Abstract
Black shales, perhaps owing to their seemingly homogeneous nature, are mainly interpreted on the basis of their unusual geochemical characteristics. Their physical record is mostly neglected as a source of information. It has now been amply demonstrated, experimentally and analytically, that many black shales were deposited by physical processes comparable
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to those that shape coarser grained sediments, i.e. transport and deposition by currents may play a role as well. Black shales are not necessarily the product of sedimentation in quiet, stagnant water and, therefore, the a priori interpretation of such conditions is not valid. These lithologies are more varied than their generic name implies and, in addition to geochemical data, their physical record is an important source of independent information. Black shales are common sedimentary products during periods associated with supercontinent breakup, large-scale volcanic activity and high sea levels. Such periods are associated with greenhouse climates, enhanced nutrient supply to seawater, both a result of volcanism and its byproducts, and extensive shelves. Thus, the overall tendency of marine sediments, in particular in relatively shallow water settings, to become enriched in organic matter during specific periods in Earth’s history, may be ascribed to global forcing mechanisms. These periods are referred to as oceanic anoxic events—not implying, however, global ocean anoxia. It is often asserted that the deposition of black shales during oceanic anoxic events was global and synchronous. The geographical distribution of black shales shows this not to be the case. Storage of organic carbon in geographically definite areas of the seafloor has, nevertheless, led to perturbations in the global carbon cycle, which, in turn, have been recorded in sediments globally as excursions in the carbon isotope record irrespective of lithology. Black shale deposition was regionally favoured in basins with physiographies and hydrological regimes that promoted organic productivity and organic matter preservation, i.e. vigorous overturning circulation, estuarine circulation patterns and enhanced Ekman-induced upwelling. Whether marine sediments are enriched in organic matter or not, regardless of global and regional forcing mechanisms, ultimately depends on local sedimentary processes. These control the type and proportion of sedimentary components in the final sediment and, therefore, its organic richness and its petrological characteristics. Different combinations of these processes may lead to very different sedimentary products. Cretaceous oceanic red beds, although a different sedimentary product, are similarly largely dependent on local processes. The global mechanisms that promoted their deposition in specific time intervals remain enigmatic, but the laterally and vertically diachronous rubefaction of different lithologies in marine sedimentary sequences shows that—if synchronous global mechanisms exist that can force the system towards the full oxidation of marine sediments—the local response can nonetheless be diachronous. Thus, palaeoceanographical interpretations and correlations based on the appearance of such rubefaction in a sedimentary sequence do not allow inferences regarding the exact timing of the global mechanisms behind their formation. The products of marine sedimentation at any given location are the result of a complex interplay of global, regional and local factors and processes. Even though global and regional forcing mechanisms may explain the temporal distribution of black shales and red beds, their geographical distribution and physical characteristics underline the importance of local processes
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