Marine Economic Deposits
Petroleum source rocks, phosphate, iron and manganese ores, and gas hydrates commonly have formed in marine environments. Many of these deposits are related to low levels or absence of oxygen in the water column, which occur in three general environments, upwelling and expanded oxygen-minimum zones and stagnant basins. Upwelling zones are regions of high biologic productivity in the oceans. Decay of the large amount of organic material produced in upwelling zones commonly creates low-oxygen conditions (e.g., Summerhayes, 1983; Demaison and Moore, 1980; Parrish, 1982). When water is warm, oxygen solubility is lower and the oxygen-minimum zone can expand. In stagnant basins, anoxia is created by the consumption of oxygen through organic decay, but the process occurs much more slowly than in upwelling zones and extraordinary amounts of organic matter are not required. Phosphate may be restricted to high-productivity environments, such as upwelling zones. Petroleum source rocks and manganese can be associated with upwelling, but also can form in low-oxygen conditions in stagnant basins and at times of expanded oxygen-minimum zones. Iron ores resulting from the oxidation of glauconite are indirectly related to low oxygen because glauconite is formed under low-oxygen conditions.
Because upwelling has been important in the formation of many of the economic deposits in the marine realm, a general discussion of upwelling and related conditions is appropriate.
Upwelling zones are regions of high biologic productivity in the oceans. The productivity results when the nutrient-rich waters from below the photic zone are carried continuously upward by vertical currents. The
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Burgeoning interest in paleoclimatology has been spurred by growing awareness of the control of paleoclimates on the formation of economic deposits. In past studies, paleoclimatic patterns were derived empirically from biogeographic patterns, and to a lesser extent, from the distributions of sedimentary paleoclimatic indicators, such as coals. The problems with this approach are numerous. In early studies, the paleoclimatic patterns appeared to make very little sense because they were reconstructed on modern continental positions. Even after the acceptance of continental drift, problems arose when the paleoclimatic indicators were poorly dated or when geologists chose paleoclimatic indicators from too long a time period, during which major paleoclimatic changes could have occurred. More recently, qualitative and quantitative models of paleoclimate have proved useful for understanding the distributions of climatically significant geologic data. These models are founded on basic principles of atmospheric and oceanic circulation as applied to global paleogeography, including reconstructed plate positions. With climate models, geologists can formulate hypotheses about the paleoclimatic patterns that might be expected during the various intervals in Earth history and test those hypotheses with the paleoclimatic indicators.