Palaeozoic black shales: how much should we trust the Recent to reconstruct the Past?

Organic-carbon-rich sediments were widely deposited during multiple intervals of Mesozoic and Palaeozoic time or even earlier; on the contrary, sediments rich in organic carbon are today restricted to small areas along continental margins and have rarely accumulated during the Cenozoic. Global marine deposits document that episodes of accumulation of OC-rich sediments occurred in different regions and at different times.These episodes were linked to climatic and palaeoceanographic perturbations that resulted in massive fluctuations in hydrologic and nutrient cycles and in ocean chemistry and that recurred throughout geologic time.The whole Palaeozoic is punctuated by a profusion of episodes of black shale deposition that represent a common and not unusual sediment for that time. Furthermore, the abundance of organic matter does not, per se, imply black shales. The Palaeozoic, in fact, is also characterized by fossiliferous OC-rich limestones, e.g. the Silurian–Devonian “Orthoceras limestones” bordering northern Gondwana. However, the paucity of survivingPalaeozoic and earlier black shale sections makes it difficult to impossible to recognize the internal structure of global events that are common in younger OC-rich sedimentary sequences. Going ever deeper into the past, in fact, two factors appear playing a more and more fundamental role: preservation and time resolution. OC-rich sediments, either in form of black shales or limestones, do not necessarily reflect periods of elevateddeposition of high organic matter but may paradoxically simply represent times of better organic matter preservation. Then, even well-dated sequences do not offer the high resolutionrecords needed to fully document or delineate short-time processes. In the Palaeozoic the length of individual biozones is generally on the order of millions of years, which is in the same range as third-order sea-level changes. Thus, an important question in Palaeozoic sequences is whether episodes occur at different scales or belong to cycles of diverse order.Also according to this premise, too often was exasperate the use of the uniformitarianism principle in which models or opinions derived from recent examples are simplistically applied to any of the older “timeboxes”. In actuality, physical and biological conditions (e.g., oxygen and CO2) have strongly varied through time. Palaeozoic black shales were clearly deposited in a CO2-dominated setting (see Berner, 1994, 1998), whereas youngerdeposits reflect a lower concentration of the same gas. Again, the nature of primary producers is not yet completely defined for pre-Jurassic production of organic matter.Furthermore, palaeogeographic scenarios reveal completely different worlds in terms of land masses, oceans, palaeolatitudes, etc. According to this, any attempt to model the deposition of OC-rich sediments through the Phanerozoic must necessarily be tuned with all these variables. Another relevant point is that some of the Phanerozoic OC-rich sediments are defined as global events, like the Cretaceous OAE1a and OAE2, but some othersappear to have had a more restricted and even localized significance. These differences require the application of different approaches in search of possible interpretations and perhaps diverse mechanisms leading to the deposition of OC-rich sequences.Finally, many of the most significant black shale episodes in the Palaeozoic strictly match with major crises in the history of life. Understanding what drives global diversity may be used to explain processes, such as mass extinctions, that control diversity and turnover at a variety of geographic and temporal scales.The main issues described here need to be further investigated and are certainly worth answering. The Scientific Communi