CHANGING OCEANIC CONDITIONS THROUGH THE LATE ORDOVICIAN GLACIATION INTO THE EARLIEST SILURIAN GLOBAL ANOXIC EVENT: EVIDENCE FROM BLACK SHALES AND NITROGEN ISOTOPES

The spatial and temporal pattern of water column anoxia through the Late Ordovician-early Silurian is reconstructed from the distribution of black shales preserved globally in three time-slices: late Katian, Hirnantian and early Rhuddanian. In the the late Katian, the geographic occurrence of black shales suggests that anoxia developed mainly in response to conditions associated with high global sea level and localized oceanic upwelling or restriction within semi-isolated deep basins, or both. By contrast, black shales became geographically very restricted during the mid-Hirnantian, commonly attributed to the effects of Hirnantian glaciation, such as cooling and increased thermohaline circulation. After the glaciation, in the Rhuddanian, black shales returned over a wide range of paleolatitudes and paleogeographic settings. In many regions anoxic conditions persisted for several million years. The widespread distribution and persistence of anoxic conditions signifies a global oceanic anoxic event (OAE). The cause of the Rhuddanian OAE is uncertain. It may have been initiated by the release of nutrients and fresh water from retreating Gondwana ice sheets together with the more enduring effects of sea level rise associated with oceanic warming and the spread of nutrient-rich waters from the deep oceans into deep shelf regions. These processes may have worked together to trigger the onset of persistent dysoxia-anoxia at both low and high paleolatitudes, aided by enhanced phosphorus recycling and nitrogen replete conditions under anoxic conditions. New nitrogen isotope data indicate that nitrogen-fixing cyanobacteria were key components of the primary producing communities during the black-shale dominated intervals in the Katian and Rhuddanian. This supports the hypothesis that these intervals of high sea level, warm climate and widespread anoxia represented N-limited productivity conditions that differed markedly from those of the Hirnantian glacial interval. In addition, the distribution of black shales during the Rhuddanian OAE suggests that anoxic ocean waters occurred at both low and high paleolatitudes. Downwelling of oxygen deficient water masses in high-latitude regions may have contributed to the greater spread of anoxia through the deep oceans of the world during the Rhuddanian.