Links between early Paleozoic oxygenation and the Great Ordovician Biodiversification Event (GOBE): A review

Abstract

The Great Ordovician Biodiversification Event (GOBE) represents a four-fold increase of genus-level diversity that post-dates the diversification of the ‘Cambrian explosion’ by 40–50 Myr. A major increase in atmospheric oxygen (O2) levels is thought to be a leading cause of the ‘Cambrian explosion’ that lowered metabolic costs associated with skeletal and collagen biomineralization. The cause(s) of the GOBE, however, is (are) less well understood, and may include a cooling climate, increased nutrient availability, higher sea levels and increased ecospace, and further oxygenation of shallow marine environments. Atmospheric O2 levels are difficult to quantitatively estimate, particularly whether oxygenation was a plausible driver of the GOBE, because O2 estimates are hampered in part by the coarse time resolution of redox proxy records (e.g., iron-based geochemical data) and isotope mass balance models. Newly published high-resolution geochemical trends are used to better constrain the timing and degree of oxygenation across the GOBE, which include traditional methods such as stable carbon (δ13C) and sulfur (δ34S) isotope trends to estimate O2 levels on a global scale, and local-scale redox-sensitive proxies such as iodine (I/Ca) and trace metal concentrations and isotopes (e.g., Mo and U). Taken together, geochemical evidence suggests that Ordovician environments became progressively more oxygenated following the end of recurrent anoxic events and extinctions of late Cambrian and Early Ordovician marine faunas. A sluggishly circulating Cambrian global ocean could have maintained an oxycline in the water column that impinged upon the continental shelf, which extended into wide expanses of shallow shelf settings during relative sea level rises that caused positive δ13C and δ34S excursions and mass extinctions. Ordovician oxygenation roughly corresponds to cessation of positive δ13C excursions (i.e., shallow water anoxia) and the major pulses of biodiversification that comprise the GOBE, suggesting that oxygenation was an important driver of Ordovician biodiversity. Anoxic conditions below an oxycline that impinged upon the continental shelf likely persisted until the Devonian when a cooler climate invigorated circulation and ventilated the global ocean, with the possible exception of some isolated basins that were persistently anoxic.