Biogeochemical Perturbations

It is also often taken for granted that many of the Earth s subsystems are exposed to free oxygen (O2), leading to a range of one-way reactions of reduced materials (such as organic carbon or metal sulfides) to an oxidized form. As pointed out many times in earlier chapters, the oxidation-reduction status of the planet is the consequence of the dynamic interactions of biogeochemical cycles. As is the case with the acid-base balances, there is considerable sensitivity to perturbations of "redox" conditions, sometimes dramatically as in the case of bodies of water that suddenly become anaerobic because of eutrophication. Another extreme... 

Zinc has only been measured accurately in open ocean by a few investigators [239,604-607]. Few data are available because of very low zinc concentrations in seawater and the ubiquitous sources of zinc contamination. The uncertainty of all zinc measurements prior to these investigations, and the paucity of reliable data since, have left little information for the environmental chemist to unravel the biogeochemical behaviour of zinc or to detect waters perturbed by anthropogenic inputs. 

Considerable evidence exists that human activities have already perturbed parts of the interlinked global biogeochemical cycles of the nutrients, micronutrients, carbon, and O2. Some of these perturbations are the consequence of climate change and others are associated with changes in the rates of input of nutrients and iron to the sea. As... 

Marine chemists have taken increasingly more sophisticated approaches towards modeling seawater composition. The goal of these models is to understand the biogeochemical controls on seawater composition well enough that the effects of future perturbations can be predicted. As described next, the first modeling efforts were based on a series of reactions that were assumed to reach equilibrium the next efforts took a steady-state approach as the composition of seawater was thought to have been relatively constant over time. 

Value of Isotopic Labeling. The experiments described demonstrate the usefulness of stable-isotope additions in understanding trophic interactions and biogeochemical fluxes in whole ecosystems. Using stable isotopes as chemical tracers in natural, undisturbed systems is especially helpful in interpreting the results of perturbation experiments. 

It is important to maintain time series of biogeochemical distributions in the Black Sea. Much is learned about the oceanography of a system when you can watch its response to a perturbation. Two important perturbations we want to continue to watch are climatic forcing and eutrophication. 

In the last 150 years the anthropogenic emission of sulfur has increased dramatically, primarily due to combustion processes [1]. In the 1950s anthropogenic emission surpassed natural emission and the atmospheric sulfur cycle is one of the most perturbed biogeochemical cycles [1,2]. The oceans are the largest natural source of atmospheric sulfur emissions, where sulfur is emitted in a reduced form, predominantly as dimethyl sulfide (DMS) and to a much lesser extent carbonyl sulfide (OCS) and carbon disulfide (CS2) [3]. Ocean emitted DMS and CS2 are initially oxidised to OCS, which diffuses through the troposphere into the stratosphere where further oxidation to sulfur dioxide (SO2), sulfur trioxide (SO3) and finally sulfuric acid (H2SO4) occurs [1-4]. 

The Century biogeochemical model was developed to study the impact of climate and atmospheric perturbations on soil organic matter and ecosystem dynamics. Century is a general model of plant-soil interactions that incorporates simplified representations of key processes relating to carbon assimilation and turnover (Figure 16). 

In this chapter, we examine the validity of the assumption of coordination equilibrium between metals and organic ligands for both natural and analytical conditions. In so doing, we consider several factors the initial distribution of metal and ligand species, the nature of the perturbation, the rates and mechanisms of the reactions involved in the reestablishment of coordination equilibrium, and the rates of competing biogeochemical processes. We do not consider here the issues of catalysis or surface, biological, and photochemical reactions that may be important in natural waters. For discussion of these issues, the reader is referred to other chapters in this volume. 

We have only recently understood the phenomena that control rainwater pH in the natural, unpolluted environment. As pointed out in Section 10.9, these appear to be mainly the cycles of sulfur and nitrogen compounds. A model of the unperturbed system is necessary in order to understand and predict the changes that occur when strong sulfur- and nitrogen-adds are added, as well as to foresee the complex effects that perturbed rainwater pH has on other biogeochemical cycles and ecosystem processes. 

Modeling the response of the oceanic Si inventory to perturbation and consequences for atmospheric CO. -Global Biogeochemical Cycles, 16, 1071,... 

Continuous increase in the global population and its industrial and agricultural activities have created four major perturbations on the coupled system of the biogeochemical cycles of the bioessential elements C, N, P, and S. These changes have led to major alterations in the exchanges of these elements between the land and sea. The perturbations are ... 

Figure 1 Major perturbations on the Earth system over the past 300 years and projections for the future (a) emissions of CO2 and (b) gaseous N and S from fossil-fuel burning and land-use activities (c) application of inorganic N and P in chemical fertilizers to cultivated land (d) loading of highly reactive C, N, and P into rivers and the coastal ocean from municipal sewage and wastewater disposal and (e) rise in mean global temperature of the lower atmosphere relative to 1700. Revised after Ver LM, Mackenzie FT, and Lerman A (1999) Biogeochemical responses of the carbon cycle to natural and human perturbations Past, present, and future. American Journal of Science 299 762-801.

Ver LM, Mackenzie FT, and Lerman A (1999) Biogeochemical responses of the carbon cycle to natural and human perturbations Past, present, and future. American Journal of Science 299 762-801. 

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