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UW OCEAN 400 - Chemical Oceanography

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1Chapter 1: Introduction James W. Murray (9/27/04) Univ. Washington ______________________________________________________________________ Chemical oceanography is the study of everything about the chemistry of the ocean and is based on the distribution and dynamics of elements, isotopes, atoms and molecules. This ranges from fundamental physical, thermodynamic and kinetic chemistry to two-way interactions of ocean chemistry with biological, geological and physical processes. It encompasses both inorganic and organic chemistry. The cornerstones of progress are breakthroughs in analytical chemistry. If this definition seems broad consider that the field also includes study of atmospheric and terrestrial (and some contend extraterrestrial) processes. If oceanography is a wagon wheel, chemical oceanography sits at the middle, and is the most interdisciplinary of all the options. Chemical oceanography includes processes that occur on a wide range of spatial and temporal scales; from global to regional to local to microscopic spatial dimensions and time scales from geological epochs to glacial-interglacial to millennial, decadal, interannual, seasonal, diurnal and all the way to microsecond time scales. Some of the various physical and biological processes and their temporal and spatial scales are summarized in Fig. 1-1 (courtesy of Tommy Dickey, UCSB). Fig. 1-12The advantages of the chemical perspective include: 1. Hugh information potential due to the large number of elements (93), isotopes (260), naturally occurring radioisotopes (78) and compounds (innumerable) present in the ocean. 2. Chemical measurements are specific, reproducibile and predictable (statistically meaningful) for major elements, trace elements, gases and isotopes. Advances in the field have frequently been linked to advances in the analytical chemistry. 3. Quantitative treatments are possible (stoichiometries, balances, predictions of reaction rates and extents, age estimates, paleotemperatures). All the normal rules of chemistry apply. 4. Chemical changes reflect controlling processes and the integrated net effect of multiple previous events. This is important because most of the ocean is inaccessible to direct observation, and past environmental records are preserved in marine sediments. Chemical Oceanography is the most interdisciplinary of all the sub-disciplines of this interdisciplinary science. - Chemical components of the ocean influence the density of seawater and thus effect its circulation. - Oceanic-crustal coupling control the distribution of major ions (> 1 mg kg-1) in seawater on time scales of 104 to106 years. Thus, we can about weathering and other crustal processes from ocean chemistry. - Biological processes in the ocean are controlled by the chemistry. At the same time biological processes are an important control on chemical distributions. The synergy between biology and chemistry has led to a whole new thriving subdiscipline called Biogeochemistry. - Chemical components are tracers of physical, biological, geological and chemical processes that make up the ocean that we observe. Understanding what controls chemical distributions helps us understand ocean dynamics. - The Oceans are the ultimate reservoir of anthropogenic chemical perturbations. - Chemical components of marine sediments and glacial ice provide clues necessary to unravel the history of past ocean chemistry and ocean-atmosphere dynamics. Understanding the past should help us predict the future.3How and why is this field relevant? As the world enters the 21st century there will increased focus on cycling of carbon. We are all aware that CO2 and other greenhouse gases are increasing in the atmosphere. There is no better example to show than the classic data from C.D. Keeling (1976) showing the seasonal oscillations and the steady annual increase of CO2 at the Mauna Loa Observatory (Fig. 1-2). Most experts conclude we are already witnessing the impact of this increase as global warming and the signal is expected to become increasingly more pronounced between now and the year 2050. This is within the lifetime of students reading this book. Thus, an important focus of education in chemical oceanography should be on the concentrations of carbon species and their controlling mechanisms. Fig. 1-2 Mauna Loa CO2 The ocean carbon cycle influences atmospheric CO2 via changes in the net air-sea CO2 flux that are driven by differences in the partial pressure of CO2, PCO2 , between the surface ocean and atmosphere. The inventory of dissolved CO2 in the oceans is 50-60 times greater than that in the atmosphere, so a small perturbation of the ocean carbon cycle can result in a substantial change in the concentration of CO2 in the atmosphere. The regional and vertical partitioning of carbon in the ocean is dominated by two interdependent carbon pumps that deplete the surface ocean of total CO2 relative to deep water. Because the solubility of CO2 increases with decreasing temperature, the SOLUBILITY PUMP transfers CO2 to the deep sea during formation of cold deep water at high latitudes. This is a link of the ocean carbon cycle to physical processes. At the same time the BIOLOGICAL PUMP removes carbon from surface waters by settling of organic and inorganic carbon derived from biological production. The biological pump transports organic carbon and CaCO3 to deeper water where much if it is dissolved or remineralized increasing the total CO2 reservoir in the deep sea that is isolated from the atmosphere. If there was no biological pump, atmospheric CO2 would be about three4times higher than it is today. This is a link of the ocean carbon cycle to biological processes. An axiom of geology is that the present is the key to the past. In oceanography we also ask the slightly different question: Is the past the key to the future? We know that in the geological past there have been big excursions in atmospheric chemistry and the earth's temperature. There was a natural variability in climate in the past that has extended to the present. How can we correctly predict the effects of global warming if we do not understand this natural variability? The immediate fundamental questions that need answering are: • What has happened to the carbon dioxide that has been emitted by human activities? • How will atmospheric carbon dioxide concentrations evolve in the future? To answer these questions we need a sound mechanistic


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