Aquatic chemistry is becoming both a rewarding and substantial area of inquiry and is drawing many prominent scientists to its fold. Its literature has changed from a compilation of compositional tables to studies of the chemical reactions occurring within the aquatic environments. But more than this is the recognition that human society in part is determining the nature of aquatic systems. Since rivers deliver to the world ocean most of its dissolved and particulate components, the interactions of these two sets of waters determine the vitality of our coastal waters. This significant vol ume provides not only an introduction to the dynamics of aquatic chem istries but also identifies those materials that jeopardize the resources of both the marine and fluvial domains. Its very title provides its emphasis but clearly not its breadth in considering natural processes. The book will be of great value to those environmental scientists who are dedicated to keeping the resources of the hydrosphere renewable. As the size of the world population becomes larger in the near future and as the uses of materials and energy show parallel increases, the rivers and oceans must be considered as a resource to accept some of the wastes of society. The ability of these waters and the sediments below them to accommodate wastes must be assessed continually. The key questions relate to the capacities of aqueous systems to carry one or more pollutants."

Metal pollution in the aquatic environment

The anoxic aquatic environment is a mass of water so depleted in oxygen that virtually all aerobic biologic activity has ceased. Anoxic conditions occur where the demand for oxygen in the water column exceeds the supply. Oxygen demand relates to surface biologic productivity, whereas oxygen supply largely depends on water circulation, which is governed by global climatic patterns and the Coriolis force. Organic matter in sediments below anoxic water is commonly more abundant and more lipid-rich than under oxygenated water mainly because of the absence of benthonic scavenging. The specific cause for preferential lipid enrichment probably relates to the biochemistry of anaerobic bacterial activity. Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows. 1. Large anoxic lakes: Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition. Conversely, lakes in temperate climates tend to be well oxygenated. 2. Anoxic silled basins: Only those landlocked silled basins with positive water balance tend to become anoxic. Typical are the Baltic and Black Seas. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Anoxic conditions in silled basins on oceanic shelves also depend upon overall climatic and water-circulation patterns. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, both at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds. 3. Anoxic layers caused by upwelling: These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is often sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression. 4. Open-ocean anoxic layers: These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a reduced scale, to worldwide "oceanic anoxic events" which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia. Geochemistry, assisted by paleogeography, can greatly help petroleum exploration by identifying paleoanoxic events and therefore widespread oil source bed systems in the stratigraphic record. Recognition of the proposed anoxic models in ancient sedimentary basins should help in regional stratigraphic mapping of oil shale and oil source beds.

Anoxic Environments and Oil Source Bed Genesis

[1] Sedimentary molybdenum, [Mo]s, has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic-rich marine facies deposited under oxygen-depleted conditions A detailed analysis of [Mo]s–total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo]s, while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo]s-TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction In the silled-basin environments, increasing restriction is correlated with a systematic decrease in [Mo]s/TOC ratios, from ∼45 ± 5 for Saanich Inlet to ∼45 ± 1 for the Black Sea This variation reflects control of [Mo]s by [Mo]aq, which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the “basin reservoir effect”) The temporal dynamics of this process are revealed by high-resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long-term trends toward lower [Mo]s/TOC ratios following development of water column stratification and deepwater anoxia Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo]aq availability and enhanced Mo transport to the sediment-water interface via Fe-Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (ie, more sulfidic conditions) These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (eg, silled basins) and are not valid for low-oxygen facies in open marine settings such as continent-margin upwelling systems

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H.H. Veeh

Papers

Accumulation of uranium in sediments and phosphorites on the South West African shelf