production in the coastal ocean has doubled, and resulted in an accumulation of biomass in all compartments of the system. The coastal ocean became more heterotrophic in response to the dominant perturbation of the increased flux of terrestrial organic matter via the rivers. Contrary to expectations, denitrification rates do not increase and the denitrification efficiency of the coastal ocean system decreases. This suggests that the coastal ocean is not likely to self-regulate the effects of human-induced perturbations on nutrient cycles. Finally, simulation results for a future condition where the rates of riverine organic and inorganic C and N inputs to the coastal ocean continue to increase at their current exponential growth patterns indicate that the proximal coastal ocean could become increasingly heterotrophic, the organic matter content could increase, and primary production could be enhanced. These changes could potentially cause noxious blooms to occur and become a generalized phenomenon of the proximal coastal ocean. There is a strong likelihood that unless human-derived inputs are regulated at the source, substantial biogeochemical modification of the global coastal ocean will occur because cyclic processes within the coastal ocean system are not rapid enough to dissipate the effects of the perturbations.

Century-scale nitrogen and phosphorus controls of the carbon cycle

Abstract In recent decades, humans have become a very important force in the Earth system, demonstrating that emissions (gaseous, liquid, and solid) are the cause of many of our environmental issues. These emissions are responsible for major global reorganizations of the biogeochemical cycles. The oceans are now a net sink of atmospheric CO2, whereas in their preindustrial state they were a source; the trophic state of the coastal oceans is progressively moving toward increased heterotrophy; and the terrestrial realm is now vacillating between trophic states, whereas in preindustrial times it was autotrophic. In this paper, we present model calculations that underscore the role of human-induced perturbations in changing Earths climate, specifically the role of anthropogenic nitrogen and phosphorus in controlling processes in the global carbon cycle since the year 1850 with projections to the year 2035. Our studies show that since the late 1940s emissions of nitrogen and phosphorus have been sequestered in the terrestrial living phytomass and groundwater. This nutrient-enhanced fertilization of terrestrial biota, coupled with rising atmospheric CO2 and global temperature, has induced a sink of anthropogenic CO2 that roughly balances the emission of CO2 owing to land use change. In the year 2000, for example, the model-calculated terrestrial biotic sink was 1730 Mtons C/year, while the emission of CO2 from changes in land use was 1820 Mtons C/year, a net flux of 90 Mtons C/year emitted to the atmosphere. In the global aquatic environment, enhanced terrestrial inputs of biotically reactive phosphorus (about 8.5 Mtons P/year) and inorganic nitrogen (about 54 Mtons N/year), have induced increased new production and burial of organic carbon in marine sediments, which is a small sink of anthropogenic CO2. It is predicted that the response of the global land reservoirs of C, N, and P to sustained anthropogenic perturbations will be maintained in the same direction of change over the range of projected scenarios of global population increase and temperature change for the next 35 years. The magnitude of change is significantly larger when the global temperature increase is maximum, especially with respect to the processes of remobilization of the biotically important nutrients nitrogen and phosphorus.